Daniel NOCERA
Artificial Photosynthesis
MIT Researchers Inch Toward Photosynthesis in a Beaker
Anne Trafton : Major Discovery from MIT Primed to
Unleash Solar Revolution
Jonathan Fahey : Solar Energy, All Night Long
Daniel Nocera, et al.: US Patent #
6,863,781 : Process for Photocatalysis...
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http://web.mit.edu/newsoffice/2001/nocera.html
( August 30, 2001 )
CAMBRIDGE, Mass. -- In a step toward creating energy from sunlight as plants do, MIT researchers have invented a compound that produces hydrogen gas with the help of a catalyst and a zap of light.
The researchers, Professor Daniel G. Nocera of chemistry and former MIT graduate student Alan F. Heyduk, reported their discovery in the August 31 issue of Science. Creating a molecule to replace a leaf -- essentially, photosynthesis in a beaker -- could provide a cheap, clean future energy source, Professor Nocera said.
"We have been seeking a future alternative fuel source by studying the principles that govern the conversion of photon energy into chemical potential," he said. "Our strategy is to use the energy of sunlight to drive reactants uphill to energy-rich products, thus harnessing the sun's energy to create a renewable energy source in the future."
Splitting chemical bonds
Nocera and Heyduk created a compound based on the metal rhodium. When the rhodium photocatalyst is dissolved in solution, the researchers add to it a hydrogen-containing acid (also called a hydrohalic acid -- one example is hydrochloric acid), and shine light on it.
"In the leaf, sugar and oxygen are energy-rich products. In our beaker, the sought-after fuels are hydrogen and a halogen, produced catalytically from the photochemical splitting of hydrohalic acid," Nocera said.
The structure of the rhodium compound allows it to break the hydrohalic acid's chemical bonds. Hydrogen gas, with a byproduct of bromides and chlorides, is produced. The by-products are chemically trapped and recycled into the reaction.
While not as complete and efficient as photosynthesis, this system comes close to the ideal use of a molecular catalyst as part of a homogeneous reaction for which scientists have been searching for more than three decades.
Plant power
The African violet on your windowsill converts sunlight into a high-energy fuel over and over again in a process so complicated that scientists have yet to come close to duplicating it. Attempts to mimic it so far have not produced an energy conversion process efficient enough to compete with fossil fuels.
However, future generations will require alternatives to limited petroleum-based fuels. If scientists can make hydrogen, it could combine with the oxygen in the air to make water. This is the process that fuel cells use to generate energy. Within fuel cells, which are now being produced by various manufacturers for vehicles and buildings, it would take a photocatalyst and solar energy to start the reaction all over again from water.
In the work reported in Science, the MIT researchers' goal was to trap the photon energy in a structurally well-defined molecule and control the subsequent reactions to convert light into hydrogen. Previously, the closest scientists have come to achieving this goal is to use photocatalysts that are solids, which need massive surface areas.
In a chemical reaction, the trick is to design a system where the energy needed to break a chemical bond is compensated by the absorption of a photon. The payoff: an alternative, clean fuel source produced with the help of sunlight.
"Heyduk and Nocera have taken fundamental ideas of photochemistry and
harnessed them to achieve a long sought-after but elusive goal: molecular-based
photocatalytic production of a useable fuel," writes James K. McCusker,
a chemistry professor at Michigan State University, in a perspective for
Science. "The importance of this work is not the specificity of the catalyst's
performance, but rather that a new door has been opened."
Putting sunlight to work
Sunlight is the perfect energy source -- readily available and free. According to the National Renewable Energy Laboratory of Golden, Colo., the sun bathes us every day with more energy than humans could use in 30 years.
Nocera says that their new process is not perfect, but it is a beginning that may re-ignite solar energy research that has been largely dormant since the 1970s.
"As it stands, we have performed half of the photosynthetic reaction by generating hydrogen. If we can now get the other half of the process to work (getting the halogen), we would have a framework for future energy production," Nocera said.
This work is supported by the National Science Foundation.
Daniel Nocera
... 
http://web.mit.edu/newsoffice/2008/oxygen-0731.html
Major Discovery from MIT Primed to Unleash Solar Revolution
---
Scientists mimic essence of plants' energy storage
system
Anne Trafton,
News Office
( July 31, 2008 )
In a revolutionary leap that could transform solar power from a marginal, boutique alternative into a mainstream energy source, MIT researchers have overcome a major barrier to large-scale solar power: storing energy for use when the sun doesn't shine.
Until now, solar power has been a daytime-only energy source, because storing extra solar energy for later use is prohibitively expensive and grossly inefficient. With today's announcement, MIT researchers have hit upon a simple, inexpensive, highly efficient process for storing solar energy.
Requiring nothing but abundant, non-toxic natural materials, this discovery could unlock the most potent, carbon-free energy source of all: the sun. "This is the nirvana of what we've been talking about for years," said MIT's Daniel Nocera, the Henry Dreyfus Professor of Energy at MIT and senior author of a paper describing the work in the July 31 issue of Science. "Solar power has always been a limited, far-off solution. Now we can seriously think about solar power as unlimited and soon."
Inspired by the photosynthesis performed by plants, Nocera and Matthew Kanan, a postdoctoral fellow in Nocera's lab, have developed an unprecedented process that will allow the sun's energy to be used to split water into hydrogen and oxygen gases. Later, the oxygen and hydrogen may be recombined inside a fuel cell, creating carbon-free electricity to power your house or your electric car, day or night.
The key component in Nocera and Kanan's new process is a new catalyst that produces oxygen gas from water; another catalyst produces valuable hydrogen gas. The new catalyst consists of cobalt metal, phosphate and an electrode, placed in water. When electricity -- whether from a photovoltaic cell, a wind turbine or any other source -- runs through the electrode, the cobalt and phosphate form a thin film on the electrode, and oxygen gas is produced.
Combined with another catalyst, such as platinum, that can produce hydrogen gas from water, the system can duplicate the water splitting reaction that occurs during photosynthesis.
The new catalyst works at room temperature, in neutral pH water, and
it's easy to set up, Nocera said. "That's why I know this is going to work.
It's so easy to implement," he said.
'Giant leap' for clean energy
Sunlight has the greatest potential of any power source to solve the world's energy problems, said Nocera. In one hour, enough sunlight strikes the Earth to provide the entire planet's energy needs for one year.
James Barber, a leader in the study of photosynthesis who was not involved in this research, called the discovery by Nocera and Kanan a "giant leap" toward generating clean, carbon-free energy on a massive scale.
"This is a major discovery with enormous implications for the future
prosperity of humankind," said Barber, the Ernst Chain Professor of Biochemistry
at Imperial College London. "The importance of their discovery cannot be
overstated since it opens up the door for developing new technologies for
energy production thus reducing our dependence for fossil fuels and addressing
the global climate change problem."
'Just the beginning'
Currently available electrolyzers, which split water with electricity and are often used industrially, are not suited for artificial photosynthesis because they are very expensive and require a highly basic (non-benign) environment that has little to do with the conditions under which photosynthesis operates.
More engineering work needs to be done to integrate the new scientific discovery into existing photovoltaic systems, but Nocera said he is confident that such systems will become a reality.
"This is just the beginning," said Nocera, principal investigator for the Solar Revolution Project funded by the Chesonis Family Foundation and co-Director of the Eni-MIT Solar Frontiers Center. "The scientific community is really going to run with this."
Nocera hopes that within 10 years, homeowners will be able to power their homes in daylight through photovoltaic cells, while using excess solar energy to produce hydrogen and oxygen to power their own household fuel cell. Electricity-by-wire from a central source could be a thing of the past.
The project is part of the MIT Energy Initiative, a program designed to help transform the global energy system to meet the needs of the future and to help build a bridge to that future by improving today's energy systems. MITEI Director Ernest Moniz, Cecil and Ida Green Professor of Physics and Engineering Systems, noted that "this discovery in the Nocera lab demonstrates that moving up the transformation of our energy supply system to one based on renewables will depend heavily on frontier basic science."
The success of the Nocera lab shows the impact of a mixture of funding sources - governments, philanthropy, and industry. This project was funded by the National Science Foundation and by the Chesonis Family Foundation, which gave MIT $10 million this spring to launch the Solar Revolution Project, with a goal to make the large scale deployment of solar energy within 10 years.
http://www.forbes.com/energy/2008/07/30/nocera-solar-power-biz-energy-cz_jf_0731solar.html
07.31.08, 2:30 PM ET
Solar Energy, All Night Long
Jonathan Fahey
MIT professor Daniel G. Nocera has long been jealous of plants. He desperately wanted to do what they do -- split water into hydrogen and oxygen and use the products to do work. That, he figures, is the only way we humans can solve our energy problems; enough energy pours down from the sun in one hour to power the planet's energy needs for a year.
In January, only a month after reevaluating his methodology in the face of a frustratingly slow process, he finally found a way. "For six months now I've been looking at the leaves and saying 'I own you guys!'"
Nocera's discovery -- a cheap and easy way to store energy that he thinks will be used to change solar power into a mainstream energy source -- will be published in the journal Science on Friday. "This is the nirvana of what we've been talking about for years," said Nocera, the Henry Dreyfus Professor of Energy at MIT. "Solar power has always been a limited, far-off solution. Now we can seriously think about solar power as unlimited--and soon."
Plants catch light and turn it into an electric current, then use that energy to excite catalysts that split water into hydrogen and oxygen during what is called photosynthesis' light cycle. The energy is then used during the dark cycle to allow the plant to build sugars used for growth and energy storage.
Nocera and Matthew Kanan, a postdoctoral fellow in Nocera's lab, focused on the water-splitting part of photosynthesis. They found cheap and simple catalysts that did a remarkably good job. They dissolved cobalt and phosphate in water and then zapped it with electricity through an electrode. The cobalt and phosphate form a thin-film catalyst around the electrode that then use electrons from the electrode to split the oxygen from water. The oxygen bubbles to the surface, leaving a proton behind.
A few inches away, another catalyst, platinum, helps that bare proton become hydrogen. (This second reaction is a well-known one, and not part of Nocera and Kanan's study.)
The hydrogen and oxygen, separated and on-hand, can be used to power a fuel cell whenever energy is needed.
"Once you put a photovoltaic on it," he says, "you've got an inorganic leaf."
James Barber, a biochemistry professor at Imperial College London who studies artificial photosynthesis but was not involved in this research, called the discovery by Nocera and Kanan a "giant leap" toward generating clean, carbon-free energy on a massive scale.
"This is a major discovery with enormous implications for the future prosperity of humankind," he said. "The importance of their discovery cannot be overstated."
Nocera's discovery arose from frustration. Disappointed with the pace of his lab's progress, Nocera and his team decided in December to question some of the basic assumptions they had made in setting up earlier experiments.
Chemists, it turns out, are always worrying about the stability of their catalysts and end up doing backflips to try to synthesize materials that won't corrode. Photosynthesis, though, is so violently reactive that the catalysts involved break down every 30 minutes. The leaf has to constantly rebuild them. Maybe, thought Nocera, instead of fighting corrosion, he should work with it. "It's a bias a lot of scientists have. We want something to be structurally stable. But all it has to be is functionally stable."
This thinking led Nocera to try his cobalt-phosphate mixture. He knew it wouldn't hold together, but he thought it might still work. Sure enough, Nocera's catalyst breaks down whenever the electricity is cut, but it assembles itself again when electricity is reapplied.
Nocera's discovery is still a science experiment. It needs plenty of engineering before it can be a useful device. The cobalt and phosphate at the center of Nocera's work is cheap and plentiful, but the hydrogen reaction uses platinum, which is rare and expensive. The electrode needs to be improved so the oxygen-making process can speed up. And the system needs to be integrated into some kind of electricity-producing device, ideally powered by solar or wind on one end and a fuel cell on the other.
But splitting the oxygen away from the water was the hard part, and Nocera has done it. "Now we can start thinking about a totally distributed solar [photovoltaic] system," he said. "We couldn't have a solar economy unless it could produce energy 24/7. Now we can."
His hope is that because unlike traditional electrolysis devices, which are expensive and require toxic alkaline solutions, his system is so cheap, simple and benign that scientists and engineers around the world will be able to improve it quickly.
For his part, Nocera says he will work to understand and improve both sides of his new discovery. His lab will try to learn every detail about just how his catalyst is making the oxygen. And he is going to work with his engineering colleagues at MIT to try to integrate his storage device into systems that he hopes one day will power homes and cars all day and all night.
http://v3.espacenet.com/textdoc?DB=EPODOC&IDX=US2003201161&F=0
US Patent # 6,863,781 Process for Photocatalysis and Two-Electron Mixed-Valence Complexes
( March 8, 2005 )Abstract --- Embodiments for the invention include a process for the production of hydrogen comprising a protic solution, a photocatalyst capable of a two-electron reduction of hydrogen ions; and a coproduct trap. The embodiment includes exposing the reaction medium to radiation capable of photoexciting the photocatalyst to produce hydrogen. The protic solution may comprise at least one of hydrohalic acid, a silane, and water, and the hydrohalic acid may be hydrochloric acid, hydrogen bromide, hydrogen fluoride or hydrogen iodide. The present application also describes novel transition metal compounds. Embodiments of the compounds include a compound comprising two transition metal atoms, wherein the transition metal atoms are in a two-electron mixed valence state and at least one transition metal is not rhodium; and at least one ligand capable of stabilizing the transition metal atom in a two-electron mixed valence state.
Current U.S. Class: 204/157.52 ; 204/157.15
Current International Class: B01J 31/18 (20060101); B01J 31/16 (20060101); B01J 35/00 (20060101); C07F 9/00 (20060101); C07F 9/24 (20060101); C01B 3/04 (20060101); C01B 3/00 (20060101); C07F 15/00 (20060101); B01J 31/26 (20060101); B01J 31/30 (20060101); C07C 001/00 (); C01B 003/00 ()
References Cited [Referenced By] -- U.S. Patent Documents : 5223634 June 1993 Gratzel et al.Other References
MacQueen et al., "Competitive Hydrogen Production and Emission Through the Photochemistry of Mixed-Metal Bimetallic Complexes", Inorganic Chemistry (no month, 1990), vol. 29, No. 12, pp. 2313-2320.* .
Heyduk, A.F., Nocera D.G.: "Hydrogen produced from hydrohalic acid solutions by a two-electron mixed valence photocatalyst" Science, American Association for the Advancement of Science, US., vol. 293, Aug. 31, 2001, pp. 1639-1641, XP002217879. .
Heyduk, A.F., Nocera D.G.: "Hydrido, halo, hydrido-halo complexes of two-electron mixed valence diiridium cores" Journal of the American Chemical Society American Chemical Society, Washington, D.C., US, vol. 122, 2000, pp. 9415-9426 Xp002217880, no month. .
Heyduk, A.F., Nocera D.G.: "A novel two-electron mixed-valence Ir(II)-Ir(O) complex" Chemical Communications, Royal Society of Chemistry, GB, 1999, pp. 1519-1520, XP002217881, no month..Description
TECHNICAL FIELD
Embodiments of the present invention relate to photocatalytic chemical processes. Embodiments of the present invention relate to the production of hydrogen from a hydrohalic acid solution using a homogenous photocatalyst. Embodiments of the present invention also include novel two-electron mixed-valence complexes.
DESCRIPTION
Photocatalytic production of chemical products offers an inexpensive method of driving a chemical reaction toward the production of molecules comprising more energy than the raw materials. Photocatalysts having excitation energies that fall within the spectrum of solar radiation at the surface of earth are of particular interest. The potential of solar chemistry in the generation of energy-rich molecules from inexpensive energy-poor raw materials has been extensively researched. Embodiments of the present invention comprise transition metal complexes capable of mediating multielectron transformations. These multielectron transformations allow activation of small molecules. Embodiments of the present invention comprise catalysts with a multinuclear core transition metal complex in a two electron mixed valence state. The two-electron mixed-valence state may mediate chemical processes, such as atom transfer, bond activation, and substrate functionalization, for example.
The multicore transition metal complexes may be stabilized by the chemical composition of the ligands coordinated with the multinuclear transition metal core. For example, the electronic and steric properties of bridging bis(phosphine)amine ligands favor disproportionation of valence-symmetric binuclear cores. Such properties in the bis(difluorophosphine)methyl amine ("dfpma") ligand stabilizes Rh.sup.0 --Rh.sup.II cores. The disproportionation of embodiments of the present invention include new ligands to stabilize two-electron mixed-valence complexes were developed by manipulation of the stereo-electronic effects engendered by phosphine and nitrogen functionalization.
An embodiment of the present invention is a process having a reaction medium comprising a protic solution, a photocatalyst capable of a two-electron reduction of hydrogen ions, and a coproduct trap. The reaction medium may be exposed to radiation capable of photoexciting the photocatalyst to produce hydrogen. The reaction medium may comprise any protic solution, such as a hydrohalic acid of hydrochloric acid, hydrogen bromide, hydrogen fluoride, and hydrogen iodide. A coproduct is any compound or atom produced other than the desired product, for example, the desired product may be hydrogen. A coproduct trap is any material, such as an atom or chemical compound, which may attach, react or bind with the coproduct to prevent the coproduct from interfering with the desired reaction. For example, the coproduct trap may be a compound capable of bonding with a halogen atom, such as, for example, tetrahydrofuran, dihydroanthracene, a silane, isopropanal and 2,3 dimethylbutadiene.
The photocatalyst of the present invention may be any photocatalyst capable of a two electron reduction of hydrogen ions and comprises a multinuclear transition metal core. An embodiment of the photocatalyst of the present invention is a multinuclear transition metal core which is capable of a two electron reduction of hydrogen atoms. The photocatalyst may also comprise a ligand that is capable of stabilizing a two-electron mixed-valence state of the binuclear transition metal core.
A further embodiment of the present invention includes a reaction medium comprising a protic solution; a photocatalyst comprising a binuclear transition metal complex and a ligand capable of supporting the photocatalyst in a two-electron mixed-valence state; and a coproduct trap. The photocatalyst may then be photoexcited to produce hydrogen from the reaction medium. The ligand may be, for example, a diphosphazane ligand or a ligand having a strongly .PI.-acidic phosphine group.
Another embodiment of the present invention comprises a process involving exciting at least two photocatalysts to an active state wherein the photocatalysts comprise two rhodium atoms; contacting the photocatalysts with hydrogen ions and halogen ions of a hydrohalic acid solution, thereby producing hydrogen and a photocatalyst in a mixed valence state comprising halogen atoms; and irradiating the photocatalyst in a mixed valence state to eliminate at least a portion of the halogen atoms. The embodiment may include a hydrogen ion and a halogen ion binding to the photocatalysts prior to reacting to produce hydrogen and an unidentified photocatalytic intermediate. It is theorized that the photocataltic intermediate reacts with additional hydrogen ions and halogen ions to form a photocatalyst comprising four halogen atoms and additional hydrogen. The process may then be continued by further radiation of the photocatalyst to eliminate the halogen atoms and absorbing the halogen atoms in a halogen-atom trap. Preferably, the photocatalyst is excited by irradiation with sunlight.
An additional embodiment of the present invention includes a compound comprising two iridium atoms, wherein the iridium atoms are in a two-electron mixed-valence state, and at least one ligand. The iridium compound may be stabilized by at least one ligand, such as, for example, a diphosphazane ligand.
BRIEF DESCRIPTION OF THE DRAWINGS
The features and advantages of embodiments of the present invention may be better understood by reference to the accompanying figures, in which:
FIG. 1(a) is a thermal ellipsoid plot (50% probability) for Rh.sub.2.sup.0,II (dfpma).sub.3 Cl.sub.2 (PEt.sub.3) (1-PEt.sub.3) and FIG. 1(b) is a thermal ellipsoid plot (50% probability) for Rh.sub.2.sup.0,II (dfpma).sub.3 Br.sub.2 (PPh.sub.3) (2-PPh.sub.3), and wherein the hydrogen atoms and solvent molecules have been omitted for clarity;
FIG. 2(a)-(d) are the following NMR spectra of dirhodium dfpma complexes: FIG. 2(a) is the .sup.19 F NMR of Rh.sub.2.sup.II,II (dfpma).sub.3 Cl.sub.4 (4) in CDCl.sub.3 ; FIG. 2(b) is the .sup.19 F{.sup.31 P} NMR spectra of Rh.sub.2.sup.II,II (dfpma).sub.3 Cl.sub.4 (4) in CDCl.sub.3 ; FIG. 2(c) is the .sup.19 F NMR of Rh.sub.2.sup.0,II (dfpma).sub.3 Cl.sub.2 (PPh.sub.3) ("1-PPh.sub.3") in CDCL.sub.3 ; and FIG 2(d) .sup.19 F {.sup.31 P} NMR spectra of Rh.sub.2.sup.0,II (dfpma).sub.3 Cl.sub.2 (PPh.sub.3) (1-PPh.sub.3) in CDCl.sub.3 (wherein the signals at -74 and -68 ppm in spectra 2(b) and 2(d) are artifacts due to the .sup.31 P decoupling pulse, and the insets in FIG. 2(a) and FIG. 2(c) show the Neumann Projection structures of 4 and 1-PPh.sub.3, respectively);
FIG. 3 is a thermal ellipsoid plot (50% probability) for Rh.sub.2 (dmpma).sub.2 Cl.sub.2 (.mu.-CO).sub.2 (6) wherein the hydrogen atoms have been omitted for clarity;
FIG. 4 is the thermal ellipsoid plot (50% probability) for Rh.sub.2 (dppma).sub.2 Cl.sub.2 (.mu.-CO).sub.2 (9) wherein the hydrogen atoms and a CH.sub.2 Cl.sub.2 solvent molecule have been omitted and only the ipso carbons of the phenyl ring are shown for clarity;
FIG. 5(a) is the thermal ellipsoid plot (50% probability) for Rh.sub.2 (dppma).sub.3 (CO).sub.2 (12) and FIG. 5(b) is the thermal ellipsoid plot (50% probability) for Rh.sub.2 (dppma).sub.2 Cl.sub.4 (CO).sub.2 (13) wherein the hydrogen atoms and solvent molecules have been omitted and only the ipso carbons of the phenyl ring are shown for clarity;
FIG. 6 is a thermal ellipsoid plot (50% probability) for Rh.sub.2.sup.0,II (tfepma).sub.3 Cl.sub.2 (CO) (16) wherein for clarity, only the methylene carbons of the CH.sub.2 CF.sub.3 groups are shown;
FIG. 7(a) is a thermal ellipsoid plot (50% probability) for syn-Rh.sub.2.sup.II,II (tfepma).sub.3 Cl.sub.4 (19) and FIG. 17(b) is a thermal ellipsoid plot (50% probability) for anti-Rh.sub.2.sup.II,II (tfepma).sub.3 Cl.sub.4 (20), wherein the hydrogen atoms and solvent molecules have been omitted for clarity;
FIG. 8 includes the solution absorption and solid-state emission spectra of syn-Rh.sub.2.sup.II,II (tfepma).sub.3 Cl.sub.4 (19) and anti-Rh.sub.2.sup.II,II (tfepma).sub.3 Cl.sub.4 (20);
FIG. 9 is a thermal ellipsoid plot for Rh.sub.2.sup.0,II (tfepma).sub.3 Cl.sub.2 (21) from preliminary X-ray data wherein for clarity, only the methylene carbons of the CH.sub.2 CF.sub.3 groups are shown;
FIG. 10 is the thermal ellipsoid plot (50% probability) for Rh.sub.2.sup.0,II (dfpx).sub.3 Br.sub.2 (PPh.sub.3) (22) wherein the solvent molecules and hydrogen atoms have been omitted for clarity;
FIG. 11 is a thermal ellipsoid plot (50% probability) for [ClRh(tfepx)].sub.2 (.mu.-tfepx) (23) wherein for clarity, only the methylene carbons of the CH.sub.2 CF.sub.3 groups are shown;
FIG. 12 is a thermal ellipsoid plot (50% probability) of Ir.sub.2.sup.I,III (dfpma).sub.2 Cl.sub.4 (cod) (1) wherein hydrogen atoms have been omitted for clarity;
FIG. 13 depicts the molecular structure of Ir.sub.2.sup.I,III (dppma).sub.3 Cl.sub.2 (2), taken from preliminary X-ray crystal structure data wherein for clarity, hydrogen atoms have been omitted and only the ipso carbons of the phenyl rings are shown;
FIG. 14 is a thermal ellipsoid plot (50% probability) of Ir.sub.2.sup.0,II (tfepma).sub.3 Cl.sub.2 (3) wherein for clarity, hydrogen atoms have been omitted and only the methylene carbons of the trifluoroethyl groups are shown;
FIG. 15 includes .sup.31 P and partial .sup.1 H NMR spectra of Ir.sub.2.sup.0,II (tfepma).sub.2 Cl.sub.2 (3) in d.sup.8 -THF at wherein FIG. 15(a) is at 20.degree. C. and FIG. 15(b) is at -80.degree. C.;
FIG. 16(a) is a thermal ellipsoid plot (50% probability) of Ir.sub.2.sup.0,II (tfepma).sub.3 Cl.sub.2 (MeCN) (4) and FIG. 16(b) is the thermal ellipsoid plot (50% probability) of Ir.sub.2.sup.0,II (tfepma).sub.3 Cl.sub.2 (CN.sup.t Bu) (5-CN.sup.t Bu) wherein for clarity, hydrogen atoms have been omitted and only the methylene carbons of the trifluoroethyl groups are shown;
FIG. 17 is a thermal ellipsoid plot (50% probability) of the Ir.sub.2.sup.0,II (tfepma).sub.3 (MeCN).sub.3.sup.2+ (6) cation wherein for clarity, hydrogen atoms and two PF.sub.6.sup.- anions have been omitted and only the methylene carbons of the trifluoroethyl groups are shown;
FIG. 18(a) is a thermal ellipsoid plot (50% probability) of Ir.sub.2.sup.I,III (tfepma).sub.3 Cl.sub.4 (7), and FIG. 18(b) is a thermal ellipsoid plot (50% probability) of Ir.sub.2.sup.II,II (tfepma).sub.2 Cl.sub.4 (MeCN).sub.2 (8), wherein for clarity, hydrogen atoms have been omitted and only the methylene carbons of the trifluoroethyl groups are shown;
FIG. 19 is a thermal ellipsoid plot (50% probability) of Ir.sub.2.sup.I,III (tfepma).sub.2 HCl.sub.3 (10) wherein for clarity, hydrogen atoms have been omitted and only the methylene carbons of the trifluoroethyl groups are shown;
FIG. 20 shows .sup.1 H NMR spectra of (a) the equatorial isomer of Ir.sub.2.sup.I,III (tfepma).sub.3 HCl.sub.3 (10), (b) the axial isomer of Ir.sub.2.sup.I,III (tfepma).sub.3 HCl.sub.3 (9), (c Ir.sub.2.sup.I,III (tfepma).sub.3 HCl.sub.2 (MeCN)][PF.sub.6 ].sub.2 (11), and (d) Ir.sub.2.sup.I,III (tfepma).sub.3 H.sub.2 Cl.sub.2 (12), wherein the spectra were acquired at room temperature in CD.sub.3 CN;
FIG. 21 is a thermal ellipsoid plot (50% probability) of Ir.sub.2.sup.I,III (tfepma).sub.2 H.sub.2 Cl.sub.2 (12) wherein for clarity, hydrogen atoms have been omitted and only the methylene carbons of the trifluoroethyl groups are shown;
FIG. 22 is a pictorial representation of the HOMO and LUMO of Ir.sub.2 (PH.sub.3).sub.6 Cl.sub.2 as calculated by the extended Huckel program YAeHMOP;
FIG. 23 is a qualitative potential energy diagram for MeCN exchange from [Ir.sub.2.sup.I,III (tfepma).sub.3 HCl.sub.2 (MeCN)].sup.2+ (11), illustrating the origin of the inverse isotope effect;
FIG. 24 depicts molecular structures of the homologous series of binuclear rhodium complexes wherein FIG. 24(a) depicts Rh.sub.2.sup.0,0 (dfpma).sub.3 (PF.sub.3).sub.2, FIG. 24(b) depicts Rh.sub.2.sup.0,II (dfpma).sub.3 Cl.sub.2 (PF.sub.3), and FIG. 24(c) depicts Rh.sub.2.sup.II,II (dfpma).sub.3 Cl.sub.4 ;
FIG. 25(a) is an electronic absorption spectrum of Rh.sub.2.sup.0,0 (dfpma).sub.3 (.eta..sup.1 -dfpma).sub.2, FIG. 25(b) is electronic absorption spectra of Rh.sub.2.sup.0,II (dfpma).sub.3 Cl.sub.2 (.eta..sup.1 -dfpma) and Rh.sub.2.sup.0,II (dfpma).sub.3 Br.sub.2 (.eta..sup.1 -dfpma), and FIG. 25(c) is electronic absorption spectra of Rh.sub.2.sup.II,II (dfpma).sub.3 Cl.sub.4 and Rh.sub.2.sup.II,II (dfpma).sub.3 Br.sub.4 in THF at room temperature;
FIG. 26 is a plot of the energy maximum of the d.sigma..fwdarw.d.sigma.* transition of Rh.sub.2.sup.0,0 (dfpma).sub.3 (L).sub.2 (L=PEt.sub.3, P(OMe).sub.3, P(O.sup.i Pr).sub.3, PPh.sub.3, and PF.sub.3) vs. Tolman electronic parameter;
FIG. 27 is a graph depicting spectral changes in the electronic absorption spectrum during the photolysis (.lambda..sub.exc >335 nm) of Rh.sub.2.sup.II,II (dfpma).sub.3 Br.sub.4 in THF and in the presence of 250 equiv. of dfpma at 0.degree. C. wherein the spectra were recorded over the span of 2 hours;
FIG. 28 is the .sup.19 F NMR (C.sub.6 D.sub.6) spectrum of the photoproduct obtained from the irradiation (.lambda..sub.exc.gtoreq.335 nm) of Rh.sub.2.sup.II,II (dfpma).sub.3 Br.sub.4 in the presence of excess dfpma in THF at 0.degree. C.;
FIG. 29(a) is a graph depicting the spectral changes in the electronic absorption spectrum during the photolysis (.lambda..sub.exc.gtoreq.436 nm) of Rh.sub.2.sup.II,II (dfpma).sub.3 Br.sub.4 in THF and in the presence of 250 equiv. of dfpma at 0.degree. C. wherein the spectra were recorded over the span of 15 min. and the photolysis was terminated when the 358-nm absorption was maximized, and FIG. 29(b) is a graph depicting spectral changes associated with continued irradiation of the solution resulting from the photolysis depicted in FIG. 29(a) with higher energy light (.lambda..sub.exc.gtoreq.335 nm) wherein spectra were recorded over the span of 3 hours;
FIG. 30 is a graph depicting the spectral changes associated with the monochromatic irradiation (.lambda..sub.exc =335 nm) of Rh.sub.2.sup.II,II (dfpma).sub.3 Br.sub.4 in THF at 0.degree. C.;
FIG. 31 is a graph of the .sup.1 H NMR (CDCl.sub.3) spectrum of the photoproduct obtained from the irradiation (.lambda..sub.exc >335 nm) of Rh.sub.2.sup.II,II (dfpma).sub.3 Br.sub.4 in the presence of 4.3 equiv. of 2,6-lutidine and 2.5 equiv. of PPh.sub.3 in THF at 0.degree. C., showing the methyl resonances of the Rh.sub.2.sup.0,II and Rh.sub.2.sup.0,0 photoproducts and the high frequency resonance for protonated 2,6-lutidine;
FIG. 32(a) depicts the HOMO and LUMO for Rh.sub.2.sup.0,0 (dfpma).sub.3 (PF.sub.3).sub.2, FIG. 32(b) depicts the HOMO and LUMO for Rh.sub.2.sup.0,II (dfpma).sub.3 Br.sub.2 (PF.sub.3), and FIG. 32(c) depicts the HOMO and LUMO for Rh.sub.2.sup.II,II (dfpma).sub.3 Br.sub.4 as determined at the extended Huckel level using YAeHMOP;
FIG. 33 is a graph of the UV-Vis spectral changes associated with the irradiation (300.ltoreq..lambda..sub.exc.ltoreq.400 nm) of Rh.sub.2.sup.0,0 (dfpma).sub.3 (CN.sup.t Bu).sub.2 in CH.sub.2 Cl.sub.2 containing 0.1 M HBr wherein the spectra were acquired at t.sub.irr =0, 10, 20, 30, 50, 70, 90, 120 and 180 min.;
FIG. 34(a) is a graph of UV-Vis absorption changes associated with the photolysis (.lambda..sub.exc.gtoreq.335 nm) of Rh.sub.2.sup.0,0 (dfpma).sub.3 (CO)(PPh.sub.3) in CH.sub.2 Cl.sub.2 solution containing 0.1 M HCl, and FIG. 34(b) is a graph of UV-Vis absorption changes associated with the photolysis (.lambda..sub.exc.gtoreq.335 nm) of Rh.sub.2.sup.0,0 (dfpma).sub.3 (CO)(PPh.sub.3) in CH.sub.2 Cl.sub.2 solution containing 0.1 M HBr, wherein the spectra of FIG. 34(a) were acquired at t.sub.irr =0, 2, 5, 7, 10, 15, 20 and 25 min. and the spectra of FIG. 34(b) were acquired at t.sub.irr =0, 2, 5, 7, 10, 12, 15, 20, 25 and 30 min.;
FIG. 35(a) is a graph of the changes in electronic absorption spectrum during monochromatic UV photolysis (.lambda..sub.exc =335 nm) of Rh2(dfpma).sub.3 (CO)(PPh.sub.3) in 0.1 M HCl/THF solution at 20.degree. C. wherein the spectra were collected at t.sub.irr =0, 10, 20, 30, 40, 50, 60 and 90 min., and FIG. 35(b) is a graph of changes in the electronic absorption of the continued photolysis of the sample in FIG. 35(a) using white light (.lambda..sub.exc.gtoreq.335 nm) wherein spectra of FIG. 35(b) were acquired at t.sub.irr =5, 10, 15 and 20 min. after the last spectrum of FIG. 35(a);
FIG. 36 is a graph of UV-Vis absorption changes associated with photolysis (.lambda..sub.exc.gtoreq.365 nm) of Rh.sub.2.sup.0,II (dfpma).sub.3 Br.sub.2 (CN.sup.t Bu) in CH.sub.2 Cl.sub.2 containing 0.1 M HBr wherein the spectra were acquired at t.sub.irr =0, 5, 10, 20, 30, 40, 50 and 60 min.;
FIG. 37(a) is a plot of total hydrogen collected as a function of time for UV-Vis white light irradiation (.lambda..sub.exc.gtoreq.335 nm) of Rh.sub.2.sup.0,II (dfpma).sub.3 Cl.sub.2 (PPh.sub.3) in 0.1 M HCl/THF solution at 20.degree. C., and FIG. 37(b) is a graph of UV-Vis absorption spectra of the sample of FIG. 37(a) indicating the decay of the Rh.sub.2.sup.0,II (dfpma).sub.3 Cl.sub.2 (PPh.sub.3) catalyst during irradiation;
FIG. 38 is a plot of predicted H.sub.2, D.sub.2, and HD production based on irradiation of Rh.sub.2.sup.0,II (dfpma).sub.3 Cl.sub.2 (PPh.sub.3) in d.sup.8 -THF initially containing 10 equiv. of HCl;
FIG. 39(a) is a graph of changes in electronic absorption spectrum during the monochromatic UV photolysis (.lambda..sub.exc =335 nm) of Rh.sub.2.sup.0,II (dfpma).sub.3 Cl.sub.2 (PPh.sub.3) in 0.1 M HCl/THF solution at 20.degree. C. wherein the spectra were acquired at t.sub.irr =0, 10, 20, 30 and 40 min., and FIG. 39(b) is a graph of spectral changes for the solution in FIG. 39(a) kept in the dark for five minutes and wherein the spectra were recorded 5 and 10 minutes after the last spectrum of FIG. 39(a);
FIG. 40 is a graph of absorption spectra recorded during photolysis (300 nm.ltoreq..lambda..sub.exc <400 nm) of Rh.sub.2.sup.II,II (dfpma).sub.3 Br.sub.4 at 20.degree. C. in THF solution containing 0.1 M HCl wherein the spectra were acquired at t.sub.irr =0, 1, 2, 3, 4 and 5 min.;
FIG. 41(a) is a .sup.19 F NMR spectrum for the photoreaction of Rh.sub.2.sup.II,II (dfpma).sub.3 Cl.sub.4 with HCl in d.sup.8 -THF, FIG. 41(b) is .sup.19 F NMR spectrum for the photoreaction of Rh.sub.2.sup.II,II (dfpma).sub.3 CO.sub.4 with HCl in d.sup.8 -THF after the addition of 0.2 M HCl, FIG. 41(c) is a .sup.19 F NMR spectrum for the photoreaction of Rh.sub.2.sup.II,II (dfpma).sub.3 Cl.sub.4 with HCl in d.sup.8 -THF after addition of 0.2 M HCl and irradiation (300 nm.ltoreq..lambda..ltoreq.400 nm) for 3 hr., FIG. 41(d) is a .sup.19 F NMR spectrum for the photoreaction of Rh.sub.2.sup.II,II (dfpma).sub.3 Cl.sub.4 with HCl in d.sup.8 -THF after addition of 0.2 M HCl and irradiation (300 nm.ltoreq..lambda..ltoreq.400 nm) for 18 hr., and FIG. 41(e) is a .sup.19 F NMR spectrum of independently prepared Rh.sub.2.sup.0,II (dfpma).sub.3 Cl.sub.2 in d.sup.8 -THF; and
FIG. 42 is a .sup.19 F NMR spectrum of Rh.sub.2.sup.II,II (dfpma).sub.3 Cl.sub.2 Br.sub.2 in d.sup.8 -THF from the reaction of Rh.sub.2.sup.0,II (dfpma).sub.3 Cl.sub.2 with bromine.
CHARACTERIZATION OF LIGAND EFFECTS ON BINUCLEAR CORES
Research in inorganic chemistry includes preparation of new molecules to mediate multi-electron transformations. Such transformations assist in the activation of small molecules and allow more controlled reactivity of small molecules. Excited-state valence disproportionation of symmetric quadruply-bonded dimers may be observed by transient spectroscopic techniques. The present inventors developed complexes in which two-electron mixed valency could be incorporated into ground state configurations of the transition metal complexes.
Preparation of two-electron mixed-valence compounds is accomplished through the internal disproportionation of valence symmetric metal complexes. The inventors envisioned three different approaches to internal disproportionation, depicted in Scheme 1, were envisioned to effect this reaction and determine ligands which are capable of stabilizing a two electron mixed valence state of a multinuclear transition metal core, and more specifically a binuclear transition metal core. In Case I of Scheme 1, an asymmetric ligand is constructed from disparate electron withdrawing and electron donating fragments. In such a case, the electron-withdrawing group may stabilize low-valent metal centers whereas the electron-donating group may stabilize high-valent metal centers such as an oxypiridinate ligand. Synthetically, disporoportionation by the method of Case I is challenging for at least two reasons. General strategies for the preparation of simple asymmetric bridging ligands utilize a complex multi-step procedure. Secondly, it is difficult to control the linkage isomerism of ligands that possess inherent asymmetry such as oxypyridinate ligands. Typically, two asymmetric ligands add to a binuclear metal core in a head-to-tail fashion, ##STR1##
yielding a net symmetric electronic environment.
Preparation of two-electron mixed-valence complexes may also be accomplished by induced asymmetry of the bridging ligand framework. See Cases II and III in Scheme 1. A bridging ligand between the metal centers may be used to create an induced asymmetric environment suitable for late transition metals. For example, a three atom bridging ligand, with .pi.-acid groups to coordinate the metal centers and a Lewis-basic bridgehead atom, may induce an asymmetric environment via directional lone pair donation from the bridgehead atom to one .pi.-acid group. In another embodiment, a ligand composed of hard .pi.-donating coordination groups and a Lewis .pi.-acidic bridgehead atom may lead to valence disproportionation in high-valent early transition metal complexes. Embodiments of the present invention may comprise any ligand capable of stabilizing a mixed valence state in the binuclear transition metal core. An embodiment of the present invention of a Case II ligand may be a ligand of the formula, RN(PY.sub.2).sub.2, wherein the ligand comprises a phosphazane group, P--N--P. The Y groups may be independently selected and may be any combination of groups which give at least one phosphorous atom of the phosphazane group strong .pi.-acid characteristics, preferably both phosphorous atoms have strong .pi.-acid characteristics. The Y groups may not all be identical atoms or groups and all the Y's need not contribute to the .pi.-acidity of the coordinating atoms. The Y group may be, for example, but not limited to, at least one of halogen, fluorine, chlorine, halogenated alkanes, halogenated alkenes, aryl substituted with electron withdrawing groups, and alcohols. The ligands have strong .pi.-acid characteristics, if at least a portion of the transition metal complexes are present in the two electron mixed valence state. As the strength of the .pi.-acidity of the attached ligands of binuclear transition metal complexes increase, the complexes will form an equilibrium wherein at least a portion of the complexes will exist in the two electron mixed valence state. These complexes will be photocatalytically active in the present invention. As the pi-acidity of the ligands increases the equilibrium of the complexes will shift toward a state in which all of the complexes are in the two electron mixed valence state. Additionally, the Figures and the Examples use three ligands to stabilize the mixed valence state of the photocatalyst, however, it is contemplated that a single ligand may be prepared comprising the characteristics necessary to stabilize the mixed valence state. The single ligand may be, for example, a multidentate ligand which comprises the electronic and steric properties described above. Such as, for example, the ligands described herein connected through the R group.
The R attached to a nitrogen atom of the phosphazane group may be any atom or group which allows participation of a lone pair of electrons to participate in .pi.-bonding with the phosporus atoms. The R attached to a nitrogen atom may be, for example, but not limited to, a substituted or unsubstituted C.sub.1 -C.sub.10 alkyl groups, methyl, ethyl, propyl, butyl, pentyl, hydrogen and a substituted phenyl, wherein the alkyl group may be a branched or an unbranched alkyl group.
A Case II ligand may be prepared by the reaction of PCl.sub.3 with primary amines which leads to the formation of a bis(dichlorophosphine)amine fragment as shown in Scheme 2. This reaction has been observed for a variety of starting materials, ##STR2##
including simple alkyl amines and substituted aniline derivatives. Reaction with Grignard and alkyl lithium reagents affords alkyl phosphines; rapid reaction with alcohols in the presence of tertiary amine bases gives the corresponding phosphite derivatives. Finally, metathesis with antimony trifluoride provides a high-yield route to the bis(difluorophosphine)amine derivative, which possess more .pi.-acidic phosphines of the PNP-type ligands. These synthetic routes are versatile. Electronic and steric parameters of the ligand can be finely manipulated by functionalization at both the nitrogen and phosphorus centers.
The ligands as described above may be used to stabilize internal disproportionation on any binuclear transition metal transition metal complex. The core may comprise different transition metals. The transition metals comprising the binuclear core may be independently selected from, for example, Rh, Ir, Cu, Fe, Ru, Cr, Mo, W, Co, Re, No, Mn, V, Nb, Ta, Zn, Au, Pd, Pt and Ag.
The chemistry of dfpma with Group VI metals was determined shortly after its preparation. Three ligands were observed to span d.sup.9 --d.sup.9 metal cores, as observed for Co.sub.2.sup.0,0 (dfpma).sub.3 (CO).sub.2. UV-irradiation of this complex in alkane solutions containing phosphine ligands leads to carbonyl loss and coordination of the phosphine in the axial site. An extensive redox chemistry was explored for Co.sub.2.sup.0,0 (dfpma).sub.3 (CO).sub.2. For example, oxidation with Br.sub.2 gave a four electron oxidized species, Co.sub.2.sup.II,II (dfpma).sub.3 Br.sub.4, whereas electrochemical reduction gave a two-electron reduced anionic complex, [Co.sub.2.sup.-I,-I (dfpma).sub.3 (CO).sub.2 ].
An embodiment of the present invention may also comprise a Case III ligand. A nonlimiting example of a Case III ligand is one of the formula, RN(BY.sub.1), wherein the R is as described above and the Y.sub.1 may be independently selected and may be any atom or group which contributes to hard .pi.-donating characteristic of the coordination groups. Embodiments of the present invention may comprise any ligand capable of causing internal disproportionation of the multinuclear core and capable of stabilizing a mixed valence state.
Unless otherwise indicated, all numbers expressing quantities of ingredients, time, temperatures, and so forth used in the present specification and claims are to be understood as being modified in all instances by the term "about." Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and claims are approximations that may vary depending upon the desired properties sought to be obtained by the present invention. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope ##STR3##
of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.
Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, may inherently contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements.
To extend the two-electron mixed-valence chemistry of these ligands, the steric and electronic characteristics of the PNP ligands framework that favor the valence disproportionation of symmetric binuclear cores was explored. Embodiments described herein suggest that the induced asymmetry of Case II in Scheme 1 is effective in promoting the formation of two-electron mixed-valence complexes. The electronic and the steric parameters of the bridging PNP ligand may be controlled to favor valence disproportionation resulting in two-electron mixed-valence species.
The reaction of dfpma with the rhodium(I) dimers, such as, [XRh.sup.I (PF.sub.3).sub.2 ].sub.2 or XRhP.sub.4, for example, where X=Cl, Br, I, alkyl, triflates, hydrides, and other monodentate anionic ligands, produces two-electron mixed-valence complexes Rh.sub.2.sup.0,II (dfpma).sub.3 X.sub.2 (L), in which L may be, for example, PF.sub.3 or a monodentate dfpma ligand, depending on reaction stoichiometry, with yields of ca. 25%. A more reliable and general path into the rhodium complexes was sought from [XRh.sup.I (cyclooctadiene)].sub.2 ("[XRh.sup.I (cod)].sub.2 ") starting materials. Rh.sub.2.sup.0,II (dfpma).sub.3 X.sub.2 (L) was therefore prepared by the addition of three equivalents of dfpma and one equivalent of a two electron donor ligand (for example, P(OR).sub.3, PR.sub.3, and CNR) to benzene solutions of [XRh.sup.I (cod)].sub.2, as depicted in Scheme 3. Stirring at room temperature resulted in precipitation of most of the product over the span of 24 hr. Concentration by solvent distillation followed by the addition of pentane typically provided high yields of Rh.sub.2.sup.0,II (dfpma).sub.3 X.sub.2 (L) (X=Cl ("1-L" in Scheme 3), Br ("2-L" in Scheme 3); L=P(OR).sub.3, PR.sub.3, CNR). In the case of the carbonyl adduct, Rh.sub.2.sup.0,II (dfpma).sub.3 Cl.sub.2 (CO) ("1-CO"), dfpma was reacted with the carbonyl dimer [ClRh(CO).sub.2 ].sub.2 in CH.sub.2 Cl.sub.2 rather than benzene. The CH.sub.2 Cl.sub.2 was then removed and the residue washed with 1:5 benzene/pentane to afford an orange powder that was dried briefly in vacuo. 1-CO left under vacuum for extended periods of time (a few hours) changed color from orange to green-blue. This color change was accompanied by disappearance of the carbonyl resonance in the IR spectrum, indicating loss of an axial CO ligand.
The solid-state structures for several of the two-electron mixed-valence complexes have been determined from single crystal X-ray diffraction data. Thermal ellipsoid plots for two derivatives, Rh.sub.2.sup.0,II (dfpma).sub.3 Cl.sub.2 (PEt.sub.3) ("1-PEt.sub.3 ") and Rh.sub.2.sup.0,II (dfpma).sub.3 Br.sub.2 (PPh.sub.3) ("2-PPh.sub.3 "), are shown in FIGS. 1(a) and (b). Thermal ellipsoid plots are ball and stick type illustrations of structures of chemical compounds. The 50% probability thermal motion ellipsoids of the atomic sites are derived from anisotropic temperature factor parameters to indicate the thermal motion of the atom 50% of the time. Tables 1 and 2 list a few metrical parameters of complexes comprising the dirhodium cores. The dfpma bond distances of these derivatives indicates stabilization of the two-electron mixed-valence dirhodium core. For example, contraction of the N--P bond distance is observed for the phosphorus atoms coordinated to the high-valent Rh.sup.II (avg N--P.sup.Rh(II) =1.634(4) .ANG. for 1-PEt.sub.3 and 1.633(6.ANG. for 2-PPh.sub.3) center, suggesting, but not meant to limit the present invention, that the nitrogen lone pair moderates the electronic properties of the .pi.-acid PF.sub.2 groups. A longer N--P bond distance is observed for the phosphorus coordinated to the Rh.sup.0 (avg N--P.sup.Rh(0) =1.659(4) .ANG. for 1-PEt.sub.3 and 1.684(5) .ANG. for 2-PPh.sub.3), consistent with a mechanism theory that there is little donation of the nitrogen lone pair to the PF.sub.2 group coordinated to the low-valent Rh.sup.0 center of the complex.
TABLE 1 N--P bond lengths (.ANG.) of the bridging dfpma ligands of Rh.sub.2.sup.0,II (dfpma).sub.3 Cl.sub.2 (PEt.sub.3) (1-PEt.sub.3) and Rh.sub.2.sup.0,II (dfpma).sub.3 Br.sub.2 (PPh.sub.3) (2-PPh.sub.3). N--P Bond Distances in 1-PE.sub.t3 (.ANG.) P(1)-N(1) 1.663(4) P(2)-N(1) 1.633(4) P(3)-N(2) 1.670(4) P(4)-N(2) 1.633(4) P(5)-N(3) 1.675(4) P(6)-N(3) 1.636(5) N--P Bond Distances in 2-PPh.sub.3 (.ANG.) P(1)-N(1) 1.684(5) P(2)-N(1) 1.630(6) P(3)-N(2) 1.684(5) P(4)-N(2) 1.634(6) P(5)-N(3) 1.685(6) P(6)-N(3) 1.636(6)
Other metrical parameters of interest include the bond distances of the dirhodium core for various Rh.sub.2.sup.0,II (dfpma).sub.3 X.sub.2 (L) derivatives. Lengthening of the dirhodium core bond length indicates stability in the mixed-valence structure. Several factors lead to structural perturbations along the Rh.sup.0 --Rh.sup.II axis, including, but not limited to, axial ligand coordination to the Rh.sup.0 center and halogen coordination to the Rh.sup.II center. The metrical data in Table 2 for selected Rh.sub.2.sup.0,II complexes gives insight into the effects of each of these substitutions. A lengthening of the Rh.sup.0 --Rh.sup.II bond distance occurs upon phosphine substitution. Exchange of an axial PF.sub.3 in Rh.sup.0,II (dfpma).sub.3 (Cl.sub.2)(PF.sub.3)("1-PF.sub.3 ") for PEt.sub.3 to form Rh.sup.0,II (dfpma).sub.3 (Cl.sub.2) (PEt.sub.3)("1-PEt.sub.3 ") results in a 0.0405 .ANG. increase in the Rh.sup.0 --Rh.sup.II separation; similarly, exchange of an axial .eta..sup.1 -dfpma for PPh.sub.3 in Rh.sub.2.sup.0,II (dfpma).sub.3 Br.sub.2 (.eta..sup.1 -dfpma) ("2-(.eta..sup.1 -dfpma)") and 2-PPh.sub.3 results in a 0.0403 .ANG. increase in the Rh.sup.0 --Rh.sup.II separation. Spectroscopic studies presented herein indicate that PF.sub.3 and .eta..sup.1 -dfpma exert similar stereo-electronic influence on the dirhodium core, permitting a comparison of the ligand effect of 1-PF.sub.3 with 2-(.eta..sup.1 -dfpma). Substitution of bromide for chloride in complexes with these .pi.-acidic axial phosphines results in a lengthening of the Rh.sup.0 --Rh.sup.II bond by 0.013 .ANG.. The magnitude of the perturbation exerted by halogen and phosphine substitution on the Rh.sup.0 --Rh.sup.II bond distance follows directly from the relative donor abilities of each axial ligand. Whereas exchange of bromide for chloride at the axial Rh.sup.II position results in only a small change in .sigma. donor ability, PEt.sub.3 and PPh.sub.3 are significantly better .sigma. donors than PF.sub.3 and dfpma, and accordingly weaken the metal-metal interaction to a greater extent.
TABLE 2 Rh.sup.0 --Rh.sup.II bond lengths (.ANG.) for Rh.sub.2.sup.0,II (dfpma).sub.3 X.sub.2 (L) (X = Cl (1-L), X = Br (2-L)). Bond Distances (.ANG.) 2-(.eta.1- 1-PF3a dfpma)b 1-PEt3 2-PPh3 Rh(1)-Rh(2) 2.785(1) 2.798(2) 2.8255(5) 2.8383(9) Rh(1)-P(1) 2.254(2) 2.246(7) 2.2342(13) 2.238(2) Rh(1)-P(3) 2.209(2) 2.229(7) 2.1906(13) 2.202(2) Rh(1)-P(5) 2.248(2) 2.243(6) 2.2228(13) 2.259(2) Rh(1)-P(7) 2.168(2) 2.203(6) 2.3277(13) 2.335(2) Rh(2)-X(1) 2.431(2) 2.579(3) 2.4530(12) 2.6066(10) Rh(2)-X(2) 2.385(2) 2.524(4) 2.3905(12) 2.5020(11) Rh(2)-P(2) 2.251(2) 2.241(7) 2.2434(13) 2.254(2) Rh(2)-P(4) 2.183(2) 2.185(7) 2.1610(13) 2.184(2) Rh(2)-P(6) 2.258(2) 2.247(7) 2.2420(14) 2.249(2)
The preparation of two-electron reduced binuclear rhodium complexes was achieved from [XRh(cod)].sub.2 starting materials. Treatment of [CIRh(cod)].sub.2 with three equivalents of dfpma, followed by two equivalents each of cobaltocene and a two-electron donor ligand resulted in the formation of binuclear rhodium complexes of the general formulation Rh.sub.2.sup.0,0 (dfpma).sub.3 (L).sub.2 (3-L where L may be PR.sub.3, P(OR).sub.3, CNR for example), which are structurally analogous to Co.sub.2 (dfpma).sub.3 (L).sub.2 (Scheme 4). The coordination environment is characterized by trigonal bipyramidal Rh.sup.0 centers. A formal metal-metal bond is supported by three bridging dfpma ligands; the PF.sub.2 groups of these bridging ligands define the equatorial plane of each Rh.sup.0 center. The axial position, trans to the metal-metal bond is capped by a donor ligand, electron accepting axial ligands such as, for example, PF.sub.3, phosphites or isonitriles impart substantial stability, and aryl and alkyl phosphines continuing complexes have been prepared as well. The stability of the reduced species is illustrated by sublimation of Rh.sub.2.sup.0,0 (dfpma).sub.3 (PF.sub.3).sub.2 ("3-PF.sub.3 ") and Rh.sub.2.sup.0,0 (dfpma).sub.3 (72 .sup.1 -dfpma).sub.2 ("3-(.eta..sup.1 -dfpma)") under vacuum at 110.degree. C.
The two-electron oxidized rhodium dimers may be prepared from [XRh(cod)].sub.2 by the addition of three equivalents of dfpma and five equivalents of PhICl.sub.2 or Br.sub.2 (Scheme 5). The excess oxidant may be required to drive the reaction to completion, since coordination of the dfpma resulted in the production of free 1,5-cyclooctadiene, ##STR4## ##STR5##
which in turn consumed the halogen oxidant. Use of excess oxidant afforded pure Rh.sub.2.sup.II,II (dfpma).sub.3 X.sub.4 (X=Cl (4), Br (5)) in yields of greater then 90%. The oxidized Rh.sub.2.sup.II,II (dfpma).sub.3 X.sub.4 species comprises octahedral Rh.sup.II centers connected by a metal-metal bond. Three dfpma ligands maintain a bridging coordination mode, and halogen ligands occupy axial and equatorial portions on each metal center.
In general, the .sup.1 H and .sup.19 F NMR spectra are useful for characterizing the complexes, however due to excessively broad peaks in the output, .sup.31 P NMR cannot be used to probe the dfpma backbone. The dfpma complexes were not fluxional on the NMR time scale. The reduced complexes, Rh.sub.2.sup.0,0 (dfpma).sub.3 L.sub.2 (3-L), which display D.sub.3d symmetry in solution, show the simplest NMR patterns; .sup.1 H and .sup.19 F NMR data for 3-CO, 3-PPh.sub.3, 3-CN.sup.t Bu, 3-PF.sub.3 and 3-(72 .sup.1 -dfpma) are presented in Table 3. In addition to resonances for the axial L ligands, a singlet, the electronic state in which the total spin angular momentum is zero, is observed for the methyl groups of the bridging dfpma ligands in the 2-3 ppm region. One resonance is observed for the dfpma ligands near -40 ppm in the .sup.19 F NMR spectrum, and is easily identified by a strong .vertline..sup.1 J.sub.PF +.sup.3 J.sub.PF.vertline. of 1110-1120 Hz.
TABLE 3 .sup.1 H and .sup.19 F NMR data for Rh.sub.2.sup.0,0 (dfpma).sub.3 L.sub.2 (3-L; L = CO, PPh.sub.3, CN.sup.t Bu, PF.sub.3 and .eta..sup.1 -dfpma). Spectra were acquired in C.sub.6 D.sub.6 except as noted. Compound .sup.1 H(.delta./ppm) .sup.19 F(.delta./ppm).sup.a 3-CO 2.17(s) -40.31(d, 1116Hz) 3-PPh.sub.3.sup.b 2.68(s, 9H) -43.18(d, 1113Hz) 7.45(m, 30H) 3-CN.sup.t Bu 0.956(s, 18H) -40.53(d, 1083Hz) 2.58(s, 9H) 3-PF.sub.3 2.12(s) -7.16(d, 1359Hz, 6F) -40.74(d, 1109Hz, 12F) 3-(.eta..sup.1 -dfpma) 2.29(s, 9H) -29.73(d, 1175Hz, 4F) 2.34(d, 7Hz, 6H) -40.94(d, 1116Hz, 12F) -76.28(d, 1252Hz, 4F) .sup.a Listed coupling constant is .vertline. .sup.1 J.sub.PF + .sup.3 J.sub.PF .vertline., .sup.b Spectra acquired in CDCl.sub.3.
TABLE 4 .sup.19 F NMR (.delta. and .vertline. .sup.1 J.sub.PF + .sup.3 J.sub.PF .vertline.) data for Rh.sub.2.sup.II,II (dfpma).sub.3 X.sub.4 (X = Cl (4), Br (5)), the designations F.sub.a-e refer to the Neumann Projections in FIG. 1.2. Spectra were acquired in CDCl.sub.3. Assignment 4 (.delta./ppm) 5 (.delta./ppm) F.sub.a -78.7(1164Hz) 5 -73.9(1101Hz) F.sub.c -46.6(1128Hz) 2 -44.5(1128Hz) F.sub.d -47.3(1295Hz) 0 -47.6(1320Hz) F.sub.e -54.9(1160Hz) 6 -48.9(1150Hz) F.sub.b -60.6(1200Hz) 2 -59.3(1185Hz)
TABLE 5 .sup.19 F NMR (.delta. and .vertline. .sup.1 J.sub.PF + .sup.3 J.sub.PF .vertline.) data for Rh.sub.2.sup.0,II (dfpma).sub.3 X.sub.2 (L) (X = Cl (1-L), Br (2-L)), the designations F.sub.a-e refer to the Neumann Projections in FIG. 1.2. Spectra were acquired in CDCl.sub.3. 2-P(O.sup.i Pr).sub.3 Assignment 1-PPh.sub.3 (.delta./ppm) 2-PPh.sub.3 (.delta./ppm) (.delta./ppm) F.sub.c -35.7(1047Hz) 0 -35.7(1067Hz) 4 -39.6(1089Hz) F.sub.e -40.6(1181Hz) 0 -40.4(1122Hz) 0 -40.9(1155Hz) F.sub.f -41.8(1106Hz) 0 -41.5(1187Hz) 0 -41.8(1115Hz) F.sub.b -49.5(1166Hz) 3 -46.8(1174Hz) 0 -46.5(1174Hz) F.sub.d -52.0(1121Hz) 0 -52.0(1118Hz) 1 -51.3(1117Hz) F.sub.a -79.1(1121Hz) 5 -74.1(1132Hz) 1 -73.0(1120Hz)
The .sup.1 H NMR spectra of the Rh.sub.2.sup.II,II (dfpma).sub.3 X.sub.4 (X=Cl (4), Br (5)) complexes show the resonances for the methyl groups of the bridging dfpma ligands as a multiplet at 3.2 ppm. The .sup.19 F NMR spectrum contains several first- and second-order spin systems, but simplifies to a five-line pattern upon .sup.31 P decoupling, consistent with a C.sub.2 symmetric structure for 4 and 5 in solution. For illustrative purposes the .sup.19 F and .sup.19 F{.sup.31 P} NMR spectra of 4 are displayed in FIGS. 2(a) and (b), respectively (Table 4 contains the chemical shift values and .vertline..sup.1 J.sub.PF +.sup.3 J.sub.PF.vertline. coupling constants for both 4 and 5). Notably, the fluorine atoms of the PF.sub.2 groups located trans to the halide ligands (F.sub.a in the Neumann Projection of FIG. 2(a)) are shifted to low frequency at -78.7 ppm. Four AB sub-spectra observed inside and outside of the .vertline..sup.1 J.sub.PF +.sup.3 J.sub.PF.vertline. doublet is indicative of direct (.sup.1 J.sub.PF) and indirect (.sup.3 J.sub.PF) phosphorus coupling constants of opposite sign. Similar XX'AA'X"X'" coupling patterns in the resonances at -60.6 and -46.6 ppm identify these resonances as the fluorine atoms F.sub.b and F.sub.c, respectively, coupled to F.sub.a through the nitrogen bridgehead. The remaining resonances at -47.3 and -54.9 ppm are assigned as F.sub.d and F.sub.e, respectively, each displaying only a doublet with a coupling constant given by .vertline..sup.1 J.sub.PF +.sup.3 J.sub.PF.vertline..
Interpretation of the NMR spectra for the Rh.sub.2.sup.0,II (dfpma).sub.3 X.sub.2 (L) (X=Cl (1-L), Br (2-L)) complexes follows along similar lines. The coordination of a PF.sub.2 of one dfpma ligand trans to a halogen on the octahedral Rh.sup.II center gives rise to two .sup.1 H NMR methyl resonances at 2.8 and 2.9 ppm for the bridging dfpma ligands, integrating in a 1:2 ratio, respectively. Additionally, the .sup.1 H NMR displays resonances diagnostic of the axial ligand, L, capping the Rh.sup.0 center. The .sup.19 F and .sup.19 F{.sup.31 P} NMR spectra of 1-PPh.sub.3 are presented in FIGS. 2(c) and 2(d) (representative .sup.19 F NMR data for 1-L can be found in Table 5). Several complex spin systems are observed in the .sup.19 F NMR absorption spectrum, but simplify to a six-line pattern for the dfpma ligands upon 31P decoupling. The structure of 1-PPh.sub.3 is C.sub.s symmetric in solution, as can be seen in the Neumann Projection inset into FIG. 2(c); the mirror plane is defined by the two rhodium centers and the equatorial halogen ligand. By analogy to the spectra of 4, the low frequency resonance at -79.3 ppm is indicative of a PF.sub.2 group coordinated trans to the halide ligand, thus assigned to F.sub.a ; the fluorine atoms of the Rh.sup.0 -coordinated PF.sub.2 group (F.sub.b) of this dfpma ligand are resolved at -49.5 ppm. The two Rh.sup.0 -coordinated PF.sub.2 groups (F.sub.e and F.sub.f) are observed as .vertline..sup.1 J.sub.PF +.sup.3 J.sub.PF.vertline. doublets at -40.6 and -41.8 by analogy to the Rh.sub.2.sup.0,0 complexes, 3-L. The remaining resonances at -35.7 and -52.0 ppm are assigned to the fluorine atoms of the remaining Rh.sup.II PF.sub.2 groups, F.sub.c and F.sub.c.
Bis(dimethylphosphite)methyl amine
The preparation of bis(dimethylphosphite)methyl amine (dmpma) and bis(diphenylphosphite)methyl amine (dppma) is straightforward. Treatment of MeN(PCl.sub.2).sub.2 with four equivalents of methanol or phenol and four equivalents of a tertiary amine base in ether or pentane gave the ligand and HCl salt. The dmpma and dppma ligands were purified by distillation and recrystallization, respectively.
Addition of dmpma to CH.sub.2 Cl.sub.2 solutions of [ClRh(CO).sub.2 ].sub.2 resulted in an immediate color change, affording an intense red solution. Precipitation yielded the product as an orange powder, which analyzed as Rh.sub.2 (dmpma).sub.2 Cl.sub.2 (CO).sub.2 (6). This compound has been characterized by X-ray diffraction and has the structure depicted in Scheme 6. Infrared spectroscopy showed two CO frequencies, at 1989 and 1808 cm.sup.-1, for both terminal and bridging CO ligands. However, .sup.31 P NMR spectroscopy showed a single resonance at 132.81 ppm with a .vertline..sup.1 J.sub.PRh +.sup.n J.sub.PRh.vertline. of 142 Hz. VT NMR studies suggested a fluxional dirhodium core. Cooling of CD.sub.2 Cl.sub.2 solutions of 6, led to broadening of the .sup.31 P NMR resonance and a decrease in the observed phosphorus-rhodium ##STR6##
coupling constant; however, the exchange process could not be arrested within the accessible temperature range. The solution NMR data is nevertheless consistent with the solid-state structure of 6 reported in the literature, in which a single CO ligand bridges the binuclear Rh.sup.l core.
Single crystals of 6 were obtained from CH.sub.2 Cl.sub.2 /heptane solution. Here the X-ray diffraction study gave an isomeric structure with two bridging CO groups (FIG. 3). Two dmpma ligands bridge a valence-symmetric binuclear rhodium core possessing terminal chloride ligands. A Rh--Rh separation of 2.7155(10) .ANG. and an intra-ligand P . . . P separation of 2.913(3) .ANG. indicate the presence of a formal metal-metal bond. Other parameters of note are an average Rh--P distance of 2.2867(19) .ANG. and an average P--N distance of 1.666(6) .ANG.. The Rh(1)'-Rh(1)-Cl(1) angle is 159.73(12).degree., and the Rh(1)-Cl(1) distance is normal at 2.418(4) .ANG..
Bis(diphenylphosphite)methyl amine (dppma)
Similar RhI2 products were obtained for the reaction of dppma with [ClRh(CO).sub.2 ].sub.2 in CH.sub.2 Cl.sub.2 solution, as shown in Scheme 7. Ligand addition resulted in an immediate color change from pale yellow-orange to very dark purple. Addition of cold pentane caused precipitation of a purple powder in moderate yields; elemental analysis suggested the formulation Rh.sub.2 (dppma).sub.2 Cl.sub.2 (CO).sub.2 (7). Infrared spectra obtained for KBr pellets of the product showed only a terminal carbonyl frequency at 2009 cm.sup.-1. This, taken with the intense purple color of the complex suggested a structure containing face-to-face square planar rhodium(I) centers as shown for 7 in Scheme 7. However, solutions of the complex showed both bridging and terminal carbonyl resonances, and VT NMR studies suggested an equilibrium between the face-to-face structure and a structure with one bridging carbonyl (8), as observed for 6.
In solution, 7 slowly rearranged to give a new isomer with two bridging CO ligands and two terminal halides, Rh.sub.2 (dppma).sub.2 Cl.sub.2 (.mu.-CO).sub.2 (9), characterized by a single .nu..sub.CO at 1828 cm.sup.1. The .sup.1 H and .sup.31 P NMR spectra of 9 revealed that this isomer is not fluxional in solution. The .sup.1 H NMR spectrum displayed a single methyl resonance for the bridging dppma ligands at 2.86 ppm; the protons of the phenyl rings are observed in ##STR7##
the 6.8-7.5 ppm region. A single phosphorus resonance was observed in the .sup.31 P NMR spectrum of 9 at 122.61 ppm (.vertline..sup.1 J.sub.PRh +.sup.n J.sub.PRh.vertline.=170 Hz)
Single crystals suitable for X-ray diffraction studies were obtained for 9 and the data confirmed the solution structure assignments. FIG. 4 shows a structure with two dppma ligands and two CO ligands spanning a short metal-metal separation of 2.7076(8) .ANG.. The carbonyl ligands bridge the dirhodium core in a symmetric fashion with Rh--C(1) and Rh'--C(1) distances of 1.990(13) and 2.010(13) .ANG., respectively; the Rh--C(1)-Rh' angle is 85.2(4).degree.. The axial halogen has a normal Rh--X distance of 2.3379(15) .ANG., and the Cl--Rh--Rh' angle is nearly linear at 175.64(6). The PNP backbone is symmetric with N--P distances of 1.661(5) and 1.676(5) .ANG., and the intra-ligand P . . . P separation is 2.886(3) .ANG..
Solutions of 9 decomposed slowly by loss of the carbonyl ligands. The final product observed was the chelated dimer, [ClRh(tfepma)].sub.2 (11), which was characterized by a single resonance in the .sup.31 P NMR spectrum at 91.13 ppm and an apparent J.sub.RhP coupling constant of 280 Hz. Purging solutions of 11 with carbon monoxide regenerated rhodium dimer 7. CO loss from 9 is proposed to proceed through a two-electron mixed-valence complex based on a literature report of a monocarbonyl derivative of the related bis(diphenylphosphite)ethyl amine (dppea) ligand. Single crystals of Rh.sub.2.sup.0,II (dppea).sub.2 Cl.sub.2 (CO) reportedly were obtained from CHCl.sub.3 solutions of Rh.sub.2.sup.I,I (dppea).sub.2 Cl.sub.2 (CO).sub.2 and provided the coordination environment depicted for 10 in Scheme 7. However, 10 was not explicitly isolated nor characterized as an intermediate in the conversion of 9 to 11.
The dppma ligand supported two-electron oxidation and reduction of the binuclear rhodium core, yielding Rh.sub.2.sup.0,0 and Rh.sub.2.sup.II,II products. 9 was reduced with two equivalents of cobaltocene in the presence of dppma, affording Rh.sub.2 (dppma).sub.3 (CO).sub.2 (12) as a yellow microcrystalline solid. Alternatively, 12 was directly prepared from the reaction of [ClRh(CO).sub.2 ].sub.2 with three equivalents of dppma and cobaltocene. Single crystals of 12 were obtained by cooling CH.sub.2 Cl.sub.2 /pentane solutions to -35.degree. C. The molecular structure is presented in FIG. 5(a) and pertinent metrical parameters are listed in Table 6. The metal-metal separation in 12 is long at 2.8687(8).ANG., however, it is within the range expected for a single Rh--Rh bond. The equatorial coordination planes are staggered with respect to one another, adopting a torsion angle of 35.2.degree.. A C(3)-O(7) bond distance of 1.138(6) .ANG. is relatively long, reflecting a decrease in bond order of the carbonyl ligand due to coordination to the electron-rich Rh.sup.0 center. As expected, the PNP backbone of the bridging dppma ligands is symmetric with an average P--N distance of 1.681(4) .ANG..
Oxidation of 9 with dichloroiodobenzene afforded the valence-symmetric Rh.sub.2.sup.II,II dimer Rh.sub.2 (dppma).sub.2 Cl.sub.4 (CO).sub.2 (13), presented as a thermal ellipsoid plot in FIG. 5(b). Alternatively, the reaction of [ClRh(cod)].sub.2 with PhICl.sub.2 and dppma yielded 13 directly. In 13, only two dppma ligands coordinate to the bimetallic core, and small donor ligands such as CO or isonitrile must be incorporated to yield the six-coordinate octahedral geometry at each rhodium center. The metal-metal separation is significantly contracted relative to the reduced complex, 12. A Rh--Rh' distance of 2.705(3) .ANG. in 13 is complimented by a torsion angle for the equatorial planes of only 0.9.degree.. The dppma ligands coordinate trans to one another, with average Rh--P distances of 2.300(6) .ANG.. The Rh--Cl(2) distance of 2.382(7) .ANG. is in the typical range for halogens coordinated trans to a Rh--Rh bond. The equatorial chloride and carbonyl ligands are coordinated in an anti conformation, with Rh--Cl(1) and Rh--C(1) distances of 2.365(8) and 1.95(4) .ANG., respectively. A short C(1)-O(1) bond distance of 1.09(4) reflects little decrease in the CO bond order.
TABLE 6 Selected bond lengths (.ANG.) for Rh.sub.2 (dppma).sub.3 (CO).sub.2 (12) and Rh.sub.2 (dppma).sub.2 Cl.sub.4 (CO).sub.2 (13). Selected Bond Lengths (.ANG.) Rh.sub.2 (dppma).sub.3 (CO).sub.2 Rh.sub.2 (dppma).sub.2 Cl.sub.4 (CO).sub.2 Rh-Rh' 2.8687(8) Rh-Rh' 2.705(3) Rh-P(1) 2.2911(15) Rh-Cl(1) 2.365(8) Rh-P(2) 2.2615(15) Rh-Cl(2) 2.382(7) Rh-P(3) 2.2987(15) Rh-P(1) 2.293(6) Rh-C(3) 1.886(6) Rh-P(2) 2.307(6) C(3)-O(7) 1.138(6) Rh-C(1) 1.95(4) P(1)-N(1) 1.686(4) C(1)-O(1) 1.09(4) P(2)-N(2) 1.678(3) P(1)-N(1) 1.680(17) P(3)-N(1)' 1.678(4) P(2)-N(1)' 1.674(17)
The oxidized and reduced complexes, 12 and 13, of dppma were prepared with isonitrile ligands replacing the carbonyl groups. An X-ray structure obtained on single crystals of Rh.sub.2 (dppma).sub.2 Cl.sub.4 (CN.sup.t Bu).sub.2 confirmed that it is isostructural with the carbonyl derivative, including the eclipsed geometry of the rhodium coordination spheres.
Bis(bis(trifluoroethyl)phosphite)methyl amine ("tfepma")
Based on the results of the methoxy- and phenoxy-substituted PNP ligands, a phosphorus substituent was sought with stronger .pi.-acid properties and a steric parameter larger than fluorine. Tolman's steric and electronic map for tertiary phosphine ligands shows that functionalization with halogenated alkanes and alcohols result in strong .pi.-acid characteristics. Tris(2,2,2-trichloroethyl)phosphite has a cone angle comparable to other primary alcohol derivatives, yet is a stronger .pi.-acid. Therefore, it was surmised that the 2,2,2-trifluoroethanol derivative would give a PNP ligand with enhanced .pi.-acid character while maintaining steric characteristics similar to those of dmpma.
The reaction of 2,2,2-trifluoroethanol with MeN(PCl.sub.2).sub.2 in ether/pentane mixtures at dry-ice/acetone temperatures proceeded smoothly to afford MeN[P(OCH.sub.2 CF.sub.3).sub.2 ].sub.2 (tfepma) in moderate yield after distillation. The viscous liquid was characterized by .sup.1 H NMR spectroscopy, which showed a triplet-of-doublets pattern at 2.51 ppm for the methyl protons of the methylamine bridgehead and a doublet-of-pentets pattern at 3.55 ppm for the methylene protons of the trifluoroethyl substituents. The .sup.31 P{.sup.1 H} NMR spectrum showed a singlet at 148.6 ppm and the .sup.19 F NMR spectrum showed a multiplet centered at -76.90 ppm.
Complexes with Bridging tfepma Ligands
The pale orange color of [ClRh(CO).sub.2 ].sub.2 in methylene chloride immediately changed to dark red upon dropwise addition of tfepma. Concentration by vacuum distillation followed by addition of pentane afforded a yellow-orange powder in moderate yield. Elemental analysis suggested the formulation Rh.sub.2 (tfepma).sub.2 Cl.sub.2 (CO).sub.2 (14), and infrared spectra obtained on KBr pellets of this product showed a carbonyl stretch at 2007 cm.sup.-1. As with the dmpma and dppma complexes, 14 is fluxional in solution. At room temperature, time-averaged NMR spectra were obtained: a single methyl resonance was observed at 2.94 ppm in the .sup.1 H NMR spectrum and a symmetric .sup.31 P resonance was observed at 131.16 ppm (.vertline..sup.1 J.sub.PRh +.sup.n J.sub.PRh.vertline.=149 Hz) in the .sup.31 P NMR spectrum. Based on the observed .vertline..sup.1 J.sub.PRh +.sup.n J.sub.PRh.vertline. coupling constant, a structure with two tfepma ligands spanning a Rh.sup.I.sub.2 core, axial Cl ligands, and bridging and terminal carbonyl ligands is assigned for 14 as shown in Scheme 8.
Modified forcing reaction conditions afforded a doubly CO-bridged isomer. Refluxing [ClRh(CO).sub.2 ].sub.2 with two equivalents of tfepma in toluene gave Rh.sub.2 (tfepma).sub.2 Cl.sub.2 (.mu.-CO).sub.2 (15) in quantitative yield. Infrared spectroscopy showed only a bridging carbonyl stretch at 1824 cm.sup.-1. .sup.31 P NMR spectroscopy again revealed a symmetric complex, with a single resonance at 128.4 ppm (.vertline..sup.1 J.sub.PRh +.sup.n J.sub.PRh.vertline.=164 Hz). The IR and NMR data are consistent with rearrangement of the metal core to give a structure analogous to 9. However, unlike the dppma compound, 15 is thermally robust and is not subject to further rearrangements of the metal core in the absence of other reagents: even at reflux in toluene, CO was not lost from the complex. ##STR8##
Treatment of solutions of 14 or 15 with one equivalent of tfepma gave a dark red solution from which maroon crystals were deposited upon prolonged standing. The crystals proved suitable for X-ray diffraction studies and yielded the molecular structure presented in FIG. 6 for Rh.sub.2.sup.0,II (tfepma).sub.3 Cl.sub.2 (CO) (16). As observed for the dfpma chemistry of rhodium, three tfepma ligands span the bimetallic core of 16. The two-electron mixed-valence complex comprises octahedral Rh.sup.II and trigonal bipyramidal Rh.sup.0 centers. The axial site of the Rh.sup.0 is coordinated by a CO ligand, and two chloride ligands occupy axial and equatorial sites of the Rh.sup.II. The metal-metal separation is normal at 2.7662(11) .ANG.. Rh--P bond distances are longer by an average of 0.037 .ANG. than those observed for the dfpma complexes due to the greater steric demand of the P(OCH.sub.2 CF.sub.3).sub.2 groups. Other metrical parameters for 16 are listed in Table 7, and in general suggest an expanded core coordination environment for the dirhodium center.
In solution, the two-electron mixed-valence complex was unstable with respect to loss of the axial CO ligand. The principal species observed by NMR spectroscopy was in fact the halogen-bridged Rh.sup.I.sub.2 dimer with two chelating tfepma ligands, [ClRh(tfepma)].sub.2 (17). This complex was prepared independently by the addition of two equivalents of tfepma to [ClRh(cod)].sub.2. The symmetric dimer was treated with one equiv of tfepma and one equiv of CN.sup.t Bu to reform the two-electron mixed-valence complex, Rh.sub.2.sup.0,II (tfepma).sub.3 Cl.sub.2 (CN.sup.t Bu), with an axial isonitrile ligand; however, the product was always contaminated with 17.
TABLE 7 Bond lengths (.ANG.) and angles (deg) for Rh.sub.2.sup.0,II (tfepma).sub.3 Cl.sub.2 (CO) (16). Selected Bond Lengths (.ANG.) Rh(1)-Rh(2) 2.7662(11) Rh(2)-Cl(1) 2.513(3) Rh(1)-P(1) 2.284(3) Rh(2)-Cl(2) 2.399(3) Rh(1)-P(3) 2.255(3) Rh(2)-P(2) 2.308(3) Rh(1)-P(5) 2.287(3) Rh(2)-P(4) 2.214(3) Rh(1)-C(4) 1.895(12) Rh(2)-P(6) 2.281(3) P(1)-N(1) 1.685(10) P(2)-N(1) 1.670(10) P(3)-N(2) 1.686(9) P(4)-N(2) 1.667(9) P(5)-N(3) 1.673(10) P(6)-N(3) 1.658(10) Selected Bond Angles (deg) Rh(2)-Rh(1)-C(4) 176.9(4) Cl(1)-Rh(2)-Cl(2) 91.51(10) Rh(1)-Rh(2)-Cl(1) 176.54(7) Cl(1)-Rh(2)-P(2) 82.86(10) Rh(1)-Rh(2)-Cl(2) 87.97(7) Cl(1)-Rh(2)-P(4) 87.23(10) Cl(2)-Rh(2)-P(2) 87.34(11) Cl(1)-Rh(2)-P(6) 98.51(10) Cl(2)-Rh(2)-P(4) 175.59(11) P(7)-Rh(1)-P(1) 90.0(3) Cl(2)-Rh(2)-P(6) 85.20(11) C(4)-Rh(1)-P(3) 94.5(4) P(1)-Rh(2)-P(3) 110.68(12) C(4)-Rh(1)-P(5) 90.0(3) P(1)-Rh(2)-P(5) 130.87(12) Rh(1)-C(4)-O(1) 177.8(11)
The tfepma ligand supports two-electron reduced species, Rh.sub.2.sup.0,0 (tfepma).sub.3 L.sub.2, (18-L) upon reduction of [ClRh(CO).sub.2 ].sub.2 with cobaltocene in the presence of three equiv tfepma and two equiv of L (L=CO, CN.sup.t Bu). Alternatively, cobaltocene reduced 15 to 18-L in the presence of L and one equivalent of tfepma. Spectroscopic data is consistent with a binuclear Rh.sup.0 core bridged by three tfepma ligands. The .sup.31 P NMR of 18-CN.sup.t Bu showed a single resonance at 142.08 ppm displaying a 216 Hz coupling constant. Similarly, the .sup.1 H NMR showed one methyl and one methylene resonance for the bridging tfepma ligands at 2.44 and 4.18 ppm, respectively, and a resonance for the CN.sup.t Bu ligands at 1.05 ppm.
Oxidation of the dirhodium core proceeded smoothly but yielded different structural isomers depending on reaction conditions. The reaction of tfepma and PhICl.sub.2 with [ClRh(CO).sub.2 ].sub.2 at room temperature in CH.sub.2 Cl.sub.2 afforded a bright yellow powder with the formula Rh.sub.2.sup.II,II (tfepma).sub.3 Cl.sub.4 in moderate yields. X-ray diffraction studies on single crystals of this product yielded the molecular structure depicted in FIG. 7(a), identifying the complex as the syn isomer, syn-Rh.sub.2.sup.II,II (tfepma).sub.3 Cl.sub.4 (19). If the reaction instead was conducted at reflux in toluene, an orange product was obtained, the solid-state structure as determined by X-ray diffraction identified this product as the anti isomer, anti-Rh.sub.2.sup.II,II (tfepma).sub.3 Cl.sub.4 (20). The molecular structure for 20 is presented in FIG. 7(b). Selected bond distances and angles for each complex are presented in Table 8. Comparison of the two isomers reveals a significant contraction of the metal core for 19. The Rh(1)-Rh(2) separation in the syn isomer is 2.6688(8) .ANG., whereas the metal-metal distance in 20 is 2.7152(10) .ANG.. Similarly, the metal-phosphorus and metal-halogen distances are markedly different: the Rh--P distances in 20 are longer by 0.045 .ANG. and the axial and equatorial Rh--Cl distances are elongated by 0.062 and 0.061 .ANG., respectively. The syn isomer also shows a much smaller twist about the Rh--Rh axis, with an average torsion angle of 26.8.degree. as compared to that of the anti isomer at 40.4.degree., which is almost perfectly eclipsed.
TABLE 8 Bond lengths (.ANG.) for syn-Rh.sub.2.sup.II,II (tfepma).sub.3 Cl.sub.4 (19) and anti-Rh.sub.2.sup.II,II (tfepma).sub.3 Cl.sub.4 (20). Selected Bond Lengths (.ANG.) syn-Rh.sub.2.sup.II,II (tfepma).sub.3 Cl.sub.4 anti-Rh.sub.2.sup.II,II (tfepma).sub.3 Cl.sub.4 Rh(1)-Rh(2) 2.6688(8) Rh(1)-Rh(2) 2.7152(10) Rh(1)-Cl(1) 2.401(2) Rh(1)-Cl(1) 2.456(2) Rh(1)-Cl(2) 2.334(2) Rh(1)-Cl(2) 2.398(2) Rh(2)-Cl(3) 2.385(2) Rh(2)-Cl(3) 2.455(2) Rh(2)-Cl(4) 2.342(2) Rh(2)-Cl(4) 2.399(2) Rh(1)-P(1) 2.281(2) Rh(1)-P(1) 2.223(2) Rh(1)-P(3) 2.152(2) Rh(1)-P(3) 2.271(2) Rh(1)-P(5) 2.248(2) Rh(1)-P(5) 2.329(2) Rh(2)-P(2) 2.258(2) Rh(2)-P(2) 2.275(2) Rh(2)-P(4) 2.154(2) Rh(2)-P(4) 2.216(2) Rh(2)-P(6) 2.271(2) Rh(2)-P(6) 2.319(2)
The structural differences between the syn and anti isomers of Rh.sub.2.sup.II,II (tfepma).sub.3 Cl.sub.4 are manifested in the absorption and emission spectra of each complex, presented in FIG. 8. The absorption spectra of 19 and 20 are dominated by three bands in the near-UV and visible regions. Two high-energy bands, assigned to transitions from the configurationally mixed Cl(.sigma.)/d.sub.z.sub..sup.2 .sigma. to the d.sub.z.sub..sup.2 .sigma.* LUMO, shift little between the two isomeric forms. However, the lowest energy transition shows a pronounced dependence on the geometry of the dirhodium core: the transition, of d.pi.*.fwdarw.d.sub.z.sub..sup.2 .sigma.* parentage, shifts from 352 nm in 19 to 458 nm in 20. This red shift of over 6500 cm.sup.-1 is accompanied by a decrease in the extinction coefficient of the band from 11,000 M.sup.-1 cm.sup.-1 to 4700 M.sup.-1 cm.sup.-1 for the syn and anti isomers, respectively. Excitation of solid samples of both 19 and 20 cooled to 77 K results in visible luminescence. The emission spectrum of 19 is dominated by bright, long-lived phosphorescence observed at 819 nm (.tau.=350 .mu.s). A high-energy band is also observed at 578 nm (.tau.<8 ns), but has only 1/100.sup.th of the intensity of the phosphorescent peak, suggesting facile deactivation of the .sup.1 d.sub.z.sub..sup.2 .sigma.* excited state. Conversely, both fluorescence and phosphorescence are strong for 20. A high-energy emission band at 573 nm (.tau.<8 ns) and a low-energy emission band at 857 nm (.tau.=150 .mu.s) are observed with an intensity ratio of 1:2 (the relative intensities of 19 to 20 were not determined).
The influence of structure on electronic character is further emphasized by the similarity between the electronic features of 20 and the dfpma congener, Rh.sub.2.sup.II,II (dfpma).sub.3 Cl.sub.4 (4). The absorption maxima of the two high-energy transitions are essentially identical and the low energy d.pi.*.fwdarw.d.sub.z.sub..sup.2 .sigma.* transition of 20 is red-shifted by only 640 cm.sup.-1 relative to the same transition in 4. Though negligible differences are observed in the fluorescence emission maxima, the phosphorescence of 20 is red shifted by 511 cm.sup.-1. This small low-energy shift is diagnostic of a weaker metal-metal interaction in 20, and correlates well with the observed difference in metal-metal separation for 4 and 20 (.DELTA.Rh(1)-Rh(2)=0.008(1) .ANG.).
A Chelating tfepma Ligand
The tfepma ligand may be used to stabilize a new two-electron mixed-valence rhodium platform in the absence of an axial donor ligand. [ClRh(cod)].sub.2 reacted with three equivalents of tfepma in CH.sub.2 Cl.sub.2, affording Rh.sub.2.sup.0,II (tfepma).sub.3 Cl.sub.2 (21). Dark green single crystals of 21 were obtained by diffusing pentane into a CH.sub.2 Cl.sub.2 solution of the complex. The structure presented in FIG. 9 was obtained from X-ray diffraction studies; selected bond distances and angles are presented in Table 9. The molecular structure of 21 reveals a departure from the coordination geometry observed for other diphosphazane complexes of rhodium. In the absence of a suitable donor ligand for the Rh.sup.0 center, a tfepma ligand binds in a chelating mode leaving only two tfepma ligands to bridge the bimetallic core. A consequence of this chelating ligand is the vacant axial coordination site at the Rh.sup.II center. Two phosphorus of the bridging tfepma ligands and one phosphorus of the chelating tfepma ligand define the equatorial plane of a distorted trigonal bipyramidal Rh.sup.0 center. The axial positions are capped by the other phosphorus of the chelate and the proximate Rh.sup.II center, bound at a metal-metal separation of 2.7529(19) .di-elect cons.. The Rh.sup.0 center defines the apex of a square pyramid about the Rh.sup.II center. Two phosphorus of the bridging tfepma ligands and two chloride ligands occupy cis positions of the basal plane, with P(2)-Rh(2)-P(4) and Cl(1)-Rh(2)-Cl(2) angles of 95.65(18).degree. and 86.52(18).degree., respectively.
TABLE 9 Bond lengths (.ANG.) and angles (deg) for Rh.sub.2.sup.0,II (tfepma).sub.3 Cl.sub.2 (21) from preliminary X-ray data. Selected Bond Lengths (.ANG.) Rh(1)-Rh(2) 2.7529(19) Rh(2)-Cl(1) 2.359(5) Rh(1)-P(1) 2.240(5) Rh(2)-Cl(2) 2.361(5) Rh(1)-P(3) 2.295(5) Rh(2)-P(2) 2.188(5) Rh(1)-P(5) 2.285(5) Rh(2)-P(4) 2.190(5) Rh(1)-P(6) 2.176(5) P(1)-N(1) 1.689(16) P(2)-N(1) 1.692(17) P(3)-N(2) 1.668(13) P(4)-N(2) 1.677(14) Selected Bond Angles (deg) P(5)-Rh(1)-P(1) 137.48(18) Cl(1)-Rh(2)-Cl(2) 86.52(18) P(5)-Rh(1)-P(3) 114.79(17) Cl(1)-Rh(2)-P(2) 88.15(18) P(5)-Rh(1)-P(6) 70.07(18) Cl(1)-Rh(2)-P(4) 171.09(18) P(6)-Rh(1)-Rh(2) 164.31(15)
Aniline-Derived Diphosphazanes
As a preliminary investigation of the electronic factors associated with exchanging the methyl group on the nitrogen bridgehead, 3,5-xylidene was refluxed in PCl.sub.3 to afford bis(dichlorophosphine)-3,5-xylidene (dcpx). This precursor can be further functionalized as shown in Scheme 2, yielding the same variety of phosphorus groups for metal coordination. Fluorination with SbF.sub.3 provided bis(difluorophosphine)xylidene (dfpx) in good yields after recrystallization from pentane and sublimation. Similarly, 2,2,2-trifluoroethanol reacted with dcpx to yield the corresponding trifluoroethyl derivative bis(bis(trifluoroethyl)phosphite)-3,5-xylidene (tfepx) as a crystalline solid from pentane.
Bis(difluorophosphine)-3,5-xylidene (dfpx)
Two-electron mixed-valence complexes of dfpx were prepared by the strategy presented in Scheme 3. A dirhodium core isostructural to 1 and 2 is presented in FIG. 10 for Rh.sub.2.sup.0,II (dfpx).sub.3 Br.sub.2 (PPh.sub.3) (22). The metrical parameters for the metal core of 22 are presented in Table 10, and are similar to those of the corresponding dfpma-bridged dirhodium complexes (Table 2); the N--P distances for the bridging dfpx ligands are 1.611(18) and 1.679(16) .ANG. for the phosphines coordinated to the Rh.sup.II and Rh.sup.0 centers, respectively. The aryl rings on the nitrogen bridgehead align perpendicularly to the P--N--P plane, with an average dihedral angle of 90.4.degree.. At 2.819(3) .ANG., the metal-metal separation is slightly shorter than that of the dfpma derivative; Rh--Br distances of 2.570(3) and 2.463(5) are contracted as well. Other ligand distances are undistinguished from the dfpma analog.
The absorption spectra of the homologous dfpma and dfpx complexes provided further evidence for the complete decoupling of the aryl .pi. system of dfpx from the core of the dirhodium complex. Aside from subtle changes in the peak intensities, absorption features are identical, displaying the expected three band pattern: for comparison, .lambda..sub.abs max =309, 366 and 410 nm for 2-PPh.sub.3 and .lambda..sub.abs max =314, 370 and 411 nm for 22-PPh.sub.3.
TABLE 10 Bond lengths (.ANG.) and angles (deg) for Rh.sub.2.sup.0,II (dfpx).sub.3 Br.sub.2 (PPh.sub.3) (22). Selected Bond Lengths (.ANG.) Rh(1)-Rh(2) 2.819(3) Rh(2)-Br(1) 2.570(3) Rh(1)-P(1) 2.227(6) Rh(2)-Br(2) 2.463(5) Rh(1)-P(3) 2.220(7) Rh(2)-P(2) 2.219(6) Rh(1)-P(5) 2.242(6) Rh(2)-P(4) 2.184(7) Rh(1)-P(7) 2.330(6) Rh(2)-P(6) 2.262(6) P(1)-N(1) 1.658(16) P(2)-N(1) 1.641(17) P(3)-N(2) 1.689(17) P(4)-N(2) 1.571(19) P(5)-N(3) 1.690(15) P(6)-N(3) 1.620(16) Selected Bond Angles (deg) Rh(2)-Rh(1)-P(7) 179.07(17) Br(1)-Rh(2)-Br(2) 90.79(13) Rh(1)-Rh(2)-Br(1) 178.13(9) Br(1)-Rh(2)-P(2) 95.35(17) Rh(1)-Rh(2)-Br(2) 90.68(12) Br(1)-Rh(2)-P(4) 85.61(18) Br(2)-Rh(2)-P(2) 87.7(2) Br(1)-Rh(2)-P(6) 91.98(17) Br(2)-Rh(2)-P(4) 176.35(18) P(7)-Rh(1)-P(1) 93.8(2) Br(2)-Rh(2)-P(6) 84.0(2) P(7)-Rh(1)-P(3) 92.3(2) P(1)-Rh(2)-P(3) 126.2(2) P(7)-Rh(1)-P(5) 93.1(2) P(1)-Rh(2)-P(5) 114.2(2)
Bis(bis(trifluoroethyl)phosphite)-3,5-xylidene (tfepx)
The reaction of tfepx with [ClRh(cod)].sub.2 did not provide direct access to a two-electron mixed-valence complex. Rather, a bright yellow microcrystalline product precipitated from the green solution afforded by 1:3 mixtures of the rhodium starting material and ligand in CH.sub.2 Cl.sub.2. Elemental analysis suggested the empirical formulation Rh.sub.2 (tfepx).sub.3 Cl.sub.2, consistent with the formulation of 21. However, X-ray diffraction studies revealed a divergent binuclear RhI structure, [ClRh(tfepx)].sub.2 (.mu.-tfepx) (23), shown as a thermal ellipsoid plot in FIG. 11. Here, two four-coordinate RhI fragments are tethered by a bridging tfepx ligand. A single chloride and a chelating tfepx ligand complete the distorted square planar coordination environment about each rhodium center. An average intra-ligand P--Rh--P angle of 70.38(8).degree. is acute for a square-planar metal complex, suggesting a significant degree of strain in the four-membered chelate ring. All tfepx ligands show symmetric PNP backbones, with an average N--P distance of 1.694(6) .ANG.. As observed in the structure of 22, the aryl ring of the bridging tfepx ligand is nearly orthogonal to the P(2)-N(2)-P(3) plane, adopting a torsion angle of 81.4.degree.. Conversely, the aryl rings of the chelating tfepx ligands of 23 are rotated to be coplanar with the P--N--P triad, displaying torsion angles of 14.30 and 9.10 for the tfepx ligands of the Rh(1) and Rh(2) centers, respectively. Rotation of the aryl rings into the P--N--P plane delocalizes the nitrogen lone pair into the aromatic system, shortening the N--C bond. In accord with this suggestion, the average N--C bond length for the chelating tfepx ligands is 1.446(9) .ANG. versus 1.485(9) .ANG. for the N--C bond of the bridging tfepx ligand.
Isolation of discrete two-electron mixed-valence complexes of rhodium relies on a delicate balance of stereo-electronic properties within the diphosphazane ligand framework. The results suggest that in addition to the coordination of three ligands to the binuclear core, the strong .pi.-acid phosphine groups and a polarizable lone pair on the nitrogen bridgehead atom help induce valence-symmetric cores to disproportionate.
The fluorinated diphosphazane, dfpma, rapidly reacts with Rh.sup.I starting materials to afford two-electron mixed-valence complexes in nearly quantitative yields. The strong .pi.-acid character of the difluorophosphine groups coupled with the small steric parameter facilitates coordination of three ligands to the bimetallic core. Crystal structures of various derivatives of 1 provide evidence for induced polarization of the PNP backbone as proposed in Scheme 1. A contracted N--P bond for the phosphines coordinated to the Rh.sup.II center is indicative of significant N.fwdarw.P donation, serving to stabilize the .pi.-acid phosphines coordinated to the oxidized metal center. With the N lone pair electron density channeled away from the PF.sub.2 groups bonded to the Rh.sup.0, the strong .pi. accepting properties of this phosphine are maintained and hence the reduced rhodium center is stabilized.
In complexes with only two bridging diphosphazane ligands, valence disproportion of the binuclear rhodium core is circumvented. The dmpma and dppma ligands exclusively form 2:1 complexes with divalent rhodium cores. In these cases, valence disproportionation is not observed, and only symmetric Rh.sup.l.sub.2 cores are obtained. Within the framework outlined in Scheme 1, coordination of two diphosphazane ligands defeats induced polarization of the ligand backbone. Scheme 9 shows that coordination of two diphosphazane ligands to a binuclear complex provides the system a facile comproportionation pathway to produce symmetric cores. Polarization of one PNP ligand via donation of the nitrogen lone pair to one phosphine group is countered by lone pair donation in the opposite direction for the other bridging ligand. N--P bond lengths for the Rh.sup.I complexes of dmpma and dppma support this contention: 6 shows N--P bond distances of 1.670(6) .ANG. and 1.661(6) .ANG. and 9 shows N--P distances of 1.661(5) .ANG. and 1.676(5) .ANG.. The literature report of Rh.sub.2.degree. l(dppea).sub.2 Cl.sub.2 (CO) indicates that two diphosphazanes may be able to stabilize a two-electron mixed-valence species transiently, but results presented here suggest that such a complex rapidly rearranges in solution to give symmetric species.
The driving force for valence disproportionation engendered by coordination of a third PNP ligand is illustrated by the conversion of 15 to 16 as shown in Scheme 8. In the absence of a third tfepma ligand, the Rh.sup.I. . . Rh.sup.I core of 15 is robust, withstanding refluxing toluene without rearrangement or ligand loss. However, addition ##STR9##
of one equivalent of tfepma to room temperature solutions of 15 induces valence disproportionation to the Rh.sub.2.sup.0,II species, 16. It is suggested that although the symmetrization mechanism of Scheme 9 is operative in 15, introduction of a third tfepma ligand "frustrates" the binuclear core, thus forcing the system to polarize and form the two-electron mixed-valence complex.
Interplay of the .pi.-acid phosphine with the nitrogen lone pair defines the ligands' ability to polarize the binuclear core. Diphosphazane functionalization at phosphorus and nitrogen may be used to probe these mediating effects of the nitrogen atom. The methyl amine bridgehead provides maximum participation of the nitrogen lone pair in .pi. bonding with the proximate phosphine groups. Conversely, competition for the nitrogen lone pair is possible in diphosphazanes with aniline bridgeheads. Participation of the bridgehead nitrogen lone pair in the .pi. system of an aryl group might drastically influence the ability of a diphosphazanes to stabilize two-electron mixed valence cores. Isolation of 22 in high yields suggests that the strong .pi.-accepting properties of the difluorophosphine group override the electron accepting properties of the dfpx aryl group. Rotation of the aryl group perpendicular to the P--N--P plane completely decouples the 1 system from the PNP backbone and indicates complete utilization of the nitrogen lone pair by the PF.sub.2 groups. Comparison of the absorption spectrum of 22 to the analogous dfpma complex 2-PPh.sub.3 lends further support to the electronic homology between the dfpx and dfpma ligands. The low energy d.pi.*.fwdarw.d.sigma.* transition, which should be most sensitive to changes in the electronic properties of the bridging diphosphazane, blue-shifts by only 59 cm.sup.1 upon substitution of dfpx for dfpma in 22 and 2-PPh.sub.3, respectively. Conversely, the aryl .pi. system competes effectively with the weaker .pi.-acid group bis(trifluoroethyl)phosphite for the nitrogen lone pair of tfepx. This competition forces the tfepx system to adopt a valence-symmetric configuration. Whereas the coordination of three tfepma ligands to a Rh.sub.2 core produces the two-electron mixed-valence complex 21, tfepx provides a structure for 23 with two non-interacting Rh.sup.I centers. Rotation of the aryl ring of the chelating tfepx ligands into the P--N--P plane indicates a significant degree of N.fwdarw.aryl lone pair donation. Contracted N--C bond lengths for the chelating ligands further support this contention. Apparently, in the tfepx case, interaction of the nitrogen bridgehead with the aryl .pi. system reduces the polarizability of the lone pair and thus valence disproportionation is not observed.
Manipulation of the steric and electronic properties of the phosphorus substituents of the PNP ligand set leads to varied chemical reactivity. The juxtaposition of strong .pi.-acid phosphines with the nitrogen lone pair in dfpma leads to a polarization of the PNP backbone to stabilize the two-electron mixed-valence species. Furthermore, incorporation of sequentially more .pi.-acidic phosphines revealed a step-by-step rearrangement of the binuclear Rh.sup.I core culminating with valence disproportionation and formation of a two-electron mixed-valence core. Diphosphazane ligands with small steric requirements, in addition to the requisite .pi.-acidity, incorporate a third bridging ligand into the binuclear core. At least three ligands may be required to stabilize the two-electron mixed-valence complex and overcome the thermodynamic driving force for valence-symmetric binuclear cores.
Reactivity Studies of the Two-Electron Mixed-Valence Complex Ir.sub.2.sup.0,II (tfepma).sub.3 Cl.sub.2
The chemistry of two-electron mixed valency was generalized to transition metals other than rhodium. Ongoing interest in hydrogen photocatalysis, suggested investigation of the two-electron mixed-valence chemistry of iridium to acquire an understanding of the hydrido and hydrido-halo chemistry of this new class of compounds. Ir.sub.2.sup.0,II (tfepma).sub.3 Cl.sub.2, was used for consideration of the stability of third-row metal-hydride bonds, to systematically investigate the reaction chemistry of H.sub.2, HX and X.sub.2 with M.sub.2.sup.0,II cores.
Homogeneous metal complexes provide a useful tool for probing small molecule reactivity with spectroscopic techniques available for thermodynamic and kinetic studies. Significant advances have been made in understanding the operation of industrial catalysts through the reactivity studies of mononuclear inorganic and organometallic complexes. This insight has led to the discovery of homogeneous molecular catalysts that rival their heterogeneous brethren in both activity and long-term stability. Furthermore, the advantage of "molecular tuning," provides substrate and product selectivities unprecedented in heterogeneous catalysis.
Notwithstanding, an advantage of the heterogeneous catalyst is the cooperative reactivity afforded by adjacent metal centers available at the catalyst surface. Many industrial processes from olefin polymerization to nitrogen fixation and carbon monoxide functionalization rely on such cooperativity for efficient operation. Though interest in multi-metallic homogenous catalysts is longstanding, the chemistry of transition metal clusters is less advanced than that of mononuclear compounds. Whereas the same palette of reactions, such as oxidative-addition, reductive-elimination and atom transfer, is available to mononuclear and multinuclear metal complexes, cluster compounds are distinguished by reactivity patterns arising from cooperative effects. For example, .beta.-hydrogen elimination is a well-known reaction observed for late ##STR10##
transition metal alkyl complexes leading to the formation of a metal-hydride-olefin complex. Conversely, alkyl cluster complexes have been observed to react by .alpha.-hydrogen abstraction pathways; the proximity of the second metal center can interact in a cooperative fashion to activate the nearest C--H bond.
The logical starting point for the development of multi-metallic catalysts is with the simplest type of cluster compounds. The chemistry of bimetallic transition metal complexes largely parallels that of analogous mononuclear complexes, since their construction is often predicated on the assembly of mononuclear metal fragments. As an example, in Scheme 10, the chemistry of square planar d.sup.8 metal fragments tethered with one or more bridging ligands, typified by pyrazolate-bridged Rh.sup.I and Ir.sup.I dimers, is dominated by oxidative addition and reductive elimination. Addition occurs either across the bimetallic core to yield a d.sup.9 --d.sup.9 dimer, or is localized at one metal of the complex to yield a d.sup.8.fwdarw.d.sup.10 complex. Subsequent functionalization of substrates has been achieved, but coupling of the two metal centers impedes further reactivity, which obviates the development of an efficient catalytic cycle. Binuclear complexes that avoid the alternate making and breaking of a metal-metal bond show enhanced potential for catalyst development. For example, the A-frame derived complexes, [MM'(dppm).sub.2 (CO).sub.3 ] (M=M'=Rh, Ir; M=Ir, M'=Rh), possess a formal metal-metal bond by virtue of coordinative unsaturation that is maintained upon substrate addition. As a result, further substrate addition and functionalization reactivity is observed after the initial activation step.
The diphosphazane ligand framework provides an excellent opportunity to develop new systems for small molecule activation and transformation reactions. The coordination chemistry of dfpma, dppma and tfepma with rhodium elucidates a ligand framework capable of stabilizing a variety of metal geometries and oxidation states while preserving a metal-metal interaction. A foray into the chemistry of these ligands with iridium, has led to the preparation of a two-electron mixed-valence complex, Ir.sub.2.sup.0,II (tfepma).sub.3 Cl.sub.2. The unique electronic and coordinative structure of this compound provides a novel platform for the detailed study of small molecule activation reactions en route to catalytic functionalization schemes. The inventors conducted structure and reactivity studies of Ir.sub.2.sup.0,II (tfepma).sub.3 Cl.sub.2 with H.sub.2, HX and X.sub.2 substrates, revealing a binuclear metal complex that maintains the facile addition pathways of mononuclear complexes with the caveat of a neighboring metal center to promote cooperative activation of small molecule substrates.
Iridium Chemistry of dfpma and dppma
Building upon studies of bis(phosphine)amine ligands with rhodium, the chemistry of these ligands with iridium was developed. The reaction of dfpma (dfpma =bis(difluorophosphine)methylamine) with monomeric iridium (I) starting materials such as ClIr(CO)(PPh.sub.3).sub.2 leads to the formation of monomeric materials, ClIr(L)(dfpma) (L=CO, PPh.sub.3). Similarly, in the absence of a suitable ligand, addition of two equivalents of dfpma leads to the formation of the dfpma-chelated dimer, [ClIr(dfpma)].sub.2. Synthetic methodologies that typically provided high-yields of two-electron mixed-valence dirhodium products, invariably gave intractable oils with iridium, as illustrated by the reaction of three equivalents of dfpma and one equivalent of L (L=P(OR).sub.3, PR.sub.3, CNR) with [ClIr(cod)].sub.2, which affords orange to red oils that elude crystallization.
A two-electron mixed-valence iridium complex was obtained when dfpma was added to [ClIr(cod)].sub.2 in the presence of excess PhICl.sub.2. A yellow microcrystalline powder was obtained in moderate yield. X-ray diffraction studies on single crystals yielded the results presented in FIG. 12 and Table 11. Ir.sub.2.sup.I,III (dfpma).sub.2 Cl.sub.4 (cod) (31) is structurally divergent from the observed rhodium dfpma chemistry: two dfpma ligands span a binuclear iridium core composed of octahedral Ir.sup.I and Ir.sup.III centers rotated by 47.1.degree.. The metal-metal separation in 31 is long at 2.8657(6) .ANG., but still within the range observed for other diiridium complexes with a formal metal-metal bond. A single 1,5-cyclooctadiene ligand remains coordinated to the Ir.sup.I center, occupying an axial site and an equatorial site trans to one chloride ligand. Three chloride ligands coordinate to the iridium(III) center, adopting a facial geometry, with surprisingly similar Ir.sup.III --Cl bond distances of 2.407(3) and 2.417(3) .ANG. for the two equatorial and axial chloride ligands, respectively. The similarity in Ir.sup.III --Cl distances further indicates a weakened metal-metal bond; normally a significant trans influence is observed in the M-X distances for halogens coordinated opposite to a strong metal-metal bond. The octahedral Ir.sup.III is completed by two cis disposed PF.sub.2 groups, bonded at an average Ir--P distance of 2.170(3) .ANG.. The dfpma ligands display an induced asymmetry across the binuclear core, with an average N--P bond distance of 1.639(10) .ANG. to the Ir.sup.III -coordinated phosphorus and an average N--P bond distance of 1.660(9) .ANG. to the Ir.sup.I -coordinated phosphorus. The PF.sub.2 groups coordinated to the Ir.sup.I center adopt a transoid arrangement (P(1)-Ir(1)-P(3)=155.56(10).degree.), resulting in significantly longer Ir.sup.I --P distances of 2.281(3) .ANG.. Metal-olefin distances to the C.dbd.C centroid of the chelating 1,5-cyclooctadiene ligand are distinguished for the axial and equatorial coordination sites at 2.222(11) and 2.182(11) .ANG., respectively. In accord with these different metal-olefin distances, C(3)-C(4) and C(7)-C(8) distances of 1.381(16) and 1.356(17) .ANG., respectively, reflect a slight decrease in C--C bond order for the more tightly bound equatorial olefin.
TABLE 11 Bond lengths (.ANG.) for Ir.sub.2.sup.I,III (dfpma).sub.2 Cl.sub.4 (cod) (31). Selected Bond Lengths (.ANG.) Ir(1)-Cl(1) 2.385(3) Ir(1)-Ir(2) 2.8657(6) Ir(1)-P(1) 2.281(3) Ir(2)-Cl(2) 2.417(3) Ir(1)-P(3) 2.281(3) Ir(2)-Cl(3) 2.407(3) Ir(1)-C(3) 2.294(11) Ir(2)-Cl(4) 2.407(3) Ir(1)-C(4) 2.283(11) Ir(2)-P(2) 2.169(3) Ir(1)-C(7) 2.314(11) Ir(2)-P(4) 2.171(3) Ir(1)-C(8) 2.332(10) P(1)-N(1) 1.667(9) P(2)-N(1) 1.642(10) P(3)-N(2) 1.652(9) P(4)-N(2) 1.636(9) C(3)-C(4) 1.381(16) C(7)-C(8) 1.356(17)
To investigate whether a stronger donating, more sterically demanding ligand might displace the 1,5-cyclooctadiene ligand, [ClIr(cod)].sub.2 was treated with three equivalents of the dppma ligand (dppma=MeN[P(OPh).sub.2 ].sub.2) in the absence of excess oxidant. Insoluble molecular compounds precipitate from reactions conducted in benzene, yielding a salmon-colored powder. Analysis suggests the formulation Ir.sub.2 (dppma).sub.3 Cl.sub.2 (2), indicating complete removal of 1,5-cyclooctadiene by the three dppma ligands. X-ray quality crystals of 32 were obtained from methylene chloride solution cooled to -35.degree. C., and yielded the molecular structure depicted in FIG. 13. As observed in the structure of 31, two PNP ligands bridge the binuclear metal core, with a third chelating dppma ligand replacing the chelating 1,5-cyclooctadiene ligand of 31. Notably, in the absence of an external oxidant, the divalent Ir.sub.2 core activates a C--H bond at an ortho position of the phenyl ring of one of the bridging dppma ligands. A metal-metal distance of 2.7919(15) .ANG. is consistent with an Ir.sup.I.fwdarw.Ir.sup.II dative bond. A vacant coordination site at the Ir.sup.I metal center (P(3)-Ir(1)-P(5)=160.7(3).degree. and P(6)-Ir(1)-Ir(2)=167.57(18).degree.) is presumably occupied by the abstracted hydrogen atom from the orthometallated phenyl group. .sup.1 H NMR spectroscopy of 32 confirms the presence of a hydride ligand by the presence of a single low frequency resonance observed at -17.08 ppm. Coordination to the hydride to the Ir.sup.I center is established by an apparent doublet-of-triplet-of-doublets pattern with a .sup.2 J.sub.HP coupling constant of 218 Hz to the trans phosphorus atoms and .sup.2 J.sub.HP couplings of 17 and 12 Hz to the cis phosphorus atoms.
Preparation of Ir.sub.2.sup.0,II (tfepma).sub.3 Cl.sub.2
In an effort to avoid orthometallation of the bridging PNP ligands, the reaction chemistry of tfepma with [ClIr(cod)].sub.2 was investigated. Initial attempts to add the tfepma ligand to iridium followed the synthetic methodology used to prepare the dppm-bridged binuclear iridium complex Ir.sub.2 (dppm).sub.2 Cl.sub.2 (CO).sub.2 : the dropwise addition of a CH.sub.2 Cl.sub.2 solution of [ClIr(cod)].sub.2 to a CH.sub.2 Cl.sub.2 solution of tfepma under an atmosphere of CO resulted in the formation of an orange solution. After 48 hours, dark green, X-ray quality crystals precipitated from solution. FIG. 14 presents the molecular structure of Ir.sub.2.sup.0,II (tfepma).sub.3 Cl.sub.2 (33) as determined from the X-ray diffraction data, Table 12 presents selected bond distances and angles of the bimetallic iridium core. As observed in the structure of 32, two of the three bidentate ligands bridge the binuclear core of 33 with the third ligand binding as a chelate. The Ir.sup.0 center possesses distorted trigonal bipyramidal geometry that is typical for d.sup.9 metals. Two equatorial sites of the Ir.sup.0 center are coordinated by phosphites from the bridging tfepma ligands with the third equatorial site occupied by one end of a chelating tfepma ligand. The other end of the chelating ligand caps the axial position of the Ir.sup.0 center at a short Ir--P distance of 2.201(5) .ANG., and a metal-metal bond distance of 2.7871(8) .ANG. joins the two metal centers. The square pyramidal Ir.sup.II center is characterized by a regular basal plane tilted by 8.degree. from normal to the Ir(1)-Ir(2) vector. Two cis equatorial sites are occupied by phosphites from the bridging tfepma ligands at an average Ir--P distance of 2.192(5) .ANG., while the other two equatorial sites are occupied by chloride ligands at an average Ir--Cl distance of 2.372(5) .ANG..
TABLE 12 Bond lengths (.ANG.) and angles (deg) for Ir.sub.2.sup.0,II (tfepma).sub.2 Cl.sub.2 (33). Selected Bond Lengths (.ANG.) Ir(1)-Ir(2) 2.7876(10) Ir(2)-Cl(1) 2.379(5) Ir(1)-P(1) 2.235(6) Ir(2)-Cl(2) 2.365(5) Ir(1)-P(3) 2.274(5) Ir(2)-P(2) 2.198(5) Ir(1)-P(5) 2.268(5) Ir(2)-P(4) 2.186(5) Ir(1)-P(6) 2.201(5) P(1)-N(1) 1.667(17) P(2)-N(1) 1.693(17) P(3)-N(2) 1.649(16) P(4)-N(2) 1.686(16) Selected Bond Angles (deg) P(5)-Ir(1)-P(1) 136.7(2) Cl(1)-Ir(2)-Cl(2) 85.3(2) P(5)-Ir(1)-P(3) 115.7(2) Cl(1)-Ir(2)-P(2) 88.0(2) P(5)-Ir(1)-P(6) 69.4(2) Cl(1)-Ir(2)-P(4) 169.50(19) P(6)-Ir(1)-Ir(2) 164.04(15)
The synthesis of 33 shown in FIG. 14 can be targeted directly by reacting [ClIr(cod)].sub.2 with three equivalents of tfepma in CH.sub.2 Cl.sub.2. Dark green crystalline 33 precipitates from the dark red solution of tfepma and the iridium starting material. Alternatively, slightly higher yields are obtained if the reaction is carried out in refluxing benzene, but the necessity to recrystallize the product reduces the value of this preparative method. The reactivity of 33 is outlined in Scheme 11.
Ligand Exchange Reactions of Ir.sub.2 0.sup.,II (tfepma).sub.3 Cl.sub.2
Solvent Exchange Reactions of 33 (I of Scheme 11)
.sup.1 H NMR and .sup.31 P NMR spectroscopy reveal a fluxional coordination environment for 33 in which the diiridium complex is under dynamic solvent exchange. At room temperature, d.sup.8 -THF solutions of 33 show a .sup.31 P NMR spectrum of four distinct resonances, albeit with significant line broadening, consistent with a C.sub.s symmetric structure for 33 in solution (FIG. 15(a)). A low frequency singlet at 20.91 ppm is assigned to the axial phosphorus coordinated to the Ir.sup.0 center; the apparent triplet at 65.59 ppm is the equatorial phosphorus of the chelating tfepma ligand. The excessively broad singlet at 80.19 ppm is assigned to the Ir.sup.II -coordinated phosphites leaving the high-frequency resonance to the Ir.sup.0 -coordinated phosphites of the bridging tfepma ligands. The .sup.1 H NMR spectrum at 20.degree. C. is consistent with a symmetric complex as well, only two methyl resonances at 2.93 and 2.62 ppm are observed for the tfepma ligands, integrating in a 2:1 ration for the bridging and chelating ligands, respectively. Upon cooling to -80.degree. C., the fluxional behavior of 33 is arrested, giving the NMR spectra of FIG. 15(b). The resonance for the chelating tfepma ligand is largely unchanged in the .sup.1 H NMR spectrum, however, the resonance for the bridging tfepma ligand is split into distinct signals at 3.17 and 2.70 ppm (.DELTA.=238 Hz), indicating an inequivalence of the bridging tfepma ligand. Even more dramatic changes are observed in the .sup.31 P NMR spectrum. In addition to a significant improvement in line shape for the resonances of the chelating tfepma ligand, two distinct signals can be resolved in the 100-110 ppm region for the equatorial phosphites of the bridging tfepma ligands. The most striking feature is the separation of two very different Ir.sup.II -phosphite resonances, at 78.31 and 89.77 ppm (.DELTA.=2321 Hz), immediately suggesting the coordination of different ligands across the metal center from the phosphorus atoms.
Rate constants for the fluxional process equilibrating the Ir.sup.II -phosphites were determined by monitoring the .sup.1 H methyl resonances of the bridging tfepma ligands in d.sup.8 -THF over the temperature range of -70.degree. C. to +30.degree. C.; analysis of the Eyring plot yields activation parameters of .DELTA.H.sup.{character pullout} =13.+-.1 kcal mol.sup.-1 and .DELTA.S.sup.{character pullout} =6.6.+-.2 cal mole.sup.-1 K.sup.-1. A similar exchange process is observed in CD.sub.3 CN, and evaluation of the corresponding kinetic data reveals a strongly solvent dependent process: over a temperature range of -5.degree. C. to +30.degree. C., .DELTA.H.sup.{character pullout} and .DELTA.S.sup.{character pullout} were determined to be 25.+-.4 kcal mol.sup.-1 and 38.+-.5cal mol.sup.-1 K.sup.-1, respectively. The positive activation entropies in both solvent systems comply with a dissociative process. This, taken with the differences in chemical shift for the Ir.sup.II -phosphites suggests the equilibrium depicted by reaction (i) of Scheme 11, in which a THF or MeCN ligand reversibly binds to an equatorial Ir.sup.II site with concomitant shift of a halide to the position opposite the metal-metal bond. ##STR11##
The solvent adduct 34, see FIG. 16 has been isolated and structurally characterized; FIG. 16(a) shows a thermal ellipsoid plot for Ir.sub.2.sup.0,II (tfepma).sub.3 Cl.sub.2 (MeCN) (4-MeCN). As deduced from NMR studies, the coordination core of the parent complex, 33 in Scheme 11, is mostly unchanged, with two chelating tfepma ligands and a third tfepma ligand chelated to the Ir.sup.0 center; a slight elongation of the metal-metal separation accompanies coordination of the acetonitrile ligand. Table 13 presents other pertinent metrical parameters of the Ir.sup.0 --Ir.sup.II core of 34.
Addition of Strong .sigma.-Donors to 33 (ii of Scheme 11)
In contrast to the solution behavior of 33, .sigma.-donating ligands such as isonitriles, phosphines and even halides, coordinate the Ir.sup.II center at the axial position, trans to the metal-metal bond yielding species of the formulation Ir.sub.2.sup.0,II (tfepma).sub.3 Cl.sub.2 (L) (35-L, L=CNR, PR.sub.3, X.sup.-). As is apparent in the thermal ellipsoid plot of the CN.sup.t Bu derivative, Ir.sub.2.sup.0,II (tfepma).sub.3 Cl.sub.2 (CN.sup.t Bu) (35-CN.sup.t Bu) shown in FIG. 16(b), the Ir.sub.2.sup.0,II mixed-valence core is preserved upon addition of the donor ligand. Apart from the differing coordination positions of the MeCN and CN.sup.t Bu ligands, the X-ray crystal structure of 35-CN.sup.t Bu is largely undistinguished from that of 34. Metrical parameters of the bimetallic core are presented for comparison in Table 13. In agreement with the solid-state results, the integration ratios for the proton resonances of the axial ligands of 35-L to those of the tfepma ligands establish that only one donor ligand is incorporated into the diiridium coordination sphere. Moreover, room temperature NMR spectra of the addition products with strong .sigma.-donor ligands reveal static solution structures that are consistent with the results of X-ray crystallography. Notably, the Ir.sup.II -coordinated phosphites of the 35-L species give rise to sharp, apparent triplets in the .sup.31 P{.sup.1 H} NMR spectrum between 66 and 73 ppm. The absence of fluxional behavior is also in evidence from the sharp high frequency doublet resonances at about 95 ppm for the Ir.sup.0 -coordinated phosphites of the bridging ligands. The .sup.31 P resonance for the axial Ir.sup.0 -phosphite is sensitive to the coordination of an axial ligand at Ir.sup.II, shifting to progressively higher frequency (.delta.=35.0, 52.5 and 53.3 ppm along the series L=Br.sup.-, PEt.sub.3, CN.sup.t Bu) with increasing L .sigma.-donor strength.
TABLE 13 Selected bond lengths (.ANG.) for Ir.sub.2.sup.0,II (tfepma).sub.3 Cl.sub.2 (MeCN) (4), Ir.sub.2.sup.0,II (tfepma).sub.3 Cl.sub.2 (CN.sup.t Bu) (35- CN.sup.t Bu) and [Ir.sub.2.sup.0,II (tfepma).sub.3 (MeCN).sub.3 ][PF.sub.6 ].sub.2 (6[PF.sub.6 ].sub.2). Ir.sub.2.sup.0,II (tfepma).sub.3 Cl.sub.2 (MeCN) Ir.sub.2.sup.0,II (tfepma).sub.3 Cl.sub.2 (CN.sup.t BU) [Ir.sub.2.sup.0,II (tfepma).sub.3 (MecN).sub.3 ].sup.2+ Ir(1)-Ir(2) 2.7964(11) Ir(1)-Ir(2) 2.8088(1) Ir(1)-Ir(2) 2.7805(7) Ir(1)-P(1) 2.216(2) Ir(1)-P(1) 2.215(5) Ir(1)-P(1) 2.230(3) Ir(1)-P(3) 2.225(2) Ir(1)-P(3) 2.225(6) Ir(1)-P(3) 2.231(3) Ir(1)-P(5) 2.272(2) Ir(1)-P(5) 2.317(6) Ir(1)-P(5) 2.280(3) Ir(1)-P(6) 2.227(2) Ir(1)-P(6) 2.233(6) Ir(1)-P(6) 2.216(4) Ir(2)-P(2) 2.186(2) Ir(2)-P(2) 2.206(6) Ir(2)-P(2) 2.220(4) Ir(2)-P(4) 2.176(2) Ir(2)-P(4) 2.188(6) Ir(2)-P(4) 2.194(3) Ir(2)-Cl(1) 2.427(2) Ir(2)-Cl(1) 2.452(5) Ir(2)-N(5) 2.079(12) Ir(2)-N(4) 2.122(7) Ir(2)-Cl(2) 2.429(6) Ir(2)-N(6) 2.089(12) Ir(2)-Cl(2) 2.512(2) Ir(2)-C(4) 2.03(2) Ir(2)-N(4) 2.183(12)
Halogen Removal from 33 (iii of Scheme 11)
Halogen removal from 33 can be effected with silver or thallium hexafluorophosphate in acetonitrile solution. A highly crystalline product analyzing as [Ir.sub.2.sup.0,II (tfepma).sub.3 (MeCN).sub.3 ][PF.sub.6 ].sub.2 (6[PF.sub.6 ].sub.2) is recovered in quantitative yield. Solution NMR spectroscopy suggests preservation of the Ir.sub.2.sup.0,II core. Excepting the resonance for the PF.sub.6 anions at -143 ppm, the 31P NMR spectrum of 36 in FIG. 17 shows a four-line pattern analogous to that observed for the C.sub.s symmetric complexes 35-L. The axial phosphorus of the chelating tfepma ligand is observed as a singlet at 28 ppm, whereas the equatorial phosphorus is observed at 50 ppm as a triplet displaying a .sup.2 J.sub.PP coupling constant of 273 Hz to the doublet at 89 ppm for the Ir.sup.0 phosphorus atoms of the bridging tfepma ligands. Finally, the Ir.sup.II phosphorus atoms give a single resonance at 57 ppm. Similarly, the .sup.1 H NMR spectrum of 36 displays two resonances for the tfepma methyl groups at 2.83 and 2.70 ppm, the 2:1 integration ratio is consistent with the expected bridging/chelating coordination mode. Complete exchange of the MeCN ligands occurs upon dissolution of the solid as evidenced by a singlet at 1.96 ppm for uncoordinated MeCN. Single crystals diffraction analysis of the product confirm the preservation of the Ir.sub.2.sup.0,II core as depicted in FIG. 17. For comparison purposes, the metric parameters for the core of 36 are presented in Table 13 alongside those of Ir.sub.2.sup.0,11 complexes 34 and 35-L. Removal of the halogen ligands results in a contracted metal-metal separation of 2.7805(7) .ANG., as well as a shorter axial Ir.sup.0 --P bond. Conversely, whereas the equatorial Ir.sup.0 --P bond distances are similar in 34-36, the Ir.sup.II --P distances are elongated by nearly 0.02 .ANG. in 36.
Oxidative Addition Reactions of Ir.sub.2.sup.0,II (tfepma).sub.3 Cl.sub.2
Chlorine Addition to 3 (iv and v of Scheme 11)
Chlorine, in the form of its iodobenzene adduct, facilely adds to 33 to give the two-electron mixed-valence complex Ir.sub.2 I,III (tfepma).sub.3 Cl4 (37) as shown in reaction (iv) of Scheme 11. X-ray crystallographic studies on single crystals of 37 yielded the molecular structure depicted in FIG. 18(a). Halogen addition occurs across the metal-metal bond of 33. The Ir--Ir distance of 2.7765(8) .ANG. (Table 14) is well within the range for a metal-metal bond. Simple electron counting arguments are consistent with the formulation of an Ir.sup.I.fwdarw.Ir.sup.III dative bond where both metals assume octahedral coordination geometry. The octahedral metal centers of 37 are rotated with respect to one another by an average value of 32.degree.. The Ir.sup.III --Cl bonds are noticeably longer than those for the reduced Ir.sub.2.sup.0,II complexes, at an average value of 2.446(4) .ANG.. This result is peculiar in light of the anticipated contraction of the iridium radius upon oxidation of Ir.sup.II to Ir.sup.III. The coordination of the bridging tfepma ligands is unchanged from 33; however, the Ir--P distances associated with the chelating ligand elongate by .about.0.1 .ANG..
NMR spectra of 37 are consistent with the octahedral coordination geometry of the oxidized metal core. The distinct environments of a chelating and two bridging ligands are signified by ligand methyl resonances in the .sup.1 H NMR spectrum at 2.71 ppm and 2.88 and 2.93 ppm, respectively. The .sup.31 P{.sup.1 H} NMR spectrum of 37 consists of six separate resonances, one for each of the unique phosphites under C.sub.1 symmetry. Referring to the numbering scheme of FIG. 18(a), P(3) and P(5) are expected to be strongly coupled owing to their trans arrangement. In this context, signals at 13.14 and 74.31 ppm are ascribed to P(5) and P(3), respectively, based on a .sup.2 J.sub.PP coupling constant of 901 Hz. As in the reduced Ir.sub.2.sup.0,II complexes, a low frequency resonance at 9.24 ppm is attributed to the P(6) phosphite trans to the Ir--Ir bond. Based on the .sup.31 P COSY spectrum of 37, the 66.46-ppm resonance is ascribed to the Ir.sup.I -bound P(1) phosphite of the bridging ligand, while the signals at 54.98 and 57.11 ppm are assigned to Ir.sup.III -bound P(2) and P(4) phosphites of the bridging ligands, respectively.
TABLE 14 Selected bond lengths (.ANG.) and angles (deg) for Ir.sub.2.sup.I,III (tfepma).sub.3 Cl.sub.4 (37),.sup.a equatorial Ir.sub.2.sup.I,III (tfepma).sub.3 HCl.sub.3 (40) and Ir.sub.2.sup.I,III (tfepma).sub.3 H.sub.2 Cl.sub.2 (42). Ir.sub.2.sup.I,III (tfepma).sub.3 Cl.sub.4.sup.b Ir.sub.2.sup.I,III (tfepma).sub.3 HCl.sub.3.sup.c Ir.sub.2.sup.I,III (tfepma).sub.3 H.sub.2 Cl.sub.2.sup.d Selected Bond Lengths (.ANG.) Ir(1)-Ir(2) 2.7765(8) 2.7775(11) 2.7561(7) Ir(1)-X(1) 2.435(4) -- -- Ir(2)-Cl(2) 2.405(4) 2.450(2) 2.465(3) Ir(2)-X(3) 2.428(4) 2.418(2) -- Ir(2)-Cl(4) 2.504(4) 2.487(2) 2.511(3) Ir(1)-P(1) 2.223(4) 2.314(2) 2.300(3) Ir(1)-P(3) 2.282(4) 2.241(2) 2.259(3) Ir(1)-P(5) 2.375(4) 2.322(2) 2.271(3) Ir(1)-P(6) 2.350(4) 2.269(2) 2.278(3) Ir(2)-P(2) 2.198(4) 2.188(2) 2.160(3) Ir(2)-P(4) 2.191(4) 2.175(2) 2.277(3) Selected Bond Angles (deg) P(1)-Ir(1)-X(1) 173.3(2) -- -- P(1)-Ir(1)-P(3) 93.4(2) 100.60(8) 96.70(11) P(1)-Ir(1)-P(5) 99.9(2) 103.61(7) 93.99(11) P(2)-Ir(2)-P(4) 96.0(2) 94.05(8) 103.61(12) P(2)-Ir(2)-Cl(2) 170.7(2) 175.14(7) 169.33(12) P(2)-Ir(2)-X(3) 86.2(2) 89.13(8) -- P(6)-Ir(1)-Ir(2) 165.20(11) 167.41(6) 158.45(10) Cl(4)-Ir(2)-Ir(1) 178.07(10) 171.62(5) 176.34(8) Selected Torsion Angles (deg) X(1)-Ir(1)-Ir(2)-Cl(2) 39.4 19.2 7.2 P(1)-Ir(1)-Ir(2)-P(2) 24.4 15.2 18.5 P(3)-Ir(1)-Ir(2)-P(4) 27.0 21.9 11.5 P(5)-Ir(1)-Ir(2)-X(3) 37.9 0.7 1.6 .sup.a The bond lengths and distances presented for 37 are the average values for two crystallographically distinct but chemically equivalent molecules within the unit cell. .sup.b X(1) = X(3) = Cl. .sup.c X(1) = H, X(3) = Cl. .sup.d X(1) = X(3) = H
Upon treatment with excess oxidant, 37 loses the chelating tfepma ligand according to reaction (v) of Scheme 11. The liberated tfepma is in turn oxidized, preventing its incorporation back into the coordination sphere of the binuclear core. A bright yellow solid analyzing as Ir.sub.2 (tfepma).sub.2 Cl.sub.4 (MeCN).sub.2 (38), and possessing relatively simple NMR spectra, is obtained. The .sup.1 H NMR spectrum comprises a single methyl resonance at 2.80 ppm and four methylene resonances between 4.48 and 5.46 ppm for the tfepma ligands. The .sup.1 H NMR signal of CH.sub.3 CN is that of free ligand indicating that the bound solvent molecule is exchanged immediately upon dissolution of solid samples of the compound. The .sup.31 P NMR spectrum of 38 exhibits two resonances at 54.97 and 57.87 ppm with a simple AA'BB' coupling pattern. These NMR results point towards a symmetric complex, which is verified by X-ray diffraction studies (FIG. 18(b)). Two bridging cis-diphosphazane ligands span a valence-symmetric Ir.sub.2.sup.II,II core supporting octahedral coordination geometries that are related to each other by a 30.degree. twist about the Ir--Ir axis. A metal-metal bond distance of 2.7525(9) .ANG. signifies a formal bonding interaction that is characteristic of a d.sup.7 --d.sup.7 complexes. Each metal center is coordinated by one equatorial and one axial chloride ligand, leaving acetonitrile to complete the octahedral coordination spheres of the metal centers. The Ir--P bond distances average 2.206(5) .ANG., and the Ir--Cl bond distances are 2.432(5) and 2.476(5) .ANG. for the equatorial and axial halogens, respectively.
The lability of the acetonitrile ligands suggested that their displacement by a tfepma ligand would provide access to a symmetric Ir.sub.2.sup.II,II Cl.sub.4 complex spanned by three bridging ligands, similar to that observed for the chemistry of rhodium with dfpma and tfepma. Addition of one equivalent of tfepma to solutions of 38 in CH.sub.2 Cl.sub.2, however, leads to nearly quantitative (>90%) conversion to the Ir.sub.2.sup.I,III complex, 37, as determined with NMR spectroscopy.
Acid Addition to 33 (vi-viii of Scheme 11)
Suspensions of 33 in CH.sub.2 Cl.sub.2 readily react with anhydrous HCl gas. Analysis of the final product provides the formulation Ir.sub.2 (tfepma).sub.3 HCl.sub.3 (40), indicating oxidative addition of HCl has occurred. An Ir--H stretch in the IR spectrum at 2115 cm.sup.-1 is consistent with the presence of a terminal hydride ligand situated trans to a phosphorus atom. FIG. 19 displays the results of single crystal X-ray data analysis of 40 as a thermal ellipsoid plot; selected bond distances and angles are listed in Table 14. As observed in the halogen addition reaction, a metal-metal separation of 2.7775(11) .ANG. is consistent with an Ir.sup.I.fwdarw.Ir.sup.III formulation containing a dative bond. Interestingly, the octahedral metal centers are nearly eclipsed with an average torsion angle of only 14.degree.. A conspicuously vacant site in the X-ray crystal structure of 40 confirms the location of the hydride in the equatorial plane of the Ir.sup.I center (P(3)-Ir(1)-P(5)=153.90(7).degree. and P(6)-Ir(1)-Ir(2)=167.41(6).degree.). Moreover, subsequent to the location of all non-hydrogen atoms, the Fourier difference map showed a single peak in the vacant iridium coordination site of appropriate intensity for a hydride ligand though the proximity of the hydride to a heavy nucleus precludes the determination of an accurate Ir--H distance. Further evidence for the hydride location comes from the elongated Ir(1)-P(1) bond distance, resulting from the strong trans influence of the hydride ligand. The Ir(1)-P(1) bond distance of 2.314(2) .ANG. is 0.091(3) .ANG. longer than that of observed in the chloride analog 37. Excluding Ir(1)-P(1), all other metal-ligand bond distances of 40 are similar to those observed in 37.
The results of NMR spectroscopy are consistent with the X-ray structural studies. A single hydride resonance is observed in the .sup.1 H NMR spectrum of 40 (FIG. 20(a)) at -10.24 ppm. A 178 Hz .sup.2 J.sub.PH coupling constant confirms the coordination of a phosphorus trans to the hydride is maintained in solution. The three different tfepma ligand environments are distinguished in the .sup.1 H NMR spectrum by unique methyl resonances at 2.65, 2.77 and 2.89 ppm. .sup.31 P-.sup.1 H HMQC experiments were used to assign a .sup.31 P NMR resonance at 86.4 ppm as the Ir.sup.I phosphite coordinated trans to the hydride ligand. The high frequency shift of this resonance follows from the strong trans influence of the hydride ligand observed in the solid-state structure. Two resonances at 64.4 and 67.8 ppm in the .sup.31 P NMR spectrum are assigned to Ir.sup.III phosphites, whereas the low frequency singlet at 21.6 ppm is assigned to the axially coordinated phosphite of the chelating ligand. As in 37, a strong .sup.2 J.sub.PP of 761 Hz observed in resonances at 26.6 and 82.2 ppm points to their assignment as the trans disposed Ir.sup.I -phosphites of the chelating and bridging tfepma ligands, respectively.
At early reaction times, another isomer of Ir.sub.2 (tfepma).sub.3 HCl.sub.3 with an axial hydride ligand is observed (39 in Scheme 11). As indicated by reactions (vi) and (vii) of Scheme 11, 39 converts to 40 upon stirring in CH.sub.2 Cl.sub.2. Examination of the