Research Summary for the Period December 1, 1995 - November 30, 1998:
During this period, we have focussed our research on two areas of our proposal where we have had success. These include: (1) C-H bond cleavage reactions of trispyrazolylboraterhodium complexes, and (2) carbon-carbon bond cleavage reactions. In addition, we have made new discoveries in the areas of C-F bond activation and Cp-tethered phosphine complexes. Some important features of this work as they related to our continued studies are summarized in this report.
Our carbon-carbon bond cleavage study is based upon the notion that metal-phenyl bonds are the strongest metal-carbon bonds. Cleavage of the C-C bonds in biaryl systems will therefore give two very strong metal-aryl bonds, and consequently offers the most thermodynamically preferred situation for observing C-C cleavage. We have had success in C-C cleavage with biphenylene, a molecule with a weaker C-C bond than biphenyl. We have studied in detail the C-C cleavage of biphenylene by Pt(PEt3)3, which catalytically produces tetraphenylene, and are continuing hydrogenolysis studies using this catalyst system.
Another area that we have been investigating involves the tris-pyrazolylborate fragment [Tp'Rh(CNCH2CMe3)] and the examination of new derivatives of the type [Tp'Rh(L)]. Recent breakthroughs in synthetic methodology have allowed us to prepare generally a variety of long sought derivatives of the isocyanide species as well as to prepare derivatives in which L can be varied. This methodology has allowed us to: (1) find evidence for rearrangements of secondary alkyl hydride complexes to primary alkyl hydride complexes, (2) demonstrate that primary C-H activation of propane is kinetically preferred over secondary activation, (3) find that cyclopropane undergoes C-H activation, then C-C cleavage, then rearrangement to an h2-propylene complex, (4) observe that the methyl deuteride complex Tp'Rh(CNR)(CH3)D rearranges intamolecularly to the Rh(CH2D)H complex, (5) observe a rare example of allylic C-H bond activation. Also, we have found kinetic evidence for the direct involvement of a methane s-complex in the reductive elimination of methane from Tp'Rh(CNR)MeH.
During the course of our C-C activation studies, we have found an example of aromatic CF bond cleavage usgin Cp*Rh(PMe3)H2. The generality and selectivity of the cleavage was investigated, and its unusual mechanism elucidated. This discovery will have an impact on future examples of catalytic C-F cleavage.
Finally, we have also synthesized new Cp-tethered phosphine complexes. Some preliminary reaction chemistry of these derivatives has been investigated. These compounds are expected to have greater stability than their untethered analogs.
Final Report for the Period
December 1, 1995- November 30, 1998.
1. Tris-pyrazolylborate Studies.
During this grant cycle, we have developed methodology for the independent synthesis of a variety of new complexes that arise from C-H activation, (HBPz*3)Rh(CNCH2CMe3)(R)H. Many C-H activation products have been identified in earlier studies (1992) using a carbodiimide derivative which photodissociates with unit quantum efficiency, thereby serving as a useful precursor for the 16-electron Rh(I) fragment [(HBPz*3)Rh(CNCH2CMe3)] = [Tp'Rh(CNR)]).1 This fragment activates (via oxidative addition) a wide range of aromatic and aliphatic C-H bonds to give the corresponding aryl or alkyl hydride complexes (Scheme I). The mechanism of activation of benzene by this fragment has been studied in some detail.2
Reaction of the [Tp'Rh(CNR)] fragment with a variety of aliphatic hydrocarbons has also been investigated. We have been able to establish the relative thermodynamic stability of the alkyl and aryl hydride complexes and have determined the relative metal-carbon bond strengths.3
In the current grant period, we have developed general synthetic methods for the preparation of alkyl chloride derivatives of the trispyrazolylborate complex and have learned how to convert these cleanly into alkyl hydride complexes.4 This methodology is important in that it allows us to prepare complexes that cannot be made using the photochemical route. The new method involves the use of Cp2ZrH2 as a hydride reducing agent, converting a chloro derivative into the hydride derivative and generating Cp2ZrCl2. The zirconium product is easily removed via chromatography on silica gel, if desired, or left in the reaction mixture as an innocuous bystander. This method offers distinct advantages over the previous methods using borohydride or aluminum hydride reagents in that no work-up is required, allowing the initial products formed to be examined immediately. The reaction of the rhodium chloro complex with Cp2ZrH2 is bimolecular, and can be made to proceed more rapidly by increasing the zirconium reagent concentration.
We have now been able to cleanly and rapidly synthesize the methyl deuteride complex Tp'Rh(CNR)(CH3)D and watch it interconvert with the isomer Tp'Rh(CNR)(CH2D)H prior to the elimination of methane. The complex Tp'Rh(CNR)(CD3)H has been prepared and behaves similarly. As these rearrangements occur in benzene solvent, they indicate that an alkane 's complex' is present along the reaction coordinate for reductive elimination. We have measured the rates of approach to equilibrium for each of these complexes also, and have done a complete kinetic analysis for obtaining the critical rate constant kfH for the formation of a methane s-complex from a methyl hydride complex. The analysis gives a value of 1.41 x 10-4 s-1 for kfH and a measure of the ratio krH/krD = 1.69. The reactions involved are shown below.
Another interesting observation was made with the methyl hydride system providing the first example of kinetic evidence for a methane s- complex. The elimination of CH4 from Tp'Rh(CNR)(CH3)H was found to be slower than the elimination of CD4 from Tp'Rh(CNR)(CD3)D, consistent with an inverse kinetic isotope effect as observed in many alkane reductive elimination reactions. However the elimination of CH4 from Tp'Rh(CNR)(CH3)H was found to be different in C6H6 than in C6D6 solvent, suggesting that the solvent was somehow involved in the transition state for methane elimination. This notion is contrary to the expected pathway of first order reductive elimination of methane to generate a s-complex which would then undergo unimolecular loss of methane.
This hypothesis was tested by varying the concentration of benzene in inert C6F6 solvent. The rate of methane formation was found to decrease as the concentration of benzene decreased. Examination of a plot of kobs vs [C6D6] showed a slight curvature, but a plot of 1/kobs vs 1/[C6D6] was found to be linear, suggesting a kinetic scheme involving a unimolecular equilibrium followed by a rate determining bimolecular step.
A scheme that is consistent with these observations is shown below. The reversible formation of the methane s-complex is proposed to form an h2-Tp' complex, since Rh(I) is d8and would favor a square planar geometry. Bimolecular substitution with benzene leads to an h2-C6D6 complex that would then activate the benzene C-H bond. This is the first example of kinetic evidence for displacement of a methane s-complex (c.f. Jensen's h2-H2 displacement by toluene5).
A second important question we have been able to address is the kinetic selectivity for activation of primary vs secondary C-H bonds in an alkane such as propane. Irradiation of the carbodiimide complex Tp'Rh(CNR)(PhN=C=NR) in liquid propane at -50 °C generated only the primary product Tp'Rh(CNR)(n-propyl)H. It was not possible to say, however, if the secondary activation product Tp'Rh(CNR)(i-propyl)H had also been produced but rearranged to the primary derivative under the reaction conditions ... until now. We have been able to show that the primary activation product is indeed the only product formed by synthesizing the secondary complex and demonstrated that the rate of rearrangement to the primary complex is slow under the reaction conditions employed in the photolysis experiment. It is remarkable that everyone tends to take this type of rearrangement for granted, yet there it only one bona fide example in the literature of such an alkane 2° to 1° rearrangement (the Bergman Cp*Ir(PMe3)(pentyl)H system6).
The experiment performed involved the synthesis of the isopropyl chloride complex Tp'Rh(CNR)(i-propyl)Cl from Tp'Rh(CNR)Cl2 plus isopropyl Grignard. Treatment of this product with Cp2ZrH2 in benzene solvent leads to the clean formation of the i-propyl derivative, which then rearranges to the n-propyl derivative, which finally forms the more stable phenyl hydride product. In modeling the kinetic scheme, however, it was necessary to include a pathway for the direct conversion of the i-propyl hydride complex to the phenyl hydride complex. The reactions involved and the modeling of the distribution of species are indicated in the figure below.
We have also investigated a series of reactions of the reactive Tp'Rh(CNR) fragment with cyclopropane. Photochemical generation of this fragment in liquid cyclopropane using the carbodiimide precursor leads to the clean formation of the C-H activation product. Upon standing, this product rearranges to the C-C insertion product in benzene solution (see following scheme). The fact that the phenyl hydride complex is not produced indicates an intramolecular rearrangement, once again arguing for the intermediacy of a s-alkane complex. The C-H insertion product has been converted to the chloro derivative and characterized by X-ray diffraction. The C-C insertion product has also been characterized by X-ray diffraction. The metallocyclobutane then rearranges to an h2-propene complex.
In addition to this C-H and C-C activation chemistry, we have found that the Rh(I) fragment reacts with t-butylethylene and ethylene to give vinylic C-H activation products. The vinyl hydride complex rearranges intramolecularly to the h2-ethylene complex, but the t-butyl-ethylene complex loses the olefin to generate the phenyl hydride complex. Reaction of the Rh(I) fragment with propene and isobutylene leads to formation of the allylic C-H activation products. The former rearranges intramolecularly to the h2-propene complex, whereas the latter loses the olefin and activates benzene.
The activation of the vinylic C-H bond of t-butylethylene and its direct elimination in benzene to give the phenyl hydride complex allow this substrate to be used for the comparison of the C-H and Rh-C bond strengths. The kinetic preference of the Rh(I) fragment for benzene over olefin activation was independently measured as 10:1. In addition, the activation and elimination of isobutylene allow the inclusion of an allylic C-H bond in this analysis. The results are indicated in the plot below, which shows two important points: (1) the correlation is not linear, but curved, and (2) the slope of the best line through the data is not 1. This latter point means that equilbrium constants for different hydrocarbons will vary tremendously. The curvature of the line means that there will be a thermodynamic preference for one hydrocarbon relative to another if it lies above a reference line of slope 1.
We have also completed a project investigating the reaction of [(C5Me5)Rh(PMe3)] with a variety of 1,3-disubstituted benzenes. We have noted a trend in the activation parameters for the reductive elimination of arenes in the series of complexes (C5Me5)Rh(PMe3)(3,5-C6H3R2)H. The trend in DH and DS values shows an entropy/enthalpy compensation that tends to compress DG values for arene elimination, indicating that all eliminations proceed by way of a similar mechanism.7
2. C-C Bond Cleavage Studies
A new area for study that we have initiated is our work with C-C bond cleavage. This field has received little attention until recently, and we believe that we have discovered a strategy for tackling this difficult problem. Our hypothesis is that metal complexes should prefer to add to the stronger C-C bonds found in biaryls, since two strong metal-aryl bonds will be formed in the product. Initial studies have been carried out with biphenylene, and we have discovered that the rhodium fragment [Cp*Rh(PMe3)] inserts first into the C-H bond, and then into the aryl-aryl C-C bond as indicated below.8
We have also explored the use of the bis-olefin complexes Cp*M(C2H4)2 where M = Rh or Co for reactivity with biphenylene. Thermal reaction of the rhodium complex also gives a product in which C-C bond cleavage has occurred, and in which a second metal has coordinated to the metallacyclopentadiene. These complexes react with a variety of small molecules, including PMe3, CO, and H2. The latter results in the formation of biphenyl, thereby accomplishing a net hydrogenolysis of the C-C bond of biphenylene although the reaction is not catalytic.9
We have also found that these complexes can serve as catalysts for the carbonylation of C-C bonds in the presence of CO. Ketones are formed where C-C bonds once existed. While the reaction only has been demonstrated with biphenylene, it will be extended to other molecules as well.9
We have recently initiated studies with a platinum(II) complex that can cleave the C-C bond of biphenylene. Reaction of Pt(PEt3)3 with biphenylene leads initially to the formation of the C-C insertion product Pt(PEt3)2(2,2'-biphenyl). Upon further heating, this complex goes on to a complex dinuclear product. In the presence of excess biphenylene, the complex is a catalyst for the C-C cleavage and coupling to generate an 8-membered ring product at 120 °C.
We have performed detailed mechanistic studies of the C-C bond cleavage and forming reactions by this system. Under catalytic conditions (130 °C, excess biphenylene), the only species observed are the two intermediates 1 and 2. Kinetic studies of this system indicate that reversible loss of phosphine from 1 is followed by oxidative addition of a biphenylene C-C bond, generating 2. 2 slowly undergoes reductive elimination (without prior phosphine loss) to generate [Pt(PEt3)2], which reacts with more biphenylene to give 1.10
The platinum C-C insertion complex can also undergo reaction with dihydrogen. In the presence of H2, complex 1 undergoes hydrogenation to give biphenyl rather than addition of a second molecule of biphenylene. The platinum hydride product, Pt(PEt3)2H2, is a catalyst for hydrogenolysis of biphenylene. In this case, reaction of biphenylene with the dihydride appears to be faster than addition of hydrogen to 1, so a slightly different mechanistic pathway occurs during the hydrogenolysis than for the formation of tetraphenylene.11
In summary, the cleavage of C-C bonds looks to be an exciting new area for discovery and development. We will extend our studies with biphenylene to other biaryl systems with stronger C-C bonds, with the ultimate target being the cleavage of biphenyl. New catalysts will be developed for hydrogenolysis, and reactions with other small molecules (CO, acetylenes, CO2, CNR, nitriles) will be examined as a means of transforming the bis-aryl metal complexes that are formed.
3. C-F Bond Cleavage Studies
In the process of examining substrates for C-C cleavage, we found that perfluorobiphenyl reacts with Cp*Rh(PMe3)H2 to give Cp*Rh(PMe3)(perfluorobiphenyl)H and HF. The reaction is fairly clean in pyridine solvent (95% yield), but displays unusual kinetics reminescent of autocatalysis. The reaction was found to be fairly general for perfluoroaromatics, with similar reactions being observed with perfluorobenzene, perfluoronaphthalene, and pentafluorobenzene.
Investigation of this reaction has led to the proposal of a new mechanism for C-F bond cleavage. We have found that deprotonation of the rhodium dihydride leads to an anion that can undergo nucleophilic aromatic substitution on the perfluoroarene, generating the organometallic product plus fluoride ion. Fluoride ion can then act as a base to deprotonate more of the rhodium dihydride, continuing the catalytic cycle. Addition of free fluoride ion results in a dramatic increase in the rate of the reaction.12 In the absence of fluoride ion, the HF produced can slowly decompose some of the starting material, producing fluoride ion in the process, and thereby making the reaction appear 'autocatalytic'. We believe that this mechanism may be general, and may replace the uphill electron transfer proposals that are in the literature.
4. Tethered Cyclopentadienyl Phosphine Complexes
One limitation we have found in using Cp*Rh(PMe3) derivatives is their lack of long term thermal stability. When these compounds are subjected to temperatures in excess of 80-100 °C for periods of greater than a week, substantial quantities of Cp*Rh(PMe3)2 are formed. This complex can only be formed by the decomposition of half of the initial rhodium complex(es) present, and is quite dead toward further reaction. One theory for the formation of this product is that loss of phosphine from Cp*Rh(PMe3)XY compounds occurs on a limited basis under these conditions (control experiments show that PMe3 is not generally labile in these derivatives), and that the free PMe3 reacts with coordinatively unsaturated [Cp*Rh(PMe3)] to give the dead-end product.
We have initiated preparation of a new class of thermally stable analogs of these compounds by working with tethered phosphine derivatives. The ligand (C5H4SiMe2CH2PPh2) has been reported in the literature by Schore,13,14 and we have found that it reacts with [RhClL2]2 where L = CO or C2H4 to give (C5H4SiMe2CH2PPh2)RhL. The ethylene, CO, and PMe3 complexes have been prepared and X-ray structural studies confirms that the phosphine is chelated to the same metal center as the C5H4 ring. Earlier complexes containing this ligand were found to bridge two metal centers.14 Other tethered ligands in the literature include C5Me4HCH2CH2PPh2,15 C5H5CH2CH2PPh2,16 and C5Me4HCH2CH2NMe2.17
We have also examined the reaction of the ethylene complex with hydrogen. The product of this reaction is not the simple dihydride, (C5H4SiMe2CH2PPh2)RhH2, but appears to be a dinuclear product in which the tethered ligand now bridges two metal centers in a dinuclear complex. Preliminary studies indicate that the 1H NMR coupling pattern for this complex is quite complicated and is inconsistent with a mononuclear complex. Further studies are required. The desired mononuclear dihydride was prepared independently by treatment of the ethylene complex with I2 followed by NaAl(OR)2H2.
We have begun examine the reactions of the mononuclear complexes
for C-H cleavage reactions. Photolysis of the dihydride complex
in benzene solvent should lead to the generation of a reactive
16-electron fragment that can insert into C-H bonds. The product
formed by benzene activation is apparently not very stable, as
no product was observed. The related complex Cp*Rh(PMePh2)PhH
has a half life for benzene loss of 17 h at 23 °C, and the
ligand is closer to a Cp derivative than a Cp* derivative. Examination
of polyfluoronated aromatic substrates where the metal-carbon
bond is stronger provided more stable C-H activation adducts.
In the presence of C6F5H, the C-H oxidative
addition product was isolated. In C6F6
solvent, the eta-2 complex (C5H4SiMe2CH2PPh2)Rh(h2-C6F6)
was obtained and structurally characterized.18
5. Publications appearing or in press during the current grant period, 1995-98:
(1) Hessell, E. T.; Jones, W. D., "Synthesis and structure of rhodium complexes containing a photolabile .eta.2-carbodiimide ligand. 1,3-Dipolar cycloaddition of phenyl azide to Tp'Rh(CNR)2 [Tp' = hydrotris(3,5-dimethylpyrazolyl)borate]," Organometallics 1992, 11, 1496-505.(2) Jones, W. D.; Hessell, E. T., "Mechanism of benzene loss from Tp'Rh(H)(Ph)(CN-neopentyl) in the presence of neopentyl isocyanide. Evidence for an associatively induced reductive elimination," J. Am. Chem. Soc. 1992, 114, 6087-95.(3) Jones, W. D.; Hessell, E. T., "Photolysis of Tp'Rh(CN-neopentyl)(h2-PhN:C:N-neopentyl) in alkanes and arenes: kinetic and thermodynamic selectivity of [Tp'Rh(CN-neopentyl)] for various types of carbon-hydrogen bonds," J. Am. Chem. Soc. 1993, 115, 554-62.(4) Wick, D. D.; Jones, W. D., "Synthesis and Characterization of (Tris(3,5-Dimethylpyrazolyl)borato)rhodium Alkyl and Vinyl Chloride Complexes," Inorg. Chem. 1997, 36, 2723-2729.(5) Lee, D. W.; Jensen, C. M., "Substitution of .eta.2-Dihydrogen by Toluene and Alkanes in IrXH2(H2)(PPri3)2 Complexes," J. Am. Chem. Soc. 1996, 118, 8749-8750.(6) Wax, M. J.; Stryker, J. M.; Buchanan, J. M.; Kovac, C. A.; Bergman, R. G., "Reversible C-H Insertion/Reductive Elimination in (h5-Pentamethylcyclopentadienyl)(trimethylphosphine)-iridium Complexes. Use in Determining Relative Metal-Carbon Bond Energies and Thermally Activating Methane," J. Am. Chem. Soc. 1984, 106, 1121-1122.(7) Selmeczy, A. D.; Jones, W. D.; Osman, R.; Perutz, R., "Substituent Effects on Reductive Elimination from Disubstituted Aryl Hydride Complexes: Mechanistic and Thermodynamic Considerations," Organometallics 1995, 14, 5677-85.(8) Perthuisot, C.; Jones, W. D., "Catalytic Hydrogenolysis of an Aryl-Aryl Carbon-Carbon Bond with a Rhodium Complex," J. Am. Chem. Soc. 1994, 116, 3647-8.(9) Perthuisot, C.; Edelbach, B. L.; Zubris, D. L.; Jones, W. D., "C-C Activation in Biphenylene. Synthesis, Structure, and Reactivity of (C5Me5)2M2(2,2'-biphenyl) (M = Rh, Co)," Organometallics 1997, 16, 2016-2023.(10) Edelbach, B. L.; Lachicotte, R. J.; Jones, W. D., "Mechanistic Investigation of Catalytic Carbon-Carbon Bond Activation and Formation by Platinum and Palladium Phosphine Complexes," J. Am. Chem. Soc. 1998, 120, 2843-2853.(11) Edelbach, B. L.; Vicic, D. A.; Lachicotte, R. J.; Jones, W. D., "Catalytic Hydrogenolysis of Biphenylene with Platinum, Palladium, and Nickel Phosphine Complexes.," Organometallics 1998, submitted.(12) Edelbach, B. L.; Jones, W. D., "Mechanism of Carbon-Fluorine Bond Activation by (C5Me5)Rh(PMe3)H2," J. Am. Chem. Soc. 1997, 119, 7734-7742.(13) Schore, N. E., "Synthesis of (C6H5)2PCH2SiMe2C5H4Li: A novel Heterodifunctional System for the Directed Linkage of Dissimilar Transition Metal Fragments," J. Am. Chem. Soc. 1979, 101, 7410-7412.(14) Schore, N. E.; Benner, L. S.; LaBelle, B. E., "Indirect Metal-Metal Linkage: Cyclic Ferrocene Complexes with a Second Metal Linked via Remote Phosphine Functionality," Inorg. Chem. 1981, 20, 3200-3208.(15) Szymoniak, J.; Beszncon, J.; Cormond, A.; Moise, C., "New Heterodifunctional Ligands for Organotransition-Metal Chemistry: Ph2P(CH2)nC5Me4H (n =0, 2)," J. Org. Chem. 1990, 1990, 1429-1432.(16) Lee, I.; Dahan, F.; Maisonnat, A.; Poilblanc, R., "Transition-Metal Derivatives of Cyclopentadienyl Phosphine Ligands. ((Cyclopentadienylethyl)diphenylphosphine)-rhodium and -iridium Chelated and Bridged Complexes," Organometallics 1994, 13, 1743-2750.(17) Jutzi, P.; Kristen, M. O.; Neumann, B.; Stammler, H.-G., "Rhodium and Iridium Complexes with the 1-(2-(Dimethylamino)ethyl)-2,3,4,5-tetramethylcyclopentadienyl Ligand," Organometallics 1994, 13, 3854-3861.(18) Lefort, L.; Crane, T. W.; Farwell, M. D.; Baruch, D. M.; Kaeuper, J. A.; Lachicotte, R. J.; Jones, W. D., "Synthesis and Reactions of Cp-Linked Phosphine Complexes of Rhodium," Organometallics 1998, 17, in press.