Research Plans for the Period December 1, 1997 - November 30, 1998:
This coming year, we will continue focus our research on three of the items presented in our proposal where we have had success. These include: (1) carbon-carbon bond cleavage reactions, (2) synthesis of new complexes for hydrogenolysis reactions, and (3) C-H bond cleavage reactions of trispyrazolylboraterhodium complexes. We have made progress in each of these areas over the past year, as described in our report, and will continue our studies in these areas.
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 some success in C-C cleavage with biphenylene, a molecule with a weaker C-C bond than biphenyl. The success has to do with the use of several new platinum based metal systems for C-C cleavage. We are nearing completion of the C-C cleavage of biphenylene by Pt(PEt3)3, which catalytically produces tetraphenylene. This work will be written up for publication, and we will continue hydrogenolysis studies using this catalyst system. We have found that Pt(PEt3)2H2 can act as a catalyst, and are pursuing the mechanism of this C-C bond hydrogenolysis. We will extend the studies to include palladium and nickel complexes for comparison. We will also continue studies of complexes of the type [Pt(chelating phosphine)]. Control of the bite angle of the chelate has been found to be an important factor in tailoring the reactivity of the PtL2 fragment, and we will take advantage of knowledge of these factors.
Another area that we will be investigating involves a continuation of our studies with 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. We will first look at the analogous phosphine systems for comparison with the isocyanide system. We will also look at rearrangements of alkyl hydride complexes that we were unable to prepare until now. We plan on extending the C-H activation work to include acetylenes.
Progress Report for the Period
December 1, 1996- November 30, 1997.
1. Tris-pyrazolylborate Studies.
We have continued to develop our C-H activation studies with the reactive trispyrazolylborate system, (HBPz*3)Rh(CNCH2CMe3)(R)H. Many C-H activation products have been identified in earlier studies 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)]). 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. 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.
Recently, 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. 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. All equilibrium and rate constants have been determined.
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, suggesting a bimolecular mechanism.
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 a methane s-complex.
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, 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.
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.
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.6
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.
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.
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. 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.
Publications appearing or in press during the current grant period,
December 1, 1995 - November 30, 1997: