Metal-Carbon a-Bonds

Complexes containing Metal-Carbon a-Bonds.— The compounds investigated during the period covered by this Report display a variety of different chemical and stereochemical types, with Rh-C and Ir-C r-bond distances in the range 2.00— 2.20 A. 

In the octahedral cation (55) of tran -[IrCl(CO)(PPh3)2(FC,H8NNH]-IBF4] the arylazo-ligand is co-ordinated to the metal atom through one nitrogen atom and also via a metal-o-carbon bond. The Ir-C(arylazo), 

12a g Einstein, A. B. Gilchrist, G. W. Raynet-Canham, and D. Sutton, J. Amer, 

Figiue 19 The molecular structure of [IrCl P(OPh)3 P(OPh)a(OCeH4) a] with the OPh groups omitted. The estimated standard deviations are 0.004 A for the Ir-Cl and Ir-P, and 0.04 A for the Ir-C bond lengths 

In the molecule [Rh(C9HjF30a) C8(CF3) P (57) the metal atom and the (CoHjFeOa) ligand form a bicyclo-fragment, with the Rh-C o-bond length and the two Rh-O distances being 2.095(9), 2.152(7), and 2.165(6) A, 

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The electrosynthesis of metalloporphyrins which contain a metal-carbon a-bond is reviewed in this paper. The electron transfer mechanisms of a-bonded rhodium, cobalt, germanium, and silicon porphyrin complexes were also determined on the basis of voltammetric measurements and controlled-potential electrooxidation/reduction. The four described electrochemical systems demonstrate the versatility and selectivity of electrochemical methods for the synthesis and characterization of metal-carbon o-bonded metalloporphyrins. The reactions between rhodium and cobalt metalloporphyrins and the commonly used CH2CI2 is also discussed. 

Metalloporphyrins containing a metal-carbon a-bond are currently limited to complexes with eight different transition metals (Ti, Ni, Fe, Ru, Co, Rh, Ir and In) and seven different non-transition metals (Al, Ga, In, Tl, Si, Ge, and Sn). These compounds have been the subject of several recent reviews(1-33 which have discussed their synthesis and physicochemical properties. 

The synthesis of metalloporphyrins which contain a metal-carbon a-bond can be accomplished by a number of different methods(l,2). One common synthetic method involves reaction of a Grignardreagent or alkyl(aryl) lithium with (P)MX or (PMX)2 where P is the dianion of a porphyrin macrocycle and X is a halide or pseudohalide. Another common synthetic technique involves reaction of a chemically or electrochemically generated low valent metalloporphyrin with an alkyl or aryl halide. This latter technique is similar to methods described in this paper for electrosynthesis of cobalt and rhodium a-bonded complexes. However, the prevailing mechanisms and the chemical reactions... 

Two aspects of porphyrin electrosynthesis will be discussed in this paper. The first is the use of controlled potential electroreduction to produce metal-carbon a-bonded porphyrins of rhodium and cobalt. This electrosynthetic method is more selective than conventional chemical synthetic methods for rhodium and cobalt metal-carbon complexes and, when coupled with cyclic voltammetry, can be used to determine the various reaction pathways involved in the synthesis. The electrosynthetic method can also lead to a simultaneous or stepwise formation of different products and several examples of this will be presented. 

Dozens of electrochemical and spectroelectrochemical papers on transition metal and main group metal-carbon a-bonded metallo-porphyrins were published between 1984 and 1987 and a summary of these results are well covered in three recent reviews(1-3). Therefore, a characterization of chemically synthesized metal-carbon porphyrins will not be discussed in this paper. 

Rhodium Porphyrins. Chemical syntheses of [CPDRh32 and (P)Rh(R) complexes are well known(4-11). Electrochemical techniques have also been used to synthesize dimeric metal-metal bonded [(TPP)RhJ 2 as well as monomeric metal-carbon a-bonded (TPP)Rh(R) and (0EP)Rh(R)(12-16). The electrosynthetic and chemical synthetic methods are both based on formation of a highly reactive monomeric rhodium(II) species, (P)Rh. This chemically or electrochemically generated monomer rapidly dimerizes in the absence of another reagent as shown in Equation 1. 

KADISHETAL. Metalloporphyrins Containing Metal-Carbon a-Bonds 455... 

KADISHETAL. Metalloporphyrins Containing Metal—Carbon a-Bonds 457... 

Cobalt Porphyrins. The primary synthetic method for generating cobalt porphyrins with a metal carbon a-bond is to react a chemically or electrochemically generated cobalt(I) anion, [(P)Co] , with an alkyl or an aryl halide(19-26). [(P)Co] is stable and... 

In summary, the four chemical systems described in this paper demonstrate the versatility and selectivity of electrochemical methods for synthesis and characterization of metal-carbon a-bonded metalloporphyrins. The described rhodium and cobalt systems demonstrate significant differences with respect to their formation, stability and to some extend, reactivity of the low valent species. On the other hand, properties of the electroche-mically generated mono-alkyl or mono-aryl germanium and silicon systems are similar to each other. 

After the discovery by Fischer and Maasbol of the first stable carbene complexes in 1964, i.e., [(CO)5W =C(OMe)R ] [21], generation of related metaUacumulene derivatives [M]=C(=C) =CR2 (n > 0) was obviously envisaged. Thus, it is presently well-established that stabilization of these neutral unsaturated carbenes by coordination to a transition metal center is possible by the use of the lone pair of electrons on the carbenic carbon atom, via formation of a metal-carbon a-bond (electron back-donation from the metal fragment to the carbon ligand may strengthen this bond). This has allowed the development of a rich chemistry of current intense interest due to the potential applications of the resulting metallacumulenic species in organic synthesis, as well as in the construction of molecular wires and other nanoelectronic devices [22]. 

Infrared and nuclear magnetic resonance data for the U(C5H5)3R complexes are consistent with a metal-carbon a bond, and this fact has been confirmed by the structural analysis of tricyclopentadienylphenyethynyluranium(IV) (72). The molecular geometry (Fig. 13) about the uranium is a distorted tetrahedron with... 

The structures found for the products of the various reactions are often themselves of interest for example, the many cases where the olefin group becomes attached to the metal by a metal-carbon a-bond form a useful body of knowledge, and structural studies have proved an apparent case of the Wagner-Meerwein rearrangement in Pt(IV) and Au(III) complexes. 

The discussion by Braterman and Cross of reductive elimination from square planar or octahedral complexes is a special case of frontier orbital theory. A transition metal L MR, R2 is taken to lose grcwps R, R2 in a concerted step to give L M + Rj - R2. By the usual book-keeping convention, the electrons in the initial M—R a bonds are assigned to the R groups but this is of course a mere convention of naming and does not affect the argument. The in-phase combination of metal-carbon a bonds correlates... 

The insertion of a metal bound 0x0 group into a metal carbon a-bond (Eq. 19), e.g., of an alkyl or aryl, has great appeal as an elementary step in the oxygenation of organic molecules. 

We infer that final products, C, D and E, stem from the same metal-carbon a-bonded intermediate B, and their relative amounts are due to kinetic factors. Carrying out the same reaction starting from an E geometric derivative, the EZ isomerization is, as... 

Formation of Transition Metal Complexes with Metal-Carbon a-Bonds Properties of Complexes with Metal-Carbon a-Bonds... 

A. Heterolysis of the Metal-Carbon a-Bond Homolysis of the Metal-Carbon a-Bond Oxidation of Lm iM +1-R Followed by Homolysis P-Hydride Shift Reactions P-Elimination Reactions P-Elimination of Carboxylates CO Insertion/Methyl Migration... 

Complexes with Metal-carbon a-Bonds Formed in Redox Processes Between Transition Metal Complexes and Organic Substrates... 

Among the catalysts used are Lewis acids991 and phosphine-nickel complexes.992 Certain of the reverse cyclobutane ring openings can also be catalytically induced (8-40). The role of the catalyst is not certain and may be different in each case. One possibility is that the presence of the catalyst causes a forbidden reaction to become allowed, through coordination of the catalyst to the -it or a bonds of the substrate.993 In such a case the reaction would of course be a concerted 2S + 2S process. However, the available evidence is more consistent with nonconcerted mechanisms involving metal-carbon a-bonded intermediates, at least in most cases.994 For example, such an intermediate was isolated in the dimerization of nor-bornadiene, catalyzed by iridium complexes.995... 

The insertion of CO into metal-carbon a bonds has been reviewed.585-590 Carbonylation of alkyl platinum(II) complexes usually requires elevated temperatures, although at higher temperatures the reaction is reversible (equation 211).591 With PtMe2(dppe) insertion occurs into only one of the Pt—Me bonds. For complexes PtX(Ar)L2, carbonylation follows pseudo first-order kinetics. Rates are decreased by addition of L to a maximum value where the carbonylation rate is independent of L. The pathway involves formation of a five-coordinate intermediate PtX(Ar)(CO)L2, followed by dissociation to form PtX(Ar)(CO)L. The migratory step to yield PtX(COAr)L is unaffected by added L. This pathway is outlined in Scheme 6.502... 

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