Carbon-Carbon Bond Formation in Dinuclear Dialkyl Complexes. Reactions of 2 (R = CH3, CH2CH3, CH2CF3) and (C11H10)Co2(CO)2(CH3)2 with Carbon Monoxide and Triphenylphosphine

Article abstract of DOI:10.1021/ja00336a026

The full details of the synthesis, thermal decomposition, and carbonylation of dialkyldicobalt dimers 2 are reported.The dinuclear dimethyl complex 2a gives acetone and cobalt clusters on thermal decomposition; the very sensitive mononuclear complex CpCo(CO)Me2 (7a) can be detected as an intermediate.The bis(trifluoroethyl) complex 2 (2c) rearranges to 7c (R = CH2CF3), which does not go on to ketone and can be isolated.Carbonylation of the dinuclear dimethyl and diethyl complexes 2a and 2b proceeds even more cleanly to ketones, giving a quantitative yield of CpCo(CO)2 as the final organometallic product.However, this reaction proceeds by a mechanism different from that of the thermal decomposition.In the carbonylation an initial very rapid reaction leads to the diacyl complex 2; this goes on to ketone at a rate faster than can be attributed to 7.Crossover experiments provide strong evidence that complexes 2a, 2b, and 2c are in rapid equilibrium with small amounts of the paramagnetic, 17-electron monomers CpCo(CO)R.The corresponding complex (C11H10)Co2(CO)2Me2 (13a), containing a bridging ligand in which two Cp rings are connected by a CH2 group, has also been synthesized.This material is more stable than 2a; it undergoes carbonylation to acetone only at temperatures near 70 deg C.It reacts with PPh3 at 25 deg C but instead of leading to ketone gives the rearranged product (14).The PPh3 + 13 <*> 14 reaction is reversible at room temperature, with an equilibrium constant of 33 M^-1.Kinetic studies on this system are consistent with a mechanism involving the reversible formation of a transient intermediate which can be trapped by PPh3.Taken together, the results require that at least three chemical pathways are accessible to complexes of type 2: reversible cleavage to paramagnetic monomers, alkyl transfer from one cobalt atom to another, and carbonylation to give dinuclear diacyl complexes, which can rapidly proceed on to form ketone.