Synthesis and Reaction Chemistry of a New Class of μ-Alkylidene Dicobalt Complexes. Crystal and Molecular Structure of (μ-Methylene)bis5-(methylcyclopentadienyl)carbonylcobalt>

Article abstract of DOI:10.1021/ja00341a028

Threatment of Na2 with gem-diiodoalkanes (RR'CI2, R = H, alkyl) leads to a class of dinuclear μ-alkyliidene complexes containing two adjacent cobalt atoms.Cis and trans isomers were observed in certain of these materials, as well as bridging and terminal carbonyls, depending upon the substituents located at the alkylidene carbon.Complexes having two alkyl substituents attached to this carbon lose one CO ligand, leading to stable unsaturated μ-alkylidene complexes.The molecular structure of (μ-(methylcyclopentadienyl)carbonylcobalt> (11) was determined by X-ray diffraction (R = 2.82percent).The crystals were triclinic (space group P1<*>), with unit cell parameters a = 8.0592 Angstroem, b = 9.5160 Angstroem, c = 9.9330 Angstroem, α = 69.678 deg, β = 74.769 deg, and γ = 83.704 deg.At elevated temperatures dimers 10 (M, M' = Co; R = H, R' = neopentyl) and 13 (M = Co; M' = Rh; R, R' = H) undergo cis-trans isomerization at a rate which is rapid on the NMR time scale.Treatment of 1 (M,M' = Co; R,R' = H) with 11 or CpRh(CO)2 leads to metal-metal exchange reactions, resulting in the formation of heteronuclear (Co-Rh) μ-alkylidene complexes. 1 reacts with ethylene to give propene and CpCo(CO)(C2H4).Mechanistic studies have demonstrated that this process is autocatalytic, and addition of other ligands (e.g., CO, C2F4, propene) retards the rate of the reaction by scavenging the unsaturated cobalt complexes which function as the autocatalysts.Finally, thermolysis of internal or disubstituted alkylidene complexes 2, 5, and 7 causes a novel rearrangement in which the dicobalt moiety migrates to the end of the alkylidene carbon chain, generating terminal alkylidene complexes having the same number of carbon atoms.Labeling studies have demonstrated that both sequential 1,2 metal shift (leading to alkene ? complexes) and allylic C-H activation (leding to ?-allyl complexes) are involved as steps in this rearrangement.