Kinetics and Mechanism of Decomposition of a Benzodicobaltacyclohexene: Reversible Dinuclear Elimination of o-Xylylene via a Dimetalla-Diels-Alder Reaction

Article abstract of DOI:10.1021/ja00356a023

A detailed mechanistic study of several reactions of the first dimetallacyclohexene, (η⁵-Cp)2Co2(μ-CO)2(μ-o-xylylene) (1), is described.The predominant reaction pathway is proposed to involve reversible cleavage of 1 In Diels-Alder fashion, into free o-xylylene (3) and the metal-metal double-bonded dimer 9.Several lines of evidence support this conclusion.Crossover experiments demonstrated that loss of 3 from 1 in its reaction with PPhMe2 to give dinuclear monophosphine adduct 2 is a dinuclear elimination process that leaves the cobalt-cobalt bond intact.Kinetic studies showed that sufficiently high concentrations of several ligands (phosphines, bis(methylcyclopentadienyl) metal-metal double-bonded dimer 10, or dimethyl acetylenedicarboxylate (DMAD)) induce decomposition of 1 at the same maximum rate, to give monophosphine adduct 2, bis(methylcyclopentadienyl)dimetallacyclohexene 5, or dinuclear DMAD adduct 16, respectively.Both the phosphine and DMAD reactions exhibited falloff in the observed rates of decomposition at lower ligand concentration.On the basis of the proposal that 1 was in thermal equilibrium with two reactive intermediates (3 and 9), theoretical results suggested, and were experimentally confirmed, that at low DMAD concentration the fallof in decomposition rate could be eliminated by lowering the concentration of 1, to again induce decomposition at the previously observed maximum rate.An Arrenius plot of data collected in this limiting rate regime, representing the rate of the retro-dimetalla-Diels-Alder reaction,gave ΔH<*> = 24.3 kcal/mol and ΔS<*> = +12.1 eu.Preparative reactions of double-bonded dimer 9 with two different o-xylylene precursors gave moderate yields of metallacycle 1, providing additional evidence for the forward dimetalla-Diels-Alder reaction.A minor reaction pathway, thermal decomposition of 1 to CpCo(o-xylylene) (12) and CpCo(CO)2 (13), was also observed.Evidence is presented suggesting that it is mechanistically related to the major decomposition pathway operating in other dinuclear dicobalt systems, involving intramolecular alkyl to cobalt migration.