Vinylidene carbonylation at a manganese–iron complex: A density functional study of mechanism

Treating the phenylvinyldene manganese complex Cp(CO)2Mndouble bond; length as m-dashC1double bond; length as m-dashC2HPh, 1, with [Fe(CO)4] yields the binuclear μ-vinylidene complex Cp(CO)2MnFe(μ-C1double bond; length as m-dashC2HPh)(CO)4, 2, that further isomerizes to the carbonylated product η4-[Cp(CO)2MnC1(CO)C2HPh]Fe(CO)3, 3. In a computational study of the mechanism using a hybrid density functional method, we considered two stereoisomers for species 2 and 3 where the phenyl group at center C2 is oriented in cis (E form) or trans (Z form) fashion to the Cp(CO)2Mn unit. Isomers 2E and 2Z were calculated to be degenerate whereas the experimentally detected species 3E is 8 kcal/mol more stable than its isomer 3Z. The two-step pathway 1 → 2Z → 3E was calculated to be the lowest-energy route with the highest activation barrier at 12 kcal/mol. The activation energy of the alternative single-step pathway 1 → 3E is 19 kcal/mol. We rationalized the stabilization of the ground state of 3E and the transition states leading to or starting from isomer 2Z as conjugation effect between the Mn–Cdouble bond; length as m-dashC metallaallene fragment and the co-planar phenyl ring.