Alkyne dichotomy and hydrogen migration in binuclear cyclopentadienylmetal alkyne complexes

Abstract

The structures and energetics of the binuclear cyclopentadienylmetal alkyne systems Cp₂M₂C₂R₂ (M = Ni, Co, Fe; R = Me and NMe₂) have been investigated using density functional theory. For the Cp₂M₂C₂(NMe₂)₂ (M = Ni, Co, Fe) systems the relative energies of isomeric tetrahedrane Cp₂M₂(alkyne) structures having intact alkyne ligands and alkyne dichotomy structures Cp₂M₂(CNMe₂)₂ in which the CC triple bond of the alkyne has broken completely to give separate Me₂NC units depending on the central metal atoms. For the nickel system Cp₂Ni₂C₂(NMe₂)₂ as well as the related nickel systems Cp₂Ni₂(MeC₂NMe₂) and Cp₂Ni₂C₂Me₂ the tetrahedrane structures are clearly preferred energetically consistent with the experimental syntheses of several stable Cp₂Ni₂(alkyne) complexes. The tetrahedrane and alkyne dichotomy structures have similar energies for the Cp₂Co₂C₂(NMe₂)₂ system whereas the alkyne dichotomy structures are significantly energetically preferred for the Cp₂Fe₂C₂(NMe₂)₂ system. The potential energy surfaces for the Cp₂M₂(MeC₂NMe₂) and Cp₂M₂C₂Me₂ systems (M = Co, Fe) are complicated by low-energy structures in which hydrogen migration occurs from the alkyne methyl groups to one or both alkyne carbon atoms to give Cp₂M₂(C₃H₃NMe₂) and Cp₂M₂(C₃H₃Me) derivatives with bridging metalallylic ligands, Cp₂M₂(CH₂CCHNMe₂) and Cp₂M₂(CH₂CCHMe) with bridging allene ligands, as well as Cp₂M₂(CH₂CH–CNMe₂) and Cp₂M₂(CH₂CH–CHMe) with bridging vinylcarbene ligands. For the Cp₂M₂C₂Me₂ (M = Co, Fe) systems migration of a hydrogen atom from each methyl group to an alkyne carbon atom can give relatively low-energy Cp₂M₂(CH₂CH–CHCH₂) structures with a bridging butadiene ligand. Five transition states have been identified in a proposed mechanism for the conversion of the Cp₂Co₂/MeCCNMe₂ system to the cobaltallylic complex Cp₂Co₂(C₃H₃NMe₂) with intermediates having agostic C–H–Co interactions and an activation energy barrier sequence of 13.1, 17.0, 15.2, and 12.0 kcal mol⁻¹.