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Alkyne dichotomy and hydrogen migration in binuclear cyclopentadienylmetal alkyne complexes
Huidong Li1,2, Jinfeng Luo1, Haoyu Chen1
1School of Science, Key Laboratory of High Performance Scientific Computation, Xihua University Chengdu 610039 China.
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 C[triple bond, length as m-dash]C 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(CH2[double bond, length as m-dash]C[double bond, length as m-dash]CHNMe2) and Cp2M2(CH2[double bond, length as m-dash]C[double bond, length as m-dash]CHMe) with bridging allene ligands, as well as Cp2M2(CH2[double bond, length as m-dash]CH-CNMe2) and Cp2M2(CH2[double bond, length as m-dash]CH-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(CH2[double bond, length as m-dash]CH-CH[double bond, length as m-dash]CH2) structures with a bridging butadiene ligand. Five transition states have been identified in a proposed mechanism for the conversion of the Cp2Co2/MeC[double bond, length as m-dash]CNMe2 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.
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