1,3-Metal-Carbon Bonding and Alkyne Metathesis: DFT Investigations on Model Complexes of Group 4, 5, and 6 Transition Metals

The formation of metallacyclobutadienes (MCBs) from chloro-ligated alkylidyne complexes of group 4, 5, and 6 transition metals (MCln(C3H3)) has been studied at the BP86/def2-TZVPP level. All the MCBs showed M-C-beta distances (similar to 2.1 angstrom) very close to M-C-alpha distances (1.8-2.0 angstrom), suggesting a bonding interaction between the metal and the beta-carbon (1,3-MC bond). Energy decomposition analysis using C-2v symmetric structures revealed that a b(2) orbital composed of mainly metal d(pi) and C-beta p(pi) overlap and an agostic a(1) orbital contributed to the orbital interaction of the 1,3-MC bond. The bond order of the 1,3-MC bond is a minimum of 0.26 for M = Cr and a maximum of 0.43 for M = Ta. Further, all the MCBs showed a characteristic delta orbital interaction through an a(2) orbital, which contributed to the double-bond character of M-C-alpha bonds (bond order 1.27-1.44). Although the formation of b(2) and a(2) orbitals increased the M-C interactions, they significantly reduced the pi interactions within the C3H3 fragment (C-C bond order 1.09-1.18). 1,3-MC bonding suggested a planar tetracoordinate configuration for C-beta, as the C-alpha-C-beta bonds possessed largely formal C-sp2-C-sp2 single-bond character. Electron density analysis showed a catastrophic character of the 1,3-MC bond. In groups 4 and 5, MCBs were more stable than the isomeric eta(3)-structures (metallatetrahedranes). A mechanistic study on the reaction between acetylene and alkylidyne complex MClnCH showed that a nearly barrierless and exothermic pathway exists for MOB formation (exothermic value 75-102 kcal/mol for groups 4 and 5; 6 27 kcal/mol for group 6). The rich metathesis chemistry associated with Mo and W is attributed mainly to the moderate activation energy required for the alkyne disproportionation step of metathesis. A mechanistic possibility other than Katz's is also proposed for alkyne metathesis that showed that the 1,3-MC bonded MCB complex can act as a metathesis catalyst by reacting with alkyne to form a bicyclic intermediate and subsequently disproportionating to yield the alkyne and the MCB. For this mechanism to be effective, rearrangement of the bicyclic intermediate to a more stable cyclopentadienyl complex has to be prevented.