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

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journal contribution
posted on 22.10.2012, 00:00 by Cherumuttathu H. Suresh, Gernot Frenking
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–Cβ distances (∼2.1 Å) very close to M–Cα distances (1.8–2.0 Å), suggesting a bonding interaction between the metal and the β-carbon (1,3-MC bond). Energy decomposition analysis using C2v symmetric structures revealed that a b2 orbital composed of mainly metal dπ and Cβ pπ overlap and an agostic a1 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 δ orbital interaction through an a2 orbital, which contributed to the double-bond character of M–Cα bonds (bond order 1.27–1.44). Although the formation of b2 and a2 orbitals increased the M–C interactions, they significantly reduced the π interactions within the C3H3 fragment (C–C bond order 1.09–1.18). 1,3-MC bonding suggested a planar tetracoordinate configuration for Cβ, as the Cα–Cβ bonds possessed largely formal Csp2–Csp2 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 η3-structures (metallatetrahedranes). A mechanistic study on the reaction between acetylene and alkylidyne complex MClnCH showed that a nearly barrierless and exothermic pathway exists for MCB 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.