Terminal Vanadium−Neopentylidyne Complexes and Intramolecular Cross-Metathesis Reactions to Generate Azametalacyclohexatrienes

Abstract

Four-coordinate vanadium complexes containing a terminal neopentylidyne functionality have been prepared by two consecutive α-hydrogen abstraction reactions both of which were induced by one-electron oxidations. Among these vanadium−alkylidyne complexes are the neutral and the cation (Nacnac)V⋮C^tBu(OTf) and [(Nacnac)V⋮C^tBu(THF)]⁺, respectively (Nacnac- = [Ar]NC(CH₃)CHC(CH₃)N[Ar], Ar = 2,6-(CHMe₂)₂C₆H₃). The vanadium−alkylidynes have been characterized by ¹H, ¹³C, ⁵¹V NMR spectroscopy and single-crystal X-ray diffraction and are consistent with a short V⋮C bond. These alkylidynes were found to transform to azametalacyclohexatriene systems via an intramolecular cross-metathesis reaction. Kinetic studies of the transformation of (Nacnac)V⋮C^tBu(OTf) in C₇D₈ reveal the formation of the azametalacyclohexatriene to be independent of solvent (toluene vs THF) and the reaction to be first order in vanadium (k = 3.30(5) × 10^-5 s^-1 at 80 °C, with activation parameters ΔH^⧧= 25.4(3) kcal/mol, ΔS^⧧ = −6(3) cal/molK). High-level DFT calculations on the full model suggest an intramolecular mechanism invoking only one transition state. The overall thermodynamic driving force for the reaction (ΔG) in solution phase was estimated to be −21.3 kcal/mol.