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

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⋮CtBu(OTf) and [(Nacnac)V⋮CtBu(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⋮CtBu(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.

通过两步连续的单电子氧化诱导的α-氢提取反应，成功制备了带有末端新戊炔基官能团的四配位钒配合物。这类钒卡拜（vanadium alkylidyne）配合物包含中性物种(Nacnac)V≡CtBu(OTf)与阳离子物种[(Nacnac)V≡CtBu(THF)]+，其中Nacnac配体阴离子（Nacnac- = [Ar]NC(CH3)CHC(CH3)N[Ar]，Ar = 2,6-二异丙基苯基(2,6-(CHMe2)2C6H3)）。上述钒卡拜配合物已通过1H、13C、51V核磁共振（NMR）波谱及单晶X射线衍射完成结构表征，其V≡C键长较短，与表征结果相符。研究发现，这类钒卡拜配合物可通过分子内交叉复分解反应转化为氮杂金属环己三烯体系。以氘代甲苯（C7D8）为溶剂，对(Nacnac)V≡CtBu(OTf)的转化过程开展动力学研究，结果表明氮杂金属环己三烯产物的生成与溶剂种类无关（甲苯与四氢呋喃（THF）对比），且反应对钒物种为一级反应（80 ℃下速率常数k = 3.30(5) × 10^-5 s^-1，活化焓ΔH⧧= 25.4(3) kcal/mol，活化熵ΔS⧧ = -6(3) cal/(mol·K)）。针对完整模型的高水平密度泛函理论（DFT）计算显示，该反应遵循仅涉及单一过渡态的分子内反应机制。溶液相中该反应的整体热力学驱动力（吉布斯自由能变ΔG）经估算为-21.3 kcal/mol。