CH Bond Activation of Methane by a Transient η2‑Cyclopropene/Metallabicyclobutane Complex of Niobium

This study challenges the problem of the activation of a CH bond of methane by soluble transition metal complexes. High pressure solution NMR, isotopic labeling studies, and kinetic analyses of the degenerate exchange of methane in the methyl complex [TpMe2NbCH3(c-C3H5)­(MeCCMe)] (1) are reported. Stoichiometric methane activation by the mesitylene complex [TpMe2Nb­(CH2-3,5-C6H3Me2)­(c-C3H5) (MeCCMe)] (2) giving 1 is also realized. Evidence is provided that these reactions proceed via an intramolecular abstraction of a β-H of the cyclopropyl group to form either methane or mesitylene from 1 or 2, respectively, yielding the transient unsaturated η2-cyclopropene/metallabicyclobutane intermediate [TpMe2Nb­(η2-c-C3H4) (MeCCMe)] A. This is followed by its mechanistic reverse 1,3-CH bond addition of methane yielding the product.本研究针对可溶性过渡金属配合物活化甲烷碳氢键的科学问题展开探究。本文报道了针对甲基配合物[TpMe2NbCH3(c-C3H5)(MeCCMe)]（1）中甲烷简并交换行为所开展的高压溶液核磁共振（NMR）、同位素标记研究与动力学分析。同时，本文实现了通过均三甲苯配合物[TpMe2Nb(CH2-3,5-C6H3Me2)(c-C3H5)(MeCCMe)]（2）进行计量型甲烷活化以生成化合物1的反应。研究证据显示，上述两类反应均通过分子内夺取环丙基的β氢原子完成：从化合物1或2中分别生成甲烷或均三甲苯，同时得到瞬态不饱和η²-环丙烯/金属双环丁烷中间体[TpMe2Nb(η2-c-C3H4)(MeCCMe)]（中间体A）。随后该中间体通过甲烷的1,3-碳氢键加成发生逆向机理过程，最终得到目标产物。