3d过渡金属物种活化甲烷的研究已有较多报道，但人们对3d前过渡金属物种与甲烷反应体系的报道非常少，与之相关的甲烷活化机理的认识仍然非常有限。在本工作中，我们通过气相质谱实验和密度泛函理论计算证实了VB+离子可以在热碰撞条件下活化甲烷产生氢气和碳硼化合物，由于强的静电相互作用，甲烷活化优先发生在VB+离子的V原子位点。甲烷的活化转化涉及二态反应性，在反应的入口处需要经历从高自旋六重态到低自旋四重态的自旋反转。由于V―CH3以及B―H化学键较强，H3C―H键断裂以V―B单元协同插入而非单个V或B原子插入C―H键的方式进行。对VB+活化甲烷的机理认识可以为新型3d过渡金属催化剂活化甲烷的研究提供理论基础。
Methane activation by transition metal species has been extensively investigated over the past few decades. It is observed that ground-state monocations of bare 3d transition metals are inert toward CH4 at room temperature because of unfavorable thermodynamics. In contrast, many mono-ligated 3d transition metal cations, such as MO+ (M = Mn, Fe, Co, Cu, Zn), MH+ (M = Fe, Co), and NiX+ (X = H, CH3, F), as well as several bis-ligated 3d transition metal cations including OCrO+, Ni(H)(OH)+, and Fe(O)(OH)+ activate the C―H bond of methane under thermal collision conditions because of the pronounced ligand effects. In most of the above-mentioned examples, the 3d metal atoms are observed to cooperate with the attached ligands to activate the C―H bond. Compared to the extensive studies on active species comprising of middle and late 3d transition metals, the knowledge about the reactivity of early 3d transition metal species toward methane and the related C―H activation mechanisms are still very limited. Only two early 3d transition metal species HMO+ (M = Ti and V) are discovered so far to activate the C―H bond of methane via participation of their metal atoms. In this study, by performing mass spectrometric experiments and density functional theory calculations, we have identified that the diatomic vanadium boride cation (VB+) can activate methane to produce a dihydrogen molecule and carbon-boron species under thermal collision conditions. The strong electrostatic interaction makes the reaction preferentially proceed the V side. To generate experimentally observed product ions, a two-state reactivity scenario involving spin conversion from high-spin sextet to low-spin quartet is necessary at the entrance of the reaction. This result is consistent with the reported reactions of 3d transition metal species with CH4, in which the C―H bond cleavage generally occurs in the low-spin states, even if the ground states of the related active species are in the high-spin states. For VB+ + CH4, the insertion of the synergetic V―B unit (rather than a single V or B atom) into the H3C―H bond causes the initial C―H bond activation driven by the strong bond strengths of V―CH3 and B―H. The mechanisms of methane activation by VB+ discussed in this study may provide useful