Synthesis, Structure, and Dynamics of Molybdenum Imido Alkyne Complexes

The monomeric alkyne complexes (η2-alkyne)Mo(NPh)(o-(Me3SiN)2C6H4) (3) have been
synthesized by the displacement of isobutylene from (η2-isobutylene)Mo(NPh)(o-(Me3SiN)2C6H4) (2). The alkyne fragment in these complexes is oriented perpendicular to the MoN
bond of the cis imido ligand, as confirmed by an X-ray structural analysis of 3e. The
deshielded nature of the chemical shifts of the α-carbons and terminal protons of the alkyne
fragments in these complexes strongly suggests the participation of the alkyne π⊥ electrons
in the Mo−alkyne interaction. The alkyne fragment in 3 rotates freely about the Mo−alkyne
bond, resulting in the fluxional behavior of these complexes at room temperature. An
activation barrier of 13.2 kcal/mol for the alkyne rotation was measured using VT NMR
spectroscopy. Computational studies using a two-layer ONIOM model, and the B3LYP hybrid
functional, provided insight into the Mo−alkyne bonding. The transition state for alkyne
rotation has been calculated and is characterized by a parallel orientation of the alkyne
fragment to the cis imido ligand. A natural bond orbital (NBO) population analysis reveals
that alkyne π⊥ donation to Mo is more extensive in the transition state than in the ground
state. Weaker Mo−N(imido) bonds are also observed in the transition state, because π
donation from the alkyne ligand competes with imido π donation.

单分子烷炔配合物（η2-烷炔）Mo(NPh)(o-(Me3SiN)2C6H4) (3) 已通过将异丁烯从（η2-异丁烯）Mo(NPh)(o-(Me3SiN)2C6H4) (2) 中置换而合成。这些配合物中的烷炔片段与顺式亚氨基配体的Mo-N键垂直排列，这一点通过3e的X射线晶体结构分析得到证实。这些配合物中烷炔片段的α碳和端基氢的化学位移去屏蔽特性强烈暗示烷炔π⊥电子在Mo-烷炔相互作用中的参与。烷炔片段在3中围绕Mo-烷炔键自由旋转，导致这些配合物在室温下表现出动力学不稳定性。使用VT NMR光谱法测得烷炔旋转的活化能为13.2 kcal/mol。采用双层ONIOM模型和基于B3LYP杂化函数的计算研究为Mo-烷炔键合提供了见解。已计算烷炔旋转的过渡态，其特征为烷炔片段与顺式亚氨基配体平行排列。自然键轨道（NBO）分析揭示，在过渡态中烷炔π⊥向Mo的捐赠比基态更为广泛。过渡态中观察到的Mo-N(imido)键也相对较弱，因为烷炔配体的π捐赠与亚氨基π捐赠存在竞争。