Mechanism of Molybdenum-Mediated Carbon Monoxide Deoxygenation and Coupling: Mono- and Dicarbyne Complexes Precede C–O Bond Cleavage and C–C Bond Formation

Deoxygenative coupling of CO to value-added C≥2 products is challenging and mechanistically poorly understood. Herein, we report a mechanistic investigation into the reductive coupling of CO, which provides new fundamental insights into a multielectron bond-breaking and bond-making transformation. In our studies, the formation of a bis­(siloxycarbyne) complex precedes C–O bond cleavage. At −78 °C, over days, C–C coupling occurs without C–O cleavage. However, upon warming to 0 °C, C–O cleavage is observed from this bis­(siloxycarbyne) complex. A siloxycarbyne/CO species undergoes C–O bond cleavage at lower temperatures, indicating that monosilylation, and a more electron-rich Mo center, favors deoxygenative pathways. From the bis­(siloxycarbyne), isotopic labeling experiments and kinetics are consistent with a mechanism involving unimolecular silyl loss or C–O cleavage as rate-determining steps toward carbide formation. Reduction of Mo­(IV) CO adducts of carbide and silylcarbyne species allowed for the spectroscopic detection of reduced silylcarbyne/CO and mixed silylcarbyne/siloxycarbyne complexes, respectively. Upon warming, both of these silylcarbynes undergo C–C bond formation, releasing silylated C2O1 fragments and demonstrating that the multiple bonded terminal MoC moiety is an intermediate on the path to deoxygenated, C–C coupled products. The electronic structures of Mo carbide and carbyne species were investigated quantum mechanically. Overall, the present studies establish the elementary reactions steps by which CO is cleaved and coupled at a single metal site.

将一氧化碳（CO）脱氧偶联为高附加值C≥2产物是极具挑战性的研究课题，其反应机理目前仍不甚明晰。本文中，我们针对一氧化碳还原偶联反应开展了系统的机理研究，为这类多电子断键与成键的转化过程提供了全新的基础认知。在本研究中，双（硅氧基卡拜）配合物的生成先于C-O键断裂过程。在-78 ℃条件下反应数日后，体系仅发生C-C偶联而未出现C-O键断裂。但当温度升至0 ℃时，该双（硅氧基卡拜）配合物即可发生C-O键断裂。硅氧基卡拜（siloxycarbyne）/一氧化碳物种可在更低温度下发生C-O键断裂，这表明单硅基化修饰以及电子云密度更高的钼（Mo）中心更有利于脱氧反应路径。基于该双（硅氧基卡拜）配合物的同位素标记实验与动力学研究结果均支持如下反应机理：以单分子硅基解离或C-O键断裂作为碳化物生成过程的决速步骤。对碳化物与硅基卡拜（silylcarbyne）物种的四价钼（Mo(IV)）一氧化碳加合物进行还原反应后，我们分别通过光谱手段检测到了还原态硅基卡拜/一氧化碳配合物以及混合硅基卡拜/硅氧基卡拜配合物。升温后，这两类硅基卡拜均会发生C-C键形成反应，释放出硅基化的C₂O₁片段，同时证明了具有多重键的端基钼-碳三键（Mo≡C）结构单元是生成脱氧C-C偶联产物过程中的关键中间体。我们通过量子力学方法研究了钼基碳化物与卡拜物种的电子结构。综上，本研究明确了一氧化碳在单金属位点上发生断裂与偶联的全部基元反应步骤。