CO Coupling Chemistry of a Terminal Mo Carbide: Sequential Addition of Proton, Hydride, and CO Releases Ethenone
CO Coupling Chemistry of a Terminal Mo Carbide: Sequential Addition of Proton, Hydride, and CO Releases Ethenone
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DOI：
10.1021/jacs.9b07743
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发表时间：
2019-10-02
影响因子：
15
通讯作者：
Agapie, Theodor
中科院分区：
文献类型：
--
作者：
Buss, Joshua A.;Bailey, Gwendolyn A.;Agapie, Theodor
Fischer 和 Tropsch 最初提出的一氧化碳 (CO) 氢化连锁反应机制涉及碳化物衍生的表面亚甲基的 C-C 偶联。迄今为止，尚不清楚能够捕获这些复杂化学步骤的单分子系统。在此，我们演示了将质子和氢化物顺序添加到源自 CO 的末端 Mo 碳化物中。所得阴离子亚甲基在低温（-78 摄氏度）下与 CO (1 atm) 偶联，释放乙烯酮。重要的是，以氢化物（H- = 2e(-) + H+）形式同步传递两个还原当量和亲电子试剂，促进了碳炔前体形成亚烷基，并在比之前描述的强单电子还原剂和亲电子试剂更温和的条件下实现偶联化学。热力学测量将促进从碳化物形成亚甲基的水度和酸度要求考虑为相对于 H-2 的异裂在能量上可行。直接碳化物羰基化实验证明，C-C 偶联之前亚甲基的形成对于有机产物的释放至关重要。光谱研究、单甲硅烷基化模型系统和量子力学计算提供了对该反应序列的机械细节的深入了解，该反应序列是费托合成初始阶段的罕见模型。
The mechanism originally proposed by Fischer and Tropsch for carbon monoxide (CO) hydrogenative catenation involves C-C coupling from a carbide-derived surface methylidene. A single molecular system capable of capturing these complex chemical steps is hitherto unknown. Herein, we demonstrate the sequential addition of proton and hydride to a terminal Mo carbide derived from CO. The resulting anionic methylidene couples with CO (1 atm) at low temperature (-78 degrees C) to release ethenone. Importantly, the synchronized delivery of two reducing equivalents and an electrophile, in the form of a hydride (H- = 2e(-) + H+), promotes alkylidene formation from the carbyne precursor and enables coupling chemistry, under conditions milder than those previously described with strong one-electron reductants and electrophiles. Thermodynamic measurements bracket the hydricity and acidity requirements for promoting methylidene formation from carbide as energetically viable relative to the heterolytic cleavage of H-2. Methylidene formation prior to C-C coupling proves critical for organic product release, as evidenced by direct carbide carbonylation experiments. Spectroscopic studies, a monosilylated model system, and Quantum Mechanics computations provide insight into the mechanistic details of this reaction sequence, which serves as a rare model of the initial stages of the Fischer-Tropsch synthesis.