Department of Chemistry and Polymer Science, Stellenbosch University, Private Bag X1, Stellenbosch, 7602 Western Cape, South Africa.
Inorg Chem. 2021 Jan 18;60(2):782-797. doi: 10.1021/acs.inorgchem.0c02799. Epub 2021 Jan 7.
This combined experimental and computational study builds on our previous studies to elucidate the reaction mechanism of methanol oxidation by Os oxido/hydroxido species (in basic aqueous media) while accounting for the simultaneous formation of Os species a comproportionation reaction between Os and Os. UV-Vis spectroscopy kinetic analyses with either CHOH or the deuterated analogue CDOH as a reducing agent revealed that transfer of α-carbon-hydrogen of methanol is the partial rate-limiting step. The resulting relatively large KIE value of approximately 11.82 is a combination of primary and secondary isotope effects. The Eyring plots for the oxidation of these isotopologues of methanol under the same reaction conditions are parallel to each other and hence have the same activation enthalpy [Δ° = 14.4 ± 1.2 kcal mol (CHOH) and 14.5 ± 1.3 kcal mol (CDOH)] but lowered activation entropy (Δ°) from -12.5 ± 4.1 cal mol K (CHOH) to -17.1 ± 4.4 cal mol K (CDOH). DFT computational studies at the PBE-D3 level with QZ4P (Os) and pVQZ (O and H) basis sets provide clear evidence to support the data and interpretations derived from the experimental kinetic work. Comparative DFT mechanistic investigations in a simulated aqueous phase (COSMO) indicate that methanol and Os first associate to form a noncovalent adduct bound together by intermolecular H-bonding interactions. This is followed by spin-forbidden α-carbon-hydrogen transfer (not O-H transfer) from methanol to Os by means of HAT, which is found to be the partial rate-limiting step. Without the organic and inorganic fragments dissociating from each other during the entire stepwise redox reaction (in order to avoid formation of highly energetically unfavorable monomer species), the HAT step is followed by PT and then ET before the final product monomers formaldehyde and Os dissociate from each other. DFT-calculated Δ° is within 5 kcal mol of the experimentally obtained value, while the DFT Δ° is three times larger than that found from the experiment.
本研究结合实验和计算，在前人研究的基础上阐明了在碱性水介质中 Os 氧化/羟物种（Os oxido/hydroxido species）催化甲醇氧化的反应机理，同时考虑了 Os 物种的同时形成（一种 Os 与 Os 的comproportionation 反应）。用 CHOH 或氘代类似物 CDOH 作为还原剂的 UV-Vis 光谱动力学分析表明，甲醇的α-碳氢键的转移是部分速率限制步骤。由此产生的约 11.82 的较大 KIE 值是一级和二级同位素效应的组合。在相同反应条件下，这些甲醇同位素物氧化的 Eyring 图彼此平行，因此具有相同的活化焓[Δ°= 14.4 ± 1.2 kcal mol（CHOH）和 14.5 ± 1.3 kcal mol（CDOH）]，但活化熵降低（Δ°）从-12.5 ± 4.1 cal mol K（CHOH）降至-17.1 ± 4.4 cal mol K（CDOH）。在 PBE-D3 水平下使用 QZ4P（Os）和 pVQZ（O 和 H）基组进行的 DFT 计算研究为实验动力学工作提供的数据和解释提供了明确的证据支持。在模拟水相（COSMO）中的比较 DFT 机理研究表明，甲醇和 Os 首先缔合形成非共价加合物，通过分子间氢键相互作用结合在一起。然后通过 HAT 从甲醇到 Os 发生禁阻自旋的α-碳氢键转移（不是 O-H 转移），这被发现是部分速率限制步骤。在整个逐步氧化还原反应中，没有有机和无机片段彼此解离（以避免形成高能不利的单体物种），HAT 步骤之后是 PT 和 ET，然后是最终产物单体甲醛和 Os 彼此解离。DFT 计算的Δ°与实验获得的值相差 5 kcal mol 以内，而 DFT 的Δ°比实验值大 3 倍。
