Morrow Mollie C, Machan Charles W
Department of Chemistry, University of Virginia, PO Box 400319, Charlottesville, Virginia 22904-4319, United States.
JACS Au. 2025 Jul 17;5(7):3424-3432. doi: 10.1021/jacsau.5c00494. eCollection 2025 Jul 28.
Increasing interest in alternative methods for fuel generation and chemical synthesis has resulted in an increased focus on the development of electrocatalysts for energy relevant small molecule transformations, such as the oxidation of methanol. Partial methanol oxidation is a crucial step in the generation of the commodity chemical formaldehyde, and its complete oxidation to carbon dioxide can also serve as the anodic reaction in direct methanol fuel cells. We report a coelectrocatalytic system comprised of an oxo-centered triruthenium cluster ( ) as the catalyst, with the electro-generated -phthalimido--oxyl (PINO) radical species acting as a redox mediator. Only a mild Brønsted base, 2,6-lutidine, is required to achieve an electrocatalytic response. The cocatalytic system demonstrates remarkable cooperativity, shifting the oxidation potential of MeOH ( ) less positive by ca. 0.5 V compared to the intrinsic response of the complex. Controlled potential electrolysis on a model substrate, 4-trifluoromethylbenzyl alcohol, demonstrates selective production of the two-electron, two-proton aldehyde product with a Faradaic efficiency of 79 ± 11% at a rate of 3.14 s. The rate of cocatalysis is 50-fold greater than the intrinsic activity of and 26-fold greater than that of PINO alone under otherwise identical conditions. Mechanistic studies reveal the oxidation of a -alkoxide species as the potential-determining step, while two possible rate-determining steps are identified depending on the substrate. A preference for sterically uninhibited electron-rich benzyl alcohol substrates suggests that a H atom transfer from the -alkoxide adduct to PINO is rate-determining, while the lack of an observed kinetic isotope effect using deuterated MeOH suggests the oxidation of the -alkoxide species is both rate- and potential-determining for cocatalysis.
对燃料生成和化学合成替代方法的兴趣日益浓厚，这使得人们更加关注用于与能源相关的小分子转化的电催化剂的开发，例如甲醇的氧化。部分甲醇氧化是商品化学品甲醛生成过程中的关键步骤，其完全氧化为二氧化碳也可作为直接甲醇燃料电池的阳极反应。我们报道了一种共电催化体系，该体系由以氧为中心的三钌簇（ ）作为催化剂，电生成的 -邻苯二甲酰亚胺基- -氧基（PINO）自由基物种作为氧化还原介质。仅需一种温和的布朗斯特碱2,6 - 二甲基吡啶即可实现电催化响应。该共催化体系表现出显著的协同作用，与 配合物的固有响应相比，甲醇（ ）的氧化电位正移减少了约0.5 V。在模型底物4 - 三氟甲基苄醇上进行的控制电位电解表明，在3.14 s的速率下，以79±11%的法拉第效率选择性生成双电子、双质子醛产物。在其他相同条件下，共催化速率比 的固有活性高50倍，比单独的PINO高26倍。机理研究表明，一种 -醇盐物种的氧化是电位决定步骤，而根据底物不同确定了两个可能的速率决定步骤。对空间位阻不受抑制的富电子苄醇底物的偏好表明，从 -醇盐加合物到PINO的氢原子转移是速率决定步骤，而使用氘代甲醇未观察到动力学同位素效应表明 -醇盐物种的氧化对于共催化既是速率决定步骤也是电位决定步骤。
