Montgomery Charlotte L, Ertem Mehmed Z, Chevalier Leo, Dempsey Jillian L
Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, United States.
Chemistry Division, Brookhaven National Laboratory, Upton, New York 11973, United States.
J Am Chem Soc. 2024 Nov 6;146(44):30020-30032. doi: 10.1021/jacs.4c01716. Epub 2024 Oct 23.
We report the two-electron, one-proton mechanism of cobalt hydride formation for the conversion of [CoCp(PN)(CHCN)] to [HCoCp(PN)]. This complex catalytically converts CO to formate under CO reduction conditions, with hydride formation as a key elementary step. Through a combination of electrochemical measurements, digital simulations, theoretical calculations, and additional mechanistic and thermochemical studies, we outline the explicit role of the PN ligand in the proton-coupled electron transfer (PCET) reactivity that leads to hydride formation. We reveal three unique PCET mechanisms, and we show that the amine on the PN ligand serves as a kinetically accessible protonation site en route to the thermodynamically favored cobalt hydride. Cyclic voltammograms recorded with proton sources that span a wide range of p values show four distinct regimes where the mechanism changes as a function of acid strength, acid concentration, and timescale between electrochemical steps. Peak shift analysis was used to determine proton transfer rate constants where applicable. This work highlights the astute choices that must be made when designing catalytic systems, including the basicity and kinetic accessibility of protonation sites, acid strength, acid concentration, and timescale between electron transfer steps, to maximize catalyst stability and efficiency.
我们报道了将[CoCp(PN)(CHCN)]转化为[HCoCp(PN)]过程中氢化钴形成的双电子、单质子机制。在CO还原条件下，该配合物催化CO转化为甲酸盐，其中氢化物的形成是关键的基元步骤。通过电化学测量、数字模拟、理论计算以及其他机理和热化学研究相结合的方法，我们概述了PN配体在导致氢化物形成的质子耦合电子转移（PCET）反应性中的明确作用。我们揭示了三种独特的PCET机制，并表明PN配体上的胺在通往热力学有利的氢化钴的过程中作为动力学上可及的质子化位点。用跨越广泛p值范围的质子源记录的循环伏安图显示了四个不同的区域，其中机理随酸强度、酸浓度以及电化学步骤之间的时间尺度而变化。在适用的情况下，使用峰移分析来确定质子转移速率常数。这项工作突出了在设计催化系统时必须做出的精明选择，包括质子化位点的碱性和动力学可及性、酸强度、酸浓度以及电子转移步骤之间的时间尺度，以最大限度地提高催化剂的稳定性和效率。
