Moberg Megan E, Reid Amelia G, Dickie Diane A, Machan Charles W
Department of Chemistry, University of Virginia, PO Box 400319, Charlottesville, VA 22904-4319, USA.
Dalton Trans. 2024 Oct 22;53(41):16849-16860. doi: 10.1039/d4dt01981d.
Homogeneous earth abundant transition-metal electrocatalysts capable of carbon dioxide (CO) reduction to generate value-added chemical products are a possible strategy to minimize rising anthropogenic CO emissions. Previously, it was determined that Cr-centered bipyridine-based NO complexes for CO reduction are kinetically limited by a proton-transfer step during C-OH bond cleavage. Therefore, it was hypothesized that the inclusion of pendent relay groups in the secondary coordination sphere of these molecular catalysts could increase their catalytic activity. Here, it is shown that the introduction of a pendent methoxy group favorably impacts a pre-equilibrium protonation prior to the catalytic resting state, resulting in a significant increase in catalytic activity without a loss of product selectivity for generating carbon monoxide (CO) from CO. Interestingly, combining the pendent methoxy group with a cationic acid causes a positive shift of the catalytic reduction potential of the system, while maintaining increased activity and quantitative selectivity. This work suggests that tuning the secondary coordination sphere with respect to cationic proton sources can result in activity improvements by modifying the kinetic and thermodynamic aspects of proton transfer in the catalytic cycle.
能够将二氧化碳（CO）还原以生成增值化学产品的均相富地球过渡金属电催化剂，是将不断上升的人为CO排放降至最低的一种可能策略。此前已确定，用于CO还原的以Cr为中心的联吡啶基NO配合物在C-OH键断裂过程中受到质子转移步骤的动力学限制。因此，有人推测在这些分子催化剂的二级配位球中引入侧链中继基团可以提高其催化活性。在此表明，引入侧链甲氧基对催化静止状态之前的预平衡质子化有有利影响，从而在不损失从CO生成一氧化碳（CO）的产物选择性的情况下显著提高催化活性。有趣的是，将侧链甲氧基与阳离子酸结合会使系统的催化还原电位正向移动，同时保持活性增加和定量选择性。这项工作表明，通过调节二级配位球相对于阳离子质子源的情况，可以通过改变催化循环中质子转移的动力学和热力学方面来提高活性。
