Karak Pirudhan, Mandal Sanajit Kumar, Choudhury Joyanta
Organometallics & Smart Materials Laboratory, Department of Chemistry, Indian Institute of Science Education and Research Bhopal, Bhopal 462 066, India.
J Am Chem Soc. 2023 Apr 5;145(13):7230-7241. doi: 10.1021/jacs.2c12883. Epub 2023 Mar 21.
Biomimetic NAD(P)H-type organic hydride donors have recently been advocated as potential candidates to act as metal-free catalysts for fuel-forming reactions such as the reduction of CO to formic acid and methanol, similar to the natural photosynthesis process of fixing CO into carbohydrates. Although these artificial synthetic organic hydrides are extensively used in organic reduction chemistry in a stoichiometric manner, translating them into catalysts has been challenging due to problems associated with the regeneration of these hydride species under applied reaction conditions. A recent discovery of the possibility of their regeneration under electrochemical conditions a proton-coupled electron-transfer pathway triggered intense research to accomplish their catalytic use in electrochemical CO reduction reactions (eCORR). However, success is yet to be realized to term them as "true" catalysts, as the typical turnover numbers (TONs) of the eCORR processes on inert electrodes for the production of formic acid and/or methanol reported so far are still in the order of 10-10; thus, sub-stoichiometric only! Herein, we report a novel class of structurally engineered heterohelicene-based organic hydride donor with a proof-of-principle demonstration of catalytic electrochemical CO reduction reaction showing a significantly improved activity with more than stoichiometric turnover featuring a 100-1000-fold enhancement of the existing TON values. Mechanistic investigations suggested the critical role of the two cationic imidazolium motifs along with the extensive π-conjugation present in the backbone of the heterohelicene molecules in accessing and stabilizing various radical species involved in the generation and transfer of hydride, multielectron-transfer steps in the electrochemical process.
仿生NAD(P)H型有机氢化物供体最近被认为是潜在的候选物，可作为无金属催化剂用于燃料形成反应，如将CO还原为甲酸和甲醇，类似于将CO固定为碳水化合物的自然光合作用过程。尽管这些人工合成的有机氢化物在有机还原化学中以化学计量的方式被广泛使用，但由于在应用反应条件下这些氢化物物种的再生问题，将它们转化为催化剂一直具有挑战性。最近发现它们在电化学条件下再生的可能性——一种质子耦合电子转移途径引发了大量研究，以实现它们在电化学CO还原反应（eCORR）中的催化应用。然而，要将它们称为“真正的”催化剂尚未成功，因为到目前为止报道的在惰性电极上用于生产甲酸和/或甲醇的eCORR过程的典型周转数（TONs）仍在10-10的量级；因此，仅为亚化学计量！在此，我们报道了一类新型的基于结构工程化杂环烯的有机氢化物供体，并对催化电化学CO还原反应进行了原理验证，显示出显著提高的活性，具有超过化学计量的周转，现有TON值提高了100-1000倍。机理研究表明，两个阳离子咪唑鎓基序以及杂环烯分子主链中存在的广泛π共轭在获取和稳定参与氢化物生成和转移的各种自由基物种、电化学过程中的多电子转移步骤方面起着关键作用。
