Studies of Cobalt-Mediated Electrocatalytic CO₂ Reduction Using a Redox-Active Ligand

The cobalt complex [CoIIIN4H(Br)2]+ (N4H = 2,12-dimethyl-3,7,11,17-tetraazabicyclo-[11.3.1]-heptadeca-1(7),2,11,13,15-pentaene) was used for electrocatalytic CO2 reduction in wet MeCN with a glassy carbon working electrode. When water was employed as the proton source (10 M in MeCN), CO was produced (fCO= 45% ± 6.4) near the CoI/0 redox couple for [CoIIIN4H(Br)2]+ (E1/2 = −1.88 V FeCp2+/0) with simultaneous H2 evolution (fH2= 30% ± 7.8). Moreover, we successfully demonstrated that the catalytically active species is homogeneous through the use of control experiments and XPS studies of the working glassy-carbon electrodes. As determined by cyclic voltammetry, CO2 catalysis occurred near the formal CoI/0redox couple, and attempts were made to isolate the triply reduced compound (“[Co0N4H]”). Instead, the doubly reduced (“CoI”) compounds [CoN4] and [CoN4H(MeCN)]+ were isolated and characterized by X-ray crystallography. Their molecular structures prompted DFT studies to illuminate details regarding their electronic structure. The results indicate that reducing equivalents are stored on the ligand, implicating redox noninnocence in the ligands for H2 evolution and CO2 reduction electrocatalysis.本研究采用钴配合物[CoIIIN4H(Br)2]+（其中N4H = 2,12-二甲基-3,7,11,17-四氮杂双环-[11.3.1]-十七碳-1(7),2,11,13,15-五烯），在含水乙腈（MeCN）体系中开展电催化CO2还原实验，以玻碳工作电极（glassy carbon working electrode）作为工作电极。当以水作为质子源（乙腈中浓度为10 M）时，在[CoIIIN4H(Br)2]+的CoI/0氧化还原对附近生成一氧化碳（CO），其法拉第效率fCO=45%±6.4，该氧化还原对的半波电位E1/2 = -1.88 V vs 二茂铁阳离子/中性对（FeCp2+/0），同时伴随氢气析出（fH2=30%±7.8）。此外，通过对照实验与玻碳工作电极的X射线光电子能谱（X-ray photoelectron spectroscopy, XPS）分析，我们证实催化活性物种为均相物种。经循环伏安法（cyclic voltammetry）测定，CO2催化反应发生于形式CoI/0氧化还原对附近；我们尝试分离三重还原产物“[Co0N4H]”，但最终分离得到双重还原的“CoI”类化合物[CoN4]与[CoN4H(MeCN)]+，并通过X射线晶体衍射（X-ray crystallography）对其完成结构表征。其分子结构推动了密度泛函理论（Density Functional Theory, DFT）研究，以阐明其电子结构细节。研究结果显示，还原当量储存于配体之上，这表明在氢气析出与CO2还原电催化过程中，配体具备氧化还原非惰性特性。