A Cobalt(III)-Hydroxo Complex Bearing a Pentadentate Amidate Ligand as an Electrocatalyst for Water Oxidation

A Co(III)-hydroxo complex, [Co-III(dpaq)OH]ClO4 (1-OH) bearing a pentadentate ligand, H-dpaq, (H-dpaq=(2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate]) catalyses water oxidation in mildly alkaline medium (pH 8.0) at a potential of 1.4 V-NHE with an average Turn-Over-Frequency (TOFmax) of 2.8x10(4) s(-1) and faradaic efficiency of 88 %. Post-electrolysis characterization of the electrode rules out the formation of any heterogeneous electroactive species. Electrochemical results and theoretical calculations confirm the occurrence of both metal and ligand centered PCET processes during anodic scanning. The resulting formally Co(V)-oxo/oxyl intermediate undergoes water nucleophilic attack to install the O-O bond. The role of axial ligand in water oxidation by Co(III)-dpaq system has been examined by comparing the reactivity of the Co-hydroxide complex (1-OH) with that of its chloride-ligated counterpart, [Co-III(dpaq)Cl]Cl (1-Cl). The results confirm the ability of the Co-dpaq complexes to bind water/or water derived ligands over chloride or non-aqueous solvents. The interplay of ligand redox non-innocence and sigma-donating ability of the N5-carboxamido ligand helps to store oxidizing equivalents and triggers O-O bond formation.