Valence‐Tautomerism‐Driven Aromatic Nucleophilic Substitution in Cobalt‐Bound Tetrabromocatecholate Ligands – Influence of Positive Charge at the Ligand Backbone on Phenoxazinone Synthase Activity

Abstract

The mononuclear CoIII complex [Co(Br3Cat‐py)2(py)2]Cl·8H2O (1, py = pyridine, Br3pyCatH2 = 3,5,6‐tribromo‐4‐pyridiniumcatechol) was synthesized through the reaction of CoCl2·6H2O with tetrabromocatechol in the presence of an excess of pyridine at elevated temperature. The solid‐state structure was determined by X‐ray crystallography, which disclosed the valence‐tautomerism‐induced aromatic nucleophilic substitution of the tetrabromocatecholate ligands by pyridine. A cyclic voltammetry study revealed the redox flexibility of both the transition‐metal centre and the coordinated redox‐active ligand in this complex. Moreover, 1 exhibits moderately strong catalytic activity and mimics the function of phenoxazinone synthase. A detailed kinetic study disclosed that the extra positive charges at the ligand backbone resulting from the substitution of the Br atoms by pyridyl groups could play an anchoring role for the stabilization of the complex–substrate intermediate. This process resembles those in biological systems, in which the extraordinary catalytic activities of metalloenzymes in various biochemical processes are associated with substrate recognition and the stabilization of the reactive intermediates through diverse supramolecular forces. DFT calculations at the M06‐2X/6‐31+G* level of theory were used to support the mechanistic aspects related to both the redox‐driven aromatic nucleophilic substitution and the biomimetic catalysis.