Cobalt Corroles as Electrocatalysts for Water Oxidation: Strong Effect of Substituents on Catalytic Activity

Abstract

Two Co(III) complexes (1Py2 and 2Py2) of new corrole ligands H3L1 (5,15-bis(p-methylcarboxyphenyl)-10-(o-methylcarboxyphenyl)corrole) and H3L2 (5,15-bis(p-nitrophenyl)-10-(o-methylcarboxyphenyl)corrole) with two apical pyridine ligands have been synthesized and thoroughly characterized by cyclic voltammetry, UV-vis-NIR, and EPR spectroscopy, spectroelectrochemistry, single-crystal X-ray diffraction studies, and DFT methods. Complexes 1Py2 and 2Py2 possess much lower oxidation potentials than cobalt(III)-tris-pentafluorophenylcorrole (Co(tpfc)) and similar corroles containing pentafluorophenyl (C6F5) substituents, thus allowing access to high oxidation states of the former metallocorroles using mild chemical oxidants. The spectroscopic (UV-vis-NIR and EPR) and electronic properties of several oxidation states of these complexes have been determined by a combination of the mentioned methods. Complexes 1Py2 and 2Py2 undergo three oxidations within 1.3 V vs FcH+/FcH in MeCN, and we show that both complexes catalyze water oxidation in an MeCN/H2O mixture upon the third oxidation, with kobs (TOF) values of 1.86 s-1 at 1.29 V (1Py2) and 1.67 s-1 at 1.37 V (2Py2). These values are five times higher than previously reported TOF values for C6F5-substituted cobalt(III) corroles, a finding we ascribe to the additional charge in the corrole macrocycle due to the increased oxidation state. This work opens up new possibilities in the study of metallocorrole water oxidation catalysts, particularly by allowing spectroscopic probing of high-oxidation states and showing strong substituent-effects on catalytic activity of the corrole complexes.