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

Abstract

Two Co(III) complexes (<b>1Py</b>_<b>2</b> and <b>2Py</b>_<b>2</b>) of new corrole ligands <b>H</b>_<b>3</b><b>L1</b> (5,15-bis(<i>p</i>-methylcarboxyphenyl)-10-(<i>o</i>-methylcarboxyphenyl)corrole) and <b>H</b>_<b>3</b><b>L2</b> (5,15-bis(<i>p</i>-nitrophenyl)-10-(<i>o</i>-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 <b>1Py</b>_<b>2</b> and <b>2Py</b>_<b>2</b> possess much lower oxidation potentials than cobalt(III)-<i>tris</i>-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 <b>1Py</b>_<b>2</b> and <b>2Py</b>_<b>2</b> 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 <i>k</i>_obs (TOF) values of 1.86 s^-1 at 1.29 V (<b>1Py</b>_<b>2</b>) and 1.67 s^-1 at 1.37 V (<b>2Py</b>_<b>2</b>). 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.