Investigation of Ligand Substituent Effects in Manganese Pyridinophane Complexes: Implications for Oxygen-Evolving Catalysis

Abstract

A series of Mn(II) complexes of differently substituted pyridinophane ligands, (Py$_2$NR$_2$)MnCl$_2$ (R = $^i$Pr, Cy) and [(Py$_2$NR$_2$)MnF$_2$](PF$_6$) (R =$^ i$Pr, Cy, $^t$Bu) are synthesized and characterized. The electrochemical properties of these complexes are investigated by cyclic voltammetry, along with those of previously reported (Py$_2$NMe$_2$)MnCl$_2$ and the Mn(III) complex [(Py$_2$NMe$_2$)MnF$_2$](PF$_6$). The electronic structure of this and other Mn(III) complexes is probed experimentally and theoretically, via high-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy ab initio quantum chemical theory (QCT), respectively. These studies show that the complexes contain relatively typical six-coordinate Mn(III). The catalytic activity of these complexes toward both H$_2$O$_2$ disproportionation and H$_2$O oxidation has also been investigated. The rate of H$_2$O$_2$ disproportionation decreases with increasing substituent size. Some of these complexes are active for electrocatalytic H$_2$O oxidation; however this activity cannot be rationalized in terms of simple electronic or steric effects.