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

A series of Mn(II) complexes of differently substituted pyridinophane ligands, (Py₂NR₂)MnCl₂ (R = ⁱPr, Cy) and [(Py₂NR₂)MnF₂](PF₆) (R = ⁱPr, Cy, ^tBu) are synthesized and characterized. The electrochemical properties of these complexes are investigated by cyclic voltammetry, along with those of previously reported (Py₂NMe₂)MnCl₂ and the Mn(III) complex [(Py₂NMe₂)MnF₂](PF₆). 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 H₂O₂ disproportionation and H₂O oxidation has also been investigated. The rate of H₂O₂ disproportionation decreases with increasing substituent size. Some of these complexes are active for electrocatalytic H₂O oxidation; however this activity cannot be rationalized in terms of simple electronic or steric effects.