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.