A series of peroxomanganese(III) complexes supported by tetradentate aminopyridyl ligands: detailed spectroscopic and computational studies.

A set of four Mn(II)(L(7)py(2)(R)) complexes, supported by the tetradentate 1,4-bis(2-pyridylmethyl)-1,4-diazepane ligand and derivatives with pyridine substituents in the 5 (R = Br) and 6 positions (R = Me and MeO), are reported. X-ray crystal structures of these complexes all show the L(7)py(2)(R) ligands bound to give a trans complex. Treatment of these Mn(II) precursors with either H(2)O(2)/Et(3)N or KO(2) in MeCN at -40 degrees C results in the formation of peroxomanganese complexes Mn(III)(O(2))(L(7)py(2)(R)) differing only in the identity of the pyridine ring substituent. The electronic structures of two of these complexes, Mn(III)(O(2))(L(7)py(2)(H)) and Mn(III)(O(2))(L(7)py(2)(Me)), were examined in detail using electronic absorption, low-temperature magnetic circular dichroism (MCD) and variable-temperature variable-field (VTVH) MCD spectroscopies to determine ground-state zero-field splitting (ZFS) parameters and electronic transition energies, intensities, and polarizations. DFT and TD-DFT computations were used to validate the structures of Mn(III)(O(2))(L(7)py(2)(H)) and Mn(III)(O(2))(L(7)py(2)(Me)), further corroborating their assignment as peroxomanganese(III) species. While these complexes exhibit similar ZFS parameters, their low-temperature MCD spectra reveal significant shifts in electronic transition energies that are correlated to differences in Mn-O(2) interactions among these complexes. Taken together, these results indicate that, while the Mn(III)(O(2))(L(7)py(2)(H)) complex exhibits symmetric Mn-O(peroxo) bond lengths, consistent with a side-on bound peroxo ligand, the peroxo ligand of the Mn(III)(O(2))(L(7)py(2)(Me)) complex is bound in a more end-on fashion, with asymmetric Mn-O(peroxo) distances. This difference in binding mode is rationalized in terms of the greater electron-donating abilities of the methyl-appended pyridines and suggests a simple way to modulate Mn(III)-O(2) bonding through ligand perturbations.