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A Series of Peroxomanganese(III) Complexes Supported by Tetradentate Aminopyridyl Ligands: Detailed Spectroscopic and Computational Studies

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posted on 03.03.2010, 00:00 by Robert A. Geiger, Swarup Chattopadhyay, Victor W. Day, Timothy A. Jackson
A set of four [MnII(L7py2R)]2+ 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 L7py2R ligands bound to give a trans complex. Treatment of these MnII precursors with either H2O2/Et3N or KO2 in MeCN at −40 °C results in the formation of peroxomanganese complexes [MnIII(O2)(L7py2R)]+ differing only in the identity of the pyridine ring substituent. The electronic structures of two of these complexes, [MnIII(O2)(L7py2H)]+ and [MnIII(O2)(L7py2Me)]+, 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 [MnIII(O2)(L7py2H)]+ and [MnIII(O2)(L7py2Me)]+, 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−O2 interactions among these complexes. Taken together, these results indicate that, while the [MnIII(O2)(L7py2H)]+ complex exhibits symmetric Mn−Operoxo bond lengths, consistent with a side-on bound peroxo ligand, the peroxo ligand of the [MnIII(O2)(L7py2Me)]+ complex is bound in a more end-on fashion, with asymmetric Mn−Operoxo 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 MnIII−O2 bonding through ligand perturbations.