Steric and Electronic Influences on the Structures of Peroxomanganese(III) Complexes Supported by Tetradentate Ligands

We report the generation and characterization of two peroxomanganese(III) complexes supported by the L7py24-Cl and L7iso-q2 ligands {L7py24-Cl = 1,4-bis[(4-chloro-2-pyridyl)methyl]-1,4-diazepane and L7iso-q2 = 1,4-bis[(2-isoquinolinyl)methyl]-1,4-diazepane} and describe X-ray structures of corresponding manganese(II) compounds. Ground and excited state properties of these peroxomanganese(III) complexes, as well as previously reported [MnIII(O2)(L7py24-Me)]+ and [MnIII(O2)(L7q2)]+ species {L7py24-Me = 1,4-bis[(4-methyl-2-pyridyl)methyl]-1,4-diazepane and L7q2 = 1,4-bis[(2-quinolinyl)methyl]-1,4-diazepane} were probed using low-temperature electronic absorption, magnetic circular dichroism (MCD), and variable-temperature, variable-field MCD spectroscopy. These data sets afford electronic transition energies and estimates of ground-state zero-field splitting parameters, which permit a detailed comparison of electronic structure. These data support the proposal that all complexes share a similar geometry, consisting of a side-on peroxomanganese(III) moiety coordinated by the tetradentate ligand in a trans fashion. However, differences in d-d transition energies offer conclusive evidence that, among this series of complexes, it is the location, and thus the steric influence, of the pyridine substituent that modulates the electronic and geometric structure of the MnIII-O2 unit. Within this series, perturbations in electronic properties of the supporting ligand have little impact on d-d transition energies. Models of peroxomanganese(III) complexes developed using density functional theory (DFT) computations support this proposal, and time-dependent DFT computations qualitatively reproduce the experimental trend in d-d transition energies.