Publication 693

Inorg. Chem. 50 (20), 10190-10203, 2011 DOI: 10.1021/ic201168j
Geometric and Electronic Structures of Peroxomanganese(III) Complexes Supported by Pentadentate Amino-Pyridine and -Imidazole Ligands

Department of Chemistry and Center for Environmentally Beneficial Catalysis, University of Kansas, Lawrence, Kansas 66045, United States
Laboratoire Stress Oxydant et Detoxication, CNRS URA 2096 and CEA/iBiTec-S/SB2SM, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France
Laboratoire d’Electrochimie Moléculaire, UMR 7591 CNRS, Université Paris Diderot, Sorbonne Paris Cité, 15 rue Jean-Antoine de Baïf, F-75205 Paris Cedex 13, France

Three peroxomanganese(III) complexes [Mn^III(O₂)(mL₅²)]⁺, [Mn^III(O₂)(imL₅²)]⁺, and [Mn^III(O₂)(N4py)]⁺ supported by pentadentate ligands (mL₅² = N-methyl-N,N',N'-tris(2-pyridylmethyl)ethane-1,2-diamine, imL₅² = N-methyl-N,N',N'-tris((1-methyl-4-imidazolyl)methyl)ethane-1,2-diamine, and N4py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) were generated by treating Mn(II) precursors with H₂O₂ or KO₂. Electronic absorption, magnetic circular dichroism (MCD), and variable-temperature, variable-field MCD data demonstrate that these complexes have very similar electronic transition energies and ground-state zero-field splitting parameters, indicative of nearly identical coordination geometries. Because of uncertainty in peroxo (side-on ?² versus end-on ?¹) and ligand (pentadentate versus tetradentate) binding modes, density functional theory (DFT) computations were used to distinguish between three possible structures: pentadentate ligand binding with (i) a side-on peroxo and (ii) an end-on peroxo, and (iii) tetradentate ligand binding with a side-on peroxo. Regardless of the supporting ligand, isomers with a side-on peroxo and the supporting ligand bound in a tetradentate fashion were identified as most stable by >20 kcal/mol. Spectroscopic parameters computed by time-dependent (TD) DFT and multireference SORCI methods provided validation of these isomers on the basis of experimental data. Hexacoordination is thus strongly preferred for peroxomanganese(III) adducts, and dissociation of a pyridine (mL₅² and N4py) or imidazole (imL₅²) arm is thermodynamically favored. In contrast, DFT computations for models of [Fe^III(O₂)(mL₅²)]⁺ demonstrate that pyridine dissociation is not favorable; instead a seven-coordinate ferric center is preferred. These different results are attributed to the electronic configurations of the metal centers (high spin d⁵ and d⁴ for Fe^III and Mn^III, respectively), which results in population of a metal-peroxo s-antibonding molecular orbital and, consequently, longer M–O_peroxo bonds for peroxoiron(III) species.