Structural Characterization of a Series of N5-Ligated Mn(IV)-oxo Species

Analysis of extended X-ray absorption fine structure (EXAFS) data for the Mn(IV)-oxo complexes [Mn(IV)(O)((DMM)N4py)](2+), [Mn(IV)(O)(2pyN2B)](2+), and [Mn(IV)(O)(2pyN2Q)](2+) ((DMM)N4py = N,N-bis(4-methoxy-3,5-di methyl-2-pyridylm ethyl)-N-bis(2-pyridyl)methylamine; 2pyN2B = (N-bis(1-methyl-2-benzimidazolyl)methyl-N-(bis-2-pyridylmethyl)amine, and 2pyN2Q = N,N-bis(2-pyridyl)-N,N-bis(2-quinolylmethyl)methanamine) afforded Mn=O and Mn-N bond lengths. The Mn=O distances for [Mn(IV)(O)((DMM)N4py)](2+) and [Mn(IV)(O)(2pyN2B)](2+) are 1.72 and 1.70 Å, respectively. In contrast, the Mn=O distance for [Mn(IV)(O)(2pyN2Q)](2+) was significantly longer (1.76 Å). We attribute this long distance to sample heterogeneity, which is reasonable given the reduced stability of [Mn(IV)(O)(2pyN2Q)](2+). The Mn=O distances for [Mn(IV)(O)((DMM)N4py)](2+) and [Mn(IV)(O)(2pyN2B)](2+) could only be well-reproduced using DFT-derived models that included strong hydrogen-bonds between second-sphere solvent 2,2,2-trifluoroethanol molecules and the oxo ligand. These results suggest an important role for the 2,2,2-trifluoroethanol solvent in stabilizing Mn(IV)-oxo adducts. The DFT methods were extended to investigate the structure of the putative [Mn(IV)(O)(N4py)](2+)·(HOTf)(2) adduct. These computations suggest that a Mn(IV)-hydroxo species is most consistent with the available experimental data.