Manganese(IV)-oxo complexes are often invoked as intermediates in Mn-catalyzed C-H bond activation reactions. While many synthetic MnIV-oxo species are mild oxidants, other members of this class can attack strong C-H bonds. The basis for these reactivity differences is not well understood. Here we describe a series of MnIV-oxo complexes with N5 pentadentate ligands that modulate the equatorial ligand field of the MnIV center, as assessed by electronic absorption, electron paramagnetic resonance, and Mn K-edge X-ray absorption methods. Kinetic experiments show dramatic rate variations in hydrogen-atom and oxygen-atom transfer reactions, with faster rates corresponding to weaker equatorial ligand fields. For these MnIV-oxo complexes, the rate enhancements are correlated with both i) the energy of a low-lying 4E excited state, which has been postulated to be involved in a two-state reactivity model, and ii) the MnIII/IV reduction potentials.

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