Characterization of monomeric Mn~(II/III/IV)-hydroxo complexes from X- and Q-band dual mode electron paramagnetic resonance (EPR) spectroscopy

摘要

Manganese-hydroxo species have been implicated in C-H bond activation performed by metalloenzymes, but the electronic properties of many of these intermediates are not well characterized. The present work presents a detailed characterization of three Mn~n-OH complexes (where n = II, III, and IV) of the tris[(N′-tert-butylureaylato)-N-ethylene]aminato ([H _3buea]~(3-)) ligand using X- and Q-band dual mode electron paramagnetic resonance (EPR). Quantitative simulations for the [Mn ~(II)H_3buea(OH)]~(2-) complex demonstrated the ability to characterize similar MnII species commonly present in the resting states of manganese-containing enzymes. The spin states of the Mn ~(III) and Mn~(IV) complexes determined from EPR spectroscopy are S = 2 and 3/2, respectively, as expected for the C_3 symmetry imposed by the [H_3buea]~(3-) ligand. Simulations of the spectra indicated the constant presence of two Mn~(IV) species in solutions of [Mn~(IV)H_3buea(OH)] complex. The simulations of perpendicular- and parallel-mode EPR spectra allow determination of zero-field splitting and hyperfine parameters for all complexes. For the Mn~(III) and Mn~(IV) complexes, density functional theory calculations are used to determine the isotropic Mn hyperfine values, to compare the excited electronic state energies, and to give theoretical estimates of the zero-field energy.