X-Band Electron Paramagnetic Resonance Comparison of Mononuclear Mn^IV-oxo and Mn^IV-hydroxo Complexes and Quantum Chemical Investigation of Mn^IV Zero-Field Splitting

X-band electron paramagnetic resonance (EPR) spectroscopy was used to probe the ground-state electronic structures of mononuclear Mn^IV complexes [Mn^IV(OH)₂(Me₂EBC)]²⁺ and [Mn^IV(O)(OH)(Me₂EBC)]⁺. These compounds are known to effect C-H bond oxidation reactions by a hydrogen-atom transfer mechanism. They provide an ideal system for comparing Mn^IV-hydroxo versus Mn^IV-oxo motifs, as they differ by only a proton. Simulations of 5 K EPR data, along with analysis of variable-temperature EPR signal intensities, allowed for the estimation of ground-state zero-field splitting (ZFS) and ⁵⁵Mn hyperfine parameters for both complexes. From this analysis, it was concluded that the Mn^IV-oxo complex [Mn^IV(O)(OH)(Me₂EBC)]⁺ has an axial ZFS parameter D (D = +1.2(0.4) cm^-1) and rhombicity (E/D = 0.22(1)) perturbed relative to the Mn^IV-hydroxo analogue [Mn^IV(OH)₂(Me₂EBC)]²⁺ (|D| = 0.75(0.25) cm^-1; E/D = 0.15(2)), although the complexes have similar ⁵⁵Mn values (a = 7.7 and 7.5 mT, respectively). The ZFS parameters for [Mn^IV(OH)₂(Me₂EBC)]²⁺ were compared with values obtained previously through variable-temperature, variable-field magnetic circular dichroism (VTVH MCD) experiments. While the VTVH MCD analysis can provide a reasonable estimate of the magnitude of D, the E/D values were poorly defined. Using the ZFS parameters reported for these complexes and five other mononuclear Mn^IV complexes, we employed coupled-perturbed density functional theory (CP-DFT) and complete active space self-consistent field (CASSCF) calculations with second-order n-electron valence-state perturbation theory (NEVPT2) correction, to compare the ability of these two quantum chemical methods for reproducing experimental ZFS parameters for Mn^IV centers. The CP-DFT approach was found to provide reasonably acceptable values for D, whereas the CASSCF/NEVPT2 method fared worse, considerably overestimating the magnitude of D in several cases. Both methods were poor in reproducing experimental E/D values. Overall, this work adds to the limited investigations of Mn^IV ground-state properties and provides an initial assessment for calculating Mn^IV ZFS parameters with quantum chemical methods.