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X-band EPR studies were performed to define the ground-state zero-field splitting (ZFS) and 55 Mn hyperfine parameters of MnIV-oxo and MnIV-hydroxo adducts. The robustness of DFT and CASSCF/NEVPT2 methods for computing ZFS parameters for mononuclear MnIV centers was assessed using a test set of seven complexes. For these complexes the DFT computations, which are often of limited utility for transition-metal ZFSs, reproduced the experimental parameters better than the CASSCF/NEVPT2 method.
X-band electron paramagnetic resonance (EPR) spectroscopy was used to probe the ground-state electronic structures of mononuclear MnIV complexes [MnIV(OH)2(Me2EBC)]2+ and [MnIV(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 MnIV-hydroxo versus MnIV-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 MnIV-oxo complex [MnIV(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 MnIV-hydroxo analogue [MnIV(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 [MnIV(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 MnIV 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 MnIV 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 MnIV ground-state properties and provides an initial assessment for calculating MnIV ZFS parameters with quantum chemical methods.
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