Oxo- and Hydroxomanganese(IV) Adducts: A Comparative Spectroscopic and Computational Study

The electronic structures of the bis(hydroxo)manganese(IV) and oxohydroxomanganese(IV) complexes [MnIV(OH)2(Me2EBC)]2+ and [MnIV(O)(OH)(Me2EBC)]+ were probed using electronic absorption, magnetic circular dichroism (MCD), and variable-temperature, variable-field MCD spectroscopies. The d−d transitions of [MnIV(OH)2(Me2EBC)]2+ were assigned using a group theory analysis coupled with the results of time-dependent density functional theory computations. These assignments permit the development of an experimentally validated description for the π and σ interactions in this complex. A similar analysis performed for [MnIV(O)(OH)(Me2EBC)]+ reveals that there is a significant increase in the ligand character in the Mn π* orbitals for the MnIVO complex relative to the bis(hydroxo)manganese(IV) complex, whereas the compositions of the Mn σ* orbitals are less affected. Because of the steric features of the Me2EBC ligand, we propose that H-atom transfer by these reagents proceeds via the σ* orbitals, which, because of their similar compositions among these two compounds, leads to modest rate enhancements for the MnIVO versus MnIVOH species.