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

The electronic structures of the bis(hydroxo)manganese(IV) and oxohydroxomanganese(IV) complexes [Mn-IV(OH)(2)(Me2EBC)](2+) and [Mn-IV(O)(OH)(Me2EBC)](+) were probed using electronic absorption, magnetic circular dichroism (MCD), and variable-temperature, variable-field MCD spectroscopies. The d-d transitions of [Mn-IV(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 pi and sigma interactions in this complex. A similar analysis performed for [Mn-IV(O)(OH)(Me2EBC)](+) reveals that there is a significant increase in the ligand character in the Mn pi* orbitals for the Mn-IV=O complex relative to the bis(hydroxo)manganese(IV) complex, whereas the compositions of the Mn sigma* 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 sigma* orbitals, which, because of their similar compositions among these two compounds, leads to modest rate enhancements for the Mn-IV=O versus (MnOH)-O-IV species.