Mn K-Edge X-ray Absorption Studies of Oxo- and Hydroxo-manganese(IV) Complexes: Experimental and Theoretical Insights into Pre-Edge Properties

Mn K-edge X-ray absorption spectroscopy (XAS) was used to gain insights into the geometric and electronic structures of [Mn-II(Cl)(2-)(Me2EBC)], [Mn-IV(OH)(2)(Me2EBC)](2+), and [Mn-IV(O)(OH)(Me2EBC)](+), which are all supported by the tetradentate, macrocyclic Me2EBC ligand (Me2EBC = 4,11-dimethy1-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane). Analysis of extended X-ray absorption fine structure (EXAFS) data for [Mn-IV(O)(OH)(Me2EBC)](+) revealed Mn-O scatterers at 1.71 and 1.84 angstrom and Mn-N scatterers at 2.11 angstrom, providing the first unambiguous support for the formulation of this species as an oxohydroxomanganese(IV) adduct. EXAFS-determined structural parameters for [Mn-II(Cl)(2)(Me2EBC)] and [Mn-IV(OH)(2)(Me2EBC)](2+) are consistent with previously reported crystal structures. The Mn pre-edge energies and intensities of these complexes were examined within the context of data for other oxo- and hydroxomanganese(IV) adducts, and time-dependent density functional theory (TD-DFT) computations were used to predict pre-edge properties for all compounds considered. This combined experimental and computational analysis revealed a correlation between the Mn-O(H) distances and pre-edge peak areas of Mn-IV=0 and Mn-IV-OH complexes, but this trend was strongly modulated by the Mn-IV coordination geometry. Mn 3d-4p mixing, which primarily accounts for the pre-edge intensities, is not solely a function of the Mn-O(H) bond length; the coordination geometry also has a large effect on the distribution of pre-edge intensity. For tetragonal Mn-IV=O centers, more than 90% of the pre-edge intensity comes from excitations to the Mn=0 sigma* MO. Trigonal bipyramidal oxomanganese(IV) centers likewise feature excitations to the Mn==O sigma* molecular orbital (MO) but also show intense transitions to 3d(x2-y2) and 3d(xy) because of enhanced 3d-4p(x,y) mixing. This gives rise to a broader pre-edge feature for trigonal Mn-IV=O adducts. These results underscore the importance of reporting experimental pre-edge areas rather than peak heights. Finally, the TD-DFT method was applied to understand the pre-edge properties of a recently reported S = 1 Mn-V=O adduct; these findings are discussed within the context of previous examinations of oxomanganese(V) complexes.