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(Me ₂EBC)]²⁺, and [Mn^IV(O)(OH)(Me ₂EBC)]⁺, which are all supported by the tetradentate, macrocyclic Me₂EBC ligand (Me₂EBC = 4,11-dimethyl-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 Å and Mn-N scatterers at 2.11 Å, 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(Me ₂EBC)]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=O 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 MnIV=O centers, more than 90% of the pre-edge intensity comes from excitations to the Mn=O σ* MO. Trigonal bipyramidal oxomanganese(IV) centers likewise feature excitations to the Mn=O σ* molecular orbital (MO) but also show intense transitions to 3dx²-y² and 3d_xy MOs because of enhanced 3d-4p_x,y mixing. This gives rise to a broader pre-edge feature for trigonal MnIV=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 MnV=O adduct; these findings are discussed within the context of previous examinations of oxomanganese(V) complexes.