Mn K-EdgeX-ray Absorption Studies ofOxo- and Hydroxo-manganese(IV) Complexes: Experimental and TheoreticalInsights into Pre-Edge Properties

摘要

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-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(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=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 coordinationgeometry also has a large effect on the distribution of pre-edge intensity.For tetragonal Mn^IV=O centers, more than 90% ofthe pre-edge intensity comes from excitations to the Mn=O σ*MO. Trigonal bipyramidal oxomanganese(IV) centers likewise featureexcitations to the Mn=O σ* molecular orbital (MO) butalso show intense transitions to 3dx²–y² and 3dxy MOs because of enhanced 3d-4px,y mixing. This gives rise to a broader pre-edge feature for trigonalMn^IV=O adducts. These results underscore the importanceof reporting experimental pre-edge areas rather than peak heights.Finally, the TD-DFT method was applied to understand the pre-edgeproperties of a recently reported S = 1 Mn^V=O adduct; these findings are discussed within the contextof previous examinations of oxomanganese(V) complexes.