Mn K-edge X-ray absorption studies of mononuclear Mn(III)-hydroxo complexes

Mn K-edge X-ray absorption spectroscopy experiments were performed on the solid- and solution-phase samples of [Mn^II(dpaq^R)](OTf) (R=H, Me) and [Mn^III(OH)(dpaq^R)](OTf). The extended X-ray absorption fine structure (EXAFS) data show distinct differences between the Mn^II and Mn^III-OH complexes, with fits providing metric parameters in excellent agreement with values from X-ray crystallography and density functional theory (DFT) computations. Evaluation of the EXAFS data for solid-phase [Mn^III(OH)(dpaq)](OTf) resolved a short Mn-OH bond distance of 1.79 Å; however, the short trans-amide nitrogen bond of the supporting ligand precluded the resolution of the Mn-OH bond distance in the corresponding solution-phase sample and for both [Mn^III(OH)(dpaq^Me)](OTf) samples. The edge energy also increases by approximately 2 eV from the Mn^II to the Mn^III-OH complexes. Experimental pre-edge analysis shows the Mn^II complexes to have pre-edge areas comparable to the Mn^III-OH complexes, despite the presence of the relatively short Mn-OH distance. Time-dependent density functional theory (TD-DFT) computations illustrate that Mn 3d-4p mixing, a primary contributor to pre-edge intensities, decreases by ~ 0.3% from the Mn^II to Mn^III-OH complexes, which accounts for the very similar pre-edge areas. Collectively, this work shows that combined EXAFS and XANES analysis has great potential for identification of reactive Mn^III-OH intermediates, such as those proposed in enzyme active sites.