Mn K-edge X-ray absorption studies of mononuclear Mn(III)–hydroxo complexes
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Mn K-edge X-ray absorption spectroscopy experiments were performed on the solid- and solution-phase samples of [MnII(dpaqR)](OTf) (R=H, Me) and [MnIII(OH)(dpaqR)](OTf). The extended X-ray absorption fine structure (EXAFS) data show distinct differences between the MnII and MnIII–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 [MnIII(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 [MnIII(OH)(dpaqMe)](OTf) samples. The edge energy also increases by approximately 2 eV from the MnII to the MnIII–OH complexes. Experimental pre-edge analysis shows the MnII complexes to have pre-edge areas comparable to the MnIII–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 MnII to MnIII–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 MnIII–OH intermediates, such as those proposed in enzyme active sites.
KeywordsX-ray absorption spectroscopy Manganese Coordination chemistry Density functional theory Hydroxo ligands
This work was supported by NSF Grant 1565661. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. The SSRL Structural Molecular Biology Program is supported by the DOE Office of Biological and Environmental Research, and by the National Institutes of Health, National Institute of General Medical Sciences (including P41GM103393). The contents of this publication are solely the responsibility of the authors and do not necessarily represent the official views of NIGMS or NIH. Use of Beamline 2-2 at SSRL was partially supported by the National Synchrotron Light Source II, Brookhaven National Laboratory, under US Department of Energy Contract No. DE-SC0012704. XAS experiments were supported by the Case Western Reserve University Center for Synchrotron Biosciences NIH Grant, P30-EB-009998, from the National Institute of Biomedical Imaging and Bioengineering (NIBIB). We thank Dr. Erik Farquhar at NSLS for outstanding support of our XAS experiments and for helpful conversations.
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