RESONANCE SCATTERING STUDIES OF MIXED VALENCE MANGANESE COMPLEXES V. Kezerashvili, A. Darovsky, X3 beamline, NSLS, Brookhaven National Laboratory, T. Weyhermüller, Max-Planck-Institut für Strahlenchemie, Mülheim, Germany, P. Coppens, Chemistry Department, SUNY at Buffalo
Manganese mixed-valence complexes model the catalase enzymes and photosystem II, which catalyzes the water oxidation reaction. We have studied three mixed valence complexes in an application of valence-specific diffraction, which allows site-specific assignment of oxidation state and mapping of f'(E) for individual atoms. Peaks on imaging plates were integrated with the program HIPPO1. The complexes are (a): Mn(IV)(ttacn)(u-O)2(u-MeCO2)Mn(III)(MeCO2), ttacn=1.4.7-trimethyl-1,4,7-triazacyclononane, (b): [(ttacn)Mn(II)(u-O)(u-pivalonato)2Mn(III) (ttacn)](ClO4)2, (c): [(dtne)Mn(IV)(u-O)2(u-MeCO2) Mn(III)](Bph4)2, dtne= di-1,4,7-triazacyclononane 1,2-ethylene, ph=phenyl. In (a) a definite shift in edge energy is observed, somewhat smaller, but similar to the shift for u-dioxo-(Mn(2,2'-bipyridyl)2)2 (BF4)3.3H2O (d)2. The pre-edge feature in the fluorescence spectrum is reflected in the Mn(IV), but not in the Mn(III) curve, indicating that the transition to a free bound state occurs at the former atom. For (b) and (c) the edge shifts appear much smaller, implying a smaller difference in electron binding energies, and therefore a smaller difference in net charge between the two atoms. Based on the current results the sequence of the magnitudes of the edge shifts in the III-IV complexes is (d)>(a)>(c), while the Mn-O bond asymmetries are in the order (a, 0.098Å)>(d, 0.069Å)>(c, 0.048Å).
The SUNY X3 beamline at NSLS is supported by the Division of Basic Energy Sciences of the U.S. Department of Energy (DE-FG02-86ER45231). Research carried out in part at the NSLS, supported by the U.S. DOE, Divisions of Materials Sciences and Chemical Sciences.
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