XAS STUDIES OF HIGH-VALENT FE MODEL COMPLEXES RELEVANT TO METHANE MONOOXYGENASE

Stanford University, Stanford CA

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Abstract

This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. High-valent iron intermediates are invoked as the active oxidizing species in both methane monooxygenase (MMO) and ribonucleotide reductase (RNR). In MMO, intermediate Q is thought to consist of an FeIV(mu-O)2FeIV core which converts methane to methanol. Whereas in RNR, an oxo-bridged FeIIIFeIV species, intermediate X is the active species in ribonucleotide reduction. The importance of these reactions and the potential utility of synthetic analogues in biomimetic homogeneous catalysis have driven efforts to synthesize analogues. Recent work in our laboratory has focused on the synthesis and characterization of dinuclear Fe-oxo complexes utilizing strongly pi- and sigma- donating phenolates and t-amines as donors. The resultant [LFe(mu2-O)FeL] complex (1) can be oxidized to the monocation [LFe(mu2-O)FeL]+ (1+) and to the dication [LFe(mu2-O)FeL]2+ (12+). EPR data of 1+ suggests an oxo-bridged-FeIIIFeIV formulation. However comparison of UV-vis data for 1+ and 2+ to mononuclear analogues suggests a ligand-centered oxidation occurs in both complexes, forming a diferric phenoxyl radical in 1+ and a diferric diphenoxyl radical in 12+. The greater stability of the dication relative to the monoanionic species is thought to result from a symmetric strengthening of the Fe-oxo bonds in the former (12+), as opposed to the asymmetric situation which is generated in the later (1+). As these species can only be generated at low temperature in solution, XAS provides an ideal means for characterizing the electronic structure and local geometric structure of these complexes.