Bioinspired Di-Fe Complexes: Correlating Structure and Proton Transfer over Four Oxidation States

Metalloproteins with active sites containing di-Fe cores exhibit diverse chemical reactivity that is linked to the precise transfer of protons and electrons which directly involve the di-Fe units. The redox conversions are commonly corroborated by spectroscopic methods, but the associated structural changes are often difficult to assess, particularly those related to proton movements. This report describes the development of di-Fe complexes in which the movements of protons and electrons are pinpointed during the stepwise oxidation of a di-Fe^II species to one with an Fe^IIIFe^IV core. Complex formation was promoted using the phosphinic amido tripodal ligand [poat]^3– (N,N′,N″-[nitrilotris­(ethane-2,1-diyl)]­tris­(P,P-diphenylphosphinic amido)) that provided dynamic coordination spheres that assisted in regulating both electron and proton transfer processes. Oxidation of an [Fe^II–(μ-OH)–Fe^III] complex led to the corresponding di-Fe^III species containing a hydroxido bridge that was not stable at room temperature and converted to a species containing an oxido bridging ligand and protonation of one phosphinic amido group to form [Hpoat]^2–. Deprotonation led to a new species with an [Fe^III–(μ-O)–Fe^III] core that could be further oxidized to its Fe^IIIFe^IV analogue. Reactions with phenols suggest homolytic cleavage of the O–H bond to give products that are consistent with the initial formation of a phenoxyl radicalspectroscopic studies indicated that the electron is transferred to the Fe^IV center, and the proton is initially transferred to the more sterically hindered oxido ligand but then relocates to [poat]^3–. These findings offer new mechanistic insights related to the stability of and the reactions performed by di-Fe enzymes.