MODELS FOR DIIRON OXO PROTEINS - THE PEROXIDE ADDUCT OF FE2(HPTB)(OH)(NO3)4

The complex of Fe(NO3)3 and N,N,N'N'-tetrakis(2-benzimidazolylmethyl)-2-hydroxy-1,3-diaminopropane (HPTB) is reformulated as [Fe2(HPTB)(mu-OH)(NO3)2](NO3)2 on the basis of H-1 NMR, EXAFS, X-ray diffraction, and conductivity data. This complex reacts with hydrogen peroxide to form a 1:1 adduct with a new charge-transfer band at 600 nm (epsilon = 1500 M-1 cm-1). Resonance Raman studies show two resonance-enhanced vibrations, nu-Fe-O at 476 cm-1 and nu-O-O, which appears as a Fermi doublet centered at 895 cm-1; these features shift to 457 and 854 cm-1, respectively, with the use of (H2O2)-O-18 but are not affected by D2O. H-1 NMR measurements indicate that the antiferromagnetic coupling is increased from -J = 20 cm-1 to ca. 70 cm-1 upon formation of the peroxide adduct, suggesting the introduction of a new coupling pathway. The Fe-57 Mossbauer spectrum of the peroxide complex reveals a quadrupole doublet (delta = 0.54 mm/s, DELTA-E-Q = 0.84 mm/s) distinct from that of the precursor complex (delta = 0.49 mm/s, DELTA-E-Q = 0.66 mm/s), indicating that the two irons are affected similarly by peroxide binding. Conductivity measurements in CH3CN show that the adduct is a 1:1 electrolyte. Taken together, the physical data suggest the formulation [Fe2(HPTB)(mu-eta1:eta1-O2)-(NO3)2](NO3) for the peroxide complex. Such dinuclear iron peroxide complexes may be relevant to putative intermediates in the oxygenation of the reduced forms of ribonucleotide reductase and methane monooxygenase.