Heme-Cu Binucleating Ligand Supports Heme/O-2 and Fe-II-Cu-I/O-2 Reactivity Providing High- and Low-Spin Fe-III-Peroxo-Cu-II Complexes

摘要

The focus of this study is in the description of synthetic heme/copper/O-2 chemistry employing a heme-containing binucleating ligand which provides a tridentate chelate for copper ion binding. The addition of O-2 (-80 degrees C, tetrahydrofuran (THF) solvent) to the reduced heme compound (p(ImH))Fe-II (1), gives the oxy-heme adduct, formally a heme-superoxide complex Fe-III-(O-2(center dot-)) (2) (resonance Raman spectroscopy (rR): nu(o-o), 1171 cm(-1) (Delta O-18(2), -61 cm(-1)); nu(Fe-o), 575 cm(-1) (Delta O-18(2), -24 cm(-1))). Simple warming of 2 to room temperature regenerates reduced complex 1; this reaction is reversible, as followed by UV-vis spectroscopy. Complex 2 is electron paramagnetic resonance (EPR)-silent and exhibits upfield-shifted pyrrole resonances (delta 9.12 ppm) in H-2 NMR spectroscopy, indicative of a six-coordinate low-spin heme. The coordination of the tethered imidazolyl arm to the heme-superoxide complex as an axial base ligand is suggested. We also report the new fully reduced heme-copper complex [(p(ImH))(FeCuI)-Cu-II](+) (3), where the copper ion is bound to the tethered tridentate portion of P-ImH. This reacts with O-2 to give a distinctive low-temperature-stable, high-spin (S = 2, overall) peroxo-bridged complex [(P-ImH)Fe-III-(O-2(2-))-Cu-II](+) (3a):lambda(max), 420 (Soret), 545, 565 nm; delta(pyr), 93 ppm; nu(o-o), 799 cm(-1) (Delta 18O(2), -48 cm(-1)); nu(Fe-o), 524 cm(-1) (Delta O-18(2), -23 cm(-1)). To 3a, the addition of dicyclohexylimidazole (DCHIm), which serves as a heme axial base, leads to low-spin (S = 0 overall) species complex [(DCHIm)(p(ImH))Fe-III-(O-2(2-)-Cu-II](+)(3b): lambda(max), 425 (Soret), 538 nm; delta(pyrr) 10.2 ppm; nu(o-o), 817 cm(-1) (Delta O-18(2), - 55 cm(-1)); nu(Fe-o), 610 cm (-1) (Delta O-18(2), -26 cm(-1)). These investigations into the characterization of the O-2 -adducts from (p(ImH))Fe-II (1) with/without additional copper chelation advance our understanding of the dioxygen reactivity of heme-only and heme/Cu-ligand heterobinuclear system, thus potentially relevant to O-2 reduction in heme-copper oxidases or fuel-cell chemistry.