Abstract
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 °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)(•-)) (2) (resonance Raman spectroscopy (rR): ν(O-O), 1171 cm(-1) (Δ(18)O(2), -61 cm(-1)); ν(Fe-O), 575 cm(-1) (Δ(18)O(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 (δ 9.12 ppm) in (2)H 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))Fe(II)Cu(I)](+) (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): λ(max), 420 (Soret), 545, 565 nm; δ(pyrr), 93 ppm; ν(O-O), 799 cm(-1) (Δ(18)O(2), -48 cm(-1)); ν(Fe-O), 524 cm(-1) (Δ(18)O(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): λ(max), 425 (Soret), 538 nm; δ(pyrr), 10.2 ppm; ν(O-O), 817 cm(-1) (Δ(18)O(2), -55 cm(-1)); ν(Fe-O), 610 cm(-1) (Δ(18)O(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.