Oxidative Verdoheme Formation and Stabilization by Axial Isocyanide Ligation

Abstract

The effect of isocyanides as axial ligands on the formation and stability of verdoheme by oxidation has been examined. The reaction of {Fe^III(OEPO)}₂ with t-butyl isocyanide under dioxygen-free conditions results in the formation of (t-BuNC)₂Fe^II(OEPO^•) with an electron paramagnetic resonance at g = 2.009 with a peak-to-peak separation of 23.5 G at 4 K. (OEPO is the trianion of octaethyloxophlorin and OEPO^• is the radical dianion obtained from OEPO by one-electron oxidation.) Exposure of chloroform solutions of either (2,6-xylylNC)₂Fe^II(OEPO^•) or (t-BuNC)₂Fe^II(OEPO^•) to dioxygen followed by the addition of ammonium hexafluorophosphate results in their transformation into the diamagnetic verdohemes, [(2,6-xylylNC)₂Fe^II(OEOP)](PF₆) and [(t-BuNC)₂Fe^II(OEOP)](PF₆), yields 68 and 70%, respectively. (OEOP is the anion of octaethyl-5-oxaporphyrin.) The oxidation reactions of (2,6-xylylNC)₂Fe^II(OEPO^•) and (t-BuNC)₂Fe^II(OEPO^•) have also been monitored by ¹H NMR spectroscopy. No resonances due to paramagnetic products could be detected, the reactions appear to result only in the formation of the diamagnetic verdohemes, and the products are not susceptible to further oxidation.