Stabilization of synthetic heme-superoxo complexes by hydrogen bonding: a still on-going quest

Abstract

Selected synthetic heme complexes designed during the past half century for O₂ binding studies are analyzed. Efficient O₂ carriers must avoid the autooxidation process upon O₂ binding and must lead to stable O₂ adducts. Several strategies have been successful in achieving this paramount property through the peripheral functionalization of the heme by pickets, straps, caps and tripods. The affinity of the complex for O₂ is generally investigated in solution by its binding constant, either measured or calculated, and the [Fe–O₂] species characterized by its spectroscopic (vibrational) data. When achieved, the stability of the O₂ adduct has been very often attributed to a possible hydrogen bond with a H-bond donor supplied by the chemical dressing around the binding site of the model. However, solid-state evidence remains to come, so far.