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

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.