One-site addition two-metal oxidation reactions of unsymmetrical bimetallic complexes related to dioxygen binding by hemerythrin

Hemerythrin is a respiratory protein found in certain marine invertebrates. It employs a unique mechanism for dioxygen binding. This mechanism, termed “one-site addition two-metal oxidation” involves addition of dioxygen to a di-Fe(II) active site of the protein in such a manner that it adds to one of the iron atoms and is reduced by two electrons, one derived from each of the iron atoms. The resulting oxyhemerythrin contains a di-Fe(III) site with a hydroperoxide bound end-on to one of the metals. The goal of the research described in this thesis is to achieve one-site addition two-metal oxidation reaction in a synthetic bimetallic system without restricting the research to biomimetic ligands, metals or oxidants.; Binucleating ligands that would provide 4-coordinate and 6-coordinate metal sites were designed and synthesized. The first generation of the ligands featured pyridine-rich 6-coordinate site and a salen-like 4-coordinate site connected by flexible aliphatic chains of varied lengths as well as an alkoxide positioned to bridge the two metals. However these ligands proved to be too flexible and afforded only tetrameric and oligomeric complexes. Then a (1,3,4)-oxadiazole was introduced as a bridging unit, and was shown to be suitable for preparation of binucleating ligands. An unsymmetrical ligand, featuring this bridge was designed to afford a 5-coordinate and 6-coordinate metal sites and provided di-Co(II) and di-Fe(II) complexes. These complexes possess main structural characteristics of hemerythrin active site: an open coordination position at one metal and an exogenous hydroxide bridge between the metals. The reactivity of these complexes towards two-electron oxidants was studied. NO₂ +, Br₂ and O₂ react with the di-Co(II) complex in a one-site addition two-metal oxidation reaction affording nitro, bromo, and peroxo di-Co(III) complexes respectively. This reactivity provides the first example of such a reaction type for a synthetic bimetallic system. The peroxo di-Co(III) complex is stable only at −40°C in acetonitrile solution, whereas a di-Fe(II) complex does not form a peroxo adduct in a reaction with dioxygen even at low temperature, thus pointing at some differences in steric environment between hemerythrin and the synthetic system.