Cobalt(II) dioxygen carriers derived from novel pentadentate Schiff base chelates: Synthesis, characterization and their copper(II) analogs

In the past, pentadentate Schiff base ligands have been used to prepare a number of effective cobalt(II) dioxygen carriers, some of which have been promising enough to warrant patenting. Unfortunately, optimal use of these complexes has been limited by their considerable susceptibility to autoxidation (oxidative degradation). Another feature of some of these systems, which may be less than optimal, is their incorporated, tertiary alkylamine, axial base. This base is a poor model for histidine, in part because it is incapable of pi-electron donation. On the basis of autoxidation studies with the lacunar cyclidenes, another class of synthetic oxygen carriers, it is proposed that ligand modifications, particularly the addition of steric bulk and/or electron withdrawing substituents, could produce pentadentate Schiff base dioxygen carriers which are considerably less prone to oxidative degradation. To this end, a novel, tetraphenyl substituted parent ligand and its bridged homologs have been developed. These ligands and their corresponding copper(II) and cobalt(II) complexes have been synthesized and fully characterized. Reaction of the cobalt(II) complexes with dioxygen has been observed qualitatively and investigated using physical methods. Some dioxygen binding constants and autoxidation rates are reported. Building on the methodology of Martell and coworkers, a family of pentadentate copper(II) and cobalt(II) complexes, which contain a pyridine moiety as axial base, has also been developed. These compounds and their free ligand precursors have been characterized using a variety of techniques. Dioxygen binding to the cobalt(II) complexes has been observed at low temperatures.