New Synthetic Cobalt Schiff Base Complexes as Oxygen Carriers

Abstract

The synthesis and dioxygen affinities of ten new cobalt Schiff base complexes and two new cobalt complexes obtained by reduction of Schiff bases are reported and compared in a common solvent with two well-known parent Schiff base complexes and six Schiff base complexes recently reported. The cobalt(II) Schiff base complexes investigated are (bis(3-fluorosalicylaldehyde) ethylen-ediiminato)cobalt(II) (CoFLUOMINE), (bis(3-methoxysalicylaldehyde) ethylenediiminato)cobalt(II) (Co3MeOSALEN), (bis(3-fluorosalicylaldehyde) o-phenylenediimato)cobalt(II) (Co3FSALOPHEN), disodium (bis(5-sulfosalicylaldehyde) o-phenylenediiminato)cobaltate(II) (Co5S03NaSALOPHEN), (bis(2-hydroxyacetophenone) o-phenylenediiminato)cobalt(II) (CoaCH3SALOPHEN), (bis(3-fluorosalicylaldehyde) tetramethylethylenediiminato)cobalt(II) (Co3FSALTMEN), (bis(3-methoxysaiicylaklehyde) tetramethylethylenediiminato)cobalt(II) (Co3MeOSALTMEN), (N,N'-bis(salicylidene)-2,6-bis(ami-nomethyl)pyridinato)cobalt(II) (CoSALMP), (N,N'-bis(salicylidene)-2,6-bis(aminoethyl)pyridinato)cobalt(II) (CoSALEP), (N,N'-bis(2-hydroxybenzyl)-2,6-bis(aminoethyl)pyridinato)cobalt(II) (CoBHBEPTY), (N,N'-bis(salicylidene)bis(2-amino-phenyl)methylaminato)cobalt(II) (CoMBA), and (N,.N'-bis(2-hydroxybenzyl)bis(2-aminophenyl)methylaminato)cobalt(II) (CoMHBA). Oxygenation equilibria were measured by monitoring gaseous oxygen uptake by the cobalt(II) complexes in bis(2-methoxyethyl) ether (diglyme) as solvent. The seven tetradentate Schiff base complexes were studied in the presence of large excess (ca. 50 times) concentrations of 4-methylpyridine. The substitution of methyl groups on the Schiff bases (α-substitution, acetophenone derivatives) increases the affinity for dioxygen but also increases the rate of degradation. Fluorine substitution on the aromatic rings of the Schiff bases increases the affinity of the complexes for dioxygen by promoting 2:1 (binuclear) dioxygen complex formation. Sulfonation of the aromatic rings of the Schiff base seems to increase the rate of degradation of the cobalt dioxygen complexes studied, Methoxy groups on the aromatic rings of the Schiff bases tend to increase the affinity of the cobalt(II) complex for dioxygen and favor 1:1 dioxygen complex formation. Combination of dioxygen with the cobalt(ll) complexes of the pentadentate Schiff bases seems to be partially inhibited by steric effects, which are relieved by hydrogenation of the ligands. The cobalt(II) complexes of reduced ligands have much higher affinities for dioxygen than do the complexes of the corresponding Schiff bases. © 1989, American Chemical Society. All rights reserved.