ChemInform Abstract: Coordination Modes of Histidine. Part 9. Synthesis, Stereochemistry, and Oxygenation of Cobalt(II) - Pyridoxal Model Complexes. A New Family of Chiral Dioxygen Carriers.

ChemInform Abstract

The Co(II) complexes (I)-(IV) are prepared by metal template condensation from salicylaldehyde, pyridoxal, or pyruvic acid and corresponding L- amino acids in the presence of Co(II) salts. They are characterized by UV/VIS, IR, and CD spectra, and magnetic data. The complexes are high-spin and have five- or six-coordinate structure. Their stereochemical properties are deduced from the CD spectra and by preparing some representative adducts with donor bases. The complexes (Ia) and (Ib) can bind molecular oxygen, but while the histidine derivatives react readily in any solvent, the derivatives of amino acids with nonpolar side chains require the presence of additional base molecules. The reactivity of the histidine complexes is determined essentially by the glycine-like mode of binding of the amino acid residue. In fact, the histidine complex (III) and (IV), in which the amino acid is bound as a substituted histamine, is completely unreactive to O2. Both 1:1 and 2 :1 O₂ adducts are formed by the Co - pyridoxal model complexes, but in some cases the presence of bulky, nonpolar side chains on the amino acid residues sterically hinders dimerization of 1:1 adducts and provides a reversible oxygenation behavior at room temperature. Complementary information on the evolution of l :l and 2:1 O₂ species during the oxygenation reactions can be obtained by combined ESR and optical/ CD measurements. The binuclear u-peroxo complexes (V) are isolated in the solid state and characterized spectrally (no yields given).