Transition Metal Complexes of Vitamin B6 Related Compounds. 1. Synthesis and Electronic and Structural Properties of Several Divalent First-row Transition-Metal Complexes of Pyridoxylideneamino Acids


The synthesis and electronic and structural properties of 23 divalent first-row transition-metal complexes of pyridoxylideneglycine, pyridoxylidenealanine, pyridoxylidenevaline, pyridoxylidenephenylalanine, and pyridoxylidenetryptophan are reported. The manganese(II), iron(II), cobalt(II), and nickel(II) complexes are of the type M(HPLaa)2·nH2O, where HPLaa is the monoanion of a pyridoxylideneamino acid (Schiff base). The Cu(II) complexes are of the type Cu(PLaa)·nH2O, where PLaa is the dianion of a pyridoxylideneamino acid ligand. The Mn, Fe, Co, and Ni complexes formed with any one of these ligands are generally x-ray isomorphous. The high-spin Mn, Fe, Co, and Ni compounds all exhibit electronic spectral properties which are consistent with a distorted octahedral coordination geometry whereas these properties for the Cu(II) compounds are consistent with five-coordination. This five-coordination arises through intermolecular bridging via the pyridine nitrogen and 5'-hydroxymethyl groups in pyridoxal. The octahedral coordination involves the phenolate oxygen, the azomethine nitrogen, and the weakly bonded carboxylate oxygen in each Schiff base ligand. Infrared band assignments are given for the azomethine C=N stretch at ca. 1660 cm-1, the ring-carbon phenolate oxygen stretch at ca. 1500 cm-1, and the asymmetric carboxylate stretch at ca. 1600 cm-1. The low-frequency infrared spectrum shows only a single band near 400 cm-1 which can be assigned to a metal-oxygen stretching absorption band. The 57Fe Mössbauer spectral parameters are typical of high-spin Fe(II) in a distorted octahedral ligand field. These results indicate that the pyridoxylideneamino acid ligands form, in general, coordination compounds different than the salicylideneamino acid ligands. This is consistent with the observation that the salicylideneamino acid complexes are not good vitamin B6 model complexes. The relative effectiveness of Zn2+, Ni2+, and Co2+ as catalysts in these reactions is explained in terms of their electronic and structural properties.



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