Ternary metal complexes of vitamin B6 related compounds. Crystal and molecular structures of copper(II)–2,2′-bipyridyl-pyridoxic acid and copper(II)–2,2′-bipyridyl–pyridoxic acid γ-lactone complexes

Abstract

Attempts to prepare ternary metal complexes of pyridoxylidene Schiff bases resulted in the novel oxidation of pyridoxal to pyridoxic acid (Hpa) or its γ-lactone (pal). The X-ray structures of three ternary metal complexes of the oxidised products are reported. The complexes, characterized also by i.r. spectroscopy, are [Cu(bipy)(pa)Cl]·2H₂O (1)(bipy = 2,2′-bipyridyl), [Cu(bipy)(pa)(ClO₄)](2), and [Cu(bipy)(pal)(NO₃)]NO₃(3). Crystals of (1) are monoclinic, space group P2₁/n, a= 14.342(2), b= 8.374(1), c= 17.331(2)Å, β= 112.31(1)°, and Z= 4; crystals of (2) are monoclinic, space group P2₁/c, a= 9.782(3), b= 14.440(4), c= 14.541(3)Å, β= 103.58(2)°, and Z= 4; crystals of (3) are triclinic, space group P, a= 8.388(2), b= 8.724(1), c= 15.483(2)Å, α= 99.37(1), β= 76.82(2), γ= 117.05(1)°, and Z= 2. The structures were solved by the heavy-atom method and refined to R values of 0.046, 0.039, and 0.049 for 3 030, 1 642, and 3 054 independent diffractometer data respectively. The co-ordination geometry around the metal ion is distorted square pyramidal in (1) and (3) and tetragonally elongated (4 + 1 + 1) octahedral in (2). Two oxygens from phenolic and carboxylate or carbonyl groups of pa or pal and two nitrogens from bipy form the base, while the axial site is occupied by a chloride ion in (1), perchlorate oxygen in (2), or a nitrate oxygen in (3). In (2), phenolic oxygen, which acts as the other axial site, bridges two Cu^II centres resulting in a dimer. Both pa and pal are zwitterionic and the protonated pyridine nitrogens are involved in hydrogen bonding. Partial stacking between the pyridine ring and one of the bipy rings is observed in (2) and (3).