Nickel and copper complexes of a new multidentate ligand: synthesis, characterization and magneto-structural investigations

The ligand <b>L</b>H₈Cl₂ (<b>L</b>H₆·2HCl) [(3-hydroxy-5-(hydroxymethyl)-2-methylpyridin-4-yl)methylene)]pyridine-2,6-dicarbohydrazide dichloride] was prepared from the Schiff base condensation of pyridine-2,6-dicarbohydrazide with pyridoxal hydrochloride in methanol. Reaction of <b>L</b>H₆·2HCl with CuCl₂·4H₂O or Cu(ClO₄)₂·6H₂O in a MeOH/water mixture produced two 1-D copper coordination polymers with formulae {[Cu₃(<b>L</b>H₄)Cl₃(H₂O)]Cl·H₂O}_<i>n</i> (1) and {[Cu₃(<b>L</b>H₄)Cl(H₂O)₂(ClO₄)][ClO₄]₂·2H₂O}_<i>n</i> (2) respectively. Similarly, reaction of <b>L</b>H₆·2HCl with NiCl₂·6H₂O, Ni(ClO₄)₂·4H₂O or Ni(NO₃)₂·6H₂O afforded the trimetallic complex [Ni₃(<b>L</b>H₄)(H₂O)₉]Cl₄·4H₂O (3), the 1-D polymer {[Ni₃(<b>L</b>H₄)(NO₃)(H₂O)₆](NO₃)₂·H₂O}_<i>n</i> (4) and [Ni₂(<b>L</b>H₆)(H₂O)₆](ClO₄)₄·3H₂O (5). The crystal structures of 1-5 were determined by X-ray diffraction. All complexes 1-4 exhibit the same geometric arrangement of the ligand with the central metal linked to the terminal metal ions by a <i>trans</i>-diazine linker with the diazine and pyridoxal arms coordinating in a tridentate NO₂ donor fashion, while the central metal atom is coordinated by the ligand in a tridentate fashion by the pyridine and two diazine-N atoms. Water and coordinating anions complete the coordination environment around the Cu²⁺ and Ni²⁺ ions. The ligand adopts the same geometry in 5 but is not deprotonated and does not possess a central metal atom. Magnetic measurements (5-300 K) reflect strong antiferromagnetic exchange across the 1,2-diazine bridge.