Synthesis and characterization of nickel(II) complexes with symmetrical tetradentate N2O2 naphthaldimine ligands. Their application as catalysts for the hydrogenation of cyclohexene

Ni acetate reacts with naphthaldimines containing the N2O2 donor group to yield two types of complex. The first, of molecular formula NiL (L=naphthaldimine), is free of the anions in which the ligand behaves as a dibasic tetradentate towards one Ni ion. These complexes are all diamagnetic, having square planar geometry. In the second type, the ligand behaves either as a monobasic acid towards the Ni ion or as a neutral molecule. These complexes are paramagnetic and exhibit a tetrahedral configuration around the central metal ion. The metal chelates were characterized by elemental analyses (t.g.a. and d.t.a.), and by i.r. and u.v.–vis. spectroscopy. Magnetic moments were also measured. A series of NiII–Schiff base complexes were tested for their catalytic activity in the hydrogenation of cyclohexene with molecular H2. The effects of the bridge length, amounts of solvent and cosolvent were studied. The use of H2O as a cosolvent greatly increases the yield of the reaction product. The hydrogenation product yield is linearly related to the dielectric constant of the medium.