Dinickel(ii) complexes: Preparation and catalytic activity

Abstract

Ligands (L_a–c) based on 2,7-bis(3,5-di-R-pyrazol-1-yl)-1,8-naphthyridine (a, R = H; b, R = CH₃; c, R = Ph) were prepared for the construction of a series of dinickel complexes. Treatment of L_x with NiCl₂ in an anhydrous methanol/THF solution resulted in the formation of dinuclear complexes [(L_x)(μ-Cl)₂Ni₂Cl₂(CH₃OH)₂] (3, x = a; 4, x = b; 5, x = c). These new complexes were characterized by elemental analysis, IR and UV–Vis spectroscopic techniques. The structures of complexes 3 and 4 were further confirmed by X-ray diffraction studies. Interestingly, crystals of 4 were obtained as a co-crystallization of 4 and the methanol substituted species [{(L_b)(μ-Cl)₂Ni₂Cl(CH₃OH)₃}Cl] (4′). These dinickel complexes have been tested in the catalytic homo-coupling of terminal alkynes with the use O₂ as the oxidant, showing excellent activities. A clear improvement on the catalytic activity of these complexes is observed as compared to the mono-nuclear species.