Dinickel(II) complexes: Preparation and catalytic activity

Ligands (La-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) (mu-Cl)(2)Ni2Cl2(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)(mu-Cl)(2)Ni2Cl(CH3OH)(3)}Cl] (4'). These dinickel complexes have been tested in the catalytic homo-coupling of terminal alkynes with the use O-2 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.