Transesterification by (hydroxo)nickel(II) complex in the presence of external alcohol

The nickel(II) complexes, [(tpa)Ni-II(mu-OH)(2)Ni-II(tpa)](ClO4)(2) (tpa = tris(2-pyridylmethyl)amine) (2), [Ni-II(CH3CN)-(tpa)(H2O)](ClO4)(2) (3) hav e been prepared and fully characterized. X-Ray crystallography of complex 2 and its methanol adduct, [(tpa)Ni-II(mu-OH)(2)Ni-II(tpa)](ClO4)(2).2MeOH ( (2).2MeOH) revealed a molecular structure containing two Ni centers bridged by two OH ligands. The Ni-O distances in 2.2MeOH are longer than those in 2. The MeCN ligand in complex 3 is replaced by EtCN to give [Ni-II(EtCN)(tp a)(H2O)](ClO4)(2) (4), which was characterized by X-ray crystallography. Th e kinetic study of p-NPA (p-nitrophenyl acetate, 0.1, 0.5, 1 mM) hydrolysis by complex 2 (20, 10, 6 mM) was performed in the presence of ethanol (100 mM) in MeCN solution (1 M = 1 mol dm(-3)). By following the 400 nm absorpti on derived from p-nitrophenolate ion, we obtained the second-order kinetics ; first-order in [2] and first-order in [p-NPA]. Additon of ethanol resulte d in enhancement of the second-order rate constant (from 1.6x10(-3) M-1 s(- 1) to 1.6x10(-2) M-1 s(-1) at 25 degrees C) as well as formation of a trans esterified product, ethyl acetate. The second-order rate constant of the re action in the presence of added ethanol (0.016 M-1 s(-1)) at 25 degrees C i s much larger than that of the reaction without ethanol (0.0016 M-1 s(-1)). Also observed was a transesterified product, ethyl acetate, from the react ion of 2 with p-NPA in the presence of ethanol. The pK(a) value of the liga ted water molecule in 3 was estimated to be 10.7 by kinetic measurements in 40% MeCN aqueous solution at 25 degrees C, I = 0.3.