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

Article abstract of DOI:10.1246/bcsj.72.239

The nickel(II) complexes, [(tpa)Ni(II)(μ-OH)₂Ni(II)(tpa)](ClO₄)₂ (tpa = tris(2-pyridylmethyl)amine) (2), [Ni(II)(CH₃CN)(tpa)(H₂O)](ClO₄)₂ (3) have been prepared and fully characterized. X-Ray crystallography of complex 2 and its methanol adduct, [(tpa)Ni(II)(μ- OH)₂Ni(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)(tpa)(H₂O)](ClO₄)₂ (4), which was characterized by X-ray crystallography. The 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 absorption derived from p-nitrophenolate ion, we obtained the second-order kinetics; first-order in [2] and first- order in [p-NPA]. Addition of ethanol resulted 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°C) as well as formation of a transesterified product, ethyl acetate. The second-order rate constant of the reaction in the presence of added ethanol (0.016 M^-1 s^-1) at 25°C is 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 reaction of 2 with p-NPA in the presence of ethanol. The pK(a) value of the ligated water molecule in 3 was estimated to be 10.7 by kinetic measurements in 40% MeCN aqueous solution at 25°C, I = 0.3.