Transesterification by (Hydroxo)nickel(II) Complex in the Presence of External Alcohol

Abstract

Abstract The nickel(II) complexes, [(tpa)NiII(μ-OH)2NiII(tpa)](ClO4)2 (tpa = tris(2-pyridylmethyl)amine) (2), [NiII(CH3CN)(tpa)(H2O)](ClO4)2 (3) have been prepared and fully characterized. X-Ray crystallography of complex 2 and its methanol adduct, [(tpa)NiII(μ-OH)2NiII(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 [NiII(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]. Additon of ethanol resulted in enhancement of the second-order rate constant (from 1.6 × 10−3 M−1 s−1 to 1.6 × 10−2 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 pKa 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.