Dinuclear zinc(II) complex with novel tripodal polyamine ligand: Synthesis, structure and kinetic study of carboxy ester hydrolysis

An imidazole-containing tripodal polyamine ligand N 1 -(2-aminoethyl)- N 1 -(2-imidazol-1-ylethyl)-ethane-1,2-diamine (L) was prepared and its dinuclear zinc(II) complex [Zn(L)(H 2 O)] 2 (ClO 4 ) 4 · 4 H 2 O ( 1 ) was obtained and examined as a catalyst for the hydrolysis of 4-nitrophenyl acetate (NA). X-ray crystal structure analysis of the complex revealed that the complex features a dinuclear cation unit with a Zn ⋯ Zn distance of 8.34 A and both Zn(II) centers adopt distorted trigonal-bipyramid geometry. The solution complexation investigation performed at 25 °C by means of potentiometric titration revealed that the mononuclear species [ZnL] 2+ is predominating in the pH rage of 7.0–9.7 in the solution and the p K a1 for the Zn-bound water is 8.50 ± 0.01. Complex 1 promoted hydrolysis of NA showed a second-order rate constant of 0.046 ± 0.004 M −1 s −1 at pH 9.0 in 10% (v/v) CH 3 CN aqueous solution at 25 °C. The pH-rate profile for the second-order rate constant of NA hydrolysis with complex 1 gave a sigmoidal curve. And the results show that in the hydrolysis process the two Zn(II) centers of the dinuclear deprotonated species do not cooperate with each other and the Zn-bound hydroxide servers as reactive nucleophile toward the ester.