Dinuclear zinc(II) complex with novel tripodal polyamine ligand: synthesis, structure and kinetic study of carboxy...

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).4H(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 8.34A and both Zn(II) centers adopt distorted trigonal-bipyramid geometry. The solution complexation investigation performed at 25 degrees 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 pK(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 degrees 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.