Role of trinuclear Zn(II) complexes in promoting the hydrolysis of 4-nitrophenyl acetate

Abstract

Potentiometric determination shows that trinuclear Zn(II) complexes of the four tripods 1,3,5-tri(2′,5′-diazahexyl)benzene (L1), 1,3,5-tri(2′,5′-diazaheptyl)benzene (L2), 1,3,5-tri(2′,5′-diazaoctyl)benzene (L3), and 1,3,5-tri(2′,5′-diazanonyl)benzene (L4) could be potential hydrolytic catalysts. CH3CN solutions containing [3Zn:L]T (0.5~2 × 10–3 mol·dm–3) with I = 0.10 mol·dm–3 of KNO3 and Good's buffer (10% volume fraction) were studied for the catalyzing hydrolysis of p-nitrophenyl acetate (NA, 0.5~2 × 10–3 mol·dm–3), at 298 K, in the 6.5–8.2 pH range. The observed rate constants, kobs, fit the equilibrium equation kobs = kcom [3Zn:L]T + kOH[OH–] + k0. The sigmoid pH~kcom profiles for NA hydrolysis suggest that either the Zn(II)-bound hydroxyl or the Zn(II)-bound water forms of the catalysts can be the active species. The observed second-order rate constants are 0.0082, 0.011, 0.0059, and 0.0019 mol–1·dm3·s–1 for the four Zn3L–H2O complexes (kA) and 0.342, 0.257, 0.382, and 0.091 mol–1·dm3·s–1 for the four Zn3L–OH- groups (kB), respectively. However, under the condition that [NA] = 0.5 × 10–3 mol·dm–3 and [3Zn:L1]T = 2~4 × 10–2 mol·dm–3 at pH 7.6, the observed rate constants, kobs, obey the equilibrium kobs = kcom[3Zn:L]T/(1/K′ + [3Zn:L]T). This indicates that the 3:1 complex (or its deprotonated hydroxide form) mediates NA hydrolysis by nucleophilic attack of the carboxyl center with the pre-formation of a coordination bond between the carboxyl oxygen atom and the Zn(II) ion. Comparison with other models was made, and the reasons for the high catalytic efficiency of the tripodal complexes were given.Key words: tripod, Zn(II), catalysis, NA hydrolysis, polynuclear.