STUDIES OF ARTIFICIAL HYDROLYTIC METALLOENZYMES - THE CATALYTIC CARBOXYESTER HYDROLYSIS BY NEW MACROCYCLIC POLYAMINE ZINC(II) COMPLEXES WITH A PHENOLIC PENDANT AS NOVEL NUCLEOPHILE

摘要

Zinc(II) complexes of new macrocyclic tetraamines (cyclam derivatives) having a strategically appended phenolic group, 6-(2'-hydroxy)-benzyl-1,4,8,11-tetraazacyclotetradecane (L-A), 6-(2'-hydroxy-5'-bromo)-benzyl-1,4,8,11-tetraazacyclotetradecane (L-B) and 6-(2'-hydroxy-3',5'-dibromo)-benzyl-1,4,8,11-tetraazacyclotetradecane (L-C), have been examined as catalysts for the hydrolyses of 4-nitrophenylacetate (NA). The phenolic functionalized macrocycles form 1 : 1 ZnL complexes at pH ca 5. The potentiometric pH titration of L-A, L-B and L-C-Zn-II complexes showed dissociation of a phenolic proton with pKa values of 8, 8, 8.7 and 8.5 at 298 K and I = 0.10 mol.l(-1) KNO3 for L-A, L-B and L-C Zn-II complexes respectively. In the kinetic studies using the zinc complex in 10% (v/v) CH3CN at 298 K, I = 0.10 mol.l(-1) KNO3 and pH 7.0-9.5, we proved that the coordinated phenolate can serve as a good nucleophile that effectively catalyzes NA hydrolysis. The hydrolysis rate follows the law upsilon=(k(cat)[complex]+k(OH)[OH-]+K-0)[NA]. The pH rate profile gave a sigmoidal curve with inflection points at pH 8.8, 8.7 and 8.6 for L-A, L-B and L-C, respectively, which correspond to the pKa value of the complex. The second-order (first-order each in complex and NA) rate constants are 0.056, 0.084 and 0.127 mol.l(-1).s(-1) for L-A, L-B and L-C are obviously larger than the corresponding value of 0.047 mol.l(-1).s(-1) for N-methylcyclen-Zn(II)-OH- complex catalyst. This is, to our knowledge, the first phenolate coordinated zinc complex that efficiently catalyses the hydrolysis of 4-nitrophenyl acetate (NA). The present study also proves that solvolysis of NA (i.e. water attack on the ester) does exist, but the reaction rate (k(0) = 1.12 x 10(-5) s(-1)) is rather small. (C) 1997 Elsevier Science Ltd. [References: 35]