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

Zinc(II) complexes of new macrocyclic tetraamines (cyclam derivatives) having a strategically appended phenolic group, -hydroxy)-benzyl-1,4, 8,11-tetraazacyclotetradecane (L-A), 5'-bromo)-benzyl-1,4,8,11-tetraaz acyclotetradecane (L-B) and -dibromo)-benzyl-1,4,8,11-tetraazacyclotet radecane (L-C), have been examined as catalysts for the hydrolyses of 4-nitrophenylacetate (NA). The phenolic functionalized macrocycles for m 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 prot on 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 kinet ic studies using the zinc complex in 10% (v/v) CH3CN at 298 K, I = 0.1 0 mol.l(-1) KNO3 and pH 7.0-9.5, we proved that the coordinated phenol ate can serve as a good nucleophile that effectively catalyzes NA hydr olysis. 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 in flection points at pH 8.8, 8.7 and 8.6 for L-A, L-B and L-C, respectiv ely, which correspond to the pKa value of the complex. The second-orde r (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 t han the corresponding value of 0.047 mol.l(-1).s(-1) for N-methylcycle n-Zn(II)-OH- complex catalyst. This is, to our knowledge, the first ph enolate coordinated zinc complex that efficiently catalyses the hydrol ysis 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.