POLYMERIZATION-TRIGGERED AND SOLVENT-TRIGGERED COOPERATIVITY BETWEEN COPPER(II) IONS IN THE CATALYSIS OF THE HYDROLYSIS OF AMINO ESTERS BY PYRIDINE-BASED LIGANDS

Polymeric (2) and oligomeric (4, 5) materials made of repeating units of 2,6-diaminomethylpyridine and 4,4'-diphenylmethane have been synthe sized with the number of monomeric units (n) ranging from 2 to 29. In 1:1 DMSO/water solutions, these materials are fully soluble and strong ly bind Cu-II ions. The complexes catalyze to different extents the hy drolysis of the p-nitrophenyl esters of alpha-, beta-, and gamma-amino acids. Only Cu-II complexes of polymeric 2 (n greater than or equal t o 10) are more effective catalysts than free Cu-II ions in the cleavag e of beta-amino esters. Such enhanced reactivity, which in the case of beta-alanine p-nitrophenyl ester (beta-AlaPNP) amounts to almost two orders of magnitude when the comparison is made with the Cu-II complex of monomeric ligand (N,N'-benzyl)-2,6-aminomethylpyridine (3), is obs erved in 1:1 (v/v) DMSO/H2O only when a certain degree of polymerizati on is reached (6 < n < 10). In 1:1 (v/v) CH3CH2OH/H2O the kinetic bene fits of the complexes of polymer 2 (n = 10) diminishes and vanishes in 9:1 (v/v) CH3CH2OH/H2O. Analysis of rate data suggests that two neigh boring Cu-II ions bound to the polymeric ligands cooperate for the occ urrence of the hydrolytic process: one of them coordinates the amino g roup of the substrate so that the carbonyl of the carboxylate faces th e second metal ion which delivers a bound hydroxyl acting as the nucle ophilic species. The selectivity toward beta-amino ester is likely ass ociated with a rather rigid conformation of these metallopolymers whic h places two metal centers at the appropriate distance one from the ot her. It is suggested that the onset of the metal ion cooperativity is connected to a conformational change of the metallopolymer from an ext ended to a globular structure, likely triggered by hydrophobic forces.