210410-16-5Relevant articles and documents
Polymerization- and solvent-triggered cooperativity between copper(II) ions in the catalysis of the hydrolysis of amino esters by pyridine-based ligands
Scrimin, Paolo,Tecilla, Paolo,Tonellato, Umberto
, p. 1143 - 1153 (2007/10/03)
Polymeric (2) and oligomeric (4, 5) materials made of repeating units of 2,6-diaminomethylpyridine and 4,4′-diphenylmethane have been synthesized with the number of monomeric units (n) ranging from 2 to 29. In 1:1 DMSO/water solutions, these materials are fully soluble and strongly bind CuII ions. The complexes catalyze to different extents the hydrolysis of the p-nitrophenyl esters of α-, β-, and γ-amino acids. Only CuII complexes of polymeric 2 (n ≥ 10) are more effective catalysts than free CuII ions in the cleavage of β-amino esters. Such enhanced reactivity, which in the case of β-alanine p-nitrophenyl ester (β-AlaPNP) amounts to almost two orders of magnitude when the comparison is made with the CuII complex of monomeric ligand (N,N'-benzyl)-2,6-aminomethylpyridine (3), is observed in 1:1 (v/v) DMSO/H2O only when a certain degree of polymerization is reached (6 3CH2OH/H2O the kinetic benefits 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 neighboring CuII ions bound to the polymeric ligands cooperate for the occurrence of the hydrolytic process: one of them coordinates the amino group of the substrate so that the carbonyl of the carboxylate faces the second metal ion which delivers a bound hydroxyl acting as the nucleophilic species. The selectivity toward β-amino ester is likely associated with a rather rigid conformation of these metallopolymers which places two metal centers at the appropriate distance one from the other. It is suggested that the onset of the metal ion cooperativity is connected to a conformational change of the metallopolymer from an extended to a globular structure, likely triggered by hydrophobic forces.
