Carboxyester hydrolysis promoted by Cu(II) complexes of pyridyl-amine carboxylate-pendant ligands

Multidentate ligands containing tripodal pyridyl-amine moieties tethered to a carboxylate group by alkyl linkers of varying lengths were synthesized to obtain a series of water-soluble ligands to elucidate the effects of the differing coordination environments on the properties of the resulting metal complexes. These new, water-soluble ligands, [bis-(2-pyridin-2-yl-ethyl)-amino]-acetic acid (L-1), 3-[bis-(2-pyridin-2-yl-ethyl)-amino]-propionic acid (L-2), 4-[bis-(2-pyridin-2-yl-ethyl)-amino]-butyric acid (L-3), and 6-[bis-(2-pyridin-2-yl-ethyl) -amino]-hexanoic acid (L-4), were treated with copper(II) perchlorate hexahydrate to yield the corresponding Cu(11) complexes, which have all beer characterized by X-ray crystallography. L-1 binds Cu(II) to form the tetrameric complex {[Cu(mu-(1))][ClO4] . 4H(2)O}(4) (1) in the solid state, whereas the Cu(11) complexes of ligands L-2-L-4 form long-chain one-dimensional polymeric complexes {[Cu-(mu-L-2) ][ClO4] . H2P}(n) (2), {[Cu(mu-L-3)][ClO4] . H2O](n) (3), and {[Cu(mu-L-2)][ClO4] . H2O](n) (4), respectively, in the solid state. Complexes 1-4 dissolved in 10%) (v/v) CH3CN aqueous solution were tested for their ability to promote the hydrolysis of the activated ester compound 4-nitrophenillacetate (NA), with 3 being the most active complex and I being the least active, possibly due to differences in the ability of the carboxylate moiety to act as either a general base or a nucleophile in the hydrolysis of NA as dictated by the tether length. The pK(a) values of the copper-bound aquo ligands in solution were measured by spectrophotometric titration. (c) 2006 Elsevier B.V. All rights reserved.