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

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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(II) complexes, which have all been characterized by X-ray crystallography. L 1 binds Cu(II) to form the tetrameric complex {[Cu(μ- 1)][ClO 4] · 4H 2O} 4 ( 1) in the solid state, whereas the Cu(II) complexes of ligands L 2–L 4 form long-chain one-dimensional polymeric complexes {[Cu(μ-L 2)][ClO 4] · H 2O} n ( 2), {[Cu(μ-L 3)][ClO 4] · H 2O} n ( 3), and {[Cu(μ-L 2)][ClO 4] · H 2O} n ( 4), respectively, in the solid state. Complexes 1– 4 dissolved in 10% (v/v) CH 3CN aqueous solution were tested for their ability to promote the hydrolysis of the activated ester compound 4-nitrophenylacetate (NA), with 3 being the most active complex and 1 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 p K a values of the copper-bound aquo ligands in solution were measured by spectrophotometric titration.