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 (L¹), 3-bis-(2-pyridin-2-yl-ethyl)-amino]-propionic acid (L²), 4-bis-(2-pyridin-2-yl-ethyl)-amino]-butyric acid (L³), and 6-bis-(2-pyridin-2-yl-ethyl)-amino]-hexanoic acid (L⁴), were treated with copper(II) perchlorate hexahydrate to yield the corresponding Cu(II) complexes, which have all been characterized by X-ray crystallography. L¹ binds Cu(II) to form the tetrameric complex {Cu(μ-¹)]ClO₄] · 4H₂O}₄ (1) in the solid state, whereas the Cu(II) complexes of ligands L²-L⁴ form long-chain one-dimensional polymeric complexes {Cu(μ-L²)]ClO₄] · H₂O}_n (2), {Cu(μ-L³)]ClO₄] · H₂O}_n (3), and {Cu(μ-L²)]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-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 pK_a values of the copper-bound aquo ligands in solution were measured by spectrophotometric titration.