Assembling chiral salan–copper(II) complexes into a 2D-network with carboxylic acid functionalization

Abstract

In this work, we aim to assemble a polymeric copper(II) network by ligand modifications with carboxylic acid functional groups. A chiral, reduced salen ligand, salan R,R-H21 with pendant ester functionalities has been synthesized. Its copper(II) complex, [Cu(R,R-1)], has been isolated, and its crystal structure determined by X-ray crystallography. Hydrolysing the ester groups of R,R-H21 to carboxylic acids yields R,R-H42 as the hydrochloride salt R,R-H42·2HCl. Treatment of R,R-H42·2HCl with copper(II) acetate in the presence of triethylamine yields the complex [Cu(R,R-H22)]n and X-ray structural analysis reveals that this consists of a 2D polymeric network. Each copper(II) center is bound within the near square-planar N2O2 donor set of the Schiff base ligand [R,R-H22]2− and pendant carboxylic acid groups of adjacent units coordinate through the CO2H carbonyl units in the axial sites of the copper ion to complete an axially distorted octahedral coordination environment. The sheets are stacked via interdigitation of the cyclohexyl domains, giving an overall porous assembly containing carboxylic acid functionalities. This has proven to be an effective strategy for adding the dimensionality of metal–organic complexes by carboxylic acid functionalization of ligands.