Metal-coordination interactions in the template-mediated synthesis of substrate-selective polymers: recognition of bis(imidazole) substrates by copper(II) iminodiacetate containing polymers

Metal-complexing polymer matrices capable of specific recognition and binding to metal-coordinating substrates have been prepared by template-directed polymerization. The synthesis of these materials involves preorganization of a copper-containing vinyl monomer, copper(II) [N-(4-vinylbenzyl)-imino]diacetic acid (1), with bifunctional bis(imidazole) templates (2-5) of varying geometry and subsequent polymerization with a large excess of ethylene glycol dimethacrylate as the cross-linking agent. Complexation of metal-chelating monomers with the template during polymerization directs the positioning of metal ions in the polymer matrices, while a high degree of cross-linking stabilizes the functional group arrangement. In equilibrium binding experiments with single substrates and selected substrate pairs, the polymers preferentially bind their own templates, with separation factors (α) of 1.17-1.35 and binding constants that range from 1800 to 3800 M^(-1). The capacities and affinities of the polymers for different substrates and ESR spectral analyses of the polymers loaded with substrate suggest a defined arrangement of metal ion sites in the templated materials that is absent in nontemplated polymers. The substrate selectivity likely involves some cooperative two-site coordination of the bis(imidazoles) as well as steric interactions with the binding cavities ("cavity fitting"). This template polymerization strategy is discussed from the viewpoint of designing highly specific abiotic receptors for recognition of delicate and complex biomolecules