Tailoring molecular assemblies for metal ion binding

Abstract

An investigation of a subtle, but potentially important, effect in metal coordination chemistry concerned with the spontaneous formation of host-guest assemblies between organic moieties which are themselves potential metal-ion ligands is reported. The use of a ligand 'package' of this type for metal complexation (where the package exists in solution in equilibrium with its corresponding metal complex) is potentially a contributing factor to enhanced complex stability. Initially, nmr titration studies were carried out on host-guest systems formed between carboxylic acids and both open chain and macrocyclic amine-containing ligands. The stoichiometries of the resulting adducts formed in deuterated methanol or chloroform were determined and in some instances it has proved possible to determine the corresponding step-wise binding constants. Solvent extraction experiments (water/chloroform) involving 1,4,8,11-tetrabenzyl-1,4,8,11-tetraazacyclodecane in the presence of hexadecanoic acid in the organic phase have demonstrated that the resulting host-guest 'package' can be employed for the synergistic extraction of nickel(II) and copper(II). The observed extraction behaviour of systems such as this can be interpreted in terms of the operation of an 'assembly effect'. The origins of the latter are not straight forward but may include a favourable entropy term associated with the involvement of an assembled ligand package for complex formation as well as a variable contribution reflecting overall lipophilicity considerations. Under defined conditions, an appreciation of this effect has the potential to provide an additional guide to the subtleties of metal complex formation alongside other well established 'effects' in coordination chemistry.