Progress towards water-soluble calixarene-like metal ion activated molecular receptors

The pendant donor macrocyclic ligand 1,4,7,10-tetrakis((S)-2-hydroxy-3-[4′-hydroxy-phenyloxy]propyl)-1,4,7,10-tetraazacyclododecane (hereafter, L3) has been prepared as both its diacetate and tetrahydrochloride salts. Under the influence of cadmium(II), L3 forms an eight co-ordinate complex with a cone conformation arising from the four aromatic moieties. These juxtapose to create a substantial cavity, fringed by four phenolic groups, that enables the complex to act as a receptor complex. The four phenolic groups impart a limited amount of water solubility to the ligand and its complexes. The receptor complex is able to retain two acetate anions to the extent that they are undissociated in DMF solution. Substitution of one of these by p-nitrophenolate or tetraphenylborate occurs readily, but these anions are more easily dissociated and it appears that they hydrogen bond to the outer fringe of phenolic groups rather than to the aliphatic hydroxyl groups, at the base of the cavity, which is the site occupied by the remaining acetate anion.