Templated Synthesis of Copper(II) Azacyclam Complexes Using Urea as a Locking Fragment and Their Metal-Enhanced Binding Tendencies towards Anions

Copper(II) azacyclam complexes 3(2+) and 4(2+) were obtained through a metal-templated procedure involving the pertinent open-chain tetramine, formaldehyde and a phenylurea derivative as a locking fragment. Both metal complexes can establish interactions with anions through the metal centre and the amide NH group. Equilibrium studies in DMSO by a spectrophotometric titration technique were carried out to assess the affinity of 3(2+) and 4(2+) towards anions. While the NH group of an amide model compound and the metal centre of the plain Cu-II(azacyclam)(2+) complex do not interact at all with anions, 3(2+) and 4(2+) establish strong interactions with oxo anions, profiting from a pronounced cooperative effect. In particular, 1) they form stable 4:1 and 1:2 complexes with H2PO4- ions in a step-wise mode with both hydrogen-bonding and metal-ligand interactions, and 2) in the presence of CH3COO-, they undergo deprotonation of the amido NH group and thus profit from axial coordination of the partially negatively charged carbonyl oxygen atom in a scorpionate binding mode.