Bimacrocyclic Effect in Anion Recognition by a Copper(II) Bicyclam Complex

The dicopper(II) complex of the bimacrocyclic ligand α,α′-bis(5,7-dimethyl-1,4,8,11-tetraazacyclotetradecan-6-yl)-o-xylene, 2, interacts with selected anions in dimethyl sulfoxide solution according to two different modes: (i) halides (Cl–, Br–, and I–) and N3– coordinate the two metal centers at the same time between the two macrocyclic subunits that face each other and (ii) anionic species that do not fit the bridging coordination mode (e.g., NCO–, SCN–, CH3COO–, NO3–, and H2PO4–) interact with copper(II) ions only at the “external” positions or their interaction is too weak to be detected. Occurrence of the bridging interaction is demonstrated by X-ray crystallographic studies performed on the adduct formed by [Cu2(2)]4+ with azide and by electron paramagnetic resonance investigation, as the anion coordination between the two copper(II) centers induces spin–spin coupling. Isothermal titration calorimetry experiments performed on [Cu2(2)]4+ and, for comparison, on [(5,7-dimethyl-6-benzyl-1,4,8,11-tetraa...