A di-copper(II) bis-tren cage with thiophene spacers as receptor for anions in aqueous solution

The system made of the bis-tren octaamino cage ligand with thiophene spacers (3) and CuII (1:2 molar ratio) has been studied in aqueous solution by means of potentiometric titrations. Both protonation and formation constants of the metal-containing species were determined, so that the system can be fully described at any pH value. 6.9 was chosen as the best pH value for the system to work as receptor for a series of bidentate anionic species: this is the lowest value at which only dimetallic complexes exist. In particular, [Cu2(3)(OH)]3+ is the species prevailing by far at this pH, so that a displacement equilibria takes places on binding anions, [Cu2(3)(OH)]3++A−=[Cu2(3)(A)]3++OH−. The binding process was followed by means of spectrophotometric titrations and, for the anions bound by the system, a Kobs was determined, related to the constant of the displacement equilibrium by K=Kobs[OH−]. A log Kobs value of 6.75 (±0.09) was determined for N3 −, 4.79 (±0.07) for NCO−, and 2.72 (±0.08) for NCS−. SO4 2−, NO3 −, HCO3 −, CH3COO−, Cl−, Br− and I−, were instead not bound by the system or bound with log Kobs values ≪2. The crystal and molecular structure of the complex [Cu2(3)(N3)]3+ was also determined, which evidenced the intrinsically increased length of the cage and of its CuII 2 complexes, related to the large atomic radius of the S atom of the thiophene spacers, which prevents the binding of monoatomic anions (halides).