The advantage of spectrophotometric measurement for size-selective complexing of Cu(II) with O2N2-azacrown ligands

Abstract

A comparative investigation of the interaction of Cu(II) with a series of 15- to 19-membered mixed-donor dibenzo-substituted macrocyclic ligands, each incorporating an O2N2-donor set, has been carried out using UV–Visible studies in methanol. Although a ring size effect has been reported for a related series of Ni(II) complexes, no such metal ion discrimination has been reported for Cu(II) in terms of its binding constants with 14- to 17-membered macrocycles. Employing Job’s method of continuous variation established 1:1 stoichiometry for the interaction between Cu(II) and 1–5. From UV–Visible studies applying the Benesi–Hildebrand equation, the binding constants (K) of Cu(II) with 1–5 were determined to be Cu(II)/1=3330 (±321)dm3mol−1, Cu(II)/2=33,700 (±71)dm3mol−1, Cu(II)/3=7260 (±151)dm3mol−1, Cu(II)/4=57,000 (±257)dm3mol−1 and Cu(II)/5=13,900 (±398)dm3mol−1 in methanol at 25°C, respectively. The calculated binding constants showed a saw-tooth pattern in which 4 (18-membered ring) gives the highest K value for these complexes. The thermodynamic parameters (ΔG, ΔH, and TΔS) of the respective complexes have also been determined.