Mixed Ligand Complex Formation between Monodentate Ligands and Various Tridentate Ligand-Copper(II) Complexes

Formation of mixed ligand complexes between monodentate ligands (L) and copper(II) complexes of tridentate ligands (A) has been investigated by potentiometric titration and thermal analysis. Thus, the constants, logKm, for the formation of the complexes of type CuA·L from CuA and L have been calculated from the titration curves by the method of non-linear least-squares. A refers to diethylenetriamine, dipicolylamine, 2,2′,2″-terpyridine, ethylenediamine-N-monoacetic acid, iminodiacetic acid, or pyridine-2,6-dicarboxylic acid and L to pyridine, p-phenolsulfonic acid, n-butylamine, or hydroxy group. The stability series in each system with L fixed has been interpreted in terms of the strength of A as a Lewis base, expressed by the sum of pKa (ΣpKa), as well as back-donation. In the absence of significant steric hindrance, favorable combination of donor atoms around copper(II) has been found to be between aromatic nitrogens and negative oxygens. Thermal stability of the isolated complexes of type CuA·PY, where PY is pyridine, was also in line with the stability sequence in solution.