Determination of the Formation Constants of Copper(II) Complexes Using Quantum Chemical Calculations

Gibbs free energy values obtained from quantum-chemical calculations were used to determine the formation constants of homo- and heteroligand copper(II) complexes with various ligands (amino acids, diimines, and phosphorylated dithiocarbamates) in an aqueous medium. A computationally robust, yet moderately expensive, level of theory—B3LYP/def2-TZVPPD—was employed. Solvent effects were accounted for using two models: C-PCM and SMD. A key prerequisite for obtaining reliable results is the use of a reference complex with a known formation constant that is structurally and solvation-wise similar to the compound under study. In this context, “similarity” implies identical stoichiometry, the same coordination number, the same number of water molecules in the inner coordination sphere; matching charges, however, is considerably less critical. The importance of considering conformers and isomers to obtain the most accurate values is demonstrated. The more rigid the structure and the greater the degree of similarity between the studied and reference compounds, the better the agreement between calculated and experimental data; the discrepancy can be as low as 0.1–0.2 logarithmic units. When an appropriate reference is selected, the average deviation of the calculated stability constants is less than 1 logarithmic unit.