Complexes of copper(I) with aromatic compounds in aqueous solutions

The apparent stability constants of the complexes of Cu⁺(aq) with several aromatic compounds, mainly substituted benzoic acids, were determined electrochemically. The results reveal that these complexes are surprisingly stable, 2.0 ≤ log K ≤ 4.0 at the pK_a of the benzoic acids. The stability constants depend on the pH. This dependence is considerably stronger than that predicted from the pK_a of the acids, suggesting that complexes of CuOH with the aromatic acids are also formed. Electron donating substituents increase the apparent stability constants. The results indicate that the complexes formed are due to d → π interactions between the copper d orbitals and the aromatic π system. The effect of Cu⁺(aq) on the ¹H NMR of benzoic acid supports this conclusion.