Intramolecular chelate formation involving the carbonyl oxygen of acetyl phosphate or acetonylphosphonate in mixed ligand copper(II) complexes containing also 2,2′-bipyridine or 1,10-phenanthroline. A decreased solvent polarity favours the metal ion–carbonyl oxygen recognition

Abstract

The stability constants of the mixed ligand complexes formed by Cu²⁺, 2,2′-bipyridine or 1,10-phenanthroline (= Arm) and acetyl phosphate (AcP²⁻) or acetonylphosphonate (AnP²⁻) were determined by potentiometric pH titrations in water and in water containing 30 or 50% (v/v) 1,4-dioxane (25 °C; I = 0.1 M, NaNO₃). Previous measurements with simple phosph(on)ate ligands, R-PO²⁻₃ (R being a non-interacting residue), had established log K^Cu(Arm)_{Cu(Arm)(R-PO3)}versus pK^H_H(R-PO3) straight-line plots and these were used now to prove that the Cu(Arm)(AcP) and Cu(Arm)(AnP) complexes possess a higher stability than is expected for a sole phosph(on)ate–Cu²⁺ coordination. This increased stability is attributed to the formation of six-membered chelates involving the carbonyl oxygen present in AcP²⁻ and AnP²⁻. The formation degree of the six-membered chelates in the Cu(AcP), Cu(Bpy)(AcP), and Cu(Phen)(AcP) systems is very close to 75% in all three cases. For the corresponding systems with AnP²⁻ it is shown that increasing amounts of 1,4-dioxane added to aqueous solutions favour the formation of the six-membered chelates in both the binary and the ternary complexes. It is concluded with regard to biological systems that such six-membered chelates will also be formed in mixed ligand complexes of other metal ions and that their formation degree will also be favoured by a reduced solvent polarity; both points are relevant for the situation in active-site cavities of enzymes.