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

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 (AnP(2-)) were determined by potentiometric pH titrations in water and in water containing 30 or 50% (v/v) 1,4-dioxane (25 degrees C; I = 0.1 M, NaNO3). Previous measurements with simple phosph(on)ate ligands, R-PO32- (R being a non-interacting residue), had established log K-Cu(Arm)(R-PO3)(Cu(Arm)) versus pK(H(R-PO3))(H) 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 AnP(2-). 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 AnP(2-) 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.