Stability and structure of metal ion complexes formed in solution with acetyl phosphate and acetonylphosphonate: Quantification of isomeric equilibria

The acidity constants of H-2(AcP) and H-2(AnP), where AcP2- = acetyl phosph ate and AnP(2-) = acetonylphosphonate, as well as the stability constants o f the 1:1 complexes formed between Mg2+, Ca2+, Sr2+ , Ba2+, Mn2+, Co2+, Ni2 +, Cu2+, Zn2+, or Cd2+ and AcP2- or AnP(2-) were determined by potentiometr ic pH titrations in aqueous solution (25 degrees C; I = 0.1 M, NaNO3). On t he basis of previously established log K-M(R-PO3)(M) versus pK(H(R-PO3))(H) straight-line plots for phosph(on)ate ligands (R-PO32-), which only a simp le -PO32- coordination (Sigel, H.; et al. Helv. Chim. Acta 1992, 75, 2634), it is concluded that the carbonyl oxygen atom of AcP2- and AnP(2-) partici pates in complex formation by giving rise to six-membered chelates for all of the;mentioned systems except for Ba(AnP) and Sr(AnP) the stability of wh ich corresponds to a simple -PO32- coordination. The formation degree of th e chelates formed by the alkaline earth ions and AcP2- is quite pronounced; it amounts, for example, for Mg(AcP) and Ca(AcP) to 41% +/- 5% and 48% +/- 8%, respectively. The corresponding results for Zn(AcP) and Cu(AcP) are 59 % +/- 6% and 76% +/- 4%, respectively. The formation degree of the six-memb ered Cu(AnP) chelate increases in water containing 30% or 50% (v/v) 1,4-dio xane. This may also be surmised for the other divalent metal ions under con ditions of lower solvent polarity arid poorer solvating properties than wat er. Such conditions exist in active-site cavities of enzymes, and the expec ted effects are shortly discussed. The indicated-measurements With AcP have only. become possible after the stabilities of the corresponding M(HPO4) c omplexes had been determined (Saha, A.; et al. J. Biol. Inorg. Chem; 1996, 1, 231) because AcP contains as an impurity always some phosphate; :this ph osphate content had to be qualified and its effect was carefully considered : in the evaluations of the experimental data. Here is most probably the ma in reason why previously published results regarding the stabilities of the Mg(AcP) and Ca(AcP) complexes vary widely; stability constants of the othe r M(AcP) complexes have not been determined previously.