Complexing ability of alkali metal and alkaline earth metal ions with organic phosphinate or phosphates in acetonitrile and binary solvents with protic solvents

- The coordination ability of alkali metal and alkaline earth metal ions with phosphates has been confirmed.
- The (reverse) coordination constants are evaluated in MeCN with protic solvents.
- The interaction increases in the order of Na+ < Li+ and Ba2 + < Ca2 + < Mg2 +.
- The concrete evidence in hydrolysis of phosphates and Mg2 + or Ca2 + in biochemical systems.

In acetonitrile (MeCN), the specific interactions of alkali metal (M+ = Li+ or Na+) and alkaline earth metal ions (M2 + = Mg2 +, Ca2 +, or Ba2 +) with various phosphorus anions, L−, i.e., diphenylphosphinate, diphenylphosphate, and bis(4-nitrophenyl)phosphate, have been examined by means of UV-visible spectroscopy. The formation of “reverse-coordinated” or coordinated species, M2L+ or ML+, has been observed in the presence of excess amounts of the metal ions to the anions. Between all the M+ or M2 + ions and 1.0 × 10− 3 mol dm− 3 diphenylphosphinate ion (n-Bu4N+ Ph2PO2−), both the precipitation of the non-charged species (ML or ML2) and the successive re-dissolution of the precipitates take place. The addition of the alkaline earth metal ions of just the equi-molar to L− causes almost complete dissolution of the precipitates through the soluble ML+ coordinated species. As for the diphenylphosphate ion [n-Bu4N+(PhO)2PO2−], no apparent interaction can be insisted between the alkali metal ions or Mg2 + and 5.0 × 10− 4 mol dm− 3 diphenylphosphate ion, based on just no precipitation occurrence. Only Na+ and Ba2 + can cause obvious precipitation with 5.0 × 10− 4 mol dm− 3 bis(4-nitrophenyl)phosphate. A good evidence, however, has been provided by the conductometric titration of 5.0 × 10− 4 mol dm− 3n-Bu4N+(PhO)2PO2− with LiClO4 or Mg(ClO4)2 in MeCN that the “strong” interaction still operate between L− and Li+ or Mg2 + regardless of no precipitation (nor the successive re-dissolution). The addition of protic solvents, such as water, MeOH, or EtOH, influences significantly the chemical interaction between the metal ions and the anions in MeCN. The solubility products (Ksp) and the “reverse coordination” or coordination constants (K2 = [M2L+]/([M+]2[L−], K1 = [ML+]/([M2 +] [L−]) have been evaluated for the systems.

The coordination ability of alkali metal (M+) and alkaline earth metal (M2 +) ions with diphenylphosphinate, diphenylphosphate, and bis(4-nitrophenyl)phosphate ions has been confirmed not only in sole MeCN but also in the binary solvents containing H2O, MeOH, or EtOH.54