Interaction of beryllium(II) in aqueous solution with bidentate ligands containing phosphonate groups

Abstract

The complexes formed by beryllium(II) with ligands derived from phosphonoacetic acid (H 3pa) and methylenediphosphonic acid (H 4mdp) have been investigated in aqueous solution using both potentiometric and multinuclear magnetic resonance measurements. The protonation constants of the two ligands were determined at 298 K at ionic strength 0.5 mol dm −3 made up with NaClO 4 and [(CH 3) 4N]Cl. Seven phosphonoacetate complexes [Be(pa)] −, [Be(pa) 2] 4−, [Be(Hpa)], [Be(Hpa)(pa)] 3−, [Be(Hpa) 2] 2−, [Be 3(OH) 3(pa)] and [Be 3(OH) 3(pa) 3] 6− and seven methylenediphosphonate complexes, [Be(mdp)] 2−, [Be(mdp) 2] 6−, [Be(Hmdp)] −, [Be(H 2mdp)], [Be(Hmdp)(mdp)] 5−, [Be(Hmdp) 2] 4− and [Be(H 2mdp)] 2−, have been assumed to be present as major complex species. The corresponding formation constants were determined at 25°C at ionic strength 0.5 mol dm −3 (NaClO 4). In general, mdp complexes are more stable than the pa complexes having comparable stoichiometry. Phosphonate is able to displace water from [Be(H 2O) 4] 2+ without the assistance of a chelate effect, even at very low pH values. The complexes [BeL 2] n− ,(L = pa, n = 4; L = mdp, n = 6), are the most predominant species at physiological pH values; the mdp derivative is more stable than the pa one which, in turn, is much more stable than the corresponding malonate complex. The complex K 2[Be(H 2mdp) 2]·2H 2O was isolated in the solid state from reaction of Be(OH) 2 with H 4mdp and KOH, in aqueous solution, at pH 2.60 and its composition has been confirmed through ESMS spectra.