A 31P-NMR study of the H+-MoO42−-(HP)O32−-HPO42−-(C6H5P)O32−-(CH3P)O32− system at low Motot/Ptot ratio - Formation of mixed-hetero X2M5-type polyanions

Abstract

Formation of X2M5-type heteropolyanions in the aqueous mixed-hetero H⁺-MoO₄²⁻-(HP)O₃²⁻-HPO₄²⁻-(C₆H₅P)O₃²⁻-(CH₃P)O₃²⁻ system has been studied by ³¹P-NMR at [Mo]_tot/[P_all]_tot = 1.5 at 298(1) K in 0.600 M Na(Cl) ionic medium. The -log[H⁺] range 1.4–6.2 has been covered. The pK_a values of methylphosphonate were determined from ³¹P-NMR chemical shift data in the range of 0 < -log[H⁺] < 13.2 as H(CH₃P)O₃⁻ 7.30 and H₂(CH₃P)O₃ 2.05. All four-component H⁺-MoO₄²⁻-P1-P2 subsystems (P1 and P2 are two of the four phosphorus-containing components), except H⁺-MoO₄²⁻-(HP)O₃²⁻-HPO₄²⁻, which has been studied previously, were examined instead of the full six-component system in order to simplify the evaluation of the NMR spectra. The concentrations were adjusted to [Mo]_tot = 60 mM and [P1]_tot = [P2]_tot = 20 mM in the measurements. All conceivable P2M5 and (P1)(P2)M5 heteropolyanions were identified. The following novel X2M5-type polyanions were found and their formation constants are given in logarithmic values with 3σ in parentheses: (CH₃P)₂Mo₅O₂₁⁴⁻ 69.68(7), (HP)(C₆H₅P)Mo₅O₂₁⁴⁻ 67.09(4), (C₆H₅P)PMo₅O₂₂⁵⁻ 63.23(5), H(C₆H₅P)PMo₅O₂₂⁴⁻ 68.11(4), (HP)(CH₃P)Mo₅O₂₁⁴⁻ 67.85(4), (CH₃P)PMo₅O₂₂⁵⁻ 64.14(7), H(CH₃P)PMo₅O₂₂⁴⁻ 68.89(4) and (C₆H₅P)(CH₃P)Mo₅O₂₁⁴⁻ 69.18(4). The equilibrium conditions are illustrated in distribution diagrams. Phosphite proved to bind more weakly in X2M5-type polyoxometallates than phosphate or phosphonates, which are almost equally favoured in complexation.