Equilibria and dynamics of some aqueous peroxomolybdophosphate catalysts: a potentiometric and 31P NMR spectroscopic study

Abstract

The equilibrium speciation in the pH⁺ + qMoO₄²⁻ + rH₂O₂ + sH₂PO₄⁻ + tSO₄²⁻ ⇌ (H⁺)_p(MoO₄²⁻)_q(H₂O₂)_r(H₂PO₄⁻)_s(SO₄²⁻)_t^{p−2q−s−2t} system in 0.300 M Na₂(SO₄) medium at 25 °C and [H₂O₂]_tot/[Mo]_tot > 2 has been determined from potentiometric data and ³¹P NMR integral and chemical shift data in the range 1.0 ≤ pH ≤ 9.0, 2.0 ≤ [MoO₄²⁻]_tot/mM ≤ 293, 32 ≤ [H₂O₂]_tot/mM ≤ 640, 2 ≤ [H₂PO₄⁻]_tot/mM ≤ 60 and 200 ≤ [SO₄²⁻]_tot/mM ≤ 380. Species with the following compositions were obtained: MoX₂P³⁻ (0,1,2,1,0), MoX₂P²⁻ (1,1,2,1,0), MoX₂P⁻ (2,1,2,1,0), Mo₂X₄P³⁻ (2,2,4,1,0), Mo₂X₄P²⁻ (3,2,4,1,0), Mo₃X₆P³⁻ (4,3,6,1,0), Mo₃X₆P²⁻ (5,3,6,1,0) and Mo₄X₈P³⁻ (6,4,8,1,0). The numbers and charges of molybdenum (Mo), peroxide (X), phosphate (P) and sulfate (S) in each species are given in the abbreviated formula Mo_qX_rP_sS_t^{p−2q−s−2t}. The numbers in parentheses refer to the values of p, q, r, s and t in the formula above. The following formation constants with 3σ were obtained; log β_0,1,2,1,0 = 5.16 ± 0.09, log β_1,1,2,1,0 = 12.73 ± 0.02 (pK_a = 7.63), log β_2,1,2,1,0 = 16.14 ± 0.03 (pK_a = 3.42), log β_2,2,4,1,0 = 25.03 (± 0.04), log β_3,2,4,1,0 = 29.54 ± 0.02 (pK_a = 4.51), log β_4,3,6,1,0 = 42.30 ± 0.03, log β_5,3,6,1,0 = 44.06 ± 0.08 (pK_a = 1.76), log β_6,4,8,1,0 = 57.30 ± 0.07. pK_a values for H₃PO₄ and H₂PO₄⁻ were determined to 2.00 and 6.48, respectively. Chemical exchange processes were detected by ³¹P NMR 2D EXSY and selective magnetisation transfer experiments between the complexes. An averaged lifetime, τ(X₂Mo–OP) ∼ 30 ms, can be estimated for the MoX₂P and Mo₂X₄P species at pH = 5.5 and 5 °C.