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Issue 12, 2003
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Equilibria and dynamics of some aqueous peroxomolybdophosphate catalysts: a potentiometric and 31P NMR spectroscopic study

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Abstract

The equilibrium speciation in the p H+ + q MoO42− + r H2O2 + s H2PO4 + t SO42− ⇌ (H+)p(MoO42−)q(H2O2)r(H2PO4)s(SO42−)tp  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 MoqXrPsStp  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 β0,1,2,1,0 = 5.16 ± 0.09, log β1,1,2,1,0 = 12.73 ± 0.02 (pKa = 7.63), log β2,1,2,1,0 = 16.14 ± 0.03 (pKa = 3.42), log β2,2,4,1,0 = 25.03 (± 0.04), log β3,2,4,1,0 = 29.54 ± 0.02 (pKa = 4.51), log β4,3,6,1,0 = 42.30 ± 0.03, log β5,3,6,1,0 = 44.06 ± 0.08 (pKa = 1.76), log β6,4,8,1,0 = 57.30 ± 0.07. pKa 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.

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

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Article information


Submitted
17 Jan 2003
Accepted
15 Apr 2003
First published
08 May 2003

Dalton Trans., 2003, 2512-2518
Article type
Paper

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

F. Taube, I. Andersson, S. Angus-Dunne, A. Bodor, I. Tóth and L. Pettersson, Dalton Trans., 2003, 2512
DOI: 10.1039/B300702B

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