Syntheses, molecular structures and pH-dependent monomer–dimer equilibria of Dawson α2-monotitanium(iv)-substituted polyoxometalates

Abstract

The preparation and structural characterization of a novel, solid Brønsted acid based on Dawson α₂-monotitanium(IV)-substituted polyoxometalates (POMs) are described. The free-acid form of the POM, i.e., 13H⁺-heteropolyacid with the formula H₁₃[1b]·55H₂O DH-1 (1b = [(α₂-P₂W₁₇TiO₆₁)(α₂-P₂W₁₇TiO₆₁H)(μ-O)]¹³⁻), was prepared by passing an aqueous solution containing a potassium salt precursor, K₁₄[1a]·17H₂O DK-1 (1a = [(α₂-P₂W₁₇TiO₆₁)₂(μ-O)]¹⁴⁻), through a cation-exchange resin column. Compound DK-1 was obtained by a stoichiometric reaction of mono-lacunary Dawson POM [α₂-P₂W₁₇O₆₁]¹⁰⁻ with Ti(SO₄)₂ in an aqueous solution. [Note: the abbreviations D, M, K and H stand for dimer, monomer, potassium salt and free-acid form, respectively.] Compounds DK-1 and DH-1 were characterized by using complete elemental analysis, thermogravimetric (TG) and differential thermal analysis (DTA), FTIR, solution (³¹P and ¹⁸³W) NMR spectroscopy, pH-varied ³¹P NMR spectroscopy, solid-state ³¹P CPMAS NMR, X-ray crystallography and acidity measurements in an organic solvent with a Hammett indicator for DH-1. The monomeric form, K₈[α₂-P₂W₁₇TiO₆₂]·18H₂O MK-1, was derived from DK-1. The molecular structure of 1b was composed of a dimer connected through one Ti–O–Ti bond between two α₂-mono-Ti^IV-substituted Dawson POM subunits. BVS (bond valence sum) calculation showed that one oxygen atom (O(60A)) in one of the two Dawson subunits was protonated, therefore the two subunits were unequivalent. On the other hand, the molecular structure of 1a was a Ti–O–Ti bonding dimer of two equivalent Dawson subunits. The pH-varied ³¹P NMR spectra of DK-1 and DH-1 in aqueous solutions showed that the monomer at pH 7.0, the dimer at pH 1.0–3.0, and the mono-protonated species of the dimer at pH 0.5 were the predominant species in the solutions. The Hammett acidity constant (H₀) of DH-1 in CH₃CN (−2.87) was estimated to be almost the same as that of homo-Dawson heteropolyacid H₆[P₂W₁₈O₆₂]·17H₂O (−2.77).