Syntheses, molecular structures and pH-dependent monomer-dimer equilibria of Dawson...

The preparation and structural characterization of a novel, solid Brønsted acid based on Dawson alpha2-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 = [(alpha2-P2W17TiO61)(alpha2-P2W17TiO61H)(mu-O)]13-), was prepared by passing an aqueous solution containing a potassium salt precursor, K14[1a].17H2O DK-1 (1a=[(alpha2-P2W17TiO61)2(mu-O)]14-), through a cation-exchange resin column. Compound DK-1 was obtained by a stoichiometric reaction of mono-lacunary Dawson POM [alpha2-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[alpha2-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 alpha2-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 la 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).