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 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).

Capillary Zone Electrophoresis as a Tool to Monitor the Stability and Hydrolytic Equilibria of Strandberg‐Type Polyoxometalates in Aqueous Solution

“…POM chemistry is often plagued by the stabilities and the hydrolysis equilibria of polyoxoanions in aqueous solutions. Many POMs can decompose into smaller fragments or undergo multiple condensation–hydrolysis equilibria, which depend on the properties of the aqueous medium such as concentration, pH, counterion, and so on . The rapid and complex hydrolysis equilibria in solution cast ambiguity on the solution structures of POMs, especially in catalytic and medicinal systems.…”

Base Catalysis by Mono- and Polyoxometalates

“…Since [103,104]. These protonated sites play an important role in the activation of oxidants and the catalytic oxidation abilities are strongly dependent on the protonation states [95][96][97][98][99][100]102,103].…”

Synthesis, X-ray Crystal Structure, and Photochromism of a Sandwich-Type Mono-Aluminum Complex Composed of Two Tri-Lacunary α-Dawson-Type Polyoxotungstates

“…Here, the molecular structure of α-Dawson-type tri-aluminum-substituted polyoxotungstate used as the starting complex changed as it passed through the ion exchange resin (H + -form). In general, ion exchange resins are used to obtain the free-acid type of polyoxometalates by the exchange of counter cations (including alkali metal ions and organic ammonium ions) with protons [27,28]. However, this treatment of α-Dawson-type tri-aluminum-substituted polyoxotungstate with ion exchange resin caused the cation exchange reaction and the molecular structure transformation of a monomeric, α-Dawson-type tri-aluminum-substituted polyoxotungstate to a dimeric, α-Dawson-type mono-aluminum-substituted polyoxotungstate.…”