Syntheses and Molecular Structures of Monomeric and Hydrogen-Bonded Dimeric Dawson-Type Trialuminum-Substituted Polyoxotungstates Derived under Acidic and Basic Conditions

The syntheses and molecular structures of the two types of α-Dawson-type trialuminum-substituted polyoxometalates, [B-α-H3P2W15O59{Al­(OH2)}3]6– (1) and [B-α-H3P2W15O59{Al­(OH)}2{Al­(OH2)}]216– (2), are described herein. The potassium and cesium salts of 1, K6[B-α-H3P2W15O59{Al­(OH2)}3]·14H2O (K-1), and Cs6[B-α-H3P2W15O59{Al­(OH2)}3]·13H2O (Cs-1) were formed by a stoichiometric reaction in water of trilacunary α-Dawson polyoxotungstate with aluminum nitrate under acidic conditions (pH ∼3). The potassium/sodium and tetramethylammonium/sodium salts of 2, K14Na2[B-α-H3P2W15O59{Al­(OH)}2{Al­(OH2)}]2·30H2O (KNa-2) and [(CH3)4N]14Na2[B-α-H3P2W15O59{Al­(OH)}2{Al­(OH2)}]2·39H2O (TMANa-2) were obtained under basic conditions (pH ∼9). These compounds were characterized by X-ray structure analyses, elemental analyses, thermogravimetric/differential thermal analyses, Fourier transform infrared, and solution 31P, 27Al, and 183W NMR spectroscopy. The polyoxoanion 1 is a monomeric, α-Dawson-type structure, resulting in an overall C3v symmetry, while the polyoxoanion 2 is a hydrogen-bonded dimeric structure, resulting in an overall S3 symmetry in the solid state. The pH dependence of polyoxoanions 1 and 2 in aqueous solution was also investigated by 31P NMR spectroscopy.