Formation of nanoporous and non-porous organic–inorganic hybrid materials incorporating α-Keggin phosphotungstate anion: X-ray crystal structure of a 3D polymeric complex [{Na6(C9H5O6)3(H2O)15}{PW12O40}]∞ with a ‘Ball-in-Bowl’ type molecular structure

Abstract Two different synthetic approaches to prepare inclusion complexes of Keggin anion [PW12O40]3− and [Cu3(BTC)2(H2O)3]n (also known as HKUST-1) have resulted in the formation of two different materials [Cu3(C9H3O6)2(H2O)3]2Na3PW12O40·nH2O (1) and [Na6(BTC-H2)3(H2O)15][PW12O40] (2). Compound 1 is a thermally stable, nanoporous material with a type 1 adsorption isotherm. It contains the well-known MOF material [Cu3(BTC)2(H2O)3]n with half of its pores occupied by the guest Keggin anion. The second product 2 is a crystalline, 3D coordination polymer with a composition of [Na6(BTC-H2)3(H2O)15][PW12O40] (where BTC-H3 stands for 1,3,5-benzene-tricarboxylic acid) and is a non-porous, highly water soluble and thermally unstable material. The compound 1 was prepared hydrothermally using precursors of HKUST-1 and Na3[(PW12O40)] whereas 2 was obtained at room temperature from aqueous solutions containing [Cu3(BTC)2(H2O)3]n and Na3[(PW12O40)] salts. The molecular structure of 2 is unique and features two distinct elements: an anionic Keggin ion [PW12O40]3− and a “bowl type” hexameric [Na6(C9H5O6)3(H2O)15]3+ macrocation which is formed by coordination of Na+ ions by (BTC-H2)− anions and water molecules. The macrocation extends in three dimensions due to Y type (BTC-H2)− to form a porous MOF offering triangular voids. In this framework the globular Keggin ions support themselves above these voids by coordinating to Na+ ions and render it completely nonporous. The complex has an unusual structure in the sense that the polyoxoanion is coordinated to [Na6(C9H5O6)3(H2O)15]3+ ions by using the three terminal oxygen atoms in a single M3O13 triplet, when each one of them is also bridging between two Na+ ions. In the crystal structure the POM anion is encapsulated by six octahedrally disposed (BTC-H2)− moieties. H-bonding interactions among water molecules produce supramolecular water hexamers and dimers.