Date Published: November 01, 2018
Publisher: International Union of Crystallography
Author(s): Nadiia I. Gumerova, Anatolie Dobrov, Alexander Roller, Annette Rompel.
Two new isopolytungstates, Na5Fe2.5[W12O40(OH)2]·36H2O and Na4Cu3[W12O40(OH)2]·28H2O, have been prepared and structurally characterized. The compounds exhibit a three-dimensional structure, in which the paratungstate anions coordinate to FeII or CuII ions in a polydentate mode.
The structural diversity of polyoxometalates (Pope, 1983 ▸) and their proven applications in catalysis (Wang & Yang, 2015 ▸), nanotechnology (Yamase & Pope, 2002 ▸), electrochemistry (Sadakane & Steckhan, 1998 ▸), materials science (Proust et al., 2008 ▸), molecular magnetism (Clemente-Juan et al., 2012 ▸), macromolecular crystallography (Bijelic & Rompel, 2015 ▸, 2017 ▸; Molitor et al., 2017 ▸) and medicine (Fu et al., 2015 ▸; Bijelic et al., 2018a ▸,b ▸) have encouraged the synthesis of novel polyanions with promising properties. One of the most common isopolytungstates (IPOTs) is paratungstate B, built of the [W12O40(OH)2]10− anion that is stable in aqueous acidic solution and exhibits a cluster-like construction of 12 W-centred distorted octahedra (Evans & Rollins, 1976 ▸; Pope, 1983 ▸). Due to a high surface charge density, the paratungstate anion acts as a multidentate ligand, which can coordinate alkaline (Peresypkina et al., 2014 ▸) and transition-metal cations (Radio et al., 2010 ▸, 2011 ▸; Gumerova et al., 2015 ▸), and also act as a precursor (Sokolov et al., 2012 ▸). By coordinating transition-metal cations, paratungstates can form high-dimensional extended structures, which exhibit unique catalytic (He et al., 2008 ▸; Chen et al., 2017 ▸) and magnetic properties (Li et al., 2008 ▸, 2009 ▸). So far, three paratungstates B with FeII and nine with CuII as counter-cations have been successfully synthesized and characterized by X-ray diffraction (Table 1 ▸). We present herein two novel paratungstates B, one with FeII and one with CuII, namely the double sodium–iron(II) paratungstate Na5Fe2.5[W12O40(OH)2]·36H2O (denoted Na5Fe2.5paraB) and the double sodium–copper(II) paratungstate Na4Cu3[W12O40(OH)2]·28H2O (denoted Na4Cu3paraB), which were synthesized by a convenient aqueous solution method.
The syntheses of Na5Fe2.5paraB and Na4Cu3paraB were carried out with WVI-to-MII ratios of W:Fe = 12:2 and W:Cu = 12:1.5, and a pH of 2.5 for Na5Fe2.5paraB and 4.2 for Na4Cu3paraB, which are different from previously reported conditions (Table 1 ▸) and made it possible to obtain compounds with new Fe–Na and Cu–Na compositions. The presence of NaI as counter-cation in paratungstates B, together with CuII or FeII, have been observed previously both in excess and in deficiency of the transition-metal ion in the reaction mixture, which had a pH in the range 3.5–6.5 (Table 1 ▸). This allows one to conclude that crystallization of paratungstates B as double-alkali–transition-metal salts is more preferable than crystallization of pure transition-metal paratungstates B, regardless of the starting molar ratios of the components and the pH of the reaction system.