Iron(II) and copper(II) paratungstates B: a single-crystal X-ray diffraction study

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.


Synthesis and crystallization
The reagents were used as purchased from Sigma-Aldrich without further purification.

IR spectroscopy
The title compounds were identified by IR measurements on a Bruker Vertex70 IR Spectrometer equipped with a single-reflection diamond-ATR unit (ATR is attenuated total reflectance) in the range 4000-400 cm À1 .    nitrogen flow with a heating rate of 5 K min À1 in the region from 298 to 973 K.

Elemental analysis
Elemental analysis was conducted using inductive-coupled plasma-mass spectrometry (PerkinElmer Elan 6000 ICP MS) and atomic absorption spectroscopy (PerkinElmer 1100 Flame AAS) in aqueous solutions containing 2% HNO 3 . Standards were prepared from single-element standard solutions of concentration 1000 mg l À1 (from Merck, Ultra Scientific and Analytika Prague).

Powder X-ray diffraction
Powder X-ray diffraction (PXRD) was performed on a Bruker D8 Advance diffractometer, with Cu K radiation ( = 1.54056 Å ), a Lynxeye silicon strip detector, a SolX energy dispersive detector and a variable slit aperture of 12 mm. The 2 range was 8-50 .

Refinement
In Table 2, the crystallographic characteristics of the two new paratungstates B and the experimental conditions of the data collection and refinement are reported. The positions of the independent H atoms were obtained by difference Fourier techniques and were refined with free isotropic displacement parameters.
Fixed isotropic displacement parameters for all H atoms with a value equal to 1.5U eq of the corresponding O-H group atom were assigned. Restrained distances for D-H bonds were applied to avoid short D-HÁ Á ÁH-D interactions. To force correct bonds, specified bonds were added to or removed from the connectivity list. The crystal packing of Na 5 Fe 2.5 paraB, viewed along the b axis. Colour code: {WO 6 } are light-blue octahedra and Na atoms are green, Fe yellow and O red.

Figure 3
The crystal packing of Na 5 Fe 2.5 paraB, viewed along the a axis. Colour code: {WO 6 } are light-blue octahedra and Na atoms are green, Fe yellow and O red.
The disordered water molecules in the coordination spheres of atom Na1 in Na 4 Cu 3 paraB and of atoms Na4 and Na5 in Na 5 Fe 2.5 paraB were refined with two positions with fixed occupancy factors of 0.5.
In Na 4 Cu 3 paraB, part of the disordered water molecules were not modelled and the disordered density was considered using the OLEX2 (Dolomanov et al., 2009) implementation of BYPASS (a.k.a. SQUEEZE; Spek, 2015). The modelled electron density is consistent with approximately four water molecules per unit cell.

Results and discussion
The syntheses of Na 5 Fe 2.5 paraB and Na 4 Cu 3 paraB were carried out with W VI -to-M II ratios of W:Fe = 12:2 and W:Cu = 12:1.5, and a pH of 2.5 for Na 5 Fe 2.5 paraB and 4.2 for Na 4 Cu 3 paraB, 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 Na I as counter-cation in paratungstates B, together with Cu II or Fe II , 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.
In Na 4 Cu 3 paraB, there is one-half unit of the POM, which lies on an inversion centre, in the asymmetric unit. For Na 5 Fe 2.5 paraB, there are two independent half-POM units in the asymmetric unit.
The  The crystal packing of Na 4 Cu 3 paraB, viewed along the c axis. Colour code: {WO 6 } are light-blue octahedra and Na atoms are green, Cu blue and O red.

Table 3
Selected bond length and angles (Å , ) in Na 5 Fe 2.5 paraB and Na 4 Cu 3 paraB.  (Evans & Prince, 1983). Selected bond lengths and angles are presented in Table 3. All the W atoms in [W 12 O 40 (OH) 2 ] 10À exhibit the +VI oxidation state, when applying the bond valence sum (BVS) calculations of Brown & Altermatt (1985). For Na 5 Fe 2.5 paraB and Na 4 Cu 3 paraB, we got average values of 6.01 and 6.09, respectively. BVS calculations for Fe and Cu sites show that both ions exhibit the +II oxidation state, with a value of 2.12 for Fe and 2.08 for Cu.
In the crystal structure of Na 5 Fe 2.5 paraB, the paratungstate anions act as decadentate ligands, which are linked via terminal O atoms to six Fe 2+ and four Na + cations. There are two crystallographically unique iron centres with different coordination modes (Figs. 1a and 1b  The crystal packing of Na 4 Cu 3 paraB, viewed along the a axis. Colour code: {WO 6 } are light-blue octahedra and Na atoms are green, Cu blue and O red.

Figure 6
Experimental (blue) and simulated (black) X-ray diffraction patterns of (a) Na 4 Cu 3 paraB and (b) Na 5 Fe 2.5 paraB.  (Fig. 5). The two [Na(H 2 O) 5 ] + cations, which are connected to O t of one polyanion and do not participate in the formation of sodium-copper chains, are located in 1D tunnels in the structure of Na 4 Cu 3 -paraB.
The results of the powder XRD patterns of Na 5 Fe 2.5 paraB and Na 4 Cu 3 paraB have been investigated in the solid state at room temperature (Fig. 6). The simulated powder diffraction pattern was based on the single-crystal structural data. The simulated peak positions are in good agreement with those observed. A comparison of the experimental and simulated powder diffraction patterns confirms that the POTs structures had been solved accurately and that both products consist of a single phase.
In the IR spectra of Na 5 Fe 2.5 paraB and Na 4 Cu 3 paraB, the characteristic peaks at 975, 950, 932, 867, 676 and 488 cm À1 , and at 972, 937, 926, 872, 675 and 493 cm À1 , respectively, are attributed to the W O t and W-O-W vibrations in the paratungstate anion, which are in agreement with previously reported data (Table 1; Qu et al., 2012). The slight peak displacements are due to the effects of different coordination modes of paratungstate B. The peaks at $1600 and 3400 cm À1 are attributed to the vibration of water molecules.
The disordered water molecules in Na 4 Cu 3 paraB were treated with SQUEEZE (Spek, 2015) and the exact number of water molecules was determined by TGA. The TG curve shows a three-step weight-loss process (Fig. 7). The first weight loss of 7.48% in the temperature range 25-125 C corresponds to all lattice H 2 O and water molecules from coordinating Na + and Cu 2+ . The second (2.72%) and third (3.61%) steps in the range 125-500 C correspond to 13 H 2 O molecules coordinating Na + and Cu 2+ . The total weight loss is 13.83%, which results in the formula Na 4 Cu 3 [W 12 O 40 (OH) 2 ]Á28H 2 O.