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

Two new isopolytungstates, Na₅Fe_2.5[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 Fe^II or Cu^II ions in a polydentate mode.

Abstract

Paratungstate B is a common isopolytungstate (IPOT) built of the [W₁₂O₄₀(OH)₂]¹⁰⁻ anion and exhibits a cluster-like construction of 12 W-centred distorted octa­hedra. Due to a high surface charge density, the paratungstate anion acts as a multidentate ligand forming high-dimensional extended structures, which exhibit unique catalytic and magnetic properties. Two new paradodeca­tungstate B compounds decorated by iron(II) or cop­per(II), namely Na₅Fe_2.5[W₁₂O₄₀(OH)₂]·36H₂O (Na₅Fe_2.5paraB) and Na₄Cu₃[W₁₂O₄₀(OH)₂]·28H₂O (Na₄Cu₃paraB), have been synthesized by a convenient aqueous solution method, and structurally characterized by single-crystal and powder X-ray diffraction, IR spectroscopy, elemental analysis and thermogravimetric analysis. Both compounds crystallize in the triclinic P space group. In both compounds, the [W₁₂O₄₀(OH)₂]¹⁰⁻ polyanion acts as a multidentate ligand that links transition-metal and sodium cations, forming a three-dimensional framework.

Introduction  

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 Fe^II and nine with Cu^II as counter-cations have been successfully synthesized and characterized by X-ray diffraction (Table 1 ▸). We present herein two novel paratungstates B, one with Fe^II and one with Cu^II, namely the double sodium–iron(II) paratungstate Na₅Fe_2.5[W₁₂O₄₀(OH)₂]·36H₂O (denoted Na₅Fe_2.5paraB) and the double sodium–copper(II) paratungstate Na₄Cu₃[W₁₂O₄₀(OH)₂]·28H₂O (denoted Na₄Cu₃paraB), which were synthesized by a convenient aqueous solution method.

Table 1. Fe^II- and Cu^II-containing paratungstates B [based on the Inorganic Crystal Structure Database (FIZ, Karlsruhe; http://www.fiz-informationsdienste.de/DB/icsd/www-recherche.html) and the Cambridge Structural Database (CSD; Groom et al., 2016 ▸)].

Compounds	Unit-cell parameters a, b and c (Å), and α, β and γ (°)	Volume (Å3), Z and space group	Synthesis details (source of W; W:M II ratio, with M = Fe, Cu; pH)	Reference
FeII
K6[{Fe(H2O)4}2(H2W12O42)]·15H2O	14.9967 (5), 10.3872 (3), 18.8237 (6); 90, 93.407 (1), 90	2927.1 (2), 2, P21/n	K2WO4; 12:1.4; –	Yang et al. (2003 ▸)
(H3O)2[{Fe(H2O)4Fe(H2O)3}2(H2W12O42)]·20H2O	12.1794 (4), 22.4938 (4), 11.6941 (3); 90, 105.731 (2), 90	3083.7 (1), 2, P21/c	Li2WO4; 12:1.4; –	Yang et al. (2003 ▸)
Na5[{Fe(H2O)3}2{Fe(H2O)4}0.5(H2W12O42)]·30H2O	12.121 (2), 12.426 (3), 13.247 (3); 68.33 (3), 71.33 (3), 71.44 (3)	1710.7 (6), 1, P	Na2WO4; 12:1.4; –	Yang et al. (2003 ▸)
CuII
Na8[Cu(H2O)2(H2W12O42)]·30H2O	13.081 (4), 13.160 (6), 20.127 (6); 78.294 (12), 78.524 (11), 72.593 (11)	3201.7 (17), 2, P	Na2WO4; 12:2.4; 4.8	Li et al. (2008 ▸)
KNa3[Cu(H2O)2{Cu(H2O)3}2(H2W12O42)]·16H2O	10.799 (2), 11.914 (2), 13.377 (3); 70.18 (3), 68.07 (3), 64.80 (3)	1410.9 (5), 1, P	Na2[W12O40(OH)2]; 12:2; 3.5	Li et al. (2009 ▸)
[{Na2(μ-H2O)2(H2O)6}{Cu(H2O)2}{Cu(H2O)4}2{Cu2(μ-OH)2(H2O)6}(H2W12O42)]·10H2O	10.697 (5), 12.921 (5), 13.653 (5); 73.608 (5), 75.671 (5), 67.748 (5)	1654.4 (12), 1, P	(NH4)6[W12O40]; 12:0.4; 6.2	Kong et al. (2010 ▸)
[{Na(H2O)4}2{Cu0.5(H2O)}4{Cu0.5(H2O)1.5}2(H4W12O42)]·3H2O	10.7060 (11), 12.7124 (14), 13.1664 (14); 113.7600 (10), 90.8230 (10), 111.8290 (10)	1493.8 (3), 1, P	Na2WO4; 12:3; 6.5	Gao et al. (2011 ▸)
[Na2(H2O)10][Cu4(H2O)12(H2W12O42)]·15H2O	10.1535 (2), 13.2118 (3), 13.7049 (5); 112.692 (3), 94.771 (3), 102.969 (2)	1623.15 (8), 1, P	Na2WO4; 12:36; 4	Qu et al. (2012 ▸)
Cu3(H2O)8[H6W12O42]	10.6753 (5), 12.7814 (5), 13.0976 (5); 113.737 (4), 90.433 (3), 112.560 (4)	1482.73 (12), 2, P	(NH4)6[W12O40]; 12:36; –	Chen et al. (2017 ▸)
(NH4)8[Cu(H2O)2H2W12O42]·10H2O	14.278 (5), 15.435 (5), 24.881 (5); 90, 90, 90	5483 (3), 2, Pbcn	(NH4)6[W12O40]; 12:2.5; 4.8	Zhang (2012 ▸)
Na2Cu3(CuOH)2[W12O40(OH)2]·32H2O	10.6836 (4), 12.9066 (6), 13.6475 (5); 73.561 (4), 75.685 (3), 67.666 (4)	1648.68 (12), 1, P	Na2WO4; 12:7.5; –	Radio et al. (2014 ▸)
Na2Cu5(H2O)24(OH)2[H2W12O42]·10H2O	10.7140 (8), 12.9476 (9), 13.6696 (10); 73.56, 75.73, 67.69	1661.8 (2), 1, P	Na2WO4; 12:20; 3.8	Qu et al. (2015) ▸

Experimental  

Synthesis and crystallization  

The reagents were used as purchased from Sigma–Aldrich without further purification.

Synthesis of Na₅Fe_2.5paraB  

Iron powder (0.112 g, 2 mmol) was added to a solution (15 ml) of Na₂WO₄·2H₂O (3.96 g, 12 mmol), which was acidified to pH = 2.5 with HCl (1 M). The mixture was stirred in an ultrasonic bath, giving a deep-blue solution, which was left to stand closed at room temperature. The pale-red–brown crystals which grew on the beaker walls were collected after three weeks (yield ∼2 g, ∼53%, based on W). Elemental analysis found (calculated) for Fe_2.5H₇₄Na₅O₇₈W₁₂ (%): Na 3.13 (3.03), Fe 3.71 (3.69), W 56.8 (58.32).

Synthesis of Na₄Cu₃paraB  

Sodium orthotungstate Na₂WO₄·2H₂O (5.5 g, 16.7 mmol) was dissolved in water (25 ml) and the pH was adjusted to 8 by adding dilute HNO₃ (1 M). An aqueous solution (10 ml) of Cu(NO₃)₂·3H₂O (0.5 g, 2.1 mmol) was then added dropwise, while the pH was maintained between 3.0 and 4.5 with HNO₃ (1 M). The final mixture was filtered (pH = 4.2) and allowed to stand closed at room temperature. Light-blue crystals formed within two months (yield ∼3.5 g, 69% based on W). Elemental analysis found (calculated) for Cu₃H₅₈Na₄O₇₀W₁₂ (%): Na 2.62 (2.51), Cu 5.13 (5.20), W 59.1 (60.16).

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⁻¹.

TGA measurements  

Thermogravimetric analysis (TGA) was performed on a Mettler SDTA851e Thermogravimetric Analyzer under a nitro­gen flow with a heating rate of 5 K min⁻¹ 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% HNO₃. Standards were prepared from single-element standard solutions of concentration 1000 mg l⁻¹ (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 Kα radiation (λ = 1.54056 Å), a Lynxeye silicon strip detector, a SolX energy dispersive detector and a variable slit aperture of 12 mm. The 2θ 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.

Table 2. Experimental details.

	Na4Cu3paraBM	Na5Fe2.5paraB
Crystal data
Chemical formula	Na4Cu3[W12O40(OH)2]·28H2O	Na5Fe2.5[W12O40(OH)2]·36H2O
M r	3621.13	3771.27
Crystal system, space group	Triclinic, P	Triclinic, P
Temperature (K)	100	100
a, b, c (Å)	10.6516 (5), 12.7532 (6), 13.0730 (5)	12.3758 (6), 14.7752 (7), 18.8919 (8)
α, β, γ (°)	113.771 (1), 90.443 (1), 112.502 (1)	92.9341 (14), 100.6938 (14), 94.1698 (15)
V (Å3)	1473.65 (11)	3378.1 (3)
Z	1	2
Radiation type	Mo Kα	Mo Kα
μ (mm−1)	24.53	21.02
Crystal size (mm)	0.13 × 0.07 × 0.02	0.37 × 0.07 × 0.04
Data collection
Diffractometer	Bruker APEXII CCD	Bruker D8 Venture
Absorption correction	Multi-scan (SADABS; Bruker, 2016 ▸)	Multi-scan (SADABS; Bruker, 2016 ▸)
T min, T max	0.004, 0.023	0.012, 0.044
No. of measured, independent and observed [I > 2σ(I)] reflections	11292, 5321, 4720	38068, 12318, 10876
R int	0.048	0.032
(sin θ/λ)max (Å−1)	0.602	0.602
Refinement
R[F 2 > 2σ(F 2)], wR(F 2), S	0.035, 0.098, 1.05	0.022, 0.058, 1.09
No. of reflections	5321	12318
No. of parameters	463	980
No. of restraints	100	405
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement	H atoms treated by a mixture of independent and constrained refinement
Δρmax, Δρmin (e Å−3)	2.15, −1.63	1.57, −1.25

Computer programs: APEX3 (Bruker, 2015 ▸), SAINT (Bruker, 2015 ▸), SHELXS97 (Sheldrick, 2008 ▸), shelXle (Hübschle et al., 2011 ▸), SHELXL2014 (Sheldrick, 2015 ▸), SHELXL2016 (Sheldrick, 2015 ▸), OLEX2 (Dolomanov et al., 2009 ▸) and DIAMOND (Brandenburg, 2006 ▸).

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—H⋯H—D interactions. To force correct bonds, specified bonds were added to or removed from the connectivity list.

The disordered water molecules in the coordination spheres of atom Na1 in Na₄Cu₃paraB and of atoms Na4 and Na5 in Na₅Fe_2.5paraB were refined with two positions with fixed occupancy factors of 0.5.

In Na₄Cu₃paraB, 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.

The structures have been deposited with the Inorganic Crystal Structure Database (ICSD) (http://www2.fiz-karlsruhe.de/icsd_home.html) under collection numbers 434558 and 434559.

Results and discussion  

The syntheses of Na₅Fe_2.5paraB and Na₄Cu₃paraB 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 Na₅Fe_2.5paraB 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 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.

The main structural elements of Na₅Fe_2.5paraB and Na₄Cu₃paraB are shown in Fig. 1 ▸. Both compounds consist of paradodeca­tungstate B [W₁₂O₄₀(OH)₂]¹⁰⁻ polyanions (Evans & Rollins, 1976 ▸; Pope, 1983 ▸), sodium and transition-metal cations, and additional water mol­ecules (Fig. 1 ▸). The paratungstate B units observed in Na₅Fe_2.5paraB and Na₄Cu₃­paraB are structurally identical to previously reported units (Table 1 ▸).

Figure 1.

Structural elements in Na₅Fe_2.5paraB and Na₄Cu₃paraB. (a) The [W₁₂O₄₀(OH)₂]¹⁰⁻ anion in Na₅Fe_2.5paraB connected to four Na⁺ and six Fe²⁺ ions via terminal O atoms. (b) A fragment of the infinite 1D chain in Na₅Fe_2.5paraB consisting of Na and Fe polyhedra. (c) The [W₁₂O₄₀(OH)₂]¹⁰⁻ anion in Na₄Cu₃paraB connected to six Na⁺ and six Cu²⁺ ions via terminal O atoms. (d) A fragment of the infinite 1D chain in Na₄Cu₃paraB consisting of Na and Cu polyhedra. Colour code: {WO₆} are light-blue or violet octa­hedra, {W₃O₁₄} are blue octa­hedra and {W₃O₁₃} are violet octa­hedra, and Na atoms are green, Fe orange, Cu blue and O red.

In Na₄Cu₃­paraB, there is one-half unit of the POM, which lies on an inversion centre, in the asymmetric unit. For Na₅Fe_2.5paraB, there are two independent half-POM units in the asymmetric unit.

The centrosymmetric [W₁₂O₄₀(OH)₂]¹⁰⁻ anion consists of four corner-sharing groups: two {W₃O₁₃} (violet octa­hedra in Figs. 1 ▸ a and 1c) and two {W₃O₁₄} (blue octa­hedra in Figs. 1 ▸ a and 1c) units. Each {W₃O₁₃} fragment is formed by three edge-sharing {WO₆} octa­hedra with a common O atom, while in the {W₃O₁₄} triads, the three edge-sharing {WO₆} octa­hedra are linearly connected with no common O atom to the three W atoms. In the {W₃O₁₃} groups, each octa­hedron has one terminal O atom, while in the {W₃O₁₄} units, each octa­hedron has two unshared O atoms (Figs. 1 ▸ a and 1c). The O atoms connected to the W centres can be classified into three groups. The first group is comprised of terminal O atoms (O_t), each bonded to one W atom. The second group consists of bridging O atoms (O_db), each connected to two W atoms. There are two types of O_db, one bridges two W atoms within the same {W₃O₁₃} or {W₃O₁₄} fragment (O_db1), while the other bridges two W atoms between the different {W₃O₁₃} and {W₃O₁₄} units (O_db2). The third group contains triply bridging O atoms, linked by three W atoms. The triply bridging O atoms exclusively from {W₃O₁₃} are labelled O_tb1, whereas the O atoms bridging three W atoms between {W₃O₁₃} and {W₃O₁₄} units are labelled as O_tb2.

The exact positions of the two protons in [W₁₂O₄₀(OH)₂]¹⁰⁻ were located previously on triply bridging O atoms of {W₃O₁₃} by neutron diffraction (Evans & Prince, 1983 ▸). Selected bond lengths and angles are presented in Table 3 ▸. All the W atoms in [W₁₂O₄₀(OH)₂]¹⁰⁻ exhibit the +VI oxidation state, when applying the bond valence sum (BVS) calculations of Brown & Altermatt (1985 ▸). For Na₅Fe_2.5paraB and Na₄Cu₃paraB, 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.

Table 3. Selected bond length and angles (Å, °) in Na₅Fe_2.5paraB and Na₄Cu₃paraB .

	Na5Fe2.5paraB	Na4Cu3paraB
W=Ot	1.719 (4)–1.797 (4)	1.710 (8)–1.780 (7)
W—Odb1	1.888 (4)–2.050 (2)	1.872 (7)–2.103 (7)
W—Odb2	1.826 (3)–2.166 (3)	1.805 (7)–2.098 (7)
W—Otb1	2.201 (3)–2.297 (3)	2.207 (7)–2.273 (7)
W—Otb2	1.895 (4)–2.259 (4)	1.882 (8)–2.287 (7)
W⋯W (between corner-sharing WO6)	3.649 (4)–3.878 (5)	3.377 (4)–3.688 (2)
W⋯W (between edge-sharing WO6)	3.273 (4)–3.352 (4)	3.306 (2)–3.377 (3)
M II—O (M = Fe or Cu)	2.087 (4)–2.169 (4)	1.918 (7)–2.366 (8)
NaI—O	2.302 (11)–2.606 (11)	2.345 (9)–2.519 (13)
O—W—O	70.73 (14)–104.38 (17)	154.94 (15)–177.56 (16)
	70.2 (3)–105.6 (3)	152.8 (3)–178.1 (3)

In the crystal structure of Na₅Fe_2.5paraB, the paratungstate anions act as decadentate ligands, which are linked via terminal O atoms to six Fe²⁺ and four Na⁺ cations. There are two crystallographically unique iron centres with different coordination modes (Figs. 1 ▸ a and 1b). The coordination sphere of one type of Fe²⁺ atom (Fe2) is formed by two O_t from the belt unit {W₃O₁₄} of one [W₁₂O₄₀(OH)₂]¹⁰⁻, one O_t from the capping {W₃O₁₃} group of a neighbouring polyanion and completed by three H₂O mol­ecules. The octa­hedrally coordinated second Fe²⁺ atom (Fe1) is linked by two O_t from the {W₃O₁₃} units of two neighbouring polyanions, two Na⁺ bridging O atoms and two lattice H₂O mol­ecules. The Fe1 octa­hedron and three Na(H₂O)₆ units from the infinite one-dimensional (1D) chain share a corner, thereby forming a two-dimensional sheet (2D) in the ab plane (Fig. 1 ▸ b). Neighbouring sheets are connected to each other by Fe2 cations, giving rise to a complicated three-dimensional structure (Figs. 2 ▸ and 3 ▸). It should be noted that the double sodium–iron(II) paratungstate B Na₅Fe_2.5[W₁₂O₄₀(OH)₂]·36H₂O reported in this work has the same cationic composition as reported in Na₅[{Fe(H₂O)₃}₂{Fe(H₂O)₄}_0.5(H₂W₁₂O₄₂)]·30H₂O (Yang et al., 2003 ▸) (Table 1 ▸). However, the minor difference with respect to the water content in these two structures leads to a significant change in the unit-cell parameters (Tables 1 ▸ and 2 ▸) and the motif of crystal packing (Figs. 2 ▸ and 3 ▸).

Figure 2.

The crystal packing of Na₅Fe_2.5paraB, viewed along the b axis. Colour code: {WO₆} are light-blue octa­hedra and Na atoms are green, Fe yellow and O red.

Figure 3.

The crystal packing of Na₅Fe_2.5paraB, viewed along the a axis. Colour code: {WO₆} are light-blue octa­hedra and Na atoms are green, Fe yellow and O red.

In the crystal structure of Na₄Cu₃­paraB, each paratungstate B anion is coordinated to six Cu²⁺ and six Na⁺ via O_t and therefore acts as a dodecadentate ligand (Figs. 1 ▸ c and 1d). There are three crystallographically unique copper centres with different coordination modes. Two (Cu1 and Cu2) out of three Cu²⁺ cations take part in the formation of infinite chains with alternating Na and Cu polyhedra connected by a common edge (Figs. 1 ▸ d and 4) and have different coordination environments. The Cu2 atoms are linked by four O_t atoms of the belt-fragment {W₃O₁₄} from two neighbouring [W₁₂O₄₀(OH)₂]¹⁰⁻ anions and two Na⁺ bridging H₂O mol­ecules. The coordination sphere of Cu3 consists of two O_t of the capping {W₃O₁₃} group of a neighbouring polyanion and is completed by four bridging H₂O mol­ecules. The third Cu1 atom coordinates to four O_t atoms of the belt units {W₃O₁₄} from two neighbouring [W₁₂O₄₀(OH)₂]¹⁰⁻ anions and two H₂O mol­ecules. The Cu²⁺ ions exhibit a distorted square–bipyramidal coordination geometry with elongated axial distances [2.365 (7)–2.520 (8) Å]. The three-dimensional (3D) structure of Na₄Cu₃-paraB consists of 2D sheets formed by two chains, namely {[(Na(H₂O)₂)₂W₁₂O₄₀(OH)₂]⁸⁻}_n and {[Na(H₂O)₂–Cu(H₂O)₂–Na(H₂O)₂–Cu(H₂O)₄]⁶⁺}_n parallel to the ab plane (Fig. 4 ▸). The 2D sheets are connected along the c axis by [Cu(H₂O)₄]²⁺ cations (Fig. 5 ▸). The two [Na(H₂O)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 Na₄Cu₃-paraB.

Figure 4.

The crystal packing of Na₄Cu₃paraB, viewed along the c axis. Colour code: {WO₆} are light-blue octa­hedra and Na atoms are green, Cu blue and O red.

Figure 5.

The crystal packing of Na₄Cu₃paraB, viewed along the a axis. Colour code: {WO₆} are light-blue octa­hedra and Na atoms are green, Cu blue and O red.

The results of the powder XRD patterns of Na₅Fe_2.5paraB and Na₄Cu₃paraB 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.

Figure 6.

Experimental (blue) and simulated (black) X-ray diffraction patterns of (a) Na₄Cu₃paraB and (b) Na₅Fe_2.5paraB.

In the IR spectra of Na₅Fe_2.5paraB and Na₄Cu₃paraB, the characteristic peaks at 975, 950, 932, 867, 676 and 488 cm⁻¹, and at 972, 937, 926, 872, 675 and 493 cm⁻¹, 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⁻¹ are attributed to the vibration of water mol­ecules.

The disordered water mol­ecules in Na₄Cu₃paraB 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 H₂O and water mol­ecules from coordinating Na⁺ and Cu²⁺. The second (2.72%) and third (3.61%) steps in the range 125–500 °C correspond to 13 H₂O mol­ecules coordinating Na⁺ and Cu²⁺. The total weight loss is 13.83%, which results in the formula Na₄Cu₃[W₁₂O₄₀(OH)₂]·28H₂O.

Figure 7.

Thermogravimetric curve of Na₄Cu₃paraB.

The success in synthesizing Na₅Fe_2.5paraB and Na₄Cu₃paraB shows that paratungstate B is a versatile building block, which can be modified by metal sites into high-dimensional architectures and the different connection principle of the transition metals has a big impact on the dimensionalities of the frameworks.

Supplementary Material

CCDC references: 1843518, 1843519

Funding Statement

This work was funded by Austrian Science Fund grants M2203 and P27534.