Crystal structure of hexasodium tetraserinolium paratungstate B decahydrate, [Na6{(CH2OH)2CHNH3}4][W12O40(OH)2]·10H2O

The isolation and characterization of an isopolytungstate anion [W12O40(OH)2]10– (paradodecatungstate B) with sodium and protonated serinol entities [Na6((CH2OH)2CHNH3)4]10+ as counter-cations are reported.

The title polyoxometalate-based organic-inorganic hybrid compound, [Na 6 -(C 3 H 10 NO 2 ) 4 ][W 12 O 40 (OH) 2 ]Á10H 2 O, consists of a di-3 -hydroxido-tetra-3oxido-octadeca--oxido-octadecaoxidododecatungstate (paradodecatungstate B) anion, [W 12 O 40 (OH) 2 ] 10-, and six sodium cations coordinated by the oxygen ions of the polyanions, serinol ligands protonated at the N atom, and water molecules. The centrosymmetric paratungstate B anion shows characteristic features in terms of bond lengths and angles. The three-dimensional framework structure is established by bonding of the sodium cations with oxygen ions of the paratungstate B anions and a network consisting of N-HÁ Á ÁO and O-HÁ Á ÁO hydrogen bonds of medium strength between the protonated serinol cations, water molecules and the paratungstate B anions. The title compound was also characterized by means of elemental analysis, IR spectroscopy and thermogravimetric analysis.

Chemical context
Polyoxometalates (POMs) are discrete anionic molecular clusters of metal oxide entities, which usually consist of transition metals of groups V and VI in their highest oxidation states. POMs exist at a unique interface between monomeric oxometalates and polymeric metal oxides and have a wide range of applications (Pope, 1983;Gumerova & Rompel, 2020). To date, a variety of strategies have been developed and used to build POM-based hybrid materials by varying the reaction conditions such as the type of addenda ions, organic ligands, pH, solvents, the molar ratio of the starting materials or the reaction environments. The [W 12 O 40 (OH) 2 ] 10paratungstate B anion is stable in aqueous acidic solution and has a cluster-like structure of twelve W-centered distorted octahedra {WO 6 } (Evans & Rollins, 1976;Pope, 1983). Due to its high surface charge density q/m = 0.83 (q = net charge, m = number of metal ions), the paratungstate B anion can act as a multidentate ligand for alkaline (Peresypkina et al., 2014) or transition-metal cations (Radio et al., 2010(Radio et al., , 2011Gumerova et al., 2015Gumerova et al., , 2018 and as a precursor for the synthesis of catalytically active sandwich-type polyoxotungstates (POTs) (Sokolov et al., 2012).
A search in the Cambridge Structural Database (version 5.42, update of November 2021;Groom et al., 2016) revealed that seventeen organic-inorganic hybrid paratungstates B have been structurally characterized so far. We are expanding ISSN 2056-9890 the class of hybrid paratungstates by using serinol (C 3 H 9 NO 2 ; 2-amino-1,3-propandiol), which has not previously been used in its protonated form as a counter-cation for paratungstates and can coordinate to metal cations in different ways via its -NH 2 or HOCH 2 -groups and thus influences both the structure and the properties of the compound significantly (Sifaki et al., 2021). Serinol is a very stable, readily watersoluble, non-toxic, odorless, biodegradable compound that is used as a versatile starting material in organic synthesis and as an additive for material applications, such as composite materials (Barbera et al., 2020;Andreessen & Steinbü chel, 2011). In POM synthesis, due to its amino group, serinol can be regarded as an alkoxylation ligand and/or as a buffer compound (pK a = 12.2; Chemicalbook, 2021). With its protonated amine group it can also act as a counter-cation.

Supramolecular features
The paratungstate B anion is bound to twelve Na + cations via both terminal (O t ) and bridging oxygen atoms (O db ). Each of the surrounding four Na1 cations and six Na3 cations are coordinated by the O atoms of two polyanions, while two Na2 cations are bound to one terminal oxygen atom of the polyanion and to the O atoms of two serinolium cations, which are further connected to the Na3 cations via the HOCH 2 -groups. Thus, a three-dimensional framework is established in the crystal structure by connecting paratungstate B anions through Ot-Na1-Ot, Ot-Na3-Ot and Ot-Na2-serinolium-Na3-Ot bridges (Fig. 2). An intimate network of N-HÁ Á ÁO and O-HÁ Á ÁO hydrogen bonds of medium strength between the protonated serinol ligands, polyoxoanions and water molecules consolidates the crystal packing (Table 1)  The principal building units in the crystal structure of [Na 6 ((CH 2 OH) 2 CHNH 3 ) 4 ][W 12 O 40 (OH) 2 ]Á10H 2 O. The asymmetric unit was doubled considering the inversion center, and coordination spheres for all cations were completed. Labeling of all atoms of asymmetric unit is shown; non-labeled atoms are generated by symmetry operation Àx + 1, Ày + 1, Àz + 1. The oxygen atoms in the paratungstate B anion are labeled according to their coordination mode: magenta for the terminal, blue for the double bridging, green for the triply bridging oxygen ions. Displacement ellipsoids are drawn at the 50% probability level. Color code: W, dark gray; Na, green; C, gray; N, blue; O, red.

Synthesis and crystallization
To obtain the title compound, Na 2 WO 4 Á2H 2 O (0.495 g, 1.5 mmol) was dissolved in 5 ml of distilled water and acidified with 1 M HCl to pH 3. Serinol (0.075 g, 0.8 mmol) was then added to the acidified orthotungstate solution, which increased the pH to 6.7. The reaction mixture was then heated to 363 K and stirred for 1 h, cooled to room temperature and left covered with parafilm. Colorless block-shaped crystals were filtered off after one week from the mother liquor, washed with water and ethanol and then air-dried (yield 0.12 g; 27%, based on W

Figure 2
The crystal packing of [Na 6 ((CH 2 OH) 2 CHNH 3 )   water molecules (O22, O23, O24, O28) were obtained by difference-Fourier techniques and were refined with free isotropic displacement parameters and O-H distances restrained to 0.95 (2) Å . The disordered water molecule (O30) was refined with two positions (O30A and O30B), with free occupancy factors to a total of 100%. H atoms bound to N or C atoms were placed in idealized positions (N-H = 0.91 Å and C-H = 0.99 or 1.00 Å for CH 2 and CH groups, respectively) and refined in riding modes, with U iso (H) values set to 1.5U eq (N) or to 1.2U eq (C). Three H-atom positions could not be included in the final model: two H-atom positions from the disordered water molecule (O30A and O30B), and one H atom that should be located inside the paratungstate B anion on the triply bridging O8 atom, which was previously proven by neutron diffraction (Evans & Prince, 1983).