Tetraammonium bis(metforminium) di-μ6-oxido-tetra-μ3-oxido-tetradeca-μ2-oxido-octaoxidodecavanadium(V) hexahydrate

In the title salt, all O—H and N—H groups in the cations (metforminium and ammonium) and lattice water molecules are donor groups for hydrogen bonds, giving a highly compact crystal structure.

The title compound, (NH 4 ) 4 (C 4 H 12 N 5 ) 2 [V 10 O 28 ]Á6H 2 O, crystallizes with the decavanadate anion placed on an inversion centre in space group P1. This anion is surrounded by a first shell of ammonium cations and water molecules, forming efficient N-HÁ Á ÁO and O-HÁ Á ÁO hydrogen bonds. A second shell includes metforminium monocations with a twisted geometry, also forming numerous intermolecular hydrogen bonds. The complex three-dimensional network of non-covalent interactions affords a crystal structure in which the cations and anions are densely packed.

Structure description
Metformin hydrochloride (MetfÁHCl: 1,1-dimethylbiguanide hydrochloride; Niranjana Devi et al., 2017) is the first-line therapy for type 2 diabetes. On the other hand, some anionic or cationic vanadium species, such as vanadate and vanadyl, have also been shown to be useful in the treatment of human diabetes (Domingo & Gó mez, 2016). Based on this background, several groups belonging to the Autonomous University of Puebla are involved in the synthesis of compounds including both metformin and oxidovanadate derivatives, with the hope of achieving synergistic effects (Sá nchez-Lombardo et al., 2014). The associated chemical crystallography is rather complex, because due to its basic character metformin can be found in various states of protonation (neutral, cationic or dicationic forms), while the degree of condensation for the vanadate moiety strongly depends on the pH of the reaction medium. Finally, most of these compounds are crystallized with a number of water molecules, which is unpredictable. The compound data reports reported here includes one (V 10 O 28 ) 6À anion, four ammonium cations, two metforminium(1+) cations HMetf + , and six water molecules (Fig. 1).
The (V 10 O 28 ) 6À anion is situated on an inversion centre in space group P1, and approaches the expected D 2h symmetry, which has been extensively reported (Bošnjaković-Pavlović et al., 2011). The negative charges are balanced by four NH 4 + and two HMetf + cations. The high resolution of the measured diffraction data (d min = 0.56 Å ) unequivocally establishes that there is no protonation of the decavanadate. The HMetf + monocation has its charge located mainly on N2. Furthermore, this cation is characterized by a dihedral angle of 54.85 (5) between planes C2-C4/N3-N5 and C1/N1-N3. This twisted geometry is observed in several other compounds of metforminium(1+). Indeed, metformin and its cations HMetf + and H 2 Metf 2+ are highly flexible entities: the twist angle for 93 structures recovered from the CSD (Groom et al., 2016) varies from 1 to 85 .

Figure 2
Main interactions between the (V 10 O 28 ) 6À anion (polyhedral representation) and the cations and water molecules. The strongest hydrogen bonds are represented as magenta dashed bonds (entries 7 and 19 in Table 1), while secondary hydrogen bonds are represented with thin black dashed lines.

Figure 1
The molecular entities in the structure of the title compound, with displacement ellipsoids for non-H atoms at the 50% probability level.
Cations and water molecules in the asymmetric unit are labelled.

Synthesis and crystallization
Good-quality single crystals of the title compound were obtained during the reaction between ammonium metavanadate (NH 4 VO 3 , 1.117 g, 9.5 mmol; Pé rez-Benítez & Bernè s, 2018) and metformin hydrochloride (MetfÁHCl, 0.497 g, 3 mmol; Niranjana Devi et al., 2017) in 100 ml of distilled water and 6 ml of acetic acid 5% v/v. In a typical procedure, the ammonium metavanadate was dissolved by heating in a water bath and then metformin hydrochloride was added and stirred until its dissolution. The water bath was removed and once the mixture cooled down to room temperature, the acetic acid was added. The homogeneous solution was slowly evaporated during several days at ambient conditions, which allowed the separation of reaction by-products by fractional crystallization, being the main products

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2.  Special details Refinement. All H atoms, with exception of the methyl groups in the HMetf + cation, were refined with free coordinates and isotropic displacement parameters calculated as U iso (H) = 1.2 or 1.5×U eq (carrier atom). The geometry for the three water molecules was restrained, with target bond lengths O-H = 0.85 (2) Å and H···H separations of 1.34 (2) Å. Methyl H atoms were included using a riding model with U iso (H) = 1.5×U eq (carrier atom).