Bis{[amino(iminiumyl)methyl]urea} tetrakis{2-[(dimethylamino)(iminiumyl)methyl]guanidine} di-μ6-oxido-tetra-μ3-oxido-tetradeca-μ2-oxido-octaoxidodecavanadium(V) tetrahydrate

In the crystal structure of the title compound, numerous N—H⋯O and O—H⋯O hydrogen bonds of medium strengths connect metforminium and guanylurea cations and centrosymmetric decavanadate(V) anions into a three-dimensional network structure.

The title compound, (C 4 H 12 N 5 ) 4 (C 2 H 7 N 4 O) 2 [V 10 O 28 ]Á4H 2 O, is a by-product obtained by reacting ammonium metavanadate(V), metformin hydrochloride and acetic acid in the presence of sodium hypochlorite, at pH = 5. The crystal structure comprises a decavanadate(V) anion (V 10 O 28 ) 6lying on an inversion centre in space group P1, while cations and solvent water molecules are placed in general positions, surrounding the anion, and forming numerous N-HÁ Á ÁO and O-HÁ Á ÁO hydrogen bonds. Metforminium (C 4 H 12 N 5 ) + and guanylurea (C 2 H 7 N 4 O) + cations display the expected shape. Interestingly, in physiology the latter cation is known to be the main metabolite of the former one. The reported structure thus supports the role of sodium hypochlorite as an oxidizing reagent being able to degrade metformin hydrochloride to form guanylurea.

Structure description
Metformin hydrochloride (MetfÁHCl: 1,1-dimethylbiguanide hydrochloride) is one of the most commonly prescribed medications for the treatment of type 2 diabetes (Maruthur et al., 2016). On the other hand, coordination compounds of vanadium, including polyoxidovanadates resulting from the condensation of the vanadate anion, likewise exhibit an antidiabetic effect, among other biological activities of interest in medicinal applications (Thompson et al., 2009;Rehder, 2020). We are involved in studies about the chemical crystallography of compounds including both types of antidiabetic species. In data reports this context, we report here the crystal structure of a compound including a decavanadate(V) anion, metforminium cations, and a degradation product of the latter, guanylurea cation (1-carbamoylguanidinium).

Figure 2
Main interactions between the decavanadate(V) anion (polyhedral representation) and the first shell including six cations and four water molecules (ball-and-stick representation). Hydrogen bonds are represented by blue dashed lines, and the label associated to each hydrogen bond refers to its entry in Table 1.

Figure 1
The structures of the molecular entities of the title compound, with displacement ellipsoids drawn at the 40% probability level. The centrosymmetric anion is shown, while the content for cations and water molecules is limited to the asymmetric unit.
ecules serve both as donor and acceptor groups for hydrogen bonding, and indeed form the strongest intermolecular contacts in the crystal structure, providing cohesion between the (001) layers in which anions and cations are located (Fig. 3). Experimental conditions used for the synthesis of the title compound were very close to those used for the synthesis of (HMetf) 2 (NH 4 ) 4 [V 10 O 28 ]Á6H 2 O, for which we previously reported the crystal structure (Polito-Lucas et al., 2021). The only difference is that sodium hypochlorite, NaOCl, was present in the reaction medium. At pH < 7, the hypochlorite anion OCl À reacts with the metforminium cation, to form guanylurea (Armbruster et al., 2015). Aqueous NaOCl or solid NaOClÁ5H 2 O are commonly used in such oxidation processes in organic synthesis (Kirihara et al., 2017). On the other hand, in physiology guanylurea is known to be the main metabolite of metformin, through a biodegradation pathway (Tassoulas et al., 2021), and both molecules raise a serious problem of anthropogenic contamination, since high concentrations are found in waste water (Tisler & Zwiener, 2019;Poursat et al., 2019;Tucker & Wesolowski, 2020). The title compound highlights the fact that bleach, also present in waste water, has the ability to degrade metformin to guanylurea. However, the question as to whether the decavanadate(V) anion (or any other vanadium-containing species) promotes or inhibits metformin degradation remains open.

Synthesis and crystallization
Orange good-quality single crystals of the title compound were obtained during the reaction between ammonium metavanadate (NH 4 VO 3 , 1.50 g, 12.1 mmol) and metformin hydrochloride (MetfÁHCl extracted from a commercial brand; 1.70 g, 10.2 mmol) in 50 ml of distilled water, 20 ml of 5% v/v acetic acid (commercial vinegar) and 2 ml of 5% v/v sodium hypochlorite (commercial bleach). In a typical procedure, NH 4 VO 3 was dissolved by gently heating in a water bath followed by addition of MetfÁHCl and stirring until dissolution. The water bath was removed, and once the mixture cooled down to room temperature, CH 3 COOH and NaOCl solutions were added. A yellow-orange homogeneous solution was obtained, and pH = 5 was measured. The solution then was evaporated at ambient conditions and the two major products, (H 2 Metf) 3 [V 10 O 28 ]Á8H 2 O (Sá nchez-Lombardo et al., 2014) and (HMetf) 4 (HGu) 2 [V 10 O 28 ]Á4H 2 O (estimated yields of ca 30 and 10%, respectively), were separated by fractional crystallization over the course of 5 to 10 d.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2.

Funding information
Funding for this research was provided by: Consejo Nacional de Ciencia y Tecnología (grant No. 268178).

Figure 3
Part of the crystal structure, viewed down the a axis, emphasizing the positions of water molecules (space-fill representation).