Syn and anti conformers of diammonium aquabis(malonato)oxidovanadate(IV) in an anhydrate crystal

In the anhydrate crystal of diammonium aquabis(malonato)oxidovanadate(IV), (NH4)2[VO(C3H2O4)2(H2O)], two conformers (syn or anti conformation on the equatorial plane) of complex cations are detected. The DFT calculations for the isomers indicate a slight influence of the conformation on their thermodynamic stability. The anionic complexes interact with adjacent anions and counter-cations through hydrogen bonds, and the hydrogen bonds lead to a structure with alternate stacking of layers consisting of either anti or syn isomers.

The asymmetric unit of the title anhydrate compound, (NH 4 ) 2 [VO(C 3 H 2 O 4 ) 2 -(H 2 O)], consists of two independent complex anions and four ammonium cations. In the complex anions, the V IV atoms are each coordinated by two malonate ligands, one water molecule and one oxide O atom in a distorted octahedral geometry. The equatorial plane is formed by the malonate O atoms, while the axial positions are occupied by water and oxide O atoms. The difference between the two independent complexes is the relative conformation of the malonate ligands. The two ligands in one complex anion are in a syn conformation, while in the other they adopt an anti conformation. In the crystal, the complex anions interact with the counter-cations and adjacent anions through O-HÁ Á ÁO, N-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds. Stacks of alternating layers consisting of either anti or syn isomers, formed with the aid of the hydrogen bonding, are observed. DFT calculations for the anti and syn isomers show a similar thermodynamic stability to each other. The crystal used for this analysis was an inversion twin with the ratio of the twin components being 0.270 (13):0.730 (13).

Structural commentary
The asymmetric unit of the title compound contains two crystallographically independent mononuclear complexes and four counter NH 4 + cations. In each complex the water molecule occupies the trans position to the oxido O atom, and two malonate ligands coordinate to the V IV center occupying an equatorial plane. Although all six-membered V/O/C-C/O chelate rings in the complexes adopt boat conformations, the whole conformation on the equatorial plane is in either a syn conformation or an anti conformation (Yuksel et al., 2008); in the syn conformer the two malonate ligands are related to each other by a pseudo twofold rotation axis along the V-O bond, while in the anti conformer they are related by an pseudo inversion centre near the V atom (Fig. 1). The corresponding coordination bonds in both conformational isomers show similar distances to each other, and atom V1 in the syn isomer and atom V2 in anti isomer are located 0.35 and 0.29 Å out of the O3/O4/O8/O7 and O13/O14/O18/O17 planes, respectively. These crystallographic data suggest no influence of the anti and syn conformations on the coordination geometry around the V IV centre.
Density functional theory (DFT) calculations based on the optimized geometrical parameters were performed at the UB3LYP/6-31G(d) level as implemented in GAUSSIAN09 (Frisch et al., 2009). Their structural parameters were extracted from the corresponding X-ray crystallographic data, and the positions of the hydrogen atoms were optimized, while the positions of all other atoms were fixed at their original positions. The results indicate little influence of the conformations on their thermodynamic stability. The calculated sum of electronic and thermal free energies for these isomers show a slight difference (ca 11 kJ mol À1 ); the energies of the anti and syn isomers are À5062702 and À5062713 kJ mol À1 , respectively.

Supramolecular features
The syn isomer interacts with adjacent seven adjacent ammonium cations via N-HÁ Á ÁO hydrogen bonds and with four other syn isomers and one anti isomer via O-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds (Table 1 and Fig. 2). On the other hand, the anti isomer interacts with nine adjacent ammonium cations and two anti isomers (Fig. 2). These hydrogen bonds lead to the construction of layers consisting of either anti or syn isomers expanding parallel to the ab plane; the two different layers stack alternately, as depicted in Fig. 3. Molecular structures of (a) syn isomer and (b) anti isomer. Displacement ellipsoids are drawn at the 50% probability level. The NH 4 + countercations have been omitted for clarity.

Synthesis and crystallization
To an aqueous solution of malonic acid (3.0 g, 29 mmol in 3 ml) was added a concentrated aqueous ammonia solution (0.5 ml  Table 1 Hydrogen-bond geometry (Å , ).

Figure 3
A packing diagram showing the alternating stacking structure. Hydrogen atoms have been omitted for clarity.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The crystal studied was an inversion twin with a ratio of the twin components of 0.270 (13)

Funding information
We thank Saitama University for the support to TY by the 'High-grade Global Education Program for Sciences', which is supported financially by the Japan Science and Technology Agency in the 'Global Science Campus' program. Data collection: APEX2 (Bruker, 2014); cell refinement: SAINT (Bruker, 2014); data reduction: SAINT and XPREP (Bruker, 2014); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2016 (Sheldrick, 2015b); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012); software used to prepare material for publication: XCIF (Bruker, 2014).