Crystal structure and Hirshfeld surface analysis of 4,4′-(propane-1,3-diyl)bis(4H-1,2,4-triazol-1-ium) pentafluoridooxidovanadate(V)

In the structure of the title salt, second-order Jahn–Teller distortion of the coordination octahedra around V ions is reflected by coexistence of short V—O bonds and trans-positioned long V—F bonds, with four equatorial V—F distances being intermediate in magnitude. Hydrogen bonding of the anions is restricted to F-atom acceptors only, with particularly strong N–H⋯F interactions [N⋯F = 2.5072 (15) Å] established by axial and cis-positioned equatorial F atoms.


Chemical context
Significant second-order Jahn-Teller distortions are inherent to the coordination octahedra of the [M v OF 5 ] 2À series (M v = V, Nb, Ta) of ions (Ok et al., 2006;Welk et al., 2002). The resulting polar symmetry of the anions could be exploited as the origin of bulk polarity when imprinted on the structures of non-centrosymmetric coordination and hydrogen bonded solids (Halasyamani, 2010). Such supramolecular synthesis with oxofluoride building blocks extends existing approaches for the development of non-centrosymmetric crystals, which attract significant interest for electro-optical applications (Gautier & Poeppelmeier, 2013).
One can anticipate that [VOF 5 ] 2À systems will show this effect to a particular extent since the vanadium ions experience a much larger out-of-centre displacement towards an apical O-ligand compared with their Nb and Ta analogues (Ok et al., 2006). This feature generates a larger dipole moment as well as mitigating against orientational disorder of the anions in crystal structures (Sharko et al., 2018). However, the supramolecular behaviour of the [VOF 5 ] 2À anions is less predictable and it is strikingly different from that of the most extensively examined Nb and Ta systems. Welk et al. (2000) noted the very weak O-coordinating ability of the [VOF 5 ] 2À anions serving as F-donor ligands only but the hydrogen-bond acceptor ability of the O atoms is less addressed. Distal interactions of the C-HÁ Á ÁO type are relevant to the structure of (H 2 bipy) [VOF 5 ] (bipy is 4,4 0 -bipyridine; Gautier et al., 2015), but surprisingly, no hydrogen bonding at all was observed for the O atoms in (H 2 En) [VOF 5 ] (En is ethylenediamine; Rieskamp & Mattes, 1976). In addition, the possible competitiveness of the O atoms with respect to other weak hydrogen-bond acceptors does not appear to have been considered so far.

Structural commentary
The molecular structure of the title compounds is shown in Fig. 1. The distorted coordination octahedra around the V ions comprise very short V1-O1 bonds of 1.5767 (12) Å and long bonds with trans-positioned F1 ligands [V1-F1 = 2.0981 (9) Å ], which define the local polar axis of the anion. Four equatorial V-F bonds [mean 1.8295 (9) Å , Table 1] are intermediate in length. That the anion geometry is sensitive to the hydrogen-bond environment is evidenced by the elongation of the V1-F4 bonds [1.8913 (9) Å ], with the F4 atoms involved in a strong N-HÁ Á ÁF interaction ( Table 2). The central ion deviates from the centroid of its six ligand atoms by d = 0.242 Å towards the O-vertex. This is reminiscent of the geometrical features of the [VOF 5 ] 2À anions in the salts with (H 2 bipy) 2+ (d = 0.268 Å ; Gautier et al., 2015) and (H 2 En) 2+ cations (d = 0.272 Å ; Rieskamp & Mattes, 1976).
The main geometrical parameters of the organic cations are very similar to those of the parent 1,3-propylenebitriazole ligand in complexes with metal ions (Senchyk et al., 2017). The dicationic structure, as the result of protonation of the N1 and N4 sites, is best reflected by differentiation of the angles involving the N atoms in the two triazolium rings: C-N(H)-N = 111.17 (12) and 111.79 (11) versus C-N-N(H) = 103.46 (12) and 104.11 (12) ( Table 1). A similar effect is known for the isoelectronic neutral pyrazole ring (Gospodinov et al., 2020). The protonation also results in a certain shortening of the N-N bonds [1.362 (2) Å ], as may be compared with N-N = 1.3918 (15) Å for the neutral and non-coordinated triazole rings in the adamantane derivative (Lysenko et al., 2019). The methylene linkage adopts a transgauche conformation with the corresponding torsion angles C5-C6-C7-N6 of À171.58 (12) and N3-C5-C6-C7 of À63.73 (17) . A diversity of metal complexes suggest nearlys equal occurrence of trans-gauche and all-trans sequences for the present moiety (Senchyk et al., 2017).

Supramolecular features
The three-dimensional packing of the title compound is mediated by hydrogen bonding and two kinds of stacking interactions. Two strong N-HÁ Á ÁF hydrogen bonds employ the most underbonded axial F1 atoms of the anion and the cis-

Figure 1
The molecular structure of the title compound with displacement ellipsoids drawn at the 50% probability level. Dotted lines indicate weak C-HÁ Á ÁF hydrogen bonding. positioned F4 atom (Fig. 2). Thus the primary pattern exactly follows the cis-directing preferences of the [VOF 5 ] 2À anions, as suggested by Poeppelmeier and co-workers (Welk et al., 2000;Gautier et al., 2015). More distal interactions are relevant to weaker CH donors (Table 2). In total they support nine C-HÁ Á ÁF contacts with a cut-off-limit of HÁ Á ÁF = 2.56 Å , which is the sum of the van der Waals radii of these species (Rowland & Taylor, 1996).  Rowland & Taylor, 1996). It should be stressed that even such a weak acceptor as the N atom of the cationic moiety is a preferable site for hydrogen bonding, instead of the O atom of the [VOF 5 ] 2À anion. For the aliphatic portion of the structure, C-HÁ Á ÁF interactions are longer and presumably weaker, whereas shorter HÁ Á ÁF contacts [2.32 Å ] correspond to the triazole-linked methylene groups, as these are more polarized and acidic. Primary strong N-HÁ Á ÁF bonding links the ionic counterparts into chains, which aggregate forming layers parallel to the ab plane. In a complement to the weak C-HÁ Á ÁF bonds, these layers are sustained by two types of stacks (Fig. 2). The first of these may be regarded as an interaction between the triazolium ring to the F2/F5/O1 face of the anion, with an interplanar angle of 12.60 (9) and centroid-to-centroid distance of 3.064 (2) Å . This interaction is favourable, as a kind of recently recognized anionÁ Á Á bonding (Bauzá et al., 2016) and it is responsible for the generation of a very short contact: F5Á Á ÁC3 i = 2.7296 (15) Å [symmetry code: (i) Àx + 1 2 , y À 1 2 , Àz + 1 2 ]. The second type may concern the stacking of the inversion-related triazolium rings. However, a relatively large intercentroid distance of 3.626 (2) Å and slippage angle of 64.2 (2) indicate a lack of overlap (Janiak, 2000). Taking into account also the zero contribution of CÁ Á ÁC contacts to the Hirshfeld surface of the cation (see below), one may postulate rather the ion-dipole interaction of two triazolium N-NH + sites, with the N1Á Á ÁN2 viii separation of 3.2926 (18) Å [symmetry code: (viii) Àx, Ày, Àz].
The packing of the layers extends the structure in the third dimension. For every next layer of the succession, the direction of the primary N-HÁ Á ÁF bonded chains is inclined by 56.8 to the direction of chains from the preceding layer (Fig. 3). Links between the layers represent most of the weak interactions, such as C-HÁ Á ÁN bonds and C-HÁ Á ÁF bonds with the aliphatic CH donors.

Synthesis and crystallization
The bitriazole was prepared in a yield of 33% by the acidcatalysed condensation of 1,3-diaminopropane and N,N-dimethylformamide azine (Lysenko et al., 2010

4,4′-(Propane-1,3-diyl)bis(4H-1,2,4-triazol-1-ium) pentafluoridooxidovanadate(V)
Crystal data (C 7  Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )