Crystal structure of (1,4-diphenyl-4H-1,2,4-triazol-3-yl)phenylamine difluorophosphate, and a survey of the difluorophosphate anion (PO2F2 −)

The title compound represents the difluorophosphate salt of Nitron, an analytical reagent used for gravimetrical analysis of nitrate ions. An analysis of the bond–length distribution and average bond lengths in PO2F2 tetrahedra is given.

years ago (Lange, 1929). It can be used as a precursor for obtaining difluorophosphates of several metals or other cations through metathesis reactions.
The synthesis, crystallization and structure analysis of III are reported here, together with a survey of the structural characteristics of the difluorophosphate anion present in inorganic, metal-organic and organic compounds.

Structural commentary
The asymmetric unit of III is composed of a Nitron molecule protonated at the C1 atom of the triazole ring, assuming the NHC-type tautomer II to be prevalent in Nitron itself, and a PO 2 F 2 À anion (Fig. 2). The central triazole ring (C1, C2, N1-N3), the phenyl ring attached to N2 (C9-C14) and the NHPh moiety attached to C2 (N4, C15-C20) are virtually co-planar with the r.m.s. deviation of the 18 non-H atoms being 0.0666 Å [greatest deviation 0.1250 (13) Å for the phenyl C19 atom]. The third phenyl ring (C3-C8) is inclined to the least-squares plane of the three aforementioned rings by 56.07 (3) (Fig. 2). A weak intra-molecular hydrogen bond between a phenyl C-H group (C16-H16) and the free N atom (N1) of the triazole cycle stabilizes the conformation of the molecule (Table 1).
In III, the tetrahedral difluorophosphate anion shows the characteristic bond lengths distribution (Table 2) between two shorter P-O bonds (mean 1.468 Å ) and two considerably longer P-F bonds (mean 1.554 Å ). The distortion of the anion is evident not only by the two pairs of different bond lengths but even more so by the bond angles that partly deviate considerably from the ideal value of 109.47 . Whereas the O1-P-O2 angle is enlarged by about 14 relative to the ideal value, the F1-P-F2 angle is reduced by about 12 ; the four O-P-F angles are rather similar, with a mean of 108.3 .

Supramolecular features
Aside from Coulombic interactions, the cation is hydrogenbonded by an N-HÁ Á ÁO interaction of medium strength between the amino group (N4) of the NHPh moiety and one of the O atoms (O2) of the difluorophosphate anion. The other O atom (O1) of the anion is the acceptor atom of a weak C-HÁ Á ÁO hydrogen bond with the protonated carbene C1 atom as the donor group. F atoms are not involved in hydrogen bonding, as frequently observed for related compounds containing the monofluorophosphate anion PO 3 F 2- (Weil et al., 2015). The two types of hydrogen-bonding interactions link the cations and anions into a three-dimensional network structure. Additionalstacking between the triazole ring (Cg1) and the phenyl ring C15-C20 (Cg2) with a centroid-to-centroid distance of Cg1Á Á ÁCg2(2 À x, 1 À y, 1 À z) = 3.5378 (9) Å and a slippage of 0.643 Å consolidates the packing (Fig. 3). The asymmetric unit of III, showing the molecular components with displacement ellipsoids for non-H atoms drawn at the 75% probability level. H atoms are given as spheres of arbitrary radius; N-HÁ Á ÁO hydrogen bonding between the organic cation and the inorganic anion is shown as a light-blue dashed line. Table 1 Hydrogen-bond geometry (Å , ).   Zagorac et al., 2019) and the CSD for the difluorophosphate anion or the PO 2 F 2 entity revealed the crystal structures of twelve inorganic and 30 metal-organic or organic compounds (Table 3). For a statistical analysis of bond lengths and angles within a PO 2 F 2 tetrahedron, only ordered PO 2 F 2 groups were considered. In summary, 67 independent PO 2 F 2 tetrahedra were used, leading to the following averaged bond lengths and angles: P-O = 1.459 (27) Å , P-F = 1.530 (21) Å ; O-P-O = 121.2 (2.9) , O-P-F = 108.7 (6) , F-P-F = 98.5 (2.6) . It is evident that the bond lengths and angles observed in III (  Table 3 Averaged bond lengths (Å ) and angles ( ) in PO 2 F 2 tetrahedra present in several compounds.   Table S1 of the supporting information.

Figure 3
The packing of the organic cations and the inorganic anions in the crystal structure of III in a view along [001]. Intermolecular N-HÁ Á ÁO and C-HÁ Á ÁO bonds are shown as light-blue and magenta dashed lines, andstacking interactions by green dashed lines.

Synthesis and crystallization
In a nickel crucible, P 2 O 5 (2.67 g) and NH 4 F (1.86 g) were thoroughly mixed. The open crucible was placed on a hot plate (' 420 K) where a vehement reaction took place within a few seconds. The crucible was then taken from the plate and cooled to room temperature. The obtained solid was dissolved in 50 ml water to which ammonia solution (25% wt ) was added until neutralisation. Subsequently, the pH was adjusted to ca. 5 with a few drops of glacial acetic acid. Nitron (3 g) was then added in small portions to the cooled (273 K) acetic solution under constant stirring for about two h. The formed solid was separated by suction filtration and recrystallized from diluted acetic acid solution. Storing the solution in a refrigerator at 280 K overnight resulted in the formation of light-brown crystals of the title compound with a rod-like form; yield 60%.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. The H atom attached to N1 was discernible in a difference-Fourier map and was refined freely.

3-Anilino-1,4-diphenyl-1H-1,2,4-triazol-4-ium difluorophosphate
Crystal data 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.