Crystal structures of two formamidinium hexafluoridophosphate salts, one with batch-dependent disorder

The crystal structures of two nitrogen heterocycle hexafluoridophosphate salts with terminal animinium groups are described. Unexpectedly, there is cation disorder in crystals of the morpholinoformamidinium compound obtained from ethanol recrystallization, which is not found in the original crystals.


Chemical context
The stability of N-heterocyclic carbenes and their applications in organic syntheses and in transition-metal catalysis has led in the past to intense interest in the syntheses of their precursors: cationic N-heterocyclic amidinium salts (Benhamou et al., 2011).Previously, one of us reported a simple and efficient one-pot procedure for the preparation of cyclic amidinium salts by exchange reactions of various orthoesters with primary and secondary �,!-diamines in the presence of ammonium tetrafluoridoborate or ammonium hexafluoridophosphate (Saba et al., 1991).This approach has been widely used for the preparation of cyclic amidinium salts in which the nitrogen-flanked carbon atom bears a hydrogen atom and the nitrogen atoms bear bulky substituents (for example: Funk et al., 2006;Scarborough et al., 2005).The use of orthoesters was then extended for the preparation of various acyclic amidinium hexafluoridophosphates as potential carbene precursors (Saba et al., 2005).We present here the first single-crystal structure determinations of these types of amidinium salts with N-heterocycles, viz.morpholinoformamidinium hexafluoridophosphate, 1, and pyrrolidinoformamidinium hexafluoridophosphate, 2.

Structural commentary
The crystal structure of 1 was determined from crystals obtained directly from the original preparation described in the Synthesis section, and also from crystals obtained by recrystallization from ethanol, 1(recryst).The structures of the molecular moieties with atom numbering are shown in Figs. 1  and 2.
In 1, the length of the delocalized C N bond of 1.3016 (15) A ˚for the terminal C7-N8 bond is close to but slightly less than the value of 1.3142 (13) A ˚found for the bond adjacent to the ring, N1-C7; the angle at the methine C7 atom is 125.98 (10) � .The formamidinium group N1-C7H-N8H 2 + is very close to planar with a root-meansquare (r.m.s.) deviation of the six atoms from the plane of 0.0050 A ˚.This group is not coplanar with the C2-N1-C6 plane of the morpholine group but is tilted by 13.4 (3) � from that plane.The morpholine moiety has the usual chair configuration, with the four atoms C2, C3, C5, C6 rigidly coplanar with an r.m.s.deviation of 0.0048 A ˚, and the O and N ends tilted by 52.6 (1) and 54.4 (1) � , respectively, from this plane.
Part of the sample was recrystallized from ethanol, in order to obtain larger crystals.Data from these crystals, 1(recryst), indicated essentially the same unit cell but with intensities that did not exactly match those obtained for the original crystal.This intensity difference was shown to be due to disorder in one of the hydrogen-bonded cation chains, discussed in the next section, which lowered the symmetry to space group Pca2 1 where two independent cations and anions are present.The shape of the cations are the same as found for the original crystal, albeit with somewhat less precision because of the disorder.To our knowledge, such a batch-dependent disorder is not often reported.Solvent-dependent disorder for some cobalt and zinc complexes is discussed in McCormick et al. (2018), but in that case there is solvate actually present in the crystal structures.
Fig. 3 shows the molecular structures of cations and anions for 2. Here, the lengths of the delocalized C N bonds in the two independent cations are slightly longer to the terminal nitrogen atom: The average for the terminal C N bonds, C6-N7 and C16-N17, is 1.323 (5) A ˚, while that for the C N bonds adjacent to the rings, N1-C6 and N11-C16, is 1.293 (5) A ˚.A slight difference in the delocalized C N bond lengths might be expected due to the differing inductive effects of the terminal H atoms and the ring atoms; the lower electron density expected on N1 in compound 1 due to the electron withdrawing inductive effect of the ring oxygen might cause the C N distance adjacent to the ring to be longer in 1 89 Figure 1 The asymmetric unit of 1, showing the atom numbering.Displacement ellipsoids are drawn at the 50% probability level, while displacement parameters for the H atoms are arbitrary.The minor disordered PF 6 À component is shown fainter.O atoms are colored red, N blue, C and H black, P magenta and F green.

Figure 2
The asymmetric unit of 1(recryst), showing the atom numbering.Displacement ellipsoids are drawn at the 50% probability level, while displacement parameters for the H atoms are arbitrary.The minor disordered PF 6 À component is shown fainter.Colors are as in Fig. 1.

Figure 3
The asymmetric unit of 2, showing the atom numbering.Displacement ellipsoids are drawn at the 50% probability level, displacement parameters for the H atoms are arbitrary, and atom colors as in Fig. 1.

Supramolecular features
Multiple contacts between the cations and the PF 6 À anions may be due to either electrostatic or hydrogen-bonding interactions.We have applied a 3.25 A ˚cutoff for C/N � � � F distances and a 110 o C/N-H � � � F angle for possible hydrogen bonds and these interactions are listed in Tables 1-3.
In 1, the PF 6 À anions are spaced close to half a unit cell apart in all three directions.There are chains of cations along the b-axis direction as seen in Fig. 4

Figure 4
Projection of about half of the unit cell of 1 down the a axis.The minor disorder components for the PF 6 À groups are not shown.The black dotted lines indicate hydrogen bonds in the layer shown, while the blue dotted lines indicate hydrogen bonds to anions half a cell above or below the layer shown.Atom colors are as in Fig. 1.Symmetry codes: (i) x þ 1 2 ; y À 1 2 ; z; (ii) x; y À 1; z.

Figure 5
Projection of about half of the unit cell of 1(recryst) down the a axis.The minor components for PF 6 À groups are not shown.Colors of atoms and of hydrogen bonds are as in Fig. 1 and Fig. 4, respectively.
to direction, and the symmetry of the structure is lowered from the centric space group Pbca to the noncentric space group Pca2 1 , with an interchange of the b and c axes.36.1 (4)% of the chains point in a direction opposite to that of their neighbors, as would be required by the centric space group, but the majority disorder component points in the opposite direction.The network of cation� � �anion hydrogen bonds is similar to that in 1, except that there do not appear to be any methine C-H� � �F contacts for either of the disordered cation chains.All intermolecular H� � �H contacts in 1 are >2.7 A ˚.
In 2, cations and anions are each spaced half a unit cell apart in all three directions as seen in Fig. 6.All but one of the N-H� � �F hydrogen-bonds listed in Table 3 are within sheets parallel to (101), and are shown for one of these sheets in Fig. 7 where hydrogen-bonded cation� � �anion� � �cation chains along the b axis can be seen.Alternate cations in the b-axis direction link to separate ribbons.The hydrogen-bonding pattern in Fig. 7 is not dissimilar to that for 1 shown in Fig. 4, except that the cation� � �cation hydrogen bonding in Fig. 4 is not possible in 2 due to lack of the O acceptor atom in the pyrrolidino ring.The C16-H16� � �F6 hydrogen bonds link the sheets together.The shortest H� � �H contacts are H2A� � �H13A(x À 1 2 , 3 2 À y, z , with R = Me 2 N-N N-, CHO, and two more complex aromatic sulfur containing moieties.Overall, the delocalized C-N distances average to 1.310 (11) A ˚. Entry FUMGUP (Allenstein et al., 1987) is the aldehyde derivative, where the delocalized C N distances differ slightly, by 0.02 A ˚. We did not find any examples of terminal formamidinium groups attached to nitrogen heterocycles, as in the present structures.

Database survey
In regards to intermolecular contacts, we found in the database 1825 N� � �F contacts less than 3.02 A ˚, the sum of the van der Waals radii.In the present compounds, only two N� � �F contact distances are less than 3.02 A ˚, while the others are all greater than this.Also, although there are over 35000 C� � �F contacts in the database less than 3.17 A ˚, the sum of the van der Waals radii, only the C7-H7� � �F2 contacts at 3.14 A shown in Fig. 3 meet this criterion.The weak intermolecular forces implied by the longer intermolecular distances in the present crystal structures may be correlated with the disorder in the anions, and the disorder possibilities in the cation chains.

Synthesis and crystallization
Compound 1 was prepared by heating an equimolar mixture of morpholine, triethyl orthoformate and ammonium hexafluoridophosphate.Similarly, compound 2 was made by heating an equimolar mixture of pyrrolidine, triethyl orthoformate and ammonium hexafluoridophosphate.Compound 1 precipitated out as the reaction mixture was being heated and was purified by crystallization from ethanol.Compound 2 crystallized as the reaction mixture was cooled, affording sufficiently pure crystals.
Infrared Spectra: FTIR spectra for the two compounds are shown in the supporting information.For compound 2, there are two clear NH 2 stretching frequencies at 3474 and 3380 cm À 1 .The bands at 1716 cm À 1 may be due to the resonant N-C N stretches.For compound 1 and 1(recyst), a  similar N-C N stretching frequency is seen at 1717 cm À 1 .Here, however, the spectrum in the N-H stretch region is more complex, with multiple bands below the prominent band at 3453 cm À 1 .Allenstein et al. (1987) include a review of the IR data for their aldehyde complex, with N-H stretches at 3342 and 3240 cm À 1 , and a band at 1695 cm À 1 for the asymmetric N-C N stretch; further assignments are given in more detail than covered in the present paper.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. Methylene H atoms were constrained to expected positions with C-H distances of 0.97 A ˚and displacement parameters set at 1.5U eq of the parent C atom for 1 and 2, and 1.2U eq for 1(recryst).H atoms bonded to N and the methine C atom were refined for 1.For 1(recryst) they were constrained due to the disorder [occupancy ratio of the disordered cation 0.639 (4):0.361(4)], and they were also constrained for 2, since refinements did not move them from their expected positions.N-H distances in 1(recryst) and 2 were refined, however.Structure 1(recryst) was refined as an inversion twin, although it seems more likely that the crystal had twinning about a mirror plane perpendicular to the c axis.Either twin operation has the same effect on the data analysis.
Initially, several sets of data were collected at room temperature on crystals of both compounds 1 and 2. Room temperature data from all crystals had very few intensities with I > 2�(I) at higher angles.The positions of the PF 6 À groups, which dominate the X-ray scattering, lead to whole groups of weak reflections.Even though the room-temperature data were not sufficiently adequate to define the disorder in 1(recryst), there were clear indications that the structure was not the same as in 1: R int for merged data from 1 and 1(recryst) was 16.4%, compared with R int values of 3.9% and 4.1% for the individual data sets.For this reason, data collection was repeated at low temperature.Refinement was complicated by disorder in the hexafluoridophosphate groups.In 1, a minor disorder component was twisted some 45 o about the F1A-P1-F2A axis; since the occupancy of this component refined to only 13.0 (8)%, the four F atoms F3B-F6B were refined isotropically.A similar positional disorder exists for one of the PF 6 À groups in 1(recryst) with an occupancy ratio of 0.876 (19):0.124(19), where the four F atoms F3B-F6B were refined isotropically.In 2, no disordered model appeared necessary.

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.

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.Refinement.Refined as a 2-component inversion twin.Hydrogen-bond geometry (Å, º)

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.Refinement.Refined as a 2-component inversion twin.
, linked together via N8-H8� � �O4 hydrogen bonds.N-H� � �F and C-H� � �F hydrogen bonds to PF 6 À groups on one side of the cation chain augment these cation chains to ribbons of cations and anions parallel to the b axis.Cation chains at z = 1/4 and z = 3/4 are related by a c glide, and point in opposite directions.1(recryst) shows the same general supramolecular features, Fig. 5, but in this case alternate cation chains are disordered as research communications 90 Neary et al. � Two formamidinium hexafluoridophosphate salts Acta Cryst.(2024).E80, 88-93

Figure 6
Figure 6 Projection of 2 down the b axis, showing the cations and anion components arranged in separate stacks along b.Atom colors are as in Fig. 1.

Figure 7
Figure 7 View of part of the structure of 2 projected approximately on (101), showing hydrogen-bonded cation� � �PF 6 � � �cation chains in the b-axis direction.Atom colors are as in Fig. 1.

Table 4
Experimental details.