Crystal structures of three hexakis(fluoroaryloxy)cyclotriphosphazenes

The syntheses and crystal structures of three cyclotriphosphazenes, all with fluorinated aryloxy side groups that generate different steric characteristics are reported.

The syntheses and crystal structures of three cyclotriphosphazenes, all with fluorinated aryloxy side groups that generate different steric characteristics, viz. hexakis(pentafluorophenoxy)cyclotriphosphazene, N 3 P 3 (OC 6 F 5 ) 6 , 1, hexakis[4-(trifluoromethyl)phenoxy]cyclotriphosphazene, N 3 P 3 [OC 6 H 4 (CF 3 )] 6 , 2 and hexakis[3,5-bis(trifluoromethyl)phenoxy]cyclotriphosphazene, N 3 P 3 [OC 6 H 3 (CF 3 ) 2 ] 6 3, are reported. Specifically, each phosphorus atom bears either two pentafluorophenoxy, 4-trifluoromethylphenoxy, or 3,5-trifluoromethylphenoxy groups. The central six-membered phosphazene rings display envelope pucker conformations in each case, albeit to varying degrees. The maximum displacement of the 'flap atom' from the plane through the other ring atoms [0.308 (5) Å ] is seen in 1, in a molecule that is devoid of hydrogen atoms and which exhibits a 'wind-swept' look with all the aromatic rings displaced in the same direction. In 3 an intramolecular C-H(aromatic)Á Á ÁF interaction is observed. All the -CF 3 groups in 2 and 3 exhibit positional disorder over two rotated orientations in close to statistical ratios. The extended structures of 2 and 3 are consolidated by C-HÁ Á ÁF interactions of two kinds: (a) linear chains, and (b) cyclic between molecules related by inversion centers. In both 1 and 3, one of the six substituted phenyl rings has a parallel-displaced aromaticstacking interaction with its respective symmetry mate with slippage values of 2.2 Å in 1 and 1.0 Å in 3. None of the structures reported here have solvent voids that could lead to clathrate formation.

Chemical context
Cyclic organophosphazenes have a long history as representatives of inorganic heterocyclic rings, and are also the focus of arguments about reactivity and pseudoaromaticity in inorganic systems (Allcock, 1972;Steudel, 1992). In our research program these compounds also serve another purpose -as synthesis and structural models for linear high polymeric organophosphazenes. These cyclic small molecules provide preliminary information related to intra-and intermolecular side-group interactions, which affect many polymer properties.
The structure of a fourth, related cyclotriphosphazene with six para-fluorophenoxy groups, was reported by other investigators (Wahl et al., 2012) and was independently verified by us. The non-fluorinated hexa(phenoxy)cyclotriphosphazene tetrahydrofuran solvate x-ray structure was also described earlier (Dietrich et al., 2000).

Structural commentary
The structures of 1, 2 and 3 (Figs. 1, 2 and 3) presented in this report have the hexaphenoxy-cyclotriphosphazene moiety as the common core of the molecule, and differ only in the substitutions on the phenyl rings. The cyclotriphosphazene ring in these structures exhibit varying degree of envelope The molecular structure of 1 with displacement ellipsoids drawn at the 50% probability level.

Figure 2
The molecular structure of 2 with displacement ellipsoids drawn at the 50% probability level.

Figure 3
The molecular structure of 3 with displacement ellipsoids drawn at the 50% probability level. The intramolecular C-HÁ Á ÁF interaction is indicated by dashed lines.
The molecular structure of 1 has a 'wind-swept' appearance ( Fig. 4) with all six pentafluorophenyl rings seemingly pushed in one direction with respect to the cyclotriphosphazene ring. In all the structures here, a pair of aryloxy groups is attached to each of the three phosphorous atoms of each molecule. Comparing the orientation of the rings within each pair, in 1 they are almost orthogonal to each other with the three dihedral angles being 72.3 (2), 76.1 (2) and 80.3 (2) ; in 2 they are between parallel and orthogonal with dihedral angles of 27.3 (2), 33.2 (2) and 62.6 (2) , and in 3 the dihedral angles cover the widest range: 30.2 (2), 45.1 (2) and 82.4 (2) . The trifluoro methyl groups in 2 and 3 are all positionally disordered. A C10-H10Á Á ÁF26 intramolecular interaction is observed in 3 [HÁ Á ÁF = 2.58, CÁ Á ÁF = 3.478 (4) Å , C-HÁ Á ÁF = 163 ].

Supramolecular features
With no hydrogen atom in the molecule of 1, hydrogen bonding is not feasible in that structure (see packing diagram in Fig. 5). The molecules nestle along the a-axis direction, with two adjacent rows facing in one direction and the other two in the opposite direction (Fig. 6). A weak parallel-displacedring interaction is observed between rings related by inversion symmetry [C25-C30, centroid-centroid distance = 4.030 (2) Å , slippage = 2.22 Å ].
In the extended structure of 2, the molecules form chains linked by C-H()Á Á ÁF type hydrogen bonds (Table 1, Fig. 7), along the b-axis direction. Pairs of centrosymmetrically related molecules interact with cyclic hydrogen bonds. Noring interactions are seen in this structure.

Figure 4
The molecule of 1 showing all six pentaflourophenoxy rings leaning to one side with respect to the central cyclotriphosphazene moiety.

Figure 5
Packing diagram for 1 viewed down b-axis direction.
propagating along the a and b-axis directions. In addition, pairs of molecules related by inversion centers have cyclic hydrogen bonding between them. A parallelstacking interaction is observed between one of the six phenoxy rings and its symmetry mate [C41-C46, centroid-centroid distance = 3.646 (2) Å , slippage 1.013 Å ]. Solvent-accessible voids are not present in any of the structures reported here. The solvent inclusion or lack thereof in the crystals of cyclotriphosphazene compounds has been discussed by Wahl et al. (2016 Table 2 Hydrogen-bond geometry (Å , ) for (3). Symmetry codes: (i) x þ 1; y; z; (ii) x; y þ 1; z; (iii) Àx þ 1; Ày þ 1; Àz.

Figure 8
Packing diagram for 3 viewed along the a-axis direction. The black dashed lines show intramolecular C-HÁ Á ÁF interactions, while the red ones show cyclic interaction between molecules straddling the inversion center. The green lines are for interactions leading to continuous chains along a-and b-axis directions.

Figure 6
Unit-cell contents of 1 viewed down the c-axis direction, showing the nestling of the molecules. The top two rows face opposite to the bottom two rows.

Figure 7
Packing diagram for 2 viewed along the a-axis direction. The dashed lines show intermolecular C-HÁ Á ÁF interactions -red ones for cyclic interactions between molecules straddling the inversion center and the green for interactions generating continuous chains along the b-axis direction.

Database survey
Earlier, we reported the crystal structures of a number of cyclotriphosphazenes with spirocyclic aryloxy side groups (Lee et al., 2010). These are clathrate systems that trap hydrocarbon molecules in the cage or tunnel structures. However, the structures in the present study, as well as a series of polymorph structures of hexakis(4-fluorophenoxy) cyclotriphosphazene reported by Wahl et al. (2016)

Synthesis and crystallization
Synthesis of hexakis(pentafluorophenoxy)cyclotriphosphazene (1): Sodium pentafluorophenoxide was prepared by the treatment of pentafluorophenol (15.88 g, 86 mmol) with a suspension of NaH 60% dispersion in mineral oil (3.10 g, 78 mmol) in 50 ml of dioxane. The pentafluorophenoxide was added to a stirred solution of hexachlorocyclotriphosphazene (3.00 g, 8.6 mmol) and the mixture was heated at reflux for 3 d.
Dioxane was removed from the mixture by rotary evaporation and the residue was dissolved in 100 ml dichloromethane. The solution was extracted with 3 Â 100 ml of deionized water, dried over MgSO 4 , and concentrated to $10 ml by rotary evaporation. A small amount of hexanes was added to the concentrated solution and it was chilled to 273 K via an ice bath to yield colorless blocks of 1, which were filtered and rinsed with cold hexanes then dried under vacuum.
Synthesis of hexakis(4-trifluoromethylphenoxy)cyclotriphosphazene (2): The aryloxide was prepared by treatment of 4-trifluoromethylphenol (1.63 g, 10 mmol) with a suspension of NaH [60% dispersion in mineral oil (0.39 g, 9.9 mmol)] in 50 ml of THF. To the stirred solution of 4-trifluoromethylphenoxide was added a solution of hexachlorocyclotriphosphazene (0.50 g, 1.4 mmol) in 15 ml of THF and the mixture was stirred at room temperature overnight. The purification steps of this compound were identical to those of compound 1 to yield colorless cubes of 2.
Synthesis of hexakis(3,5-bis-trifluoromethylphenoxy)cyclotriphosphazene (3): A stirred suspension of NaH [60% dispersion in mineral oil (0.19 g, 4.9 mmol)] in 25 ml of THF was treated with liquid 3,5-bis-trifluoromethylphenol (0.767 ml, 5.0 mmol) by dropwise addition. The resulting aryloxide solution was then added to a stirred solution of hexachlorocyclotriphosphazene (0.25 g,  Computer programs: SMART and SAINT (Bruker, 2001), SHELXS97 (Sheldrick, 2008), SHELXL (Sheldrick, 2015) and OLEX2 (Dolomanov et al., 2009). research communications 0.72 mmol) in 25 ml of THF and the reaction mixture was stirred at room temperature overnight. The mixture was concentrated by rotary evaporation and the residue was dissolved in 40 ml dichloromethane. The dichloromethane solution was washed with 40 ml of deionized water, followed by 20 ml of 5% HCl, and finally rinsed with 40 ml of deionized water. The organic layer was dried over MgSO 4 and the dichloromethane was removed by rotary evaporation to yield a colorless oil, which crystallized as colorless needles of 3 after standing for several hours. The crystals were rinsed with cold methanol and then dried under vacuum. The NMR data for 1-3 are as follows:

Refinement
Crystal data, data collection and structure refinement details for all three structures are summarized in Table 3. The hydrogen atoms in 2 and 3 were placed geometrically (C-H = 0.93 Å ) and refined as riding on their parent atoms with U iso (H) = 1.2U eq (C).

Hexakis(pentafluorophenoxy)cyclotriphosphazene (1)
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.

Hexakis[4-(trifluoromethyl)phenoxy]cyclotriphosphazene (2)
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.32 e Å −3 Δρ min = −0.27 e Å −3 Special details Experimental. The data collection nominally covered a full sphere of reciprocal space by a combination of 4 sets of ω scans each set at different φ and/or 2θ angles and each scan (10 s exposure) covering -0.300° degrees in ω. The crystal to detector distance was 5.82 cm. 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. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ.

Hexakis[3,5-bis(trifluoromethyl)phenoxy]cyclotriphosphazene (3)
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.28 e Å −3 Δρ min = −0.32 e Å −3 Special details Experimental. The data collection nominally covered a full sphere of reciprocal space by a combination of 4 sets of ω scans each set at different φ and/or 2θ angles and each scan (30 s exposure) covering -0.300° degrees in ω. The crystal to detector distance was 5.82 cm. 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.