Crystal structure of hexamethyl 4,4′,4′′,4′′′,4′′′’,4′′′’’-[(1,3,5,2λ5,4λ5,6λ5-triazatriphosphinine-2,2,4,4,6,6-hexayl)hexakis(oxy)]hexabenzoate

The title compound consists of a cyclotriphosphazene core and six 4-methoxycarbonylphenoxy groups. The phosphorus atoms are attached to two substituents located up and down with respect to the plane of the phosphazene ring, the central P3N3 ring having a twisted-boat conformation.


Chemical context
In the past few decades, a rich variety of cyclotriphosphazenes with interesting properties and applications have been synthesized by replacing the Cl atoms of hexachlorocyclotriphosphazene with various nucleophiles. The properties of cyclotriphosphazenes depend on the inorganic skeleton, as well as on the nature of the substituents attached to the P atoms (Patil et al., 2011). Hexakis(allyl 4-hydroxybenzoate)cyclotriphosphazene (HABC) possessing six reactive peripheral allyl groups is used as a functional phosphazenebased oligomer for the synthesis of optical resin, through radical homopolymerization of itself and copolymerization with methyl methacrylate (Guo et al., 2009). The title compound, HMPC, was obtained accidentally from the recrystallization of the crude product of HABC. Subsequently, as a retardant additive, HMPC was blended with a polymer of methyl methacrylate to obtain the flame-retardant polymer MC-PMMA. In this context, we report here the synthesis and crystal structure of HMPC.

Structural commentary
The molecule of HMPC (Fig. 1) comprises a cyclotriphosphazene core and six 4-methoxycarbonyl phenoxy groups, and each P atom is attached to two substituents. Three of the six 4-methoxycarbonylphenoxy substituents are on one side of the phosphazene ring, while the other three groups are located on the opposite side. The central phosphazene ring is slightly nonplanar, having a boat distortion, with atoms P1 and N2 lying 0.1223 (7) and 0.138 (2) Å , respectively, on the same side of the plane defined by atoms N1/N3/P2/P3, in agreement ISSN 2056-9890 with the values reported in the literature for hexakis(4formylphenoxy)cyclotriphosphazene (Patil et al., 2011).

Supramolecular features
In the title compound, there are no usual hydrogen-bonding or stacking interactions, the crystal structure being enforced by van der Waals forces only.

Database survey
In a search in the Cambridge Structural Database (Groom et al., 2016), 15 structures were found incorporating the same cyclophosphazene motif substituted by six phenoxy groups. Of these, only one structure contained alkoxycarbonylphenoxy groups bonded to each P atom of a phosphazene skeleton (Zhu et al., 2015). In that structure, the atoms of two terminal propenyl groups are disordered over two sets of sites, with refined site-occupancy ratios of 0.249 (12):0.751 (12) and 0.476 (9):0.524 (9); no intermolecular interactions were observed.

Synthesis and crystallization
All of the chemicals and solvents were of reagent grade. Hexachlorocyclotriphosphazine (HCCP) was purchased from Zhengzhou ALFA Chemical Co. Ltd, recrystallized from dry hexane and sublimated twice. Anhydrous K 2 CO 3 was activated at 413 K for 2 h. Methyl 4-hydroxybenzoate was synthesized according to the literature method of Guo et al.

(2009).
A three-necked round-bottomed flask was equipped with a nitrogen inlet, an addition funnel and a condenser. To a mixture of hexachlorocyclotriphosphazene (1.04 g, 3 mmol) and anhydrous K 2 CO 3 (3.5 g, 253 mmol) in tetrahydrofuran (50 ml), a solution of methyl 4-hydroxybenzoate (3.20 g, 21 mmol in tetrahydrofuran) was added dropwise at room temperature. The reaction mixture was heated at ca 338 K for 48 h under nitrogen and thin-layer chromatography (TLC) was used to monitor the reaction. The resulting suspension

Figure 1
The molecular structure of the title compound showing 50% probability displacement ellipsoids.
was filtered and the filtrate concentrated, leading to the formation of a pale-yellow viscous liquid. This was dissolved in 20 ml ethyl acetate and the solution added dropwise to methanol. Colourless needle-shaped crystals suitable for X-ray diffraction analysis were obtained by slow evaporation of the solvent.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms were constrained, with C-H = 0.93-0.98 Å and U iso (H) = 1.5U eq (C) for methyl H atoms and 1.2U eq (C) for other H atoms. A rotating model was used for the methyl groups. An ISOR restraint in SHELXL2014 (Sheldrick, 2015) was applied to the methyl C16 atom. Ten low-angle reflections with F o << F c , whose intensities may have been significantly reduced by the beam stop, were omitted from the final cycles of refinement.   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 )