1,1′,4,5-Tetrahydrotrispiro[1,3,2-diazaphosphole-2,2′-[1,3,5,2,4,6]triazatriphosphinine-4′,6′′-dibenzo[d,f][1,3,2]dioxaphosphepine-6′,6′′′-dibenzo[d,f][1,3,2]dioxaphosphepine] acetone monosolvate

The title compound, C26H22N5O4P3·C3H6O, has been achieved in a two-step synthesis that does not require chromatography. This molecule contains a seven-membered spirocyclic ring at two P-atom positions and a five-membered ring containing new P—N bonds at the other P-atom position. Endocyclic torsion angles about the central biphenyl C—C bonds are −41.5 (3) and −44.4 (3)°, and P—N bonds of the central P3N3 ring are within the range 1.5665 (17)–1.6171 (17) Å, while the P—O distances are in the range 1.5940 (14)–1.6041 (14) Å. One N—H group makes an intermolecular N—H⋯N hydrogen bond, forming centrosymmetric dimers, while the other N—H group makes an N—H⋯O hydrogen bond to the acetone solvent molecule. The crystal was a two-component non-merohedral twin with ratio 0.811/0.189.


Comment
The non-halogenated title compound contains two key elements phosphorus and nitrogen, which are incorporated into the backbone moiety of the molecule to achieve flame retardant properties (Bakos et al., 1982). Studies have shown that an increase in the number of phosphorus (P), nitrogen (N), or P-N bonds leads to improved flame retardancy (Drews & Barker, 1985). The substitution of two biphenol groups and ethylenediamine onto phosphazene was achieved through nucleophilic substitution in high yield. This compound was applied to cotton fabric and has shown promising preliminary results as a potential flame retardant.
The proposed efficacy and novelty of this flame retardant is based on the design of phosphorus (P3) surrounded by four nitrogen (N2, N3, N4, N5) atoms, thereby increasing the number of P-N bonds in the molecule. Incorporation of the 2,2′-biphenol moiety at P1 and P2 promotes a seven membered spirocyclic structure with endocyclic torsion angles about the central biphenol C-C bonds being nearly equal, -44.4 (3)° for C1/C6/C7/C12 and -41.5 (3)° for C13/C18/C19/C24. Both seven-membered rings have twist conformations with the P atom on the local twofold axis; however the deviation from C 2 local symmetry is substantial for both. The asymmetry parameter (Duax et al., 1976) is 14.83 (13)° for the ring containing P1 and 18.44 (14)° for that containing P2.
The utilization of ethylenediamine at the P3 position forms a five-membered ring with new P-N bonds, which lies roughly perpendicular (dihedral angle 81.42 (4)°) to the phosphazene ring. The conformation of the 5-membered ring is closest to an envelope with C25 at the flap position. C25 lies 0.554 (2) Å out of the plane of the other four atoms, and the C s asymmetry parameter is 3.44 (17)°. The N-P-N bonds have angles of 94.47 (9)° to 113.64 (9)°, which is lower than typical angles of ~118° (Allcock, 1972). The narrowing of the angle may be due to van der Waals repulsions from the four N atoms surrounding P3 (Allcock, 1972). It has been reported that N-P-N bonds with two bulky phenyl groups on P have a narrowed angle of 115° due to mutual repulsions form the bulky phenyl group (Allcock, 1972). The phosphazene ring itself is slightly nonplanar, having a boat distortion, with P2 lying 0.1982 (4) Å and N3 lying 0.161 (2) Å on the same side of the plane defined by P1, P3, N1, and N2. The phosphazene ring has P-N bond lengths ranging from 1.5665 (17) to 1.6171 (17) Å. These values are typical for phosphazene rings (Barclay et al. 2002;Olthof, 1969). This molecule has similar bond angles and bond lengths, when substituted with the same or comparable spiro molecules, as found in the literature (Allcock, 1972, Ciftci et al., 2013. Overall, the molecule has approximate C 2 symmetry, as seen in Fig. 1. Both NH groups donate intermolecular hydrogen bonds, as shown in Fig. 2. The N5-H···N3 (at 2 -x, 1 -y, 1 -z) bond leads to centrosymmetric hydrogen-bonded dimers, with graph set R 2 2 (8) (Etter, 1990), while N4 donates a hydrogen bond to acetone O. The acetone molecule forms two C-H···O hydrogen bonds with the phosphazene molecule, one as a supplementary materials donor and the other as an acceptor (Table 1).

Experimental
All reagent grade chemicals were purchased from Sigma Aldrich and were used without further purification. ESI Mass Spectra were recorded on an Agilent Technologies 6520 Accurate Mass Q-TOF LC/MS. NMR spectra were recorded using a Varian 400MHZ spectrometer. 1 H and 13 C are given in δ relative to TMS and 31 P is given δ relative to external 85% aqueous H 3 PO 4 .
A 3-neck round bottom flask was equipped with an argon inlet, an addition funnel, and a stopper. (1) was allowed to stir in CH 2 C1 2 until completely dissolved. A solution of ethylenediamine in CH 2 C1 2 was added drop-wise to the round bottom flask at 0°C and allowed to warm to room temperature. The reaction was allowed to stir for 24 h. Thin Layer Chromatography (TLC) was used to follow the reaction using Hexanes/ CH 2 C1 2 (50:50) and DCM/MeOH (90:10).
Purification was achieved by gently extracting the organic layer with warm distilled water. The organic layer was dried by anhydrous sodium sulfate and concentrated. The product (2) obtained was a white solid with a yield of 84%. The compound was tested by ESI-MS and 1 H, 13 C, 31 P -NMR using Acetone-d 6 . Crystals of (2) were grown in an NMR tube from acetone.

Refinement
H atoms on C were placed in idealized positions with C-H distances 0.95 -0.99 Å and thereafter treated as riding, with a torsional parameter refined for each methyl group. Coordinates of the NH hydrogen atoms were refined isotropically.
U iso for H were assigned as 1.2 times U eq of the attached atoms (1.5 for methyl).  The molecular structure of the title compound with atomic numbering scheme and 50% ellipsoids. The solvent molecule is not shown.  Hydrogen bonding, illustrating the centrosymmetic dimer about 1, 1/2, 1/2 and the two acetone acceptors. H atoms on C are not shown.  Refinement. Refinement of F 2 against ALL reflections, using a TWIN4 hkl file prepared by TWINABS (Sheldrick, 2004).