2,6,7-Trioxa-1-phosphabicyclo[2.2.2]octan-4-ylmethanol 1-sulfide

The title compound, C5H9O4PS, was synthesized by the reaction of pentaerythritol with thiophosphoryl chloride. In the crystal structure, the three six-membered rings all adopt boat conformations. Molecules form chains along the c axis via intermolecular O—H⋯O hydrogen bonds.

The title compound, C 5 H 9 O 4 PS, was synthesized by the reaction of pentaerythritol with thiophosphoryl chloride. In the crystal structure, the three six-membered rings all adopt boat conformations. Molecules form chains along the c axis via intermolecular O-HÁ Á ÁO hydrogen bonds.

Comment
Intumescent flame retardant systems appear as an attractive topic and represent a wide and interesting area of research (Bourbigot & Duquesne, 2007). The use of pentaerythritol (Le Bras et al., 1997) as char former in intumescent formulations which composed of three components, i.e. an acid source, a char forming agent and a blowing agent for thermoplastics is associated with migration, water solubility and other problems. Those problems were solved by synthesis of additives that concentrate the three intumescent flame retardant elements in one molecule (Fontaine et al., 2008). The compound synthesized (Ratz & Aweeting, 1964) which has little intumescence is the intermediate product of the concentrate intumescent flame retardant.
In the molecule of the title compound ( Fig.1), three six-membered rings adopt boat conformations. The bond angle of C3-C4-C5 is 112.7 (4)° which is bigger than one of sp 3 hybrid, it may be the result of the co-existence of the three six-membered rings attached at C5. The torsion angles of S1/P1/O3/C3 and O1/C1/C4/C5 are -178.7 (3)° and -178.4 (4)°, respectively. Intermolecular O-H···O hydrogen bonds link the molecules with formation chains along c axis and effective stabilized the crystal structure (Table).

Experimental
A mixture of 62.6 g (0.46 mol) pentaerythritol and 77.9 g (0.46 mol) thiophosphoryl chloride was heated at 418 K in a 250 ml round-bottomed flask equipped for reflux, protected from atmospheric moisture and equipped with magnetic stirrer.
Evolution of hydrogen chloride ceased after 5 h. The resulting cake was extracted with 150 ml boiling water and cooled to room temperature. During the extracting some material remained undissolved and collected as a heavy oil at the bottom of the flask. The aqueous solvent was separated from this oil by decantation through a folded filter. The product was crystallized from water and afforded white crystals (62 g, yield 68.7%, m.p. 431-433 K).

Refinement
H atoms were positioned geometrically (C-H = 0.97Å and O-H = 0.82 Å) and refined using a riding model, with U iso (H) = 1.2U eq (C), U iso (H) = 1.5U eq (O). Fig. 1. The asymmetric unit of title compound with the atom numbering scheme. Displacement ellipsoids are drawn at the 30% probability level. H atoms are presented as a small spheres of arbitrary radius.

Special details
Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'s 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 > 2σ(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.