3,9-Dichloro-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5.5]undecane-3,9-dione

In the title compound, C5H8Cl2O6P2, the two six-membered rings display chair conformations. The P=O bond distances are 1.444 (2) and 1.446 (2) Å. Weak intermolecular C—H⋯O hydrogen bonds are present in the crystal structure.

In the title compound, C 5 H 8 Cl 2 O 6 P 2 , the two six-membered rings display chair conformations. The P O bond distances are 1.444 (2) and 1.446 (2) Å . Weak intermolecular C-HÁ Á ÁO hydrogen bonds are present in the crystal structure.

Comment
Studies of pentaerythritol diphosphonate compounds have been significant interested. On one hand, the compounds have been reported to act as one of the most important reaction intermediates of fire retardant agents (Tanabe et al., 2005). On the other hand, it seems to be a good candidate in modifying the stability of polymers (Granzow, 1981). The findings have triggered the development of new flame retardant materials. As an extension of the work on the structural characterization of pentaerythritol diphosphonate compounds, the preparation and crystal structure of the title compound, (I), is proposed here.
The asymmetric unit of (I) contain a spiro[5.5]undecane molecule ( Fig. 1). Several compounds with similar structures have been reported previously (Heinemann et al., 1994;Zhang et al., 2006). The bond lengths and angles are within normal ranges (Allen et al., 1987;Elnagar et al., 2000). The six-membered rings of (I) have the chair conformation consistent with the steric difference in this conformation between opposite ends of the molecule. In addition, the C1-C3-C2 and C4-C3-C5 angles are in the range of 109.4 (3)-109.2 (3)°,the P-Cl bond lengths are 2.0050 (14) and 2.0047 (13) Å, respectively. In the crystal structure of (I), The non-classic C-H···O hydrogen bonds ranging from 3.099 (4) to 3.260 (4) Å contributed to the stability of the crystal packing.

Experimental
The title compound was prepared by reaction of pentaerythritol with phosphorus oxychloride in acetonitrile according to the reported procedures (Li et al., 2002). Crystals were produced at the bottom of the vessel on slow evaporation of acetic acid solution.

Refinement
All H atoms were placed geometrically with C-H = 0.99 Å and refined using a riding atom model with U iso (H) = 1.2U eq (C).   4,8, as those based on F, and R-factors based on ALL data will be even larger.