Crystal structure of 4-methyl-2,6,7-trioxa-1-phosphabicyclo[2.2.2]octane

The title compound has a bicyclo[2.2.2] structure with the P atom at the prow and the bridge-head C atom, with the bonded methyl group, at the stern. The three six-membered rings in the bicyclo[2.2.2] structure have essentially identical good boat conformations.


Chemical context
Phosphorus-based ligands bind strongly to transition metals and these complexes offer a wide range of properties due to the high volume of accessible substituents (Downing & Smith, 2004;Tolman, 1977;Joslin et al., 2012). Complexation experiments with these ligands can yield mono-or bi-nuclear complexes (van den Beuken et al., 1997). Phosphorus-based complexes are an important class of compounds in homogeneous catalysis and coordination chemistry (Downing & Smith, 2004;Kü hl, 2005). In particular, we have noted interesting studies comparing the donor ability of bicyclic phosphites and the related acyclic phosphites; the phosphorus atom in the former shows more positive charge than in the acyclic phosphites and, hence, the donor ability of bicyclic phosphites is lower than that of the related acyclic phosphites (Vande Griend et al., 1977;Joslin et al., 2012). The present work is a continuation of an investigation into the synthesis and study of bi-and tri-cyclic, penta-and hexa-coordinated phosphoranes to form anionic, neutral and zwitterionic compounds (Said et al. 1996;Timosheva, et al. 2006;Kumara Swamy & Satish Kumar, 2006). In this paper, we report the synthesis, clean isolation and crystal structure of 4-methyl-2,6,7-trioxa-1phosphabicyclo[2.2.2]octane (Tolman, 1977;Joslin et al., 2012).

Structural commentary
The molecular structure of the title compound, Fig. 1, shows a bicyclo[2.2.2] structure with the phosphorus atom as one ISSN 2056-9890 bridge-head atom and C3, with the bonded methyl group, as the other. The three six-membered rings in the bicyclo[2.2.2] structure have essentially identical, good boat conformations. The P-O bond lengths are very similar, lying in the range 1.613 (2)-1.616 (2) Å ; the O-P-O angles at the prow have angles in the range 100.17 (9)-101.34 (10) , whereas the C-P-C angles at the stern lie in the range 107.99 (17)-109.08 (18) .
A comparison between acyclic and bicyclic phosphites based on the 'hinge' effect has shown (Vande Griend et al., 1977;Joslin et al., 2012) that the O-P-O and P-O-C angles, a and b in Scheme 1, change upon ligation with a metal. Due to the steric profile of the bicyclic phosphite, the changes here in a, a 0 and b, b 0 upon metal ligation are less than in acyclic phosphites. Verkade had pointed out earlier that the porbital overlap between P and O in bicyclic phosphites is less than in acyclic phosphites, making P more positive and therefore reducing the basicity of P relative to that in acyclic phosphites (Vande Griend et al., 1977); hence, the coordination ability of acyclic phosphites is higher than that of bicyclic phosphites (Verkade, 1972). A variety of multi-cyclic phosphorus compounds including their coordination to various metals has been studied. Based on the trends found in basicity, it is expected that the title compound would show a donating ability to metal centres very similar to that of the more commonly studied bicyclic phosphite P(OCH 2 ) 3 CEt (Verkade, 1972). The average of O-P-O bond angle (a, Scheme 1) in our study is 100.7 o , whereas the average O-P-O bond angle in coordinated phosphites (a 0 , Scheme 1) is larger, e.g. in Ru{P(OCH 2 ) 3 CEt}Cl 2 , it is 102.5 o (Joslin et al., 2012), the same as in [Rh 2 I 2 (C 6 H 5 N 2 O 2 ) 2 (COMe) 2 {-P(OCH 2 ) 3 CMe} 2 ] (Venter et al., 2009); this suggests a slightly larger Tolman angle (Tolman et al., 1977) after metal ligation. In another study, the enhanced -accepting ability of the bicyclic phosphite ligand compared to the PPh 3 and other phosphine ligands was demonstrated clearly in the shorter M-P bond distances in the bicyclic phosphite complexes (Erasmus et al., 1998).

Supramolecular features
Contacts between molecules are at normal van der Waals distances, the shortest of which is H4BÁ Á ÁO6 0 , at 2.58 Å ( Table 1). The nearest neighbours of the phosphorus atom are hydrogen atoms at distances of at least 3.09 Å . A view of the packing along the b axis is shown in Fig. 2. A view of a molecule of bicyclo-P(OCH 2 ) 3 CMe, indicating the atomnumbering scheme. Displacement ellipsoids are drawn at the 30% probability level. Table 1 Hydrogen-bond geometry (Å , ).

Figure 2
A view along the crystallographic b axis.
Within each group, there is very little variation in the P-O distances. The bond angles in the bicyclic structure are quite constrained, but we do note a trend, down the four groups of increasing P-O distances, of a corresponding decrease in O-P-O angles from ca 107 to 100 .

Synthesis and crystallization
To 4.26 g (35.46 mmol) of 2-(hydroxymethyl)-2-methylpropane-1,3-diol in 70 mL of dry benzene at RT was added 4.26 g (106.38 mmoles in mineral oil 60%) of NaH in small portions over a period of 20 minutes. The mixture was stirred for 3h before 4.87 g (35.46 mmol) of PCl 3 were added dropwise over a period of 20 mins in benzene (10 mL) using a dropping funnel. The reaction mixture was stirred overnight before NaCl was removed by filtration under nitrogen cover. Benzene was removed completely under low pressure. 5 mL of diethyl ether was added, followed by 3 mL of n-hexane. The mixture was placed in deep freeze to afford the title compound as a white solid (yield 4.52 g, 86%; m.p. 369-373 K). The product was purified further by sublimation at 393 K/0.5 mm to yield crystals. 1

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2.

Special details
Experimental. CrysAlisPro, Agilent Technologies, Version 1.171.36.21 Empirical absorption correction using spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm. 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.