Tris(4-formylphenyl)phosphane oxide tetrahydrofuran hemisolvate

The title compound, C21H15O4P·0.5C4H8O, contains an ordered phosphane oxide in a general position and a tetrahydrofuran solvent molecule disordered about a twofold axis. All three aldehyde substituents are nearly coplanar with their attached benzene rings, with C—C—C—O torsion angles in the range 1.64 (17)–4.24 (19)°. All three have different conformations with respect to the P=O group, one syn, one anti, and one gauche. Two of the aldehyde substituents form intermolecular C—H⋯O contacts.

The title compound, C 21 H 15 O 4 PÁ0.5C 4 H 8 O, contains an ordered phosphane oxide in a general position and a tetrahydrofuran solvent molecule disordered about a twofold axis. All three aldehyde substituents are nearly coplanar with their attached benzene rings, with C-C-C-O torsion angles in the range 1.64 (17)-4.24 (19) . All three have different conformations with respect to the P O group, one syn, one anti, and one gauche. Two of the aldehyde substituents form intermolecular C-HÁ Á ÁO contacts.
The purchase of the diffractometer was made possible by grant No. LEQSF(1999-2000 Triphenylphosphane oxide (TPPO) has been extensively structurally studied, as a result of the high basicity of its O atom, which makes it an excellent hydrogen-bond acceptor. Its utility as a crystallization aid for molecules having hydrogenbond donors was reported by Etter & Baures (1988), which has led to its use in forming molecular cocrystals (Siegler et al., 2007). Also, it has four known polymorphs (Spek, 1987;Brock et al., 1985;Lenstra, 2007) and several known solvates (Thierbach et al., 1980;Baures & Silverton, 1990;Baures, 1991).

Experimental
To prepare (I), the precursor tris(4-formylphenyl)phosphane was first prepared following and combining elements of the procedures for the synthesis of tris(4-formylphenyl)phosphane (Bartlett et al., 1978) and bis(4-formylphenyl)dimethylsilane (Kumagai & Itsuno, 2001). A sample of 4-bromobenzaldehyde dimethyl acetal (5 ml, 29.9 mmol) was combined with 40 ml dry THF in an inert atmosphere in a round-bottom flask. The solution was brought to -78 °C under streaming N 2 , and n-butyllithium/hexanes 1.6 M (19.8 ml, 31.7 mmol) was added over approximately 1 h while stirring. The solution initially turned from colorless to light yellow, then to milky white. After 2 h, at -78 °C, PCl 3 (0.80 ml, 9.17 mmol) was added over a period of 15 minutes. When the PCl 3 was added, the solution turned orange-red. The solution was kept at -78 °C for another 1 h. Then the solution was allowed to come to room temperature over 1 h. The solvent was evaporated, leaving the crude acetal, which was dissolved in a mixture of dichloromethane and water. The solution was then washed: first with concentrated NaHCO 3 and then with brine. The organic phase was dried over Na 2 SO 4 . The organic phase was then evaporated, leaving a residue (5.24 g). The crude material was dissolved in 50 ml THF and 50 ml 2 M HCl. The solution was stirred under reflux conditions for 1 h under a stream of N 2 . To the solution, 50 ml of water and 50 supplementary materials sup-2 Acta Cryst. (2013). E69, o1362-o1363 ml of ethyl acetate were added. The organic phase was then washed, first with concentrated NaHCO 3 and then with brine, dried over Na 2 SO 4 and evaporated, leaving a residue (3.97 g). This residue, which is crude tris(4-formylphenyl)phosphane and tris(4-formylphenyl)phosphane oxide (I), was dissolved in 25% CHCl 3 / 75% ethyl acetate and applied to a silica gel column with 25% CHCl 3 / 75% ethyl acetate as the mobile phase. The column was run as a flash column. This process yielded pure tris(4-formylphenyl)phosphane, 1.51 g, 44% yield. Continuing to run the flash column produced pure tris(4-formylphenyl)phosphane oxide (I),1.92 g, 56% yield. Crystals of (I) were prepared by evaporation of a solution in THF over one week.

Refinement
H atoms were placed in idealized positions with C-H distances 0.95 -0.99 Å and thereafter treated as riding. U iso for H was assigned as 1.2 times U eq of the attached C atoms. The THF molecule is disordered about a twofold axis, and its atoms were assigned half occupancy.

Computing details
Data collection: COLLECT (Nonius, 2000); cell refinement: SCALEPACK (Otwinowski & Minor, 1997); data reduction: DENZO and SCALEPACK (Otwinowski & Minor, 1997); program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008). 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 > σ(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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (