1-(4-Chlorophenyl)-2-[tris(4-methylphenyl)-λ5-phosphanylidene]butane-1,3-dione

In the title ylide, C31H28ClO2P [common name α-acetyl-α-p-chlorobenzoylmethylenetri(p-tolyl)phosphorane], the dihedral angle between the 4-chlorophenyl ring and that of the ylide moiety is 66.15 (10)°. The geometry around the P atom is slightly distorted tetrahedral [angle range = 105.22 (8)–115.52 (9)°] and the carbonyl O atoms are syn-oriented with respect to the P atom. The ylide group is close to planar [maximum deviation from the least-squares plane = 0.006 (2) Å] and the P—C, C—C and C=O bond lengths are consistent with electron delocalization involving the O atoms.

In the title ylide, C 31 H 28 ClO 2 P [common name -acetyl--pchlorobenzoylmethylenetri(p-tolyl)phosphorane], the dihedral angle between the 4-chlorophenyl ring and that of the ylide moiety is 66.15 (10) . The geometry around the P atom is slightly distorted tetrahedral [angle range = 105.22 (8)-115.52 (9) ] and the carbonyl O atoms are syn-oriented with respect to the P atom. The ylide group is close to planar [maximum deviation from the least-squares plane = 0.006 (2) Å ] and the P-C, C-C and C O bond lengths are consistent with electron delocalization involving the O atoms.

Experimental
A mixture of parachlorobenzoyltri(paratolyl)phosphorane (0.03 mol) and acetic anhydride (0.3 mol) in dry chloroform (10-20 ml) was stirred at 60°C. The reaction was monitored by TLC. The resulting dark solution was evaporated at 80 °C (12 ml) to give a glue which was triturated with ether and the precipitated product was filtered and recrystallized using a solvent diffusion technique (yield; 65%: m.p. 459-458).

Refinement
The hydrogen atom positions were calculated and refined using a riding model technique, with C-H aromatic = 0.93 Å or C -H methyl = 0.96 Å, with U iso (H) = 1.2U eq (C)(aromatic) or 1.5U eq (C)(methyl).

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