[4-(Dimethylamino)phenyl]diphenylphosphine selenide

In the title compound, C20H20NPSe, the P atom lies in a distorted tetrahedral environment. The Tolman cone angle is 157° indicating steric crowding at this atom. In the crystal, weak C—H⋯Se interactions create linked dimeric units and C—H⋯π interactions are also observed.

In the title compound, C 20 H 20 NPSe, the P atom lies in a distorted tetrahedral environment. The Tolman cone angle is 157 indicating steric crowding at this atom. In the crystal, weak C-HÁ Á ÁSe interactions create linked dimeric units and C-HÁ Á Á interactions are also observed.
Discussed here, as part of an ongoing study, is the structure of the title compound, which is the selenium derivative of the phosphane PPh 2 (4-NMe 2 -C 6 H 4 ), where Ph = C 6 H 5 .
The title compound (see Fig. 1) crystallizes in the monoclinic space group, P 2 1 /c (Z=4), with its molecules adopting a distorted tetrahedral arrangement about the phosphorus atom. The average C-P-C and Se-P-C angles are 105.28 (11)° and 113.40 (8)° respectively. The Se-P distance is 2.1069 (7) Å which is significantly shorter than the 2.1241 (5) Å reported for the analogous SePCy 2 (4-NMe 2 -C 6 H 4 ) compound (Phasha et al., 2012). An increase of 26 Hz in the 1 J( 31 P-77 Se) NMR coupling is also observed for the title compound compared to the dicyclohexcyl analogue. This is in accordance with Bent's rule that the s-character of the phosphorus lone pair electrons will decrease with more electrondonating substituents (Bent, 1961).
To describe the steric demand of phosphane ligands a variety of models have been developed, of which the Tolman cone angle (Tolman, 1977) is still the most commonly used method. Applying this model to the geometry obtained for the title compound (and adjusting the Se-P bond distance to 2.28 Å) we calculated an effective cone angle from the geometry found in the crystal structure as 157° (Otto, 2001). This value is comparable to the cone angles calculated for the structure of the free (Lynch et al., 2003) and oxidized (Dreissig & Plieth, 1972) forms of the phosphane (calculated as 158° and 161° respectively). The orientation of the substituents for the oxidized derivative is comparable to that of the title compound, whereas the free phosphane shows substantial differences in its orientations. To illustrate this observation, the coordinates of P and ipso C-atoms of the three structures are superimposed using Mercury (see Fig. 2; Macrae et al., 2006;Weng et al., 2008a;Weng et al., 2008b). The reason for the different substituent orientations are possibly due to different interactions observed to the packing of these structures. It is also interesting to note that coordination of the phosphane to transition metals does not induce significant steric crowding, and hence a smaller cone angle, of the ligand at the coordination sphere. Data extracted for these coordination complexes from the Cambridge Structural Database shows an average cone angle of 159° (Allen, 2002; 9 observations with metals: Au, Pt, Pd, Rh and Cu).
Packing in the crystals is assisted by weak C-H···Se interactions creating linked dimeric units of the title compound. In addition C-H···π interactions are also observed (see table 1

Refinement
The aromatic and methyl H atoms were placed in geometrically idealized positions (C-H = 0.95-0.98) and allowed to ride on their parent atoms, with U iso (H) = 1.2U eq (C) for aromatic and U iso (H) = 1.5U eq (C) for methyl H atoms respectively.
Methyl torsion angles were refined from electron density.  A view of the title complex, showing the atom-numbering scheme and 50% probability displacement ellipsoids.

Figure 2
Conformational similarity between the title compound (blue), the phosphine oxide (red) and the free phosphine (green).
The root mean squared deviations (RMSD) to the title compound were 0.0279 Å (oxide derivative) and 0.0473 Å (free phosphine). Special details Experimental. The intensity data was collected on a Bruker Apex DUO 4 K CCD diffractometer using an exposure time of 20 s/frame. A total of 2352 frames were collected with a frame width of 0.5° covering up to θ = 28.66° with 99.3% completeness accomplished. 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 > 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