Tris(4-methylphenyl)phosphine selenide

In the title molecule, C21H21PSe or PSe(C7H7)3, the P atom has a distorted PSeC3 tetrahedral environment, formed by the Se atom [P=Se = 2.1119 (5) Å] and three aryl rings. Two short intramolecular C—H⋯Se contacts occur. In the crystal, weak intermolecular C—H⋯Se interactions link the molecules into zigzag double chains propagating in [100]. The previous report of this structure [Zhdanov et al. (1953 ▶). Dokl. Akad. Nauk SSSR (Russ.) (Proc. Nat. Acad. Sci. USSR), 92, 983–985] contained no geometrical data.


Comment
There has been extensive development in understanding the transition metal phosphorous bond by various groups, including our own, with various techniques such as single-crystal X-ray crystallography, multi nuclear NMR and IR (Roodt et al., 2003). As part of this systematic investigation we are now also studying selenium bonded phosphorus ligands (see Muller et al. 2008) This way there is no steric crowding effect, abeit crystal packing effects, as normally found in transition metal complexes with bulky ligands, e.g. in trans-[Rh(CO)Cl{P(OC 6 H 5 ) 3 } 2 ] coneangles variation from 156° to 167° was observed for the two phosphite ligands (Muller, et al. 2006). The J( 31 P-77 Se) coupling can also be used as an additional probe to obtain more information regarding the nature of the phosphorous bond. Reported here, as part of the above continuing study, the single-crystal structure of the compound P(4-Me-C 6 H 3 ) 3 is presented. This was done as no geometrical data are available from the CCDC (Cambridge Structural Database; Version 5.31, update of August; Allen, 2002) on the previously published structure reported by Zhdanov et al., 1953. Crystals of the title compound, (I), packs in the P2 1 /c (Z = 4) space group with the molecules lying on general positions.
All geometrical features of the molecule (Allen, 2002) are as expected with the selenium atom and the three aryl groups adopting a distorted arrangement about phosphorous (see Fig. 1 and Table 1). The cone angle was found to be 161.1° when the Se-P distance is adjusted to 2.28 Å (the default value used in Tolman, 1977).
The packing in the unit cell show Se-atoms forming dimeric units with bi-furcated H-atoms of C12. These units are propagated along the [100] direction with additional weak C-H···Se interactions (See Table 2, Fig. 2).

Experimental
SeP(4-Me-C 6 H 3 ) 3 and KSeCN were purchased from Sigma-Aldrich and used without purification. Eqimolar amounts of KSeCN and the SeP(4-Me-C 6 H 3 ) 3 compound (ca 0.04 mmol) were dissolved in the minimum amounts of methanol (10 -20 ml). The KSeCN solution was added drop wise (5 min.) to the phosphine solution with stirring at room temperature.
The final solution was left to evaporate slowly until dry to give colourless blocks.

Refinement
The aromatic and methylene H atoms were placed in geometrically idealized positions (C-H = 0.93 -0.98 Å) and constrained to ride on their parent atoms with U iso (H) = 1.2U eq (C) and U iso (H) = 1.5U eq (C) respectively, with torsion angles refined from the electron density for the methyl groups. The highest residual electron density is located 0.94 Å from Se.