Dicyclohexyl(4-isopropylphenyl)phosphane selenide

In the title compund, C21H33PSe, the Se=P bond is part of a distorted tetrahedral environment on the P atom. Both cyclohexyl groups adopt chair conformations. A cone angle of 170° was calculated using an adaptation of the Tolman model. Intermolecular C—H⋯Se and C—H⋯Cg contacts are observed (Cg is the centroid of the benzene ring).

In the title compund, C 21 H 33 PSe, the Se P bond is part of a distorted tetrahedral environment on the P atom. Both cyclohexyl groups adopt chair conformations. A cone angle of 170 was calculated using an adaptation of the Tolman model. Intermolecular C-HÁ Á ÁSe and C-HÁ Á ÁCg contacts are observed (Cg is the centroid of the benzene ring).   Table 1 Hydrogen-bond geometry (Å , ).

Related literature
Cg1 is the centroid of the C13-C18 benzene ring. Research funding from the University of Johannesburg is gratefully acknowledged.

Comment
The bonding of phosphorus to transitional metals have being investigated extensively, with several attempts to divide the properties of the phosphorus ligand into steric and electronic effects. Various techniques such as single-crystal X-ray crystallography, multi nuclear NMR and IR (Roodt et al., 2003) have been used to this extent. Recently we have also included selenium derivatives of the phosphorus compounds into this study (Muller et al., 2008). This route seems viable as the use of expensive transition metals and steric influence from other ligands in the coordination sphere are eliminated, leaving only crystal packing effects as an additional influence on the steric property of the phosphorus ligand. As part of this investigation we report here the selenium derivative of PCy 2 (4-i Pr-C 6 H 4 ) where Cy = cyclohexyl and i Pr = isopropyl.
Molecules of the title compound ( Fig. 1) adopts a distorted tetrahedral arrangement about the P atom with average C-P -C and Se-P-C angles of 106.0° and 112.7° respectively. The cone angle was found to be 170° when the Se-P distance was adjusted to 2.28 Å (the default value from Tolman, 1977). This value is ca 5° larger than previous reported values where the present phosphine was bonded to a transition metal centre (Makhoba et al., 2011;Vuba & Muller, 2012). This indicates to some extend the flexibility of this phosphine ligand and its ability to use space to enable less crowding of its substituents. Weak intermolecular C-H···Se and C-H···Cg contacts are observed (Table 1

Experimental
KSeCN (10 mg, 0.0694 mmol) and PCy 2 (4-i Pr-C 6 H 4 ) (21.96 mg, 0.0694 mmol) were both dissolved in a minimum amount of methanol (10-20 ml). The KSeCN solution was added drop wise (5 min) to the phosphine solution while stirring at room temperature. The final solution was left to evaporate slowly in order to give crystals that are suitable for single-crystal X-ray study.

Refinement
All H atoms were positioned in geometrically idealised positions with C-H = 1.00 Å, 0.99 Å, 0.98 Å and 0.95 Å for methine, methylene, methyl and aromatic H atoms respectively and constrained to ride on their parents atoms with U iso (H) = 1.2U eq , except for methyl where U iso (H) = 1.5U eq was utilized. The initial positions of methyl H atoms were located from a Fourier difference map and refined as fixed rotor. The highest residual electron density of 0.41 e Å -3 was located 0.76 Å from C19, and the deepest hole of -0.24 e Å -3 is 0.81 Å from P1. Both represent no physical meaning.   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.