2-Isobutyl-2-phosphabicyclo[3.3.1]nonane 2-selenide

The title compound, C12H23PSe, represents the first structure of a phosphine containing the bicyclic 2-phosphabicyclo[3.3.1]nonane (VCH) unit. It contains two chiral centres per molecule which can be either R,R- or S,S and crystallizes as a centrosymmetric, racemic micture of the enantiomers. The P—Se bond distance of 2.1360 (16) Å is typical for these compounds. The Tolman cone angle (2.28 Å from P) was calculated as 163°, and the effective cone angle (using the crystallographically determined P—Se bond distance) is 168°.

The title compound, C 12 H 23 PSe, represents the first structure of a phosphine containing the bicyclic 2-phosphabicyclo-[3.3.1]nonane (VCH) unit. It contains two chiral centres per molecule which can be either R,R-or S,S and crystallizes as a centrosymmetric, racemic micture of the enantiomers. The P-Se bond distance of 2.1360 (16) Å is typical for these compounds. The Tolman cone angle (2.28 Å from P) was calculated as 163 , and the effective cone angle (using the crystallographically determined P-Se bond distance) is 168 .

Comment
It is well established that the steric and electronic properties of phosphine ligands have a major influence on the chemistry of its metal species. Sevaral methods are used to quantify the electronic characteristics of phosphines, including NMR measurements of first order Pt-P, Rh-P, Se-P and P-BH 3 coupling constants (Allen &Taylor, 1982 andSteyn, 2000) and measuring of CO stretching frequencies in complexes such as [Ni(L)(CO) 3 ] (Tolman, 1977) or trans-[RhCl(CO)(L) 2 ] (Otto & Roodt, 2004).
Phosphine ligands containing bicyclic substituents have been shown to add significant benefits to the catalytic performance of several homogeneously catalysed systems (Bungu & Otto, 2007a). The best known example is the 9phosphabicyclo[3.3.1]nonane and 9-phosphabicyclo[4.2.1]nonane mixture of isomers (Phoban family of ligands) patented by Shell (Van Winkle et al., 1969) for modified cobalt hydroformylation. Sasol has reported on the use of bicyclic phosphines derived from (R)-(+)-Limonene (Lim family) , Dwyer et al., 2004 and vinylcyclohexene (VCH family)  for similar applications.
After a convenient synthetic protocol for phosphine selenides were developed (Otto et al., 2005) we extensively used the Se-P coupling constants for the quantification of electronic properties of phosphine ligands (Bungu & Otto, 2007b).
Smaller values for the coupling constants correspond with ligands of higher basicity (more electron donating). We now report the synthesis and crystallographic characterization of the title compound, (I), 2-isobutyl-2-phosphabicyclo[3.3.1]nonane 2-selenide (VCH-i Bu) which represents the first crystal structure of a VCH family member.
Compound (I) crystallizes in the monoclinic space group P2/c and consists of the VCH backbone, the iso-butyl side chain and the selenium atom coordinated to phosphorus in a tetrahedral fashion. The compound contains two chiral centres on the VCH backbone (C13 and C15) which can be R,R-or S,S (as in the arbitrarily chosen asymmetric molecule; Fig. 1) and it crystallizes as a racemic mixture on account of the centrosymmetric space group. The P atom could also be considered as chiral based on the four different substituents. All bond distances and angles are within normal ranges. Even though larger Se-P coupling constants are indicative of more effective s-orbital overlap no clear trends are evident in the Se-P bond distances.
The packing in the unit cell is governed by van der Waals forces alone since no pertinent intramolecular interactions were evident. The Tolman-(2.28 Å from P) and effective cone angles (using the crystallographically determined Se-P bond distance) were calculated (Otto, 2001) resulting in values of 163 and 168° respectively. The title compound was prepared according to the procedure described previously (Bungu & Otto, 2007b), colourless blocks of (I) were obtained by evaporation of a dichloromethane solution. 31 P (CDCl 3 ): 29.72 p.p.m. ( 1 J Se-P = 684 Hz) and 29.88 p.p.m. ( 1 J Se-P = 670 Hz).

Refinement
The H atoms were placed in geometrically idealized positions (CH = 0.98, CH 2 = 0.97 and CH 3 = 0.96 Å) and constrained to ride on their parent atoms with U iso (H) = 1.2U eq (C) for CH and CH 2 and U iso (H) = 1.5U eq (C) for CH 3 . Fig. 1

Special details
Experimental. The intensity data were collected on a Bruker X8 ApexII 4 K Kappa CCD diffractometer using an exposure time of 40 s/frame with a frame width of 0.3°; a total of 1315 frames were collected.
The crystals were of poor quality and were, as a precautionary measure, covered with Canada balsam. Consequently some intensity in the reflextions were sacrificed and the completeness is somewhat low at high angles. 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 Rfactors(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.