Crystal structure of 2-chloro-1,3-bis(2,6-diisopropylphenyl)-1,3,2-diazaphospholidine 2-oxide

The synthesis, spectroscopic and crystal structure of 2-chloro-1,3-bis(2,6-diisopropylphenyl)-1,3,2-diazaphospholidine 2-oxide are reported.

1. Chemical context 1,3,2-Diazaphospholidines are a class of N-heterocyclic phosphines (NHPs) that feature an N-P-N moiety bridged by a C 2 H 4 fragment, thus forming a five-membered ring. Derivatives are often substituted by alkyl, aryl, or halogen groups at the phosphorus position (denoted as position 2), allowing them to serve as both ligands and/or precursors in organometallic chemistry (Gudat, 2010). The title compound, 2-chloro-1,3-bis(2,6-diisopropylphenyl)-1,3,2-diazaphospholidine 2-oxide, is closely related to these compounds and its analogs are commonly used as precursor molecules for the synthesis of pharmaceuticals targeted towards immunosuppressants and chemotherapy medications (Gholivand & Mojahed, 2005). The crystal structure of the title compound is reported herein and features a saturated five-membered NHP substituted at the phosphorus position by both O and Cl atoms.

Structural commentary
The molecular structure of the title compound is shown in Fig. 1. The title compound crystallizes in the monoclinic space group P2 1 /n with one molecule present in the asymmetric unit. Bond lengths between the flanking nitrogen atoms show a statistical difference when compared to each other [P1-N1 = 1.6348 (14) Å and P1-N2 = 1.6192 (14) Å ] and is likely caused by the half-chair (or envelope) conformation of the heterocycle at the C2 position. The N-P-N bond angle of 95.60 (7) deviates significantly from an ideal tetrahedral geometry. Bond lengths between P1-Cl1 and P1-O1 are 2.0592 (7) and 1.4652 (12) Å , respectively, with a bond angle of 105.51 (5) for the O-P-Cl atoms. The isopropyl groups are oriented away from the central five-membered ring and lead to intramolecular short-contact D-HÁ Á ÁA interactions between methine atoms H9, H12, H21, and H24, and N1 and N2. Intramolecular short-contact D-HÁ Á ÁA interactions are also present for Cl1 and O1 atoms and are summarized in Table 1. The steric demands of the bulky 2,6-diisopropylphenyl groups cause the aromatic rings to twist away from the central five-membered ring with torsion angles of À75.66 (19) and 83.39 (19) for P1-N1-C3-C4 and P1-N2-C15-C20, respectively. The dihedral angles between the heterocyclic ring (all atoms) and the C3-C8 and C15-C20 aromatic rings are 76.61 (9) and 88.75 (9) , respectively.

Figure 2
The packing of the title compound, showing the formation of C-HÁ Á ÁO hydrogen bonds (red and cyan lines).

Figure 1
The molecular structure of the title compound, showing 50% probability displacement ellipsoids. H atoms have been omitted for clarity.
which features N-benzyl substituents and a cyclohexyl ring fused to the bridging ethylene C atoms.

Synthesis and crystallization
The synthesis of the title compound was achieved using a similar method as used for 2-chloro-1,3-bis(2,6-diisopropylphenyl)-1,3,2-diazaphospholidine (Caputo et al., 2008), except phosphoryl chloride was used instead of phosphorus trichloride. In a 200 ml Schlenk flask, 1.142 g (3.00 mmol, 1 eq.) of N,N 0 -bis(2,6-diisopropylphenyl)ethane-1,2-diamine were dissolved in 45 ml of THF producing a colourless solution. Separately 0.478 g (3.11 mmol, 1.04 eq.) of phosphoryl chloride and 0.959 g (9.48 mmol, 3.16 eq.) of N-methylmorpholine were dissolved in 75 ml of THF producing a colourless solution, and transferred to a 125 ml pressureequalizing dropping funnel. The diamine solution was cooled to 195 K using a liquid nitrogen/acetone bath and monitored using a thermocouple, and once cold (ca 10 minutes) the phosphoryl chloride mixture was added dropwise to the diamine solution over 30 minutes. Once the addition was complete, the colourless reaction mixture was left to stir at 195 K for 60 minutes, after which it was allowed to warm to room temperature and left to stir for two days at room temperature. The reaction was monitored by 31 P{ 1 H} NMR spectroscopy, and became pale yellow in colour with a slight amount of colourless precipitate as it proceeded. Once the starting material was completely consumed, the reaction mixture was dried in vacuo to give a pale-yellow coloured solid. Extraction of this solid with 50 ml of a 3:2 mixture of pentane:THF produced the desired product as a pale-yellow coloured solution following filtration through Celite, which when dried in vacuo afforded 0.919 g (66%) of the desired product as a faintly yellow coloured powder. Crystals of the product in the form of colourless blocks were obtained by concentrating the filtrate and storing in a 238 K freezer overnight.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms were included in geometrically idealized positions and refined using a riding model [C-H = 0.95-0.99; U iso (H) = 1.2-1.5U eq (C)]. The methyl H atoms were allowed to rotate, but not to tip, to best fit the electron density.   Data collection: APEX2 (Bruker, 2008); cell refinement: SAINT (Bruker, 2008); data reduction: SAINT (Bruker, 2008); program(s) used to solve structure: SHELXT2014 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2016 (Sheldrick, 2015b); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and PLATON (Spek, 2009); software used to prepare material for publication: publCIF (Westrip, 2010). Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.