Synthesis and crystal structure of 1,3-di-tert-butyl-2-chloro-4,4-diphenyl-1,3,2λ3,4-diazaphosphasiletidine

A new Si,Si-diphenyl-, P-chlorofunctionalized diazaphosphasiletidine has been synthesized. The crystal structure of the highly moisture-sensitive four-membered heterocyclic molecular compound, which has a remarkably elongated P—Cl bond, is described and compared with related compounds and their arrangement of molecules in the solid state.


Chemical context
Diazaphosphasiletidines are heterocyclic compounds that contain an SiN 2 P four-membered ring as the central building block. The first synthesis was described in the year 1963 (Fink, 1963) and compounds of the class have attracted considerable attention in phosphorus chemistry (e.g. Scherer et al., 1982;Frank et al., 1996;Mo et al., 2018). The Pchlorosubstituted diazaphosphasiletidines are well known members of this class and syntheses of such compounds have been described in the literature over a couple of decades (Klingebiel et al., 1976;Eichhorn & Nö th, 2000). They have found widespread use as reagents for reactions based on the P-chlorofunctionalization. Our research group, for instance, has shown that they play a crucial role in the preparation of dispirocyclic tetraphosphetes (Frank et al., 1996;Breuers et al., 2015) and diazaphosphasiletidine adducts with P-coordination Gü n et al., 2017). However, due to their high moisture sensitivity, the structural characterization of such P-chloroderivatives by X-ray diffraction remains a challenge. There are only two reports on the crystal structure of P-chlorosubstituted diazaphosphasiletidines of type Me 2 Si(NR) 2 PCl, namely 2-chloro-1,3-bis(2,4,6-trimethylphenyl)-4,4-dimethyl-1,3,2 3 ,4-diazaphosphasiletidine (A; Breuers & Frank, 2016) and 1,3-di-tertbutyl-2-chloro-4,4-dimethyl-1,3,2 3 ,4-diazaphosphasiletidine (B; Gü n et al., 2017), and there is only one report on a structure of type Ph 2 Si(NR) 2 PCl, namely 2-chloro-1,3-di-tert- ISSN 2056-9890 pentyl-4,4-diphenyl-1,3,2 3 ,4-diazaphosphasiletidine (C; Mo et al., 2018). Crystals of the first structurally characterized chloro-substituted diazaphosphasiletidine A contained approximately 12% of a second compound, namely 2-chloro-1,3-bis(2,4,6-trimethylphenyl)-4-chloro-4-methyl-1,3,2 3 ,4-diazaphosphasiletidine. With respect to this impurity, an Si,Sidiphenyl-substituted diazaphosphasiletidine (C) has successfully been introduced to preparative chemistry to avoid problems related to the content of Si,P-bis(chloro)functionalized species present in samples of the Si,Si-dimethyl derivative. However, the crystal-structure determination of C suffered from severe disorder. All the aspects mentioned before persuaded us to focus on preparation of single crystals of the title compound suitable for structure determination. After extensive attempts, we were finally able to grow single crystals by slow sublimation in vacuo and confirmed its composition and its structure via X-ray diffraction.

Database survey
In compound B, molecules are connected via very weak P-Cl bridging bonds, which leads to a weak state of dimerization. Generally, the strength of association of molecules via E-Cl bridging bonds increases from P to Bi in related diazasileditines of type Me 2 Si(NR) 2 ECl. Me 2 Si(N t Bu) 2 AsCl contains dimers and in the antimony and the bismuth analogues the molecules are connected into chains via bridging Cl atoms . In contrast, the solid-state structures of the title compound, A, C, Ph 2 Si(N t Bu) 2 AsCl (Belter, 2016) and Me 2 Si(NDipp) 2 SbCl (Ma et al., 2013) do not exhibit intermolecular EÁ Á ÁCl interactions and consist of isolated molecules.

Synthesis and crystallization
The title compound was prepared ( Fig. 3) according to generally known procedures under an argon atmosphere in oven-dried glassware using Schlenk techniques, modifying a published protocol (Eichhorn & Nö th, 2000). 5.5 g (16.8 mmol) of N,N 0 -di( t butyl)-Si,Si-diphenylsilanediamine were dissolved in 60 ml n-pentane. 13.6 ml of a n-butyllithium solution (c = 2.5 mol/l in n-hexane, 16.8 mmol) were added at 263 K. The reaction mixture was stirred for 24 h at room temperature. Cooling to 178 K and addition of 1.5 ml (16.8 mmol) PCl 3 yielded an off-white suspension. This was stirred for 3 h. After filtration and removal of the solvent under reduced pressure, the crude product was obtained as an off-white solid. Sublimation at 333 K under reduced pressure yielded colourless crystals within a couple of hours (77% yield based on PCl 3 ). Reaction scheme for the preparation of the title compound.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. Positions of the majority of the hydrogen atoms were identified via subsequent Fourier syntheses. In the refinement, a riding model was applied using idealized C-H bond lengths (0.95-0.98 Å ) as well as H-C-H and C-C-H angles. In addition, the H atoms of the CH 3 groups were allowed to rotate around the neighboring C-C bonds. The U iso values were set to 1.5U eq (C methyl ) and 1.2U eq (C ar ). To account for residual electron density in the regions of the two tert-butyl groups and for elongated anisotropic displacement ellipsoids of several carbon atoms that did not appear to be physically meaningful, a two-position disorder for each tert-butyl group was introduced with partial occupation sites for all carbon atoms but the tertiary ones C1 and C5 [occupancy ratio 0.752 (6):0.248 (6) ratio (group containing C1) and 0.878 (9):0.122 (9) ratio (C5); in Figs. 1 and 2 disorder is omitted for clarity]. Appropriate same distance and anisotropic displacement restraints and some equivalent anisotropic displacement parameters had to be applied to stabilize the geometry of the minor occupancy parts of the partial occupation site models. The correct absolute structure of the non-centrosymmetric structural model is confirmed by the Flack parameter (Table 1).  SHELXL2014/7 (Sheldrick, 2015b); molecular graphics: DIAMOND (Brandenburg, 2016); software used to prepare material for publication: SHELXL2014/7 (Sheldrick, 2015b) and 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (