Crystal structure of 1,3-di-tert-butyl-2-chloro-1,3,2-diazaphosphorinane − a saturated six-membered phosphorus nitrogen heterocycle with a partially flattened chair conformation and a long PIII—Cl bond

Sublimation in vacuo slightly above room temperature gave crystals of the P-chloro-functionalized saturated six-membered N-heterocyclic title compound 1,3-di-tert-butyl-2-chloro-1,3,2-diazaphosphorinane. In the crystal, no interactions stronger than van der Waals forces are found between the molecules that neither suffer from chair conformation disorder nor from rotational disorder of the tert-butyl groups. Characteristic structural features are the partial flattening of the ‘cyclohexane-chair’ conformation at the heteroatom side of the six-membered ring and the length of the weakened P—Cl bond [2.2869 (6) Å].


Chemical context
Over the past two decades, P-chlorofunctionalized N-heterocyclic phosphanes (NHPCls) received considerable attention, mainly as precursors of N-heterocyclic phosphenium ions (NHPs) that are valence isoelectronic compounds of the wellknown N-heterocyclic carbenes (NHCs) (Papke et al., 2017), but also as educts of tetrakis(amino)diphosphanes (e.g. Bezombes et al., 2004;Blum et al., 2016;Edge et al., 2009;Frank et al., 1996), some of which reversibly dissociate to stable phosphinyl radicals ('jack-in-the-box dipnictines'; Hinchley et al., 2001), and as starting materials in the synthesis of mixed-valent tetrakis(amino)tetraphosphetes (Breuers et al., 2015;Frank et al., 1996). Furthermore, NHPCls and NHPs have been used as ligands in transition metal complexes (Thomas et al., 2018), some of which have a potential application in catalysis (Gatien et al., 2018). In the context of NHP chemistry, the majority of compounds are five-membered cycles, and especially P-chlorofunctionalized 1,3,2-diazaphospholenes (Denk et al., 1996;Carmalt & Lomeli, 1997) ISSN 2056-9890 have gained a widespread use as precursors for 1,3,2-diazaphospholenium cations (the most prominent class of NHPs) that are weak -donors and strong -acceptors (Caputo et al., 2008;Tuononen et al., 2007). A limited number of structurally characterized examples is known for the class of P-chlorofunctionalized four-membered NHPCls Cl-P<(NR) 2 >E and the related NHPs. The fourth ring member >E, joining the class-defining Cl-P<(NR) 2 fragment, is an >SiR 2 group in most cases (e.g. Breuers & Frank, 2016;Gü n et al., 2017;Mo et al., 2018;Mo & Frank, 2019;Veith et al., 1988) but some compounds containing >C N-R (Brazeau et al., 2012), >B-Ph (Konu et al., 2008) and >As-Cl (Hinz et al., 2015) have also been synthesized and structurally characterized. In contrast to the aforementioned compounds with four-and five-membered rings, six-membered NHPs and NHPCls are less present in recent publications, although 2-chloro-1,3,2diazaphophorinanes H 2 C<(CH 2 NR) 2 >P-Cl, for instance, have been known since the early 1970s Nifant'ev et al., 1977). Temperature-dependent dynamical NMR investigations showed that in solution these substances are not subject to a fast conformation change, like the ring-inversion process of cyclohexane, and that in the predominant conformation the chloro substituent is expected to be in the axial position and the residues on the nitrogen atoms are oriented 'diequatorial'. This gives rise to a quite complex 1 H-NMR spectrum with an AA 0 KK 0 QTX pattern (X = P, AA 0 KK 0 = C 4 and C 6 protons, Q and T = C 5 protons; Hutchins et al., 1972). Furthermore, the number and position of the signals in the 1 H-NMR spectrum are dependent on concentration, which was attributed to intermolecular chlorine-exchange mechanisms. Even though this parent class of six-membered NHPCls has been known for quite some time, no crystal structure analysis has thus far been reported. Herein, we present the crystal structure of the title compound that allows for a structural comparison with the most closely related four-or five-membered NHPCls known, on one hand, and with phospha-and 1,3,2-dioxaphosphacyclohexane derivatives, on the other hand.

Structural commentary
The molecular structure of 1 in the crystal is shown in Fig. 1. The molecule does not suffer from conformational disorder, which is often recognized in the solids of saturated N-heterocyclic compounds. The main characteristics of the molecule are: (i) the partially flattened chair conformation of the central six-membered heterocycle (displayed in more detail in Fig. 2) with an angle of 53.07 (15) between the plane defined by the carbon atoms and the best plane of C1, C3, N1 and N2, and an angle of 27.96 (7) between the latter plane and the plane defined by the nitrogen and phosphorus atoms; (ii) the equatorial orientation of both tert-butyl groups, enforced by the approximate trigonal-planar coordination of the nitrogen atoms [sums of angles 356.2 (N1) and 355.8 (N2)], in combination with the axial orientation of the chloro substituent ( Fig. 2) [out of plane angle: 106.83 (5) ]; (iii) the length of the P1-Cl1 bond, 2.2869 (6) Å , is substantially longer than the standard single bond (2.02 Å ; Brown, 2016) and the longest bond found in a six-membered NHPCl so far. Chair conformation of the molecule (H atoms are omitted for clarity); note the cyclohexane-like conformation at the 'carbon-atom side' [folding angle 53.07 (15) as compared to 54.5 (6) in the ordered, monoclinic phase of C 6 H 12 (Kahn et al., 1973)] and the 'semi-flattened' conformation [folding angle 27.96 (7) ] at the 'phosphorus/nitrogen-atom side'.

Supramolecular features
Inspection of the intermolecular distances gives no evidence for interactions stronger than van der Waals forces in the crystal of 1. The closest contact is given between Cl1 and the methylene group of the neighbouring molecule containing C1 at a ClÁ Á ÁC distance of 3.7134 (18) Å , symmetry related by the c glide plane (symmetry code: x, 1 2 À y, 1 2 + z). Fig. 3 shows the packing of the molecules in the crystal. Space group-symmetry gives rise to an appealing wave-like pattern.

Synthesis and crystallization
The title compound was prepared under an argon atmosphere in oven-dried glassware using standard Schlenk techniques, modifying a published procedure (Nifant'ev et al., 1977) by including a lithiation step. 3.75 g (20.1 mmol) of N,N 0 -di-tertbutyl-1,3-propanediamine were dissolved in a mixture of diethyl ether and n-hexane (35 ml/55 ml). 16 ml of an n-butyllithium solution (c = 2.5 mol l À1 in n-hexane, 40 mmol) were slowly added at 263 K. Half an hour later, the reaction mixture was allowed to reach room temperature and the resulting pale-yellow suspension was stirred for 16 h. 2.92 g of PCl 3 (21.3 mmol) were added dropwise over a period of 15 minutes at 195 K. To complete the reaction, the yellow reaction mixture was stirred for another hour with cooling and finally for two h at room temperature. Subsequently, the LiCl precipitate was filtered off and, after removal of the solvent under reduced pressure, the crude product was obtained as a yellow solid. Colourless block-shaped crystals suitable for X-ray structure determination were obtained by sublimation in a vacuum (3Á10 À2 mbar) at 313 K (30% yield; m.p. 327 K), by NMR analysis proved to be pure substance. 1

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. Positions of all hydrogen atoms were identified via subsequent ÁF syntheses. In the refinement, a riding model was applied using idealized C-H bond lengths (0.98-0.99 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 neighbouring C-C bonds. The U iso values were set to 1.5U eq (C methyl ) and 1.2U eq (C methylene ).  SHELXL2014 (Sheldrick, 2015b); molecular graphics: DIAMOND (Brandenburg, 2015); software used to prepare material for publication: SHELXL2014 (Sheldrick, 2015b). 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.