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

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


Chemical context
Phosphine azides possess at least one azide group attached to phosphorus and display a broad range of reactivity that is directly dependent on the other substituents attached to the P atom. One of the most interesting properties of these molecules is that both free and coordinated alkyl and aryl derivatives are much more reactive than their corresponding amino derivatives, as demonstrated by their lower thermal and photochemical stability. For example, the phosphinoazide complex Ph 2 P(N 3 )-Cr(CO) 5 readily undergoes photolysis under UV light to produce the phosphino-isocyanate complex Ph 2 P(NCO)-Cr(CO) 5 (Ocando et al., 1985), while the related bis(diisopropylamino) complex (iPr 2 N) 2 P(N 3 )-Cr(CO) 5 does not (Cowley et al., 1995). The crystal structure of the title compound is the first reported example of a structurally characterized 2-azido-1,3,2-diazaphospholidine; however, a few closely related compounds are known, such as those derived from 1,3,2-diazaphospholenes.

Structural commentary
The molecular structure of the title compound is shown in Fig. 1. It crystallizes in the monoclinic space group P2 1 /n with ISSN 2056-9890 one molecule in the asymmetric unit. The bond lengths between the P atom and its flanking N atoms are similar [P1-N4 = 1.6680 (15) Å , P1-N5 = 1.6684 (14) Å and N4-P1-N5 = 91.14 (7) ], while the phosphorus centre adopts a trigonal pyramidal geometry, with the sum of the angles at phosphorus equal to 294.14 (7) . The azide group is quasilinear [N3-N2-N1 = 176.6 (2) ], with similar N-N bond lengths [N1-N2 = 1.168 (2) Å and N2-N3 = 1.155 (2) Å ]. The phosphorus-azide bond length (P1-N1) is significantly longer [1.8547 (16) Å ] than found for atoms N4 and N5. The average sum of the bond angles at the N4 and N5 positions is 359.87 (12) , very close to an ideal trigonal planar geometry. This is a strong indication that the nominal lone pairs of atoms N4 and N5 participate in N-PÁ Á Á interactions and, when coupled with the significantly longer P1-N1 bond length, suggests a partial ionic character similar to earlier reports in acyclic structures (Cowley et al., 1995). The overall conformation of the C1/C2/N4/N5/P1 ring is well described as an envelope, with atom N5 deviating from the other atoms (r.m.s. deviation = 0.030 Å ) by À0.274 (2) Å . 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 103.69 (18) and 101.83 (17) for P1-N1-C3-C4 and P1-N2-C15-C20, respectively. The isopropyl groups are oriented away from the central five-membered ring, but the 'congested' nature of the molecule results in intramolecular short contacts between all four of the methine H atoms (H9, H12, H21 and H24) and atoms N4 and N5 (Table 1).

Supramolecular features
The only significant directional interaction in the crystal of the title compound is a long [2.69 (3) Å ] C-HÁ Á ÁN hydrogen bond to the terminal N atom of the azide group, which results in [100] chains in the crystal (Fig. 2).

Figure 2
The packing of the title compound, showing intermolecular C-HÁ Á ÁN interactions as dashed lines, which result in [100] chains.

Figure 1
The molecular structure of the title compound, showing 50% displacement ellipsoids. H atoms have been omitted for clarity.

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. Dihedral angles for the methyl H atoms were allowed to refine freely. The atomic displacement parameters of atoms N1 and N2 were constrained to be approximately equal using an EADP command.  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.