Crystal structure and Hirshfeld surface analysis of the new cyclodiphosphazane [EtNP(S)NMe2]2

A new cyclodiphosphazane, [EtNP(S)NMe2]2, was synthesized and characterized by NMR and EDX spectroscopy and single-crystal XRD. The stability of the structure is ensured only by van der Waals interactions and the their prevalence is confirmed by an analysis of the three-dimensional Hirshfeld surface (HS) and two-dimensional fingerprint plots (FP).

The cyclic compound 2,4-bis(dimethylamino)-1,3-diethylcyclodiphosphazane-2,4-dithione [systematic name: 2,4-bis(dimethylamino)-1,3-diethyl-1,3,2 5 ,4 5diazadiphosphetidine-2,4-dithione], C 8 H 22 N 4 P 2 S 2 or [EtNP(S)NMe 2 ] 2 , is member of a class of molecules that may be used, by virtue of their complexation properties, for the extraction of metals. This compound was characterized in solution by ( 1 H and 31 P) NMR, and in the solid state by energydispersive X-ray spectroscopy (EDX) and by X-ray crystallography. In the crystal, the molecule sits on an inversion centre such that the P and N atoms form a centrosymmetric cyclic P 2 N 2 arrangement. The crystal packing is dominated by van der Waals interactions. The prevalence of these interactions is illustrated by an analysis of the three-dimensional Hirshfeld surface (HS) and by two-dimensional fingerprint plots (FP). The relative contribution of different interactions to the HS indicates that the HÁ Á ÁH contacts account for 74.3% of the total HS area.
In the study of organophosphorus compounds, one of the aims is to prepare new complexing agents. Indeed, the literature shows many studies of the bidentate organophosphorus ligands HN[P(E)R 2 ] 2 (E: O, S, Se; Balazs et al. 1999;Silvestru et al. 2000;Ghesner et al. 2005;Cristurean et al. 2008) and RN[P(E)R 2 ] 2 (Benabicha et al. 1986;Ladeveze et al. 1986;Alouani et al. 2002Alouani et al. , 2007Peulecke et al. 2009), etc. All of these ligands may act as chelating agents containing both hard (N) and soft (P) elements. In addition, the flexibility of the (EPNPE) system provides a ready means of altering, and thereby possibly improving, their complexing properties. Several complexes based on these ligands have been reported, such as those described by Bennis & Alouani (2012) and by Mejri et al. (2016). ISSN 2056-9890 We report here the synthesis, characterization by ( 1 H and 31 P) NMR and energy-dispersive X-ray (EDX) spectroscopies, and a single-crystal structure of a new cyclodiphosphazane, 1,3-diethyl-2,4-dimethylamine-2,4-dithiocyclodiphosphazane, [EtNP(S)NMe 2 ] 2 (I). In order to evaluate the nature of the intermolecular interactions in the crystal packing and their associated energies, detailed analyses of Hirshfeld surfaces (HS) and fingerprint plot (FP) calculations were performed (Spackman & McKinnon, 2002;Parkin et al., 2007;Rohl et al., 2008;Spackman & Jayatilaka, 2009).

Structural commentary
The molecular structure of (I) is shown in Fig. 1, selected crystallographic data are presented in Table 1, and an EDX spectrum confirming the presence of C, N, P and S is shown in Fig. 2.
With regard to the conformation of (I), its structure differs from that of P 2 S 2 N 5 C 9 H 27 (S-NIPA) (Benabicha et al. 1986) primarily by the existence of the P 2 N 2 ring. The literature also shows several similar ligands, for example trans-[(EtNH)P(S)NEt] 2 (Hill et al. 1994) and cis-P 2 S 2 N 4 C 20 H 42 (Chandrasekaran et al. 2011). The most similar known ligand to (I) is the cyclic molecule trans-[(EtNH)P(S)NEt] 2 (Hill et al. 1994). The two molecules differ in the environments of the nitrogen atoms, which are all bound to ethyl groups in trans-[(EtNH)P(S)NEt] 2 , the peripheral carbons of which are all disordered. The molecular structure of (I). Atomic displacement parameters for the non-H atoms are drawn at the 30% probability level. Unlabelled atoms are related to labelled ones by the symmetry operation Àx + 1, Ày + 1, Àz.

Figure 2
The EDX spectrum of (I), showing the presence of C, N, P, and S.

Figure 3
Perspective view of part of the crystal structure of (I), viewed approximately down the a axis. H atoms have been omitted for clarity.

Hirshfeld surface analysis
Organic small molecule crystal packings are often dominated by a particular type of interaction, e.g. hydrogen bonding or van der Waals contacts. However, the overall crystal packing is determined by a combination of many forces, and hence all of the intermolecular interactions of a structure should be taken into account. Visualization and exploration of intermolecular close contacts of a structure is invaluable, and this can be achieved using the Hirshfeld surface (Spackman & McKinnon, 2002;Spackman & Jayatilaka, 2009). A large range of properties can be visualized on the Hirshfeld surface with the program CrystalExplorer (Wolff et al., 2012), including d e and d i , which represent the distances from a point on the HS to the nearest atoms outside (external) and inside (internal) the surface, respectively.
Intermolecular distance information on the surface can be condensed into a two-dimensional histogram of d e and d i , which is a unique identifier for molecules in a crystal structure, and is known as a fingerprint plot (Parkin et al., 2007;Rohl et al., 2008). Instead of plotting d e and d i on the Hirshfeld surface, contact distances are normalized in CrystalExplorer using the van der Waals radius of the appropriate internal (r i vdw ) and external (r e vdw ) atom of the surface: For (I), the three-dimensional HS mapped over d norm is given in Fig. 4. Contacts with distances equal to the sum of the van der Waals radii are shown in white, and contacts with distances shorter than or longer than the related sum values are shown in red (highlighted contacts) or blue, respectively. Two-dimensional FP plots showing the occurrence of all kinds of intermolecular contacts are presented in Fig. 5a.
The HÁ Á ÁH interactions are shown on the three-dimensional HS as white spots. These contacts appear in the middle of the scattered points in the two-dimensional FP (Fig. 5b) View of the three-dimensional Hirshfeld surface (HS) of (I) mapped with d norm .

Figure 6
Hirshfeld surface of (I) mapped over curvedness. three-dimensional HS (74.3%). Significant HÁ Á ÁS/SÁ Á ÁH interactions (25.5%) can also be seen, indicated by the pair of wings in the two-dimensional FP with a prominent long spike at d e + d i $ 1.9Å (Fig. 5c). The HÁ Á ÁN/NÁ Á ÁH interactions are shown on the three-dimensional HS marked with a blue spot for long contacts. These comprise only 0.2% of the total Hirshfeld surface, and are represented by two symmetrical narrow pointed spikes with d e + d i $ 2 Å (Fig. 5d). The presence of these interactions may also be shown by the Hirshfeld surface mapped as a function of curvedness (Fig. 6).

Synthesis and crystallization
All reagents and solvents were obtained from commercial sources and used without further purification. The synthesis of (I) was carried out in three steps: * Step 1: Addition of pyridine dropwise to a solution in anhydrous heptane of 2 mol of (EtNH 2 HCl) and 2 mol of PCl 3 at 268 K, gave precipitation in the form of a salt. Then, the reaction mixture was refluxed for 24 h. An oil was obtained after filtration of the pyridinium salt and evaporation of the heptane and the excess PCl 3 . This step corresponds to the formation of P 2 N 2 cycle, according to the bibliographic data (Chandrasekaran et al. 2011;Hill et al. 1994). All these operations were conducted under a nitrogen atmosphere to avoid hydrolysis of the chlorinated compounds. The yield of this step is 85% with respect to ethylammonium chloride. * Step 2: At a temperature of 263 K, 1 mol of the synthesized [EtNPCl] 2 was added dropwise to an ether solution containing 2 mol of dimethylamine, 2 mol of triethylamine and 4-dimethylaminopyridine (4-DMAP) as catalyst. After 10 h of agitation, Et 3 NHCl was precipitated. Filtration of the salt and evaporation of the ether gave an oil. All these operations were conducted under a nitrogen atmosphere. The yield of this step is 40%. * Step 3: The sulfurization of [EtNPNMe 2 ] 2 with 2 mol of sulfur gave the final product, 1,3-diethyl-2,4-dimethyl-2,4-dithio-cyclodiphosphazane (I), in a yield of about 80%.
Crystallization was carried out from ethanol by slow evaporation at room temperature. After one week, yellow single crystals suitable for X-ray diffraction analysis were obtained. A qualitative EDX analysis on some crystals confirmed the presence of C, N, P and S.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. H atoms attached to CH 3 and CH 2 groups were placed geometrically and refined using a riding model: C-H = 0.96 Å for CH 3 group with U iso (H) = 1.5U eq (C) and C-H = 0.97Å for CH 2 group with U iso (H) = 1.2U eq (C).