A triclinic polymorph of tricyclohexylphosphane sulfide: crystal structure and Hirshfeld surface analysis

The conformation found for (C6H11)3P=S in the triclinic polymorph lacks the mirror symmetry found in the orthorhombic form. Nevertheless, the conformations are in essential agreement. In the crystal, linear supramolecular chains are sustained by methine-C—H⋯S(thione) interactions.


Chemical context
Recent interest in the chemistry of phosphanegold(I) dithiocarbamate compounds stems from their potential as anticancer agents (de Vos et al. 2004;Ronconi et al. 2005;Gandin et al. 2010;Jamaludin et al. 2013;Keter et al. 2014;Altaf et al. 2015). In keeping with the increasing interest in gold compounds as potential anti-microbial agents to meet the challenges of microbes developing resistance to available chemotherapies (Glišić & Djuran, 2014) and in recognition of the potential of metal dithiocarbamates as anti-microbial agents (Hogarth, 2012), the anti-bacterial properties of phosphanegold(I) dithiocarbamates have also been explored in recent times (Sim et al., 2014;Chen et al., 2016). For example, the 'all-ethyl' compound, Et 3 PAu(S 2 CNEt 2 ), exhibits broadrange activity against Gram-positive and Gram-negative bacteria and was shown to be bactericidal against methicillinresistant Staphylococcus aureus (MRSA) . As an extension of these studies, investigations into the antimicrobial potential of related bis(phosphane)copper(I) dithiocarbamates and their silver(I) analogues were undertaken, again revealing interesting results and dependency of activity upon phosphane-and dithiocarbamate-bound substituents . During further investigations in this field, the title compound, Cy 3 P S (I), was isolated as a decomposition product from a long-term (months) recrystallization of an acetone solution containing (Cy 3 P) 2 Ag(S 2 CNEt 2 ). The crystal and molecular structures of ISSN 2056-9890 (I) are reported herein and the results compared with those of a previously determined orthorhombic polymorph, (II) (Kerr et al., 1977;Reibenspies et al., 1996). Further, a detailed comparison of the Hirshfeld surfaces for (I) and (II) is presented.

Structural commentary
The molecular structure of (I), Fig. 1, features a tetrahedrally coordinated P V centre defined by a thione-S and threecarbon atoms of the cyclohexyl substituents. The P1-C bond lengths span an experimentally distinct range of 1.8350 (14) to 1.8468 (15) Å , Table 1. The distortions from the ideal tetrahedral geometry are relatively minor with the widest angles generally involving the thione-S atom. The cyclohexyl rings, each with a chair conformation, adopt orientations so that the methine-H atom is directed towards the thione-S atom in the cases of the C1-and C13-rings, i.e. are syn, with that of the C7ring being anti.
As mentioned above, the structure of (I) has been reported previously in an orthorhombic form in two separate determinations (Kerr et al., 1977;Reibenspies et al., 1996). Data from the more recent determination, measured at 163 K (Reibenspies et al., 1996), are included in Table 1. The major difference in (II) is that the molecule lies on a crystallographic mirror plane; the 2 Â syn plus 1 Â anti-conformation of the methine-H atoms with respect to the thione-S atom persists. In (II), the P-C bond lengths are equal within experimental error. However, differences are apparent in the bond angles subtended at the P V centre whereby the angles in (II) span a wider range, i.e. 8.5 , cf. 6.3 in (I). Also, the widest angle at the P1 atom in (II) is subtended by the symmetry-related cyclohexyl rings.
An overlay diagram for (I) and (II) is shown in Fig. 2, which highlights the coincidence of the cyclohexyl ring associated with the methine-H atom having the anti-disposition with respect to the thione-S atom. Clearly, there are conformational differences apparent between the cyclohexyl rings related across the pseudo-and crystallographic mirror planes in (I) and (II), respectively.

Supramolecular features
The only directional supramolecular interactions in the crystal of (I) identified in PLATON (Spek, 2009) Table 1 Geometric parameters (Å , ) for the triclinic (I) and orthorhombic (II) polymorphs of Cy 3 P S.

Figure 1
The molecular structure of polymorph (I), showing the atom-labelling scheme and displacement ellipsoids at the 70% probability level.

Figure 2
Overlay diagram of polymorphs (I), red image, and (II), blue image. The molecules are overlapped so the three -C atoms of the cyclohexyl rings are coincident.
H7Á Á ÁS(thione) contacts, i.e. involving the anti-disposed thione-S and methine-H atoms, Table 2. These lead to a linear chain aligned along the a axis as illustrated in Fig. 3a. The chains pack with no directional interactions between them, Fig. 3b.
In the original report of polymorph (II), it was stated 'There are no unusual inter-molecular contacts' (Kerr et al., 1977); no comment on the molecular packing was made in the redetermination (Reibenspies et al., 1996). As seen from Fig. 4, supramolecular zigzag chains are evident in the molecular packing of (II), but these are sustained by weak methylene-C-HÁ Á ÁS(thione) interactions [HÁ Á ÁS i = 3.027 (2) Å , CÁ Á ÁS i = 3.938 (2) Å with the angle at H = 159 for (i) 1 + x, y, z] formed on either side of the mirror plane, so the sulfur atom forms two such contacts, and propagate along the a axis.
A more detailed analysis of the molecular packing in (I) and (II) is given in Hirshfeld surface analysis.

Hirshfeld surface analysis
In order to gain more insight into the molecular packing found in (I) and (II), the structures were subjected to a Hirshfeld surface analysis which was performed as described in a recent publication (Jotani et al., 2016).
The different shapes of Hirshfeld surfaces mapped over electrostatic potential in Fig Table 2 Hydrogen-bond geometry (Å , ).       (I) and (II). A pair of bright-red spots appearing on the Hirshfeld surface mapped over d norm near methine-H7 and thione-S1 for (I), Fig. 6, on the extremities of the molecule represent the donor and acceptor of the C-HÁ Á ÁS interaction, Table 2. They are viewed as the respective blue (positive) and red (negative) regions on the Hirshfeld surface mapped over electrostatic potential, Fig. 5. The absence of characteristic spots on the d norm -mapped Hirshfeld surfaces in the orthorhombic polymorph (II) (not shown) indicates no similar interactions within the sum of the van der Waals radii; see below. The immediate environments about reference molecules of (I) and (II) within the d norm -mapped Hirshfeld surfaces showing intermolecular C-HÁ Á ÁS interactions are displayed in Fig. 7a and b, respectively. In the crystal of (II), the zigzag chain of weak intermolecular methylene-C-HÁ Á ÁS(thione) contacts on either side of the crystallographic mirror plane is viewed as the pair of red dashed lines in Fig. 7b (see above). The overall two-dimensional fingerprint plots for (I) and (II), and those delineated into HÁ Á ÁH and SÁ Á ÁH/HÁ Á ÁS contacts (McKinnon et al., 2007) are illustrated in Fig. 8. It is interesting to note that in both polymorphs only sulfur and hydrogen atoms lie on the periphery of the Hirshfeld surfaces and contribute to interatomic contacts such as they are; the percentage contributions are as quantified in Table 3. The different relative orientations of the cyclohexane rings in the two forms are also evident through the distinct distribution of points in their respective two-dimensional fingerprint plots,  Fig. 8a, the top region, corresponding to donor interactions is stunted with respect to the lower, acceptor region. For (I), a pair of small peaks at d e + d i < 2.4 Å in the fingerprint plot delineated into HÁ Á ÁH contacts, Fig. 8b, show the contribution from short interatomic HÁ Á ÁH contacts in the molecular packing, Table 4. This contrasts the situation for (II), where the pair of peaks occur at d e + d i > 2.4 Å , i.e. at separations greater than the sum of van der Waals radii. The relative strength of the intermolecular C-HÁ Á ÁS interactions in (I) and (II) are characterized from the fingerprint plots delineated into SÁ Á ÁH/ HÁ Á ÁS contacts, Fig. 8c Table 4 Short interatomic contacts in (I).
Contact distance symmetry operation Views of the Hirshfeld surface for polymorph (I) mapped over d norm over the range À0.160 to 1.823 au.

Figure 7
Views of the Hirshfeld surfaces mapped over d norm about a reference molecule highlighting intermolecular C-HÁ Á ÁS interactions and short interatomic HÁ Á ÁH contacts as white and red dashed lines, respectively, for (a) polymorph (I) and (b) polymorph (II).
metric distribution of points in the fingerprint plot delineated into SÁ Á ÁH/HÁ Á ÁS contacts for (II) in Fig. 8c is the result of the orientation of the cyclohexane rings with respect to the crystallographic mirror plane. The upper region, corresponding to donor HÁ Á ÁS contacts, contributes 4.7% to the surface cf. 6.5% in the lower region, corresponding to SÁ Á ÁH acceptor contacts. The similarity in the molecular packing of (I) and (II) is reflected in the similarity in the physiochemical data collated in Table 5 and calculated in Crystal Explorer (Wolff et al., 2012) and PLATON (Spek, 2009). While it is noted the values are very close for (I) and (II) (Table 5), the volume of the molecule in (I) is slightly greater than that in (II), as is the surface area. However, the molecule in (II) is marginally more globular and reflecting the lack of directional interactions between molecules, allowing a closer approach, the density is greater than in (I). Nevertheless, the packing efficiency is marginally greater in (I), probably reflecting the lack of symmetry in the molecule cf. (I).

Database survey
There are a number of triorganophosphane sulfide structures in the crystallographic literature (Groom et al., 2016) with those conforming to the general formula R 3 P S being summarized here. Thus, structures have been described with fractional atomic coordinates, for example with R = Me (Tasker et al., 2005), iPr (Staples & Segal, 2001) Table 6 Geometric parameters (Å , ) for selected R 3 P S molecules. R P S S-P-C C-P-C Reference Me a 1.9664 (7) 112.88 (6) Notes: (a) The molecule has crystallographic mirror symmetry with the S1, P1 and C1 atoms lying on the plane; (b) the molecule has crystallographic threefold symmetry with the S1 and P1 atoms lying on the axis; (c) monoclinic polymorph; (d) two independent molecules in the asymmetric unit; (e) triclinic polymorph; (f) orthorhombic polymorph. polymorph), 2-tolyl (Cameron & Dahlè n, 1975), 3-tolyl (Cameron et al., 1978), 4-FPh (Barnes et al., 2007), 2-(Me 2 NCH 2 ) 3 Ph (Rotar et al., 2010), 2,4,6-Me 3 Ph (Garland et al., 2013) and 2,4,6-(OMe) 3 Ph (Finnen et al., 1994). Selected geometric data for these structures along with those for (I) and (II) are collected in Table 6. The R = Me and iPr molecules have crystallographic mirror symmetry as for (II) whereas the R = tBu compound has crystallographically imposed threefold symmetry. Two polymorphs have been found for R = Ph, and each of these features two independent molecules in the asymmetric unit. The longest P S bond length, i.e. 1.9748 (13) Å , is found in sterically encumbered (2,4,6-Me3Ph) 3 P S (Garland et al., 2013). That steric effects are not the only factors influencing the magnitude of the P S bond length is realized in the structure of Me 3 P S, with small, electron-donating groups, which has the second longest P S bond length across the series. The comments on the lack of definitive trends in the S-P-C and C-P-C bond angles made above for (I) and (II) hold true across the series although, generally, the former are wider than the latter. Interestingly, in the threefold symmetric tBu 3 P S structure, all angles are about 109 .

Synthesis and crystallization
The title compound (I) is an unexpected product from the in situ reaction of (Cy 3 P) 2 AgCl with Na[S 2 CNEt 2 ] in a 2:1 ratio. The preparation was as follows: Cy 3 P (Sigma-Aldrich; 0.6 mmol, 0.196 g) dissolved in acetone (20 ml) was added to an acetone solution (20 ml) of AgCl (Sigma-Aldrich; 0.3 mmol, 0.05 g) at room temperature. Then, Na[S 2 CNEt 2 ] (BDH, 0.3 mmol, 0.08 g) in acetone (20 ml) was added to the reaction mixture followed by stirring for 4 h. The resulting mixture was filtered, covered to exclude light and left for evaporation at room temperature. Colourless crystals were obtained after four months. Yield: 0.132 g (55%), m.p.: 437-440 K. IR (cm À1 ): (P S) 624 (s).

Tricyclohexyl-λ 5 -phosphanethione
Crystal data 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.