A monoclinic polymorph of [(Z)-N-(3-chlorophenyl)-O-methylthiocarbamato-κS](triphenylphosphane-κP)gold(I): crystal structure and Hirshfeld surface analysis

A linear geometry defined by a P,S-donor set is observed in the title polymorph; an intramolecular Au⋯O interaction is noted. The packing is consolidated by C—H⋯O and C—H⋯π interactions to generate a three-dimensional network.


Chemical context
Interest in the chemistry of phosphanegold(I) N-aryl-Oalkylthiocarbamates, i.e. compounds of general formula R 3 PAu[SC(OR 0 ) NR 00 ] (R, R 0 = alkyl, aryl; R 00 = aryl) continues owing to their recently disclosed exciting biological activities. Thus, various triphenylphosphane derivatives display excellent cytotoxicity profiles against HT-29 colon cancer cells, a particularly virulent form of cancer, and mechanistic studies have shown these to induce both intrinsic and extrinsic pathways of cell death leading to apoptosis (Yeo, Ooi et al., 2013;Ooi et al., 2015). Further, species with R 00 = p-tolyl have proven to exhibit impressive in vitro potency against Gram-positive bacteria (Yeo, Sim et al., 2013). It was during another synthesis of the title compound, (I), for further biological studies, that crystals of a new polymorph were isolated from its methanol solution. This is called form to distinguish it from the earlier triclinic form, form (Tadbuppa & Tiekink, 2010). Herein, the crystal and molecular structures of form of (I) are described along with a comparison with the parameters characterizing form . Further, a Hirshfeld surface analysis of both polymorphic forms of (I) is presented.

Structural commentary
The molecular structure of the new monoclinic form of (I), form , is shown in Fig. 1, and selected geometric parameters ISSN 2056-9890 are collected in Table 1. The gold(I) atom is coordinated in an approximately linear configuration by phosphane-P and thiolate-S atoms. Confirmation of the 'thiolate' assignment is readily seen in the relatively long C1-S1 bond length and the significant -character in the C1-N1 bond when the geometric parameters are compared with structures of related thiocarbamide molecules (Ho et al., 2005;Kuan et al., 2007); the crystal structure of the thiocarbamide precursor in (I) is not available for comparison. As is invariably observed in this class of compound, the Au-S bond length is longer than the Au-P bond. The small deviation from ideal linearity for the P-Au-S bond is related to the close approach of the oxygen atom to the gold(I) atom, i.e. 3.052 (3) Å . The pattern of bond angles about the quaternary carbon atom, C1, follow the expected trends with the widest angle involving the sulfur and doubly bonded nitrogen atom and with the narrowest angle involving the single-bonded atoms. The conformation about the formal C1 N1 bond, Table 1, is Z.
Form crystallizes in the monoclinic space group P2 1 /n with Z 0 = 1. The earlier polymorph, by contrast, crystallizes in triclinic space group P1, also with Z 0 = 1. A comparison of the key geometric parameters is given in Table 1. From these data, it is clear that there is experimental inequivalence in the bond lengths involving the gold(I) atoms, with the Au-S and Au-P bond lengths in form being marginally longer. The intramolecular AuÁ Á ÁO separation in form is also longer than the comparable separation in form , and this is correlated with a smaller deviation from a linear geometry about the gold(I) atom in . By contrast, the bond angles are, by and large, equivalent within experimental error. A significant conformational difference is evident in the molecular structures of forms and of (I). As seen from the overlay diagram shown in Fig. 2, this difference occurs as a result of a twist about the Au-S bond as seen in the values of the P1-Au-S1-C1 torsion angles of À8.4 (7) and 106.2 (7) in forms and , respectively.

Supramolecular features
Supramolecular dimers feature in the molecular packing of form of (I), which are sustained by N-aryl-C-HÁ Á ÁO(methoxy) interactions, Fig. 3a and Table 2. The dimers are connected into a three-dimensional architecture by a network of C-HÁ Á Á interactions, Fig. 3b and Table 2. Within this arrangement, centrosymmetrically related Ph 3 P ligands align to form a so-called six-fold phenyl embrace (6PE) (Dance & Scudder, 1995) featuring edge-to-face phenyl-C-HÁ Á Á(phenyl) interactions, Fig. 3c. While the interactions are too long to be considered as significant in terms of the criteria in PLATON (Spek, 2009), there are a number of such interactions, i.e. 2 Â [3.22, 3.26 and 3.29 Å ], that serve to reinforce the 6PE embrace with one pair of rings accepting two interactions each. In form of (I), the most prominent feature of the molecular packing is the formation of supramolecular chains mediated by C-HÁ Á Á interactions (Tadbuppa & Tiekink, 2010). Further analysis of the molecular packing in polymorphic (I) is given in the following Section. The molecular structure of polymorphic form of (I), showing the atomlabelling scheme and displacement ellipsoids at the 70% probability level. Table 1 Geometric data (Å , ) for (I), forms a and , and (II) b .

Figure 2
Overlay diagram of polymorphic forms (blue image) and (red) of the molecular structures of (I). Molecules have been overlapped so that the S1, O1 and N1 atoms are coincident.

Analysis of the Hirshfeld surfaces
The non-covalent interactions present in the pair of polymorphs of (I), i.e. forms and , were studied through Hirshfeld surface analysis by mapping on the normalized contact distance (d norm ) upon computation of the inner (d i ) and outer (d e ) distances of the Hirshfeld surface to the nearest nucleus (Spackman & Jayatilaka, 2009;McKinnon et al., 2007). All computation as well as generation of two-dimensional fingerprint plots were performed using Crystal Explorer 3.1 (Wolff et al., 2012). Distances involving hydrogen atoms were normalized by default to the standard neutron-diffraction bond lengths. As evident from Fig. 4 and Table 3, forms and of (I) exhibit relatively similar percentage contributions of the indicated intermolecular interactions to their Hirshfeld surfaces. However, the specific contributions to their interaction profiles are distinct as evidenced from the overall and decomposed two-dimensional fingerprint plots shown in Fig Table 2 Hydrogen-bond geometry (Å , ).

Figure 4
Percentage contribution of different close contacts to the Hirshfeld surface of forms and of (I).

Figure 3
Molecular packing in form of (I): (a) view of the supramolecular dimer sustained by C-HÁ Á ÁO contacts, shown as orange dashed lines, (b) view of the unit-cell contents shown in projection down the a axis, highlighting the C-HÁ Á Á interactions as purple dashed lines, (c) image of the sixfold phenyl (6PE) between centrosymmetrically related Ph 3 P ligands, highlighted in space-filling mode.
distinct spikes evident for form , Fig. 5c, correlating with the C-HÁ Á ÁO interactions leading to dimer formation. While beyond the sum of their respective van der Waals radii (Spek, 2009), ClÁ Á ÁH/HÁ Á ÁCl interactions make contributions to the Hirshfeld surfaces of both forms and , with the contacts, again, being shorter in form , i.e. 2.76 vs 3.00 Å , leading to more the distinct forceps in Fig. 5d. In general, the observation of generally shorter contacts in form may indicate greater crystal-packing efficiency (Lloyd et al., 2005). Table 4 collates various molecular/crystal structure descriptors for the polymorphic forms. Immediately evident is that the calculated unit-cell densities are identical but the crystal-packing efficiency (KPI; Spek, 2009) for form is marginally greater. Computation on the area-to-volume ratio between forms and revealed very little difference as did the globularity (G) and asphericity () indices. All these indicators suggest that the polymorphs arise as a result of a simple interplay between molecular conformation and crystalpacking effects.

Database survey
The most closely related structure to (I) in the crystallographic literature (Groom et al., 2016), is the R 0 = OEt analogue, i.e. (II), (Tadbuppa & Tiekink, 2009). Key geometric parameters for this structure are also included in Table 1. Non-systematic variations in parameters are noted, e.g. the Au-S bond length in (II) is intermediate between those found in the polymorphic forms of (I), and the Au-P bond length is the longest of the three structures. However, differences are small and probably can be ascribed to the influences of crystal-packing effects. research communications Figure 5 Comparison of the (a) complete Hirshfeld surface and full fingerprint plots between form and form polymorphs (top row) and the corresponding d norm surfaces and two-dimensional plots associated with (b) CÁ Á ÁH/HÁ Á ÁC, (c) OÁ Á ÁH/HÁ Á ÁO and (d) ClÁ Á ÁH/HÁ Á ÁCl contacts.
As indicated in the Chemical context, biological considerations motivate ongoing investigations into the chemistry of phosphanegold(I) N-aryl-O-alkylthiocarbamates. This notwithstanding, the relative ease of growing crystals have prompted several crystal engineering studies. Thus, correlations between AuÁ Á ÁAu (aurophilic) and solid-state luminescence responses have been made for the series of compounds, R 3 PAu[SC(OMe) NC 6 H 4 NO 2 -p] (R = Et, Cy and Ph), and bidentate phosphane analogues, Ph 2 P-(CH 2 ) n -PPh 2 for n = 1-4 and when the bridge is Fc (ferrocenyl) (Ho et al., 2006). In another study, the influence of R and Y substituents upon the molecular packing of compounds of the general formula [(Ph 2 P(CH 2 ) 4 PPh 2 ){AuSC(OR 0 ) NC 6 H 4 Y-p} 2 ] for R 0 = Me, Et or iPr and Y = H, NO 2 or Me was undertaken (Ho & Tiekink, 2007). Besides the anticipated linear P-Au-S configuration, a common feature of all the analysed structures until then was the presence of intramolecular AuÁ Á ÁO interactions, as illustrated in Fig. 1. This changed in another systematic study, this time of R 3 PAu[SC(OMe) NR 00 ], for R = Ph, o-tol, m-tol or p-tol, and R 0 ' = Ph, o-tol, m-tol, p-tol or C 6 H 4 NO 2 -p, where it proved possible to induce a conformational change in the molecule so that an intramolecular AuÁ Á Á interaction formed rather than AuÁ Á ÁO (Kuan et al., 2008); AuÁ Á Á interactions are well documented in the crystallographic literature (Tiekink & Zukerman-Schpector, 2009;Caracelli et al., 2013). For example, having R = R 00 = p-tol simultaneously activated the gold atom, making it amenable to form an AuÁ Á Á interaction with the comparatively electron-rich aryl ring. Recently, bipodal forms of the thiocarbamide ligands were prepared and complexed with phosphanegold(I) species yielding binuclear molecules also with intramolecular AuÁ Á Á interactions . Computational chemistry showed the AuÁ Á Á interactions to be more favourable, by ca 12 kcal mol À1 , than the putative AuÁ Á ÁO interaction .
Such interplay between substituents in crystal engineering endeavours, along with the observation that biological activities are acutely sensitive to substitution patterns, ensures this area of research will continue to attract significant attention.

Synthesis and crystallization
All chemicals and solvents were used as purchased without purification. All reactions were carried out under ambient conditions. Melting points were determined on a Biobase auto melting point apparatus MP300. IR spectra were obtained on a Perkin Elmer Spectrum 400 FT Mid-IR/Far-IR spectrophotometer from 4000 to 400 cm À1 ; abbreviation: s, strong.

Table 4
Physiochemical properties for forms and of (I).  Table 3 Percentage contribution of the different intermolecular contacts to the Hirshfeld surface in forms and of (I).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5. The carbon-bound H atoms were placed in calculated positions (C-H = 0.95-0.98 Å ) and were included in the refinement in the riding-model approximation, with U iso (H) set to 1.2-1.5U eq (C). The maximum and minimum residual electron density peaks of 2.04 and 1.06 e Å À3 , respectively, were located 1.01 and 0.77 Å from the Au atom.

[(Z)-N-(3-Chlorophenyl)-O-methylthiocarbamato-κS](triphenylphosphane-κP)gold(I)
Crystal data [Au(C 8  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 2.04 e Å −3 Δρ min = −1.06 e Å −3 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.