New monoclinic form of {O-Ethyl N-(4-nitrophenyl)thiocarbamato-κS}(tri-4-tolylphosphane-κP)gold(I): crystal structure and Hirshfeld surface analysis

A linear SP coordination geometry for the gold atom is found in the title structure, which also features a short intramolecular Au⋯O interaction, in contrast to a Au⋯π interaction found in the first polymorph.

The title phosphanegold(I) thiolate compound, [Au(C 9 H 9 N 2 O 3 S)(C 21 H 21 P)], is a second monoclinic polymorph (space group P2 1 /c) that complements a previously reported Cc polymorph [Broker & Tiekink (2008). Acta Cryst. E64, m1582]. An SP donor set defines an approximately linear geometry about the gold atom in both forms. The key distinguishing feature between the present structure and the previously reported polymorph rests with the relative disposition of the thiolate ligand. In the title compound, the orientation is such to place the oxygen atom in close contact with the gold atom [AuÁ Á ÁO = 2.915 (2) Å ], in contrast to the aryl ring in the original polymorph. In the crystal, linear supramolecular chains along the a-axis direction mediated by C-HÁ Á Á and nitro-OÁ Á Á interactions are found. These pack with no directional interactions between them. The analysis of the Hirshfeld surfaces for both forms of [Au(C 9 H 9 N 3 O 3 S)(C 21 H 21 P)] indicates quite distinctive interaction profiles relating to the differences in intermolecular contacts found in their respective crystals.

Chemical context
Phosphanegold(I) thiolates of the general formula R 3 PAu[SC(OR 0 ) NR 00 ], for R, R 0 = alkyl, aryl and R 00 = aryl, have proven to exhibit exciting biological activities. For example, compounds of the type Ph 3 PAu[SC(OR) NPh], R = Me, Et and i-Pr, induce G 2 /M cell cycle arrest in HT-29 cancer cells and exhibit tolerable toxicity based on experiments on zebrafish (Yeo, Ooi et al., 2013;Ooi et al., 2017). Further, in vitro mechanistic investigations point to these compounds inducing both intrinsic and extrinsic pathways of cell death leading to apoptosis. In complementary studies on compounds with R 00 = 4-tolyl, quite promising in vitro potency against Gram-positive bacteria has been revealed (Yeo, Sim et al., 2013). However, such biological potential does not extend to activity against certain Acanthamoeba castellanii (Siddiqui et al., 2017). The observed biological activity for this class of compound has necessitated synthesis and re-synthesis during the course of which various polymorphs (e.g. Yeo et al., 2016a) and solvates (e.g. Yeo et al., 2016b) have been revealed. Of particular interest has been the recent appearance of conformational polymorphs for these compounds.
Referring the conformation shown in the Scheme, most structures having the formula R 3 PAu[SC(OR 0 ) NR 00 ] display an intramolecular AuÁ Á ÁO interaction. In an exercise in crystal engineering, it was argued that by moderating the electronic properties of the phosphane-bound and thiolate-N-bound groups, it was possible to direct a change in conformation so that an intramolecular AuÁ Á Á(aryl) interaction formed instead of the AuÁ Á ÁO contact (Kuan et al., 2008). Such AuÁ Á Á(aryl) interactions are well established in the supramolecular chemistry of molecular gold compounds (Tiekink & Zukerman-Schpector, 2009;Caracelli et al., 2013) and have important implications in mechanisms associated with catalytic gold (Lin & Hammond, 2012). As mentioned above, current interest in the biological activity of this class of compounds has prompted renewed synthesis and scale-up. Recently, a conformational polymorph was discovered during a check for sample purity, via powder X-ray diffraction, for a compound, Ph 3 PAu[SC(OEt) NPh], that was originally reported in a form with an intramolecular AuÁ Á ÁO interaction (Hall & Tiekink, 1993). The new polymorph featured an intramolecular AuÁ Á Á(aryl) interaction instead, an observation ascribed to thermodynamic considerations (Yeo, Tan, Otero-de-la-Roza et al., 2016). Herein, as a continuation of structural studies of these compounds, a new polymorph for (4-tol) 3 PAu[SC(OEt) NC 6 H 4 NO 2 -4] is reported which was reported originally in space group Cc with a AuÁ Á Á(aryl) interaction (Broker & Tiekink, 2008), but now with a AuÁ Á ÁO interaction. Herein, the crystal and molecular structures of a P2 1 /c polymorph of (4-tol) 3 PAu[SC(OEt) NC 6 H 4 NO 2 -4], (I), are described complemented by an analysis of the Hirshfeld surfaces calculated for (I) and for the original Cc form, (II).

Structural commentary
The molecular structure of (I) is shown in Fig. 1 and selected interatomic parameters are collected in Table 1. The gold(I) atom is coordinated by thiolate-S and phosphane-P atoms in a near linear geometry. The P1-Au-S angle of 175.80 (3) deviates from the ideal 180 , an observation which might be ascribed to the formation of an intramolecular AuÁ Á ÁO interaction of 2.915 (2) Å , which arises as the thiolate ligand is orientated to place the oxygen atom in close proximity to the gold atom. As is usual for these compounds, the Au-S bond is longer than the Au-P bond. The C1 N1 bond length of 1.259 (4) Å is consistent with significant double character in this bond and, by implication, the presence of a thiolate-S atom. These bond-length conclusions are vindicated by a comparison of the bond lengths found in the uncoordinated molecule, i.e. EtOC( S)N(H)C 6 H 4 NO 2 -4 (Benson et al., 2006). Here, the C1 S1 and C1-N1 bond lengths are 1.672 (2) and 1.354 (3) Å , respectively, i.e. clearly shorter and longer than the related bond lengths in (I). The equivalent geometric parameters to those listed in Table 1 for the Cc polymorph (Broker & Tiekink, 2008) are equal within experimental error with one possible exception, being the P-Au-S angle, which at 174.54 (10) appears to be narrower by about 1 than the equivalent angle in (I), Table 1. Table 1 Selected geometric parameters (Å , ).

Figure 1
The molecular structure of (I) showing the atom-labelling scheme and displacement ellipsoids at the 50% probability level.

Figure 2
Overlay diagram of the molecular structures found in (I) (P2 1 /c, red image) and (II) (Cc, blue) forms of (4-tol) 3 PAu[SC(OEt) NC 6 H 4 NO 2 -4]. The molecules have been overlapped so that the P-Au-S fragments are coincident.
The central S1, O1, N1 and C1 atoms of the thiolate ligand are strictly (r.m.s. deviation of the fitted atoms = 0.0008 Å ) planar. The plane through the nitrobenzene ligand is orthogonal to the former plane, forming a dihedral angle of 89.67 (12) . Finally, the nitro group is essentially co-planar with the ring to which it is connected, forming a dihedral angle of 4.7 (4) .
The differences in conformation for (I) and (II) are starkly highlighted in the overlay diagram shown in Fig. 2. Some physical properties for the two forms, calculated in Crystal Explorer (Wolff et al., 2012) and PLATON (Spek, 2009), are included in Table 2. These data indicate significant differences between the molecules comprising polymorphs (I) and (II), most notably indicating the molecule in (II) to be more compact, spherical and to have a greater density, all parameters consistent with this being the thermodynamically more stable form.

Supramolecular features
The geometric parameters defining the identified intermolecular interactions are listed in Table 3. The key feature of the molecular packing is the formation of linear supramolecular chains along the a-axis direction, Fig. 3a. These are sustained by a combination of nitrobenzene-C-HÁ Á Á(tolyl) interactions as well as nitro-OÁ Á Á(tolyl) contacts, Fig. 3b. For the latter, the nitro group lies over the ring, with the two residues being almost parallel, forming a dihedral angle = 7.4 (2) . While comparatively rare, the latter interactions have been discussed in the crystallographic literature (Huang et al., 2008).

Analysis of the Hirshfeld surfaces
The Hirshfeld surface calculations on polymorphic (I) and (II) were performed in accord with recent related work (Jotani et al., 2017). In short, the two monoclinic polymorphs reveal quite distinctive features in their Hirshfeld surfaces. Table 2 A comparison of some physical properties between the molecules in the polymorphs of (4-tol) 3 PAu[SC(OEt) NC 6 H 4 NO 2 -4].

Molecule
Volume

Figure 3
Molecular packing in (I): (a) a view of the linear supramolecular chain sustained by nitrobenzene-C-HÁ Á Á(tolyl) interactions as well as nitro-OÁ Á Á(tolyl) contacts shown as purple and orange dashed lines, respectively (non-participating H atoms have been removed) and (b) a view of the unit-cell contents shown in projection down the a axis.

Figure 4
Views of the Hirshfeld surface mapped over d norm for (a) (I) in the range À0.003 to +1.441 au and (b) (II) in the range 0.007 to 1.513 au. Table 3 Hydrogen-bond geometry (Å , ).

D-HÁ
It is clearly evident from the Hirshfeld surfaces mapped over d norm for forms (I) and (II), Fig. 4, that the former conformation favours an intramolecular AuÁ Á ÁO contact while the latter, an intramolecular AuÁ Á Á(aryl) interaction. In addition, the tiny red spots appearing near the nitro-O2 and tolyl-C11 atoms in Fig. 4a indicate the significance of short inter-atomic CÁ Á ÁO/OÁ Á ÁC contacts, Table 4, in the packing of (I). The immediate environments about a reference molecule within the shape-index mapped surface for (I), Fig. 5a, b, and the d norm -mapped surface for (II), Fig. 5c, are consistent with (I) forming C-HÁ Á Á and N-OÁ Á Á interactions together with few short inter-atomic contacts in its packing, whereas the packing of (II) involves only a few short inter-atomic contacts, Table 4. The donor and acceptor of the C-HÁ Á Á(aryl) contact in (I) appear as blue and bright-red regions around the participating atoms and highlighted with red and yellow dotted lines in Fig. 5a. The intermolecular nitro-OÁ Á Á interaction involving both nitrobenzene-O2 and O3 atoms with the same symmetrically located tolyl ring (C17-C22) are viewed as two adjoining blue and bright-orange regions in Fig. 5b. The short inter-atomic SÁ Á ÁH/HÁ Á ÁS, CÁ Á ÁH/HÁ Á ÁC and OÁ Á ÁH/ HÁ Á ÁO contacts influential in the structure of (II) are highlighted with black, red and yellow dashed lines, respectively, in Fig.5c.
From the overall two-dimensional fingerprint plots for (I) and (II), Fig. 6a, it is apparent that the different orientations of the thiolate ligands significantly impact upon the observed features in the plots. This is also visible from the fingerprints   Table 4 Summary of short inter-atomic contacts (Å ) in (I) and (II).
Contact Distance Symmetry operation  in the relative percentage contributions from the different contacts to the Hirshfeld surfaces, as summarized in Table 5. Although HÁ Á ÁH contacts make dominant contributions of 50.1 and 55.2% to the Hirshfeld surfaces of (I) and (II), respectively, the plot area and the distribution of characteristic points within the plots indicate different propensities to form such inter-atomic contacts, Fig. 6b. The pair of small, closely situated peaks at d e + d i < 2.40 Å , i.e. the sum of two times the van der Waals radius of hydrogen, are observed for both the polymorphs and reflect short inter-atomic HÁ Á ÁH contacts, Table 4. The distinctive features of fingerprint plot delineated into CÁ Á ÁH/HÁ Á ÁC contacts, Fig. 6c, wherein the half-arrows in (I) contrast the forceps in (II) with their tips at d e + d i $ 2.8 Å and 2.9 Å , respectively, arise as the result of distinctive intermolecular interactions in the two forms: the former has a C-HÁ Á Á contact while the latter has short inter-atomic CÁ Á ÁH/HÁ Á ÁC contacts, Fig. 5c and Table 4. Thus, the short CÁ Á ÁH/HÁ Á ÁC contacts involving the nitrobenzene-H4 atom interacting with the tolyl-C12 and C13 atoms for (I), Table 4, have analogous contacts in form (II), Fig. 5b and Table 4. Although, OÁ Á ÁH/HÁ Á ÁO and SÁ Á ÁH/HÁ Á ÁS contacts make almost similar percentage contributions to the Hirshfeld surfaces for both the forms, Table 4, the distinct features in their delineated fingerprint plots, Fig. 6c and d, reflects the different types of inter-atomic contacts they form. In the respective plots for the form (I), the distribution of characteristic points are far away from the van der Waals separations indicating the absence of such short inter-atomic contacts in the packing. By contrast, the forceps-like tips at d e + d i $2.7 Å in the OÁ Á ÁH/HÁ Á ÁO delineated and the knifeedge tips at d e + d i $2.9 Å in the SÁ Á ÁH/HÁ Á ÁS delineated fingerprint plots for (II) are the result of short inter-atomic OÁ Á ÁH/HÁ Á ÁO and SÁ Á ÁH/HÁ Á ÁS contacts, Table 4. The other inter-atomic contacts summarized in Table 4 have small percentage contributions to the Hirshfeld surfaces of (I) and (II) and are considered to have negligible influence in the crystals.

Database survey
A measure of the significance of AuÁ Á Á(aryl) interactions can be seen in the polymorphic structures of ClAuP(Ph 2 )CH 2 (Ph 2 )PAuCl. In the original form, intramolecular AuÁ Á ÁAu interactions [3.34 Å ] were observed (Schmidbaur et al., 1977) but, in the more recently determined second form, intramolecular AuÁ Á Á(aryl) interactions (3.58 Å ) were formed instead (Healy, 2003). The real significance of this is that the energy of stabilization to a structure provided by AuÁ Á ÁAu interactions is comparable to that provided by conventional hydrogen bonding (Schmidbaur, 2001). This observation lead to systematic investigations into the cooperation/competition between hydrogen-bonding and AuÁ Á ÁAu interactions (Schneider et al., 1996;Schmidbaur et al., 2012) with the former often winning out owing to steric pressures associated with bringing gold centres into close proximity (Tiekink, 2014). The structures found for research communications  ClAuP(Ph 2 )CH 2 (Ph 2 )PAuCl imply that AuÁ Á Á(aryl) interactions provide comparable energies of stabilization to their crystal structures. Indeed, computational chemistry on the polymorphic system Ph 3 PAu[SC(OEt) NPh] suggested the form with the intramolecular AuÁ Á Á(aryl) contact was more than 5 kcal mol À1 stable than the form with the intramolecular AuÁ Á ÁO contact (Yeo et al., 2015). Related studies on a binuclear compound of the general formula [Et 3 PAuS(OMe) N] 2 (1,4-C 6 H 4 ) indicated that each AuÁ Á Á(aryl) interaction in the centrosymmetric molecule was more stable by more than 12 kcal mol À1 than each putative AuÁ Á ÁO contact (Yeo et al., 2015). This near equivalence in energies of different intermolecular contacts in metalcontaining species is the focus of a recent review (Tiekink, 2017).

Synthesis and crystallization
The title compound (I) was prepared following established literature procedures (Ho et al., 2006). Yellow crystals were obtained by the slow evaporation of a CH 2 Cl 2 /Et 2 O/hexane (1:1:2) solution of (I). Crystals with the same unit-cell characteristics were also isolated from benzene and ethylacetate solutions of (I). 1

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 6. The carbon-bound H atoms were placed in calculated positions (C-H = 0.94-0.98 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 1.16 and 0.78 e Å À3 , respectively, were located 0.81 and 1.28 Å from the Au atom. Owing to interference from the beam-stop, the (011) reflection was omitted from the final cycles of refinement.   N-(4-nitrophenyl) program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014/7 (Sheldrick, 2015); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012), DIAMOND (Brandenburg, 2006) and

Crystal data
[Au (C 9  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.