Crystal structure of [μ2-1,1′-bis(diphenylphosphanyl)ferrocene-κ2 P:P′]bis[(pyrrolidine-1-carbodithioato-κS)gold(I)]

The centrosymmetric molecule features a linearly coordinated AuI atom within an S(dithiocarbamate) and P(phosphane) donor set.


Chemical context
Investigations into the potential anti-cancer activity of phosphanegold(I) dithiocarbamates, R 3 PAu(S 2 CNR 0 2 ), date back over a decade (de Vos et al., 2004;Vergara et al., 2007;Jamaludin et al., 2013). These investigations are complemented by the recently reported impressive anti-microbial activity for this class of compound (Sim et al., 2014) whereby R 3 PAu[S 2 CN( i Pr)CH 2 CH 2 OH], R = Ph and Cy, exhibited specific activity against Gram-positive bacteria while the R = Et derivative displayed broad-range activity against both Gram-positive and Gram-negative bacteria. Motivated by observations that 1,1 0 -bis(diphenylphosphanyl)ferrocene (dppf) derivatives also possess biological activity (Ornelas, 2011;Braga & Silva, 2013), it was thought of interest to couple dppf with Au I dithiocarbamates. This led to the isolation of the broadly insoluble title compound, dppf{Au[S 2 CN(CH 2 ) 4 ]} 2 , (I), which was subjected to a crystal structure determination. The results of this study are reported herein along with a comparison to related species.

Structural commentary
The Fe II atom in dppf{Au[S 2 CN(CH 2 ) 4 ]} 2 , (I), is located on a centre of inversion, Fig. 1. The Au I central atom exists in the anticipated linear geometry defined by thiolate-S and phosphane-P atoms. The Au-S1 bond length is considerably longer than the i.e. 2.3378 (8) cf. 2.2580 (8) Å . The dithiocarbamate ligand is orientated to place the S2 atom in close proximity to the Au I atom. However, the resulting intramolecular AuÁ Á ÁS2 interaction is long at 3.1538 (8) Å , consistent with a monodentate mode of coordination for the dithiocarbamate ligand. The pattern of C1-S1, S2 bond ISSN 2056-9890 lengths supports this conclusion in that the strongly bound S1 atom forms a longer, i.e. weaker, C1-S1 bond [1.757 (3) Å ] cf. with C1-S2 of 1.689 (3) Å . Nevertheless, the close approach of the S2 atom to the Au I central atom is correlated with the deviation from the ideal linear geometry, i.e. S1-Au-P1 is 169.35 (3) .
Similar features are noted in related structures as outlined below in the Database survey. The pyrrolidine ring is twisted about the C2-C3 bond. Owing to being located on a centre of inversion, the Fe II atom is equidistant from the ring centroids of the Cp rings [FeÁ Á ÁCg, Cg i = 1.6566 (13) Å ] and the Cg-Fe-Cg i angle is constrained by symmetry to be 180 ; symmetry operation (i): 1 À x, Ày, 2 À z. Again, from symmetry, the Cp rings have a staggered relationship.

Supramolecular features
In the crystal packing, the most prominent interactions are of the type C-HÁ Á ÁS. Data for the phenyl-C-HÁ Á ÁS(thione) interactions are collected in Table 1. These interactions, involving the dual acceptor S2 atom, serve to assemble molecules into supramolecular layers in the bc plane, Fig. 2. The thickness of each layer corresponds to the length of the a axis, i.e. 10.9635 (4) Å , and the layers stack along this axis with no directional interactions between them, Fig. 3.

Database survey
It has been approximately 40 years since the first report of a structure related to (I), i.e. Ph 3 PAu(S 2 CNEt 2 ), by Wijnhoven et al. (1972). This serves as the archetype for approximately 20 other neutral phosphanegold(I) dithiocarbamate structures in 1144 Tan and Tiekink [Au 2 Fe(C 5 H 8 NS 2 ) 2 (C 34 H 28 P 2 )] Acta Cryst. The molecular structure of (I), showing the atom-labelling scheme and displacement ellipsoids at the 70% probability level. Unlabelled atoms are related by the symmetry operation (Àx + 1, Ày, Àz + 2).

Figure 2
A view of the supramolecular layer in the bc plane sustained by phenylthione C-HÁ Á ÁS interactions, shown as orange dashed lines. H atoms not involved in intermolecular interactions have been omitted for clarity. Table 1 Hydrogen-bond geometry (Å , ). Symmetry codes: (i) x; Ày þ 1 2 ; z þ 1 the crystallographic literature (Groom & Allen, 2014), each having a more or less linear P-Au-S arrangement. There are two structures containing the pyrrolinedithiocarbamate ligand, as in (I), but with phosphane ligands Ph 3 P [(II);  and Cy 3 P [(III); Ho & Tiekink, 2002]. From the data collated for (I)-(III) in Table 2, it is evident that the basic structural features in all three compounds are similar. There is also a closely related dppf-type structure whereby a methylene bridge has been inserted between one P atom and the Cp ring, i.e. (Ph 2 PCH 2 C 5 H 4 FeC 5 H 4 PPh 2 )[Au(S 2 CNEt 2 )] 2 Á2CHCl 3 , [(IV); Š tě pnička & Císařová , 2012]. In this analogue of (I), the Fe II atom is in a general position. While the Au 2 P 2 entity in (IV) remains approximately co-planar, as is crystallographically imposed in (I), i.e. the Au-PÁ Á ÁP-Au pseudo torsion angle is 161.82 (5) , the Au I atoms lie approximately to the same side of the molecule as opposed to the strictly anti conformation found in (I). As seen in Table 2, the selected geometric parameters in (I) and (IV) are comparable. Despite having the shortest intramolecular AuÁ Á ÁS2 contact in (IV), the deviation of the S-Au-P angle from linearity is not the greatest in this structure.

Synthesis and crystallization
Two solutions were prepared. Firstly, a solution of the sodium salt of pyrrolidine dithiocarbamate (Aldrich, 1.6 mmol) was prepared by dissolving this (0.2628 g) in methanol (25 ml Unit-cell contents shown in projection down the c axis, showing the stacking of supramolecular layers. The phenyl-thione C-HÁ Á ÁS interactions are shown as orange dashed lines. One layer is shown in spacefilling mode.  ferrocene]bis[chloridogold(I)] (synthesized by the reduction of KAuCl 4 by Na 2 SO 3 followed by the addition of a stoichiometric amount of 1,1 0 -bis(diphenylphosphanyl)ferrocene; 0.8154 g, 0.8 mmol) was prepared by dissolution in dichloromethane (75 ml). The solution containing the dithiocarbamate salt was added to the gold precursor solution. The resulting mixture was stirred for 3 h at room condition and then filtered. After a week of slow evaporation in a refrigerator, some darkyellow blocks appeared that were characterized crystallographically. M. p. 378-379 K. IR (cm À1 ): 1435 s (C-N); 1152 m, 996 m (C-S).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Carbon-bound H-atoms were placed in calculated positions (C-H = 0.95-0.99 Å ) and were included in the refinement in the riding-model approximation, with U iso (H) set to 1.2U eq (C). The maximum and minimum residual electron density peaks of 1.57 and 1.11 e Å À3 , respectively, were located 0.92 and 0.79 Å from the Au atom.  (Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

[µ 2 -1,1′-Bis(diphenylphosphanyl)ferrocene-κ 2 P:P′]bis[(pyrrolidine-1-carbodithioato-κS)gold(I)]
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.