Crystal structure of bis[μ-bis(diphenylphosphanyl)methane-κ2 P:P′]digold(I) dichloride acetone monosolvate monohydrate

The AuI atoms in the dication of the title structure, [(C6H5)2PCH2P(C6H5)2Au2]Cl2·(CH3)2C=O·H2O, show an aurophilic interaction of 2.9743 (2) Å.


Chemical context
Recent studies have highlighted the significant biological activity exhibited by phosphanegold(I) carbonimidothioates, i.e. compounds of the general formula Ph 3 PAu[SC(OR) N(aryl)]; R = alkyl. These compounds are cytotoxic and kill cancer cells by initiating apoptotic pathways (Yeo, Ooi et al., 2013;Ooi et al., 2015) and prove to be very potent to Grampositive bacteria (Yeo, Sim et al., 2013). Over and above this potential, phosphanegold(I) carbonimidothioates offer opportunities in crystal engineering (Kuan et al., 2008) and exhibit solid-state luminescence (Ho et al., 2006).

Structural commentary
The asymmetric unit of (I) comprises a [Au 2 (Ph 2 PCH 2 PPh 2 )] 2+ dication, two Cl À anions, and a solvent molecule each of acetone and water; all species are in general positions. The molecular structure of the dication is shown in Fig. 1. Two Au I atoms are bridged by two Ph 2 PCH 2 PPh 2 ligands, forming an eight-membered {-PCPAu} 2 ring. The ring has the form of a boat with the methylene-C1 and C2 atoms lying to one side of the ring and 0.589 (5) and 0.581 (5) Å , respectively, above the least-squares plane through the Au 2 P 4 atoms which have a r.m.s. deviation of 0.0849 Å . There is a transannular Au1Á Á ÁAu2 (aurophilic) interaction of 2.9743 (2) Å . This interaction is partly responsible for the deviations of the P1-Au1-P3 and P2-Au2-P4 angles from the ideal 180 , i.e. 173.24 (4) and 170.04 (4) , respectively. The Au-P bond lengths are almost equivalent, ranging from a short  to a long Au2-P4 2.3130 (12) Å . The Cl1 À anion forms a weak bridge between the two Au I atoms with Au1Á Á ÁCl1 and Au2Á Á ÁCl2 separations of 2.9492 (13) and 2.9776 (12) Å , respectively. The second Cl À anion participates in hydrogen bonding as described below in Supramolecular features.

Figure 2
Unit-cell contents of (I) shown in projection down the c axis. Intramolecular aurophilic interactions are drawn as orange dashed lines and the weak AuÁ Á ÁCl contacts are shown as black dashed lines. Intermolecular O-HÁ Á ÁCl, C-HÁ Á ÁCl1, C-HÁ Á ÁCl2, C-HÁ Á ÁO(acetone) and C-HÁ Á Á interactions are shown as blue, orange, brown, green and purple dashed lines, respectively. The acetone molecules have been highlighted in space-filling mode.

Database survey
The [Au 2 (Ph 2 PCH 2 PPh 2 )]Cl 2 salt has been characterized twice previously, originally as an acetone solvate (Schmidbaur et al., 1977) and subsequently as an acetonitrile solvate (Liou et al., 1994). Geometric data characterizing the eight-membered rings are summarized in Table 2. The most notable difference between the structure of (I) and the dications is that the latter are disposed about a centre of inversion and the eightmembered {-PCPAu} 2 rings have flattened chair conformations, with the methylene-C atoms lying to either side of the eight-membered ring. The similarity between the literature structures and the difference between these and the dication in (I) are highlighted in the overlay diagram shown in Fig. 3. The other remarkable difference between the three structurally characterized [Au 2 (Ph 2 PCH 2 PPh 2 )]Cl 2 salts relates to the mode of association between the complex Au cations and Cl À anions. As noted above and shown in Fig. 4a, the Cl1 À anion in (I) forms a weak bridge between the two Au I atoms. In the acetone solvate (Schmidbaur et al., 1977), each Cl À anion associates with one Au I atom at a distance of 2.771 (4) Å . A similar pattern is noted in the acetonitrile solvate (Liou et al., 1994), but the distances are significantly longer at 2.951 (4) Å . The close AuÁ Á ÁCl contacts appear to influence the P-Au-P angles in that those in the [Au 2 (Ph 2 PCH 2 PPh 2 )]Cl 2 salts with loosely associated Cl À anions having greater distortions from linearity, in particular for the acetone solvate (Schmidbaur et al., 1977), compared with dications characterized with noncoordinating counter-anions, namely BH 4 À , ClO 4 À (Cao et al., 2006), PF 6 À (Wu et al., 2003) and [H 3 BCN] À , Table 2.

Figure 3
Overlay diagram of the [Au 2 (Ph 2 PCH 2 PPh 2 )] 2+ dications in (I) (red image), LEKGAJ (green) and PPEAUC (blue), overlapped so that the one methylene C and the two Au I atoms are coincident.

Figure 4
Details of the weak AuÁ Á ÁCl interactions, shown as dashed black lines, in the dications of (a) (I), (b) LEKGAJ and (c) PPEAUC. For clarity, all H atoms have been removed and only the ipso-C atoms shown. stirred at 323 K for 2 h. The final product was extracted with dichloromethane (100 ml) and the solution was left for evaporation at room temperature. After 3 weeks a slurry formed. This was redissolved in a solvent mixture of acetone/ acetonitrile (1:1 v/v, 100 ml) and left for slow evaporation. Colourless crystals were obtained after 10 days. Yield: 0.213 g (43%

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 1.2-1.5U equiv (C). The water-bound H atoms were refined with O-H = 0.84AE0.01 Å , and with U iso (H) = 1.5U equiv (O). The U 33 parameter was elongated for the C93 atom. In the final refinement this was restrained to be nearly isotropic using the ISOR command in SHELXL (Sheldrick, 2015). The maximum and minimum residual electron density peaks of 3.50 and 1.82 eÅ À3 , respectively, were located 0.90 Å and 0.78 Å from the Au1 and Au2 atoms, respectively.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. The maximum and minimum residual electron density peaks of 3.50 and 1.82 e Å -3 , respectively, were located 0.90 Å and 0.78 Å from the Au1 and Au2 atoms, respectively.