Synthesis and crystal structure of dichlorido(1,10-phenanthroline-κ2 N,N′)gold(III) hexafluoridophosphate

The gold(III) atom in the title complex has a square-planar coordination environment defined by two Cl atoms and a chelating phenanthroline ligand.


Chemical context
Au III is isoelectronic with Pt II and forms compounds with similar coordination modes and structures. Therefore, the synthesis of Au III -based compounds has attracted much interest in the field of bioinorganic and medicinal chemistry after the successful application of cis-platin [cis-diamminedichloridoplatinum(II)] for cancer treatment (Siddik, 2003). Aromatic N-donors, such as 1,10-phenanthroline, are of interest given their planar structure that synergizes well with the typical square-planar coordination sphere of Au III , producing potent DNA-intercalating agents (Abbate et al., 2000;Zou et al., 2015). On the other hand, Au III compounds differ from Pt II compounds in terms of their interactions with biomolecules, their stability in biological media or their mechanism of action. A review on cytotoxic properties and mechanisms of Au III compounds with N-donors has been provided by Zou et al. (2015).
In this context we have prepared the title salt, [AuCl 2 (C 12 H 8 N 2 )]PF 6 , that was characterized by elemental and mass spectrometric analysis (ESI(+)-QTOF-MS), 1 H nuclear magnetic resonance measurements and by single crystal X-ray diffraction.

Structural commentary
All atoms in the title salt are on general positions. The Au III atom has a square-planar coordination environment, with the chlorido ligands in a cis configuration to each other. The Au III atom deviates from planarity (as determined based on the four coordinating atoms) by 0.018 Å (r.m.s.). The main bond lengths [Au-N1 = 2.032 (2), Au-N2 = 2.036 (2), Au-Cl1 = 2.251 (1) and Au-Cl2 = 2.255 (1) Å ] are in the normal ranges for this kind of complexes (see Database survey). The bite angle of the 1,10-phenanthroline ligand is 81.75 (9) , while the Cl1-Au-Cl2 angle is 89.28 (3) . Despite the highly symmetrical nature of the hexafluoridophosphate counter-ion, this unit does not show any disorder. The structures of the molecular entities of the [AuCl 2 (C 12 H 8 N 2 )]PF 6 salt are shown in Fig. 1.

Supramolecular features
The molecular packing in the crystal is shown in Fig. 2. Despite the square-planar coordination environment around Au III and the presence of the highly conjugated and planar 1,10phenanthroline ligand,interactions have little relevance to the stabilization of the crystal. The shortest -like interaction between the centroids [Cg1Á Á ÁCg2 i ; symmetry code: (i) 1 2 + x, y, 1 2 À z; Fig. 3] of two neighbouring 1,10-phenanthroline rings are associated with a distance of 4.2521 (15) Å , which is very close to the upper limit of the threshold established by Janiak (2000) for a relevant offset interaction.
The interactions between the hexafluoridophosphate counter-ion and the 1,10-phenanthroline ligands constitute the major intermolecular interactions in the crystal and can be divided into two types. The first type corresponds to an anion-donorÁ Á Á -acceptor interaction (Chifotides & Dunbar, 2013 Packing of the crystal structure of [AuCl 2 (C 12 H 8 N 2 )]PF 6 in a view along the c axis. Displacement ellipsoids are drawn at the 40% probability level.

Figure 1
The molecular entities of the title salt [AuCl 2 (C 12 H 8 N 2 )]PF 6 . Displacement ellipsoids are drawn at the 40% probability level. Hydrogen atoms are not labelled for clarity.

Database survey
A few structures of Au III -(1,10-phenanthroline) compounds have been reported in the literature with different counterions. Abbate et al. (2000) reported the monohydrate chloride structure that crystallizes in the space group type P2 1 /n, with Au-N distances of 2.033 (8)  The Au-N distances are 2.05 (1) and 2.05 (1) Å , while the Au-Cl distances are 2.290 (5) and 2.299 (5) Å . The title compound has Au-N distances similar to that of the structure reported by Abbate et al. (2000), but slightly shorter than the one by Pitteri et al. (2008). Regarding the Au-Cl distances, [AuCl 2 (C 12 H 8 N 2 )]PF 6 and the structure reported by Abbate et al. (2000) have shorter ones than that reported by Pitteri et al. (2008). Although the [AuCl 2 (C 12 H 8 N 2 )] + cations in the three structures exhibit no significant differences, their crystal packings vary greatly as a consequence of the intermolecular interactions with the different counter-ions. The structure reported by Abbate et al. (2000) has the Au III -(1,10-phenanthroline) units closer in space, with the shortest centroid-to-centroid distance being 3.820 Å , much closer than 4.2521 (15) Å observed in the title compound. Furthermore, the presence of a water molecule and the chloride counter-ion establish a classical hydrogen-bonding network, which is absent in the structure of the title compound. The structure determined by Pitteri et al. (2008) is the only one with an axial AuÁ Á ÁL interaction, namely AuÁ Á ÁBr (3.374 Å ).

Synthesis and crystallization
[AuCl 2 (C 12 H 8 N 2 )]PF 6 was synthesized by a modification of a literature protocol (Casini et al., 2010)  X-ray analysis were obtained by recrystallization from acetonitrile solution.

Solution stability
The stability of the [Au(1,10-phenanthroline)] 3+ moiety is critical for the biological properties of the compound, including cytotoxicity. The [AuCl 2 (C 12 H 8 N 2 )]PF 6 salt was dissolved in deuterated dimethylsulfoxide (DMSO-d6) and the solvent replacement was followed by 1 H NMR for 72 h (Fig. 4). 1 H NMR spectra were acquired on a Bruker Avance III 400 MHz. The labile chlorido ligands were replaced, as expected, but the [Au(1,10-phenanthroline)] 3+ moiety remained stable in the presence of the coordinating solvent (DMSO) throughout the period evaluated.

Dichlorido(1,10-phenanthroline-κ 2 N,N′)gold(III) hexafluoridophosphate
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.