Crystal structure of bromido(η6-1-isopropyl-4-methylbenzene)(7-oxocyclohepta-1,3,5-trien-1-olato-κ2 O,O′)osmium

In the title compound, the central OsII ion is ligated by a hexahaptic η6 p-cymene ring, a Br− ligand and two O atoms of a chelating tropolonate group. The p-cymene ligand presents more than one conformation, giving rise to a discrete disorder, which was modelled with two different orientations with occupancy values of 0.561 (15) and 0.439 (15).


Chemical context
The chemistry of half-sandwich organometallic Os II -arene complexes with O-donor ligands has drawn considerable interest because of their potential application as anticancer agents (Zhang & Sadler, 2017). In particular, several complexes of this type with O,O-and N,O-chelating ligands have been investigated (Hanif et al., 2010;van Rijt et al., 2009). While the complexes with N,O-ligands have shown in vitro anticancer activity comparable to Cisplatin, the benchmark anticancer metallopharmaceutical, complexes with O,Oligands exhibit low activity. This has been attributed to the poor stability of these complexes in aqueous solution and the formation of inactive hydroxy-bridged dimers (Hanif et al., 2014). The mechanism of the cytotoxic action of the Os IIarene complexes is generally thought to involve hydrolysis of the Os-X bond (where X = a halide ligand) to generate an active Os-OH 2 species, which binds to biomolecules leading to cellular dysfunction and consequently triggering apoptosis. While the anticancer activity of the Os II -arene complexes has often been compared to that of their Ru analogues, no definitive structure-activity relationship has yet been elucidated. In addition, the Os II -arene complexes appear to have an altered pharmacological profile in comparison with the ruthenium complexes (Bruijnincx & Sadler, 2009). As part of our studies in this area, single-crystal X-ray diffraction was used to determine the structure of the title compound, (I).

Structural commentary
The molecular structure of (I) is shown in Fig. 1 and selected geometrical data are presented in Table 1. The complex adopts a 'three-legged piano-stool' geometry, where the 6 -coord- ISSN 2056-9890 inated arene ring is present as the seat, and the two O atoms of the tropolonate ligand along with the bromido ligand as the three legs of the stool.
The tropolonato anion is chelated to the metal centre, forming a five-membered OsO 2 C 2 ring, which is almost planar, with the tight bite angle [76.3 (2) ] of the tropolonate chelate resulting in a distorted pseudo-octahedral coordination sphere. The rigid tropolonate ligand backbone is made up of an almost planar seven-membered ring consisting of conjugated sp 2 carbon atoms. The Os-O bond lengths [2.071 (6) and 2.088 (6) Å ] are similar to those of the related ruthenium compound (ca 2.1 Å ) published previously (Dwivedi et al., 2016). The isobidentate nature of the OsO 2 C 2 moiety is evidence of delocalization of the C O bonds of the tropolone ligand upon coordination [C11-O1 = 1.303 (11), C17-O2 = 1.299 (11)Å ]. The aromatic ring of the p-cymene ligand appears almost planar, with the displacement of the arene ring centroid from the Os II center [1.676 Å ] being comparable with other similar complexes (Peacock et al., 2007;Kandioller et al. 2013).

Supramolecular features
In the crystal, the coordinated O atoms of the tropolonate ligand accept weak C-HÁ Á ÁO interactions (Table 2) from the p-cymene ring in the range 2.40-2.72 Å , which contribute to the crystal packing. In addition, the bromide ion acts as a hydrogen-bond acceptor, forming C-HÁ Á ÁBr hydrogen bonds with a C-H group from the arene ring of an adjacent molecule. There is also astacking interaction between the tropolone ligands with the plane-to-plane distances of the stacked aromatic ring moieties at 3.895 Å (Fig. 2

Figure 2
Detail of the packing of (I) showing aromaticstacking between the seven-membered rings as a blue dashed line.

Figure 1
The molecular structure of (I) showing 50% displacement ellipsoids. Only one orientation of the disordered benzene ring is shown.

Synthesis and crystallization
All synthetic procedures were carried out using standard Schlenk techniques under an atmosphere of argon. The osmium dimer [Os( 6 -p-cymene)Br 2 ] 2 (1.037 g, 1.07 mmol) and sodium tropolonate (0.448 g, 3.11 mmol) were suspended in methanol (100 ml). The suspension was stirred at room temperature overnight to give a dark-brown solution. The solution was filtered and the solvent was removed in vacuo.
The residue was extracted with CH 2 Cl 2 (80 ml). The solvent was removed under reduced pressure to give the title compound as a red-brown solid. Yield 72% (0.807 g, 1.54 mmol). Red blocks of (I) were obtained by slow evaporation from a concentrated dichloromethane solution at room temperature over several days.

Special details
Experimental. Numerical integration absorption corrections based on indexed crystal faces were applied using the XPREP routine (Bruker, 2016) 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.