Crystal structure of (2,2′-bipyridine-κ2 N,N′)bis(3,5-di-tert-butyl-o-benzoquinonato-κ2 O,O′)ruthenium(II)

In the title compound, the RuII ion has a distorted octahedral RuN2O4 coordination environment defined by two 3,5-di-tert-butyl-o-benzosemiquinone ligands and one 2,2′-bipyridine ligand. In the crystal, the compounds are linked by C—H⋯O and π–π stacking interactions, resulting in a layer structure.


Chemical context
The coordination chemistry of o-quinone ligands has been a subject of interest since the beginning of the century, but only within the past decade have detailed studies on the composition and properties of o-quinone complexes been carried out. It has been reported that o-quinone derivatives are noninnocent electroactive ligands that can be found as neutral quinones, radical semiquinones or dianionic catecholates (Lever et al., 1988). The coordination chemistry of ruthenium complexes has been studied over the past few decades because of their versatile and diverse applications in molecular catalysis (Pagliaro et al., 2005;Ramakrishna & Bhat, 2011) and bioinorganic chemistry (van Rijt & Sadler, 2009). Ruthenium complexes with two o-quinone derivatives and one 2,2 0 -bipyridine (bpy) ligand, namely [Ru(bpy)(C 6 H 4 O 2 ) 2 ] and [Ru(bpy)(C 14 H 20 O 2 ) 2 ] (title compound), have been investigated by using various experimental techniques (Lever et al., 1988). Although the ruthenium metals in these complexes potentially could be in the (II), (III) or (IV) oxidation state, according to the oxidation states of the two o-quinone ligands, the state of the metals was confirmed to be bivalent by photoelectron spectroscopy. In order to obtain ruthenium(III) species, it was necessary to oxidize the complexes by silver perchlorate in non-aqueous media. Lever et al. (1988) concluded that the complexes are best regarded as Ru II (bpy)-(sq) 2 (sq: semiquinone anion-radical) with significant mixing of metal and ligand orbitals through Ru-sq back-donation, which results in elongation of the C-O bonds of o-quinone ligands. This elongation has been demonstrated for [Ru(bpy)(C 6 H 4 O 2 ) 2 ] by X-ray single crystal analysis, but the structure of the title compound has not previously been characterized. ISSN 2056-9890

Figure 1
The molecular structure of the title compound with the atom-labelling scheme. Displacement ellipsoids are drawn at the 40% probability level.

Figure 2
A packing view of the title compound with the C-HÁ Á ÁO andinteractions shown as dashed lines.

Synthesis and crystallization
3,5-Di-tert-butyl-o-benzoquinone (0.2 g, 0.90 mmol) was added to 20 ml dry methanol and then triethylamine (0.181 g, 1.8 mmol) was added dropwise and the resultant mixture was stirred for 5 min. Ru(bpy) 2 Cl 2 (0.288 g, 0.45 mmol) was then added to the solution and the contents were refluxed for 6 h. After refluxing, the reaction mixture was cooled down to room temperature and the contents were filtered off. The obtained residue was washed with cold methanol and dried in vacuo (yield: 0.160 g, 70%). Slow evaporation of a solution of the compound in a CH 2 Cl 2 -MeOH mixture (1:1, v/v) yielded single crystals suitable for X-ray diffraction. Crystals of title compound gave no EPR signal at room and liquid nitrogen temperatures, and thus are diamagnetic.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. H atoms of the methyl groups were located in a difference Fourier map and refined as part of rigid rotating groups, with C-H = 0.96 Å and U iso (H) = 1.5U eq (C). The remaining (aromatic) H atoms were placed geometrically and refined using a riding model, with C-H = 0.93 Å and U iso (H) = 1.2U eq (C).

(2,2′-Bipyridine-κ 2 N,N′)bis(3,5-di-tert-butyl-o-benzoquinonato-κ 2 O,O′)ruthenium(II)
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.