Crystal structure of a dinuclear ruthenium(II) complex with a bent CO2 2− bridge

In the complex cation of the title compound, two RuII atoms are bridged via the carbon and oxygen atoms of an anionic CO2 2– carbonite ligand, resulting in an unsymmetrical dinuclear structure.


Chemical context
Carbon dioxide is an undesirable by-product of the burning of fossil fuels and hence a significant pollutant responsible for climate change. There is considerable interest in using CO 2 as a renewable energy source, capturing and reducing its atmospheric concentration to yield carbon-neutral fuels. However, because CO 2 is thermodynamically stable, its activation and conversion to useful chemicals or fuels are challenging. At present, particular attention has been paid to transition metal catalysts for the activation of CO 2 (Vogt et al., 2018). An understanding of the molecular and crystal structures and vibrational spectroscopic properties of CO 2 ligands bonded to transition metal catalysts is essential because these reveal information concerning the intermediates of the catalytic activation of CO 2 (Gibson, 1999). Many transition metal compounds containing CO 2 or derivatives thereof have been isolated and identified so far. CO 2 ligands can coordinate not only in 1 -C and 2 -C,O modes in mononuclear complexes, but also in bridging modes (Gibson, 1996(Gibson, , 1999. A binuclear complex containing a bridging CO 2 ligand is bonded to one metal by carbon and bonded to the other metal center by one (: 2 mode) or two oxygen (: 3 mode) atoms. Although bridging CO 2 complexes can be synthesized in various ways, a particularly unusual method is the formation of anionic CO 2 2À -bridged dimers by ISSN 2056-9890 the action of water and oxygen on a ruthenium complex containing an unstable formyl ligand . This formyl complex can be obtained from the corresponding dicarbonyl precursor (Toyohara et al., 1995). Therefore, we used this convenient method to synthesize a dimer directly from the stable dicarbonyl precursor and further clarified the crystal structure of the solvated dimer.

Structural commentary
An X-ray structural analysis of the solvent-free dimer [Ru 2 (CO) 2 (C 10 H 8 N 2 ) 4 (: 2 -C,O-CO 2 )] 2+ has previously been performed by Gibson et al. (1996). In their model, the CO 2 2À bridged anion was disordered in both the PF 6 À and BPh 4 À salts, which is not the case here. The title compound consists of two {Ru(CO)(bpy) 2 } 2+ units (bpy = 2,2 0 -bipyridine) singly bridged by a : 2 -C,O carbonite ion, leading to an unsymmetrical dinuclear structure for the resulting cation ( Fig. 1, Table 1). The coordination environment around each Ru II atom is approximately octahedral, and the two terminal CO groups point in the same direction. The Ru1-N1 bond, which is trans to the carbonite carbon, is relatively long [2.154 (4) Å ], suggesting a strong trans influence of the CO 2 2À anion. Although the O-C-O angle in the anionic CO 2 2À bridge [122.4 (5) ] has a typical value observed for this type of bridging anion (Gibson et al., 1997(Gibson et al., , 1998, the lengths of the two C-O bonds [1.269 (9) Å for C1-O1 and 1.254 (7) Å for C1-O2] are almost identical with the difference (Á = 0.015 Å ) being much smaller than those of analogous singly anionic CO 2 -bridged Ru II dimers (0.065 and 0.084 Å ; Gibson et al., 1997Gibson et al., , 1998. The interatomic C2Á Á ÁO2 and C23Á Á ÁO2 distances between carbonyl ligands of 2.853 (6) and 2.818 (7) Å , respectively, are notably shorter than the sum of the van der Waals radii for the atoms involved. Additionally, there are intramolecular C-HÁ Á ÁO and aromaticcontacts, with a centroid-to-centroid distance of 3.889 (3) Å present in the complex cation (Table 2). These interactions may contribute to the unusual C-O bond-length distribution in the bridging CO 2 2À anion described above. The vibrational spectra of the terminal carbonyl groups are useful indicators of the electronic states around the central metal atoms or cations in metal complexes (Oyama et al., 2009). The introduction of the anionic CO 2 2À ligand into the {Ru(CO)(bpy) 2 } 2+ unit results in a large redshift (ca 100 cm À1 ) for the C O group in the IR spectrum, which suggests significant differences in the electron density around the Ru II cations. This IR band indicates that the carbonite ion has a strong electron-donating ability compared to those of the terminal carbonyl ligands.

Supramolecular features
In the crystal structure, additional solvent molecules are incorporated, viz. an acetronitrile and a disordered diethyl ether molecule (occupancy 0.5) per formula unit. There are weak C-HÁ Á ÁF and C-HÁ Á ÁO hydrogen bonds between the complex cation and/or the solvent molecules (CH 3 CN and Et 2 O) and the PF 6 À anions, leading to the formation of a three-dimensional supramolecular network structure (

Figure 1
The molecular structure of the complex cation in the title compound, with atom labels and displacement ellipsoids for non-H atoms drawn at the 50% probability level.
[Ru(bpy) 2 (CO) 2 ](PF 6 ) 2 (10 mg, 0.013 mmol) was dissolved in CH 3 CN (1 ml), followed by the addition of aqueous NaBH 4 (2 eq.) at 253 K. The reaction mixture was stirred for 2 d, and then an excess of Et 2 O was added to the solution at the same temperature. Yellow-orange single crystals gradually formed from the solution when it was allowed to stand at 253 K, yielding X-ray quality crystals. The crystals were obtained in 48% yield (4 mg). The spectroscopic data for the solvent-free compound are consistent with those of Gibson et al. (1996).
riding model with U iso (H) = 1.2U eq (C). The equatorial F atoms of one of the PF 6 À anions are disordered over two sets of sites with an occupancy ratio of 0.908 (7):0.092 (7). The minor components were refined with isotropic displacement parameters. The same applies for the diethyl ether solvent molecule, the central O atom of which is disordered over an inversion centre. The maximum and minimum residual electron density peaks of 3.14 and 2.41 e Å À3 are located 0.77 and 0.73 Å , respectively, from atom Ru1. Data collection: PROCESS-AUTO (Rigaku, 1998); cell refinement: PROCESS-AUTO (Rigaku, 1998); data reduction: