Crystal structure of a mononuclear RuII complex with a back-to-back terpyridine ligand: [RuCl(bpy)(tpy–tpy)]+

In this first crystal structure of an Ru complex with 6′,6"-bis(pyridin-2-yl)-2,2′:4′,4":2",2"’-quaterpyridine, a ‘half’ of the ligand (one of the two terpyridyl units) is N^N^N mer-coordinated, whereas the other is free and adopts a trans,trans conformation about the interannular C—C bonds. The crystal packing features π–π stacking interactions between tpy–tpy ligands.


Chemical context
Aqueous homogeneous photocatalysis by supramolecular assemblies is a powerful concept in the development of sunlight-driven catalytic schemes for renewable energy applications (Herrero et al., 2011;Li et al., 2012;Raynal et al., 2014). In our recent efforts in this area, we have introduced alcohol-oxidation photocatalysts based on dinuclear Ru complexes (Chen et al., , 2011. One of these systems is the chromophore-catalyst dyad [(tpy)Ru(tpy-tpy)Ru(bpy)-(H 2 O)] 4+ , in which the well-defined photosensitizer {(tpy)Ru(tpy)} and catalyst {(tpy)Ru(bpy)(H 2 O)} moieties are linked by the single covalent bond between the back-to-back terpyridines (tpy-tpy). In this and other related photocatalysts containing the {(tpy)Ru(bpy)(L)} moiety (L = H 2 O or Cl À ), the aqua species is typically formed by easy ligand substitution from its chlorido precursor in water Davidson et al., 2015;Jakubikova et al., 2009;Li et al., 2015). Therefore, the mononuclear chlorido complex 1 reported here was initially prepared and isolated as an intermediate in the synthesis of the dinuclear precatalyst [(tpy)Ru(tpy-tpy)-Ru(bpy)(Cl)] 3+ . In addition to catalysis, the bridging tpy-tpy ligand finds relevance to the construction of donor-acceptor complexes with applications in charge/energy transfer and molecular (opto)electronics (Wild et al., 2011). Surprisingly, however, the crystal structure reported here is the first for an Ru II complex.

Structural commentary
The hexafluoridophosphate salt of the monocationic complex (1ÁPF 6 ) crystallizes in the triclinic (P1) space group. The structure of 1 is shown in Figs. 1 and 2, and selected data are summarized in Table 1. The complex has a distorted octahedral geometry at the metal due to the restricted bite angle of its meridionally coordinating tridendate ligand (a tpy moiety). The N1-Ru-N3 angle of 159.32 (16) is very similar to those of bis-terpyridyl Ru II complexes (Chen et al., 2013a;Jude et al., 2013), and far from the ideal angle of 180 . The bidentate bpy ligand has a cis configuration, with the N4-Ru-N5 angle of 79.04 (16) in agreement with those found in similar chlorido Ru II -bpy complexes (Chen et al., 2011(Chen et al., , 2013b (Chen et al., 2013a;Jude et al., 2013). For the bidentate ligand, the Ru-N distance is 2.075 (4) Å for N5 but only 2.028 (4) Å for N4, reflecting the increased Ru II !N bpy -backbonding interaction at the coordinating atom trans to the -donor Cl À ligand (Chen et al., 2013b). The Ru-Cl distance of 2.3982 (16) Å is nearly the same as those observed previously (Chen et al., 2013b;Jude et al., 2009). As expected, the free (uncoordinated) 'half' of tpytpy adopts a trans,trans conformation about the interannular C-C bonds (Constable et al., 1993). Unlike the coordinating half of tpy-tpy, the rings of the free tpy moiety are only approximately coplanar, with angles of 20.9 (3) and 13.3 (3) between adjacent rings.

Figure 2
Two views of a 2Â2Â2 crystal packing diagram of 1ÁPF 6 . Displacement ellipsoids are drawn at the 50% probability level. H atoms are omitted for clarity.

Synthesis and crystallization
Compound 1ÁPF 6 was prepared by slow dropwise addition of a DMF solution of cis-Ru(bpy)(DMSO) 2 Cl 2 into a solution of the tpy-tpy ligand (also in DMF) at reflux. The reaction solution was refluxed for another 2.5 h and then cooled down to room temperature. After evaporation of the solvent on a rotavap, water was added to dissolve the solid and excess NH 4 PF 6 was added to form the precipitate, which was filtered off and dried under vacuum. Further purification was performed by column chromatography using alumina and a mixture of acetonitrile/toluene (1:2) as the eluant. The product was collected from the first band. The solvent was evaporated and the dark-red solid was collected and dried under vacuum (yield: 30%). Analysis calculated for C 40 H 28 N 8 F 6 PClRu: C,53.25;H,3.13;N,12.42. Found: C,52.71;H,3.12;N,11.86. Single crystals for X-ray structural analysis were grown by slow diffusion of diethyl ether into acetonitrile solutions of the complexes in long thin tubes.

Other Characterization
The identity of the complex [Ru(Cl)(bpy)(tpy-tpy)] + was also characterized in MeCN solutions by other techniques. Mass spectra (ESI-MS: m/z 757) are in agreement with the formulation for the cation, i.  Chen et al., 2009), which is consistent with the slightly more electronwithdrawing nature of tpy-tpy compared to tpy.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All carbon-bound hydrogen-atom positions were idealized and set to ride on the atom they were attached to, with C-H = 0.93 Å (aromatic) and U iso (H) = 1.2U eq (C). Each atom in the anion was modeled in two positions, with site occupancies tied to 1.0. A total of 48 temperature-factor restraints were used to force convergence. The SQUEEZE routine in PLATON (van der Sluis & Spek, 1990;Spek, 2015) was used to treat disordered solvent molecules. The given chemical formula and other crystal data do not take into account the solvent. The final refinement included anisotropic temperature factors on all non-hydrogen atoms. Data collection: APEX2 (Bruker, 2007); cell refinement: SAINT-Plus (Bruker, 2007); data reduction: SAINT-Plus (Bruker, 2007); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: Mercury (Macrae et al., 2008); software used to prepare material for publication: publCIF (Westrip, 2010).