Crystal structure and redox potentials of the tppz-bridged {RuCl(bpy)}+ dimer

The dinuclear complex [(bpy)(Cl)Ru(tppz)Ru(Cl)(bpy)](PF6)2 has been synthesized as a catalyst precursor and characterized by X-ray crystallography and cyclic voltammetry.


Chemical context
The design and synthesis of electrochemically and photochemically active ruthenium(II)-polypyridine complexes have been of continued interest in the development of homogeneous electrocatalysis and photocatalysis toward watersplitting schemes for renewable energy applications (Yamazaki et al., 2010;Herrero et al., 2011;Jurss et al., 2012). In our previous work, we introduced Ru dyads in which a lightharvesting Ru moiety (chromophore) and a multi-electron/ multi-proton redox-active Ru moiety (catalyst) were linked by back-to-back terpyridine (tpy-tpy) or tetrapyridylpyrazine (tppz) ligands to give modular light-driven oxidation catalysts with a varying extent of charge delocalization between the Ru centers (Chen et al., 2009(Chen et al., , 2013. In such catalysts containing the {(tpy/tppz)Ru(bpy)(L)} moiety (L = H 2 O or Cl À ), the aqua species is typically formed by ligand substitution from its chloro precursor in water (Davidson et al., 2015b;Matias et al., 2016). Therefore, the chloro complex reported here was initially prepared and isolated as an intermediate in the synthesis of binuclear precatalysts based on the {Ru(tppz)Ru} structural framework . In addition to catalysis, the bis-tridentate tppz ligand finds relevance to the assembly of donor-acceptor metal complexes with electron/ ISSN 2056-9890 energy-transfer properties for potential applications in molecular (opto)electronic devices (Davidson et al., 2015a;Fantacci et al., 2004;Nagashima et al., 2014Nagashima et al., , 2016Wadman et al., 2009).

Structural commentary
The hexafluoridophosphate salt of the binuclear complex [(bpy)(Cl)Ru II (-tppz)Ru II (Cl)(bpy)] 2+ (I) crystallized from an acetonitrile solution in the monoclinic (C2/c) space group. Its crystal structure is shown in Fig. 1, and selected geometrical data are summarized in Table 1. As shown in Fig. 2, the dicationic complex packs in alternating layers with the uncoordinated PF 6 À anions. The complete complex is generated by a crystallographic twofold axis bisecting the C6-C6 i and C7-C7 i [symmetry code: (i) Àx + 1, y, Àz + 3 2 ] bonds of the central pyrazine ring, although it is close to being locally centrosymmetric. The complete tppz ligand has a significantly twisted conformation, with an average angle of 42.4 between the mean planes of adjacent pyridyl rings. The metal-coordinated chloride ligands are in a trans configuration relative to each other across the {Ru(tppz)Ru} core. The two equivalent metal coordination spheres exhibit a distorted octahedral geometry at the Ru II ion due to the restricted bite angle of the bistridendate tppz ligand; the N1-Ru-N3 angle of 160.6 (3) is very similar to those of related tppz-Ru II complexes Jude et al., 2013), and significantly less than the ideal angle of 180 . The Ru atom is essentially in the equatorial mean plane formed by atoms N1, N2, N3, and N4, with a deviation of only 0.026 Å . The bidentate bpy ligand has a cis configuration, with the N4-Ru-N5 angle of 78.4 (3) , in agreement with those found in similar chlorido Ru II -bpy complexes (Chen et al., 2013;Rein et al., 2015). The N5 atom of bpy is arranged trans to the chloride ligand in a nearly linear N-Ru-Cl fashion [172.6 (2) ]. The distances of the two Ru-N bonds for bpy are 2.053 (8) and 2.090 (8) Å , with the shorter bond opposite to Ru-Cl reflecting the increased Ru II !N bpy -backbonding interaction at the coordinating atom trans to the -donor Cl À ligand (Chen et al., 2013). The Ru-Cl bond length of 2.406 (3) Å and the intramolecular RuÁ Á ÁRu separation of 6.579 (4) Å are also similar to those observed for the most closely related Ru(tppz)Ru complexes Hartshorn et al., 1999) 78.4 (3)

Figure 1
The molecular structure of the title compound, with displacement ellipsoids drawn at the 40% probability level. H atoms have been omitted for clarity, except for H13; its close contact with Cl1 is indicated by a red dotted line. [Symmetry code: (i) Àx + 1, y, Àz + 3 2 .]

Figure 2
View along the b axis of a 1 Â 2 Â 2 crystal packing diagram of I. Displacement ellipsoids are drawn at the 40% probability level. Intraand intermolecular HÁ Á ÁCl interactions (those with separations shorter than the sum of van der Waals radii) are represented by the fine dotted lines.
tppz ligand, the Ru-N bond lengths involving the outer N atoms trans to each other are 2.069 (8) and 2.070 (9) Å , whereas the Ru-N bond involving the central N atom has the much shorter length of 1.939 (7) Å as a result of both the geometric constraint imposed by such mer-arranged ligands and the stronger -acceptor ability of the pyrazine-centered bridge Jude et al., 2013). An intramolecular C13-H13Á Á ÁCl1 close contact of 2.74 Å is similar to that observed earlier for complexes containing the {RuCl(bpy)} moiety (Chen et al., 2013;Jude et al., 2008;Rein et al., 2015), although this proximity appears to be partly a consequence of geometry rather than chemically significant bonding.

Supramolecular features
In the crystal, C-HÁ Á ÁCl and C-HÁ Á ÁF interactions (Table 2) with HÁ Á ÁX distances that are shorter than the sum of van der Waals radii can be identified and appear to provide some further stabilization of the crystal packing.

Database survey
A search in the Cambridge Structural Database (Groom et al., 2016) listed only four entries for the {RuCl(bpy)(tppz)} substructure. Of these, two are mononuclear complexes [one with the Ru III oxidation state (Daryanavard et al., 2009) and another at the Ru II state (Tondreau et al., 1996)] and the other two are binuclear complexes [one with tpy instead of bpy and Cl À , and another with Me 2 bpy instead of bpy and the two Cl À ligands in a cis configuration (Hartshorn et al., 1999)].

Electrochemical characterization
Cyclic voltammograms of I in acetonitrile ( Fig. 3; top) show two metal-based oxidation processes at +0.65 and +0.94 V versus Ag/Ag + (10 mM AgNO 3 ). These processes are clearly reversible and correspond to the redox couples Ru II -Ru II / Ru II -Ru III and Ru II -Ru III /Ru III -Ru III , respectively. The stability of the fully oxidized complex is also demonstrated by the voltammogram starting from the Ru III -Ru III species, obtained after application of +1.25 V for 100 s prior to the initial run in the cathodic direction ( Fig. 3; bottom). Two additional reversible processes are observed at À0.89 and À1.39 V, which are characteristic of the ligand-based reductions at the tppz bridge. The separation of 290 mV between the two Ru II /Ru III redox potentials gives a comproportionation constant (K c ) of about 8.0 Â 10 4 , which reflects the stabilization of the mixed-valent state Ru II -Ru III relative to its reduced and oxidized isovalent counterparts Ru II -Ru II and Ru III -Ru III (Richardson & Taube, 1984;Rocha & Toma, 2004). This K c value suggests a significant communication between the Ru centers, although electrochemical properties alone cannot be taken as conclusive evidence for electronic coupling across the bridging ligand because of possible electrostatic effects . By comparison with its precursor [Cl 3 Ru II (tppz)Ru III Cl 3 ] À , which shows a separation greater than 700 mV between the two Ru II /Ru III redox potentials and which has been well characterized as a borderline case of valence localization/delocalization Rocha et al., 2008), the electrochemical data are consistent with a charge-localized configuration in the mixed-valent species [(bpy)(Cl)Ru II (tppz)-Ru III (Cl)(bpy)] 3+ .

Synthesis and crystallization
Compound I was prepared from the mixed-valent complex (nBu 4 N)[Cl 3 Ru II (tppz)Ru III Cl 3 ] as starting material . This precursor was treated by refluxing an ethanolic    (17)  144 Symmetry codes: (i) x; Ày þ 2; z À 1 solution with two equivalents of bpy in the presence of triethylamine as a reductant and the final solid product was collected by filtration of the precipitate formed upon addition of a concentrated aqueous solution of NH 4 PF 6 to the reaction mixture. Green blocks of I were grown by the slow diffusion of diethyl ether into acetonitrile solutions of the product in long thin tubes.

Refinement
Crystal data, data collection, and structure refinement details are summarized in Table 3. Six disordered acetonitrile solvent molecules were treated using PLATON/SQUEEZE (van der Sluis & Spek, 1990;Spek, 2015) and not included in the refinement model; the stated chemical formula, molar mass, etc., do not take account of these solvent molecules. All H atoms (aromatic) were idealized and refined as riding atoms, with C-H = 0.93 Å and U iso (H) = 1.2U eq (C).

µ-2,3,5,6-Tetrakis(pyridin-2-yl)pyrazine-bis[(2,2′-bipyridine)chloridoruthenium(II)] bis(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.