Isolation and structural comparison of RuII-dnp complexes [dnp = 2,6-bis(1,8-naphthyridin-2-yl)pyridine] with axially or equatorially coordinating NCS ligands

The crystal structures of two RuII complexes bearing a tridentate polypyridine ligand and N-coordinating thiocyanato ligands at the axial or equatorial position are compared.


Chemical context
Polypyridylruthenium(II) complexes play essential roles in key technologies, such as solar energy conversion (Lewis, 2007). In particular, Ru II complexes with thiocyanate ion(s) are interesting as dye molecules for dye-sensitized solar cells (Hagfeldt et al., 2010). As a ligand, the thiocyanate group can bond to metals through the terminal nitrogen or sulfur atoms since it is ambidentate. Linkage isomeric pairs can be distinguished using spectroscopic techniques when they exist as a mixture (Brewster et al., 2011;Vandenburgh et al., 2008). However, identifying the coordinating atom (N or S) by structural analysis is more reliable when only one isomer exists.

Figure 1
Molecular structure of the complex cation in (I), with atom labels and displacement ellipsoids for non-H atoms drawn at the 50% probability level.
C40 rings, respectively] are present, with a slippage of 1.2 Å for Cg1Á Á ÁCg2. It is inferred from these results that bothinteractions are not exactly cofacial. The slippage angle is 19.2 for Cg1Á Á ÁCg2 and 16.2 for Cg3Á Á ÁCg4. As mentioned above, it is important to distinguish the coordination atom of the thiocyanato ligand because of its ambidentate coordination mode. Both S-and N-coordinated Ru II complexes containing polypyridines have been determined structurally, but the N-atom coordination is overwhelmingly dominant. These complexes can be distinguished crystallographically by the Ru-X-C bond angle (X = N or S) through the coordinating atom. For example, the Ru-S-C bond angles (for S-ligating examples) are 104-106 (Brewster et al., 2011;Homanen et al., 1996;Vandenburgh et al., 2008), whereas the Ru-N-C bond angles (for N-ligating examples) are in the range 159-179 (Brewster et al., 2011;Cadranel et al., 2012;Shklover et al., 2002;Vandenburgh et al., 2008;Zakeeruddin et al., 1997). In the present cases, the Ru-X-C bond angles of compounds (I) and (II) are 175.6 (3) and 166.03 (19) , respectively, indicating that the Ru II atoms in both compounds exhibit an N-coordination.

Supramolecular features
Additional solvent molecules are incorporated in the crystal structure of (I), i.e., a water molecule and a disordered acetone molecule (occupancy 0.5) per formula unit. Apart from Coulombic forces, there are weak C-HÁ Á ÁF hydrogen bonds between the complex cation and the PF 6 À anion (Table 1) and the acetone molecule [O1Á Á ÁO2 = 2.87 (1) Å ]. These interactions contribute to the stabilization of the packing and formation of a three-dimensional supramolecular structure (Fig. 3).
In the crystal structure of (II), weak C-HÁ Á ÁX (X = N or S) hydrogen-bonding interactions exist between the complex cation and the NCS À anion (Table 2) along with the intramolecular hydrogen bonds. Additionalinteractions [Cg5Á Á ÁCg5 i = 4.0093 (15) Å ; Cg5 is the centroid of the N5/ C17-C21 ring; symmetry code: (i) 1 À x, 1 À y, 1 À z] with a centroid slippage of 1.263 Å for Cg5Á Á ÁCg5 i are present. The slippage angle is 18.4 for Cg5Á Á ÁCg5 i . These interactions lead to the formation of a three-dimensional network structure (Fig. 4).

Figure 4
The crystal packing of compound (II) with hydrogen bonds (blue; for numerical details, see Table 2) andcontacts (green) shown as dashed lines. Ring centroids are shown as red spheres.

Synthesis and crystallization
A methanolic solution (40 ml) containing [Ru(dnp)(PPh 3 ) 2 -(H 2 O)](PF 6 ) 2 (50 mg, 0.039 mmol) (Oyama et al., 2013) and 1.1 eq. of NaSCN (10 mg) was heated under reflux for 30 min. The volume was reduced to 5 ml, and a saturated solution of KPF 6 was added. The resulting solid was filtered and washed sequentially with water and diethyl ether. The yield was 32 mg (69%). Crystals suitable for use in X-ray diffraction (XRD) studies were grown by vapor diffusion of diethyl ether into an acetone solution of (I). Fourier transform infrared (FTIR) spectroscopy using a KBr pellet showed CN at 2130 cm À1 .
For the synthesis of compound (II), a methanolic solution (20 ml) containing [Ru(dnp)(py) 2 (H 2 O)](PF 6 ) 2 (25 mg, 0.028 mmol) (Oyama et al., 2013) and 2.2 eq. of NaSCN (5 mg) was heated under reflux for 30 min. The reaction mixture was reduced to 3 ml. The addition of diethyl ether (5 ml) to the solution resulted in the formation of a precipitate of (II). The crude product was purified by column chromatography on Al 2 O 3 (eluent: acetone). The yield was 9 mg (40%). Single crystals suitable for XRD studies were obtained by recrystallization from acetone. FTIR using a KBr pellet showed CN at 2121 (ligand) and 2055 cm À1 (counter-ion). Table 3 lists the crystal data, data collection, and structure refinement details. All hydrogen atoms were placed at calculated positions [C-H = 0.93 or 0.96 Å in (I), C-H = 0.95 Å in (II)] and refined using a riding model with U iso (H) = 1.2U eq (C). The acetone solvent molecule in (I) (C41-C43, O2) is disordered over an inversion center and was refined with an occupancy of 0.5. The oxygen atom of the solvent water molecule (O3) was refined with an isotropic displacement parameter. H atoms of the coordinating and the solvate water molecules could not be localized from difference-Fourier maps. Therefore, they are not part of the model but part of the formula.

κP)ruthenium(II) hexafluoridophosphate-acetone-water (1/0.5/1) (I)
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. Refinement. Refinement was performed using all reflections. The weighted R-factor (wR) and goodness of fit (S) are based on F 2 . R-factor (gt) are based on F. The threshold expression of F 2 > 2.0 sigma(F 2 ) is used only for calculating Rfactor (gt).

thiocyanate (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. Refinement. Refinement was performed using all reflections. The weighted R-factor (wR) and goodness of fit (S) are based on F 2 . R-factor (gt) are based on F. The threshold expression of F 2 > 2.0 sigma(F 2 ) is used only for calculating Rfactor (gt).