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organic compounds
As part of a homologous series of novel polyfluorinated bipyridyl (bpy) ligands, the title compound, C16H14F6N2O2, contains the smallest fluorinated group, viz. CF3. The molecule resides on a crystallographic inversion centre at the mid-point of the pyridine Cipso—Cipso bond. Therefore, the bpy skeleton lies in an anti conformation to avoid repulsion between the two pyridyl N atoms. Weak intramolecular C—HN and C—HO interactions are observed, similar to those in related polyfluorinated bpy–metal complexes. A π–π interaction is observed between the bpy rings of adjacent molecules and this is probably a primary driving force in crystallization. Weak intermolecular C—HN hydrogen bonding is present between one of the CF3CH2– methylene H atoms and a pyridyl N atom related by translation along the [010] direction, in addition to weak benzyl-type C—HF interactions to atoms of the terminal CF3 group. It is of note that the O—CH2CF3 bond is almost perpendicular to the bpy plane.
Bipyridine (bpy) is among the most versatile of ligands in organometallics. It has been used extensively to prepare a variety of chelating compounds with different metals (Haga et al., 2000; Bain, Biebuyck & Whitesides, 1989; Vogelson et al., 2003; Chambron & Sauvage, 1986, 1987). Structures with the motif [4,4'-bis(RfCH2OCH2)-2,2'-bpy]MCl2 (Rf = CnF2n+1 or HCnF2n, M = Pd or Pt) are interesting and unusual (Lu et al., 2007; Lu, Tu, Wen et al., 2010; Lu, Tu, Hou et al., 2010). However, the X-ray crystal structure of polyfluorinated bpy compounds (Quici et al., 1999) still remains elusive. Here, we report the structure of the title compound, (I), the simplest polyfluorinated bpy in the series 4,4'-bis(RfCH2OCH2)-2,2'-bpy.
Ruthenium polypyridine complexes have been of particular interest because of their special photophysical properties (Juris et al., 1988; Kalyanasundaram, 1992). By systematically varying the substituents on different positions of the bpy ring and/or the length of the substituents, one can tune the redox and spin properties. This possibility makes them attractive for use in applications such as dye-sensitized solar cells (DSSC; Grätzel, 2001), molecular electronics and catalysis. Compound (I) has been used to prepare novel fluorinated ruthenium complexes in order to tune the electron density and electrochemical properties at the metal centre (Lagref et al., 2003; Chen et al., 2006; Slattery et al., 1994; Curtright & McCusker, 1999). Additionally, compounds derived from the alkylation of 4,4'-dimethyl-2,2'-bipyridine have also been tested for fungicidal activity against some plant diseases (Kelly-Basetti et al., 1995).
Compound (I) exhibits a crystallographic inversion centre at the midpoint of the pyridine Cipso—Cipso bond and and crystallizes in the space group P21/n. Many free bpy ligands described in the literature [Cambridge Structural Database (CSD; Allen, 2002) refcodes EDOXAW and EDOXEA (Maury et al., 2001), FOBRUK and FOBSAR (Iyer et al., 2005), KIDNAP (Vogtle et al., 1990), MILZUC (Heirtzler et al., 2002), NAMKAN02 (Zhang et al., 2003), NOFZUD (Coles et al., 1998), UHIBAO (Viau et al., 2003), VEXQAQ (Spek et al., 2000), VEXQAQ01 (Rice et al., 2002) and VOLLAJ(Butler & Soucy-Breau, 1991)] are also seen to possess a crystallographic centre of symmetry, two distinctive features being the planarity of the connected pyridyl units and the anti arrangement (Alborés et al., 2004; Iyer et al., 2005). The planar bpy C10N2 group in (I) has a weak N1···H3'–C3' hydrogen-bonding interaction, suggested by the N···H distance of 2.49 Å and angle of 100°. The two polyfluorinated side arms point to opposite sides of the bpy plane.
Fig. 1 shows the molecular structure of (I), which is the first reported example of a polyfluorinated bpy of this type. The special features are the polyfluorinated CF3CH2OCH2 tails, with the C9–O8 bond almost perpendicular to the bpy plane [85.6°(1)]; a side view is depicted in Fig. 2. There is a weak C3—H3···O8 hydrogen-bonding interaction present in the five-membered H3/C3/C4/C7/O8 system, providing support to the vertical side arm. Similar to its metal-containing counterparts (Lu, Tu, Hou et al., 2010), the C3—H3···O8 interaction shows structural parameters of a relatively small C3—C4—C7—O8 torsion angle of 21.9 (2)°, an H3···O8 distance of 2.52 Å, shorter than the sum of the van der Waals radii of H and O atoms (2.77 Å; Reference?), and a C—H···O angle of 100°.
As can be seen from Fig. 2, the packing of (I) in the solid state is mainly governed by π–π stacking, with a spacing of 3.476 (1) Å for pairs of molecules related by translation along the b axis. Weak intermolecular C9—H9···N1 hydrogen-bonding interactions are also found for the above-mentioned b-translation related pairs, with H9···N1 = 2.62 Å. From Fig. 2 and Table 1, two weak benzyl-type C–H···F hydrogen-bonding interactions have been located that give support to the stacking in the crystalline state. These involve the terminal F atoms, with H···F distances of 2.49 and 2.67 Å, respectively (see Table 1). The sum of the van der Waals radii of H and F atoms is 2.67 Å (Reference?).
In addition, the two π–π stacking directions are almost orthogonal (Fig. 3). The two normals are at a dihedral angle of 85.7°(1). The multiple supramolecular interconnections in (I) are consistent with its higher m.p. of 356 K compared with its longer polyfluorinated analogues (Rf = C2F5, m.p. 313 K; Rf = C3F7, m.p. 342 K).
In conclusion, one of the elusive polyfluorinated bpy compounds has been crystallized and structurally characterized, showing π–π stacking and weak C—H···F, C—H···O and C—H···N hydrogen-bonding interactions in the solid state.