4,4′-Bis(2,2,2-trifluoro­ethoxy­methyl)-2,2′-bipyridine

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 C_ipso—C_ipso bond. Therefore, the bpy skeleton lies in an anti conformation to avoid repulsion between the two pyridyl N atoms. Weak intramolecular C—H...N and C—H...O 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—H...N 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—H...F inter­actions to atoms of the terminal CF₃ group. It is of note that the O—CH₂CF₃ bond is almost perpendicular to the bpy plane.

S1. Comment

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(R_fCH₂OCH₂)-2,2'-bpy]MCl₂ (R_f = C_nF_2n+1 or HC_nF_2n, 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(R_fCH₂OCH₂)-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 C_ipso—C_ipso 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 (R_f = C₂F₅, m.p. 313 K; R_f = 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.

Related literature

For related literature, see: Alborés et al. (2004); Allen (2002); Bain, Biebuyck & Whitesides (1989); Butler & Soucy-Breau (1991); Chambron & Sauvage (1986, 1987); Chen et al. (2006); Coles et al. (1998); Curtright & McCusker (1999); Grätzel (2001); Haga et al. (2000); Heirtzler et al. (2002); Iyer et al. (2005); Juris et al. (1988); Kalyanasundaram (1992); Kelly-Basetti, Cundy, Pereira, Sasse, Savage & Simpson (1995); Lagref et al. (2003); Lu et al. (2007); Lu, Tu, Hou, Lin, Li & Liu (2010); Lu, Tu, Wen, Liu, Chou & Jiang (2010); Maury et al. (2001); Quici et al. (1999); Rice et al. (2002); Slattery et al. (1994); Spek et al. (2000); Viau et al. (2003); Vogelson et al. (2003); Vogtle et al. (1990); Zhang et al. (2003).

S2. Experimental

4,4'-Bis(CF₃CH₂OCH₂)-2,2'-bpy, (I), was prepared according to the general procedure of Lu et al. (2007). The crude product was further purified by vacuum sublimation or chromatography [Which specific technique was used?] to obtain (I) as a colourless solid. Recrystallization proceeded with dissolution of (I) in dimethyl sulfoxide to form a saturated solution, to which a water overlayer (5 ml) was added. Solvent diffusion over a period of a week at 298 K afforded white needle crystals of (I).

Analytical data for (I): yield 86%, m.p. 356 K; ¹H NMR (500 MHz, d-DMSO, room temperature, δ, p.p.m.): 8.68 (d, H₆, ³J_HH = 5.0 Hz, 2H), 8.38 (s, H₃, 2H), 7.41 (d, H₅, ³J_HH = 5.0 Hz, 2H), 4.83 (s, bpy-CH₂, 4H), 4.22 (q, -OCH₂CF₃, ³J_HF = 9.2 Hz, 4H); ¹⁹F NMR (470.5 MHz, d-DMSO, room temperature, δ, p.p.m.): -73.2 (t, -CH₂CF₃, ³J_HF = 8.6 Hz, 6F); ¹³C NMR (126 MHz, d-DMSO, room temperature, δ, p.p.m.): 155.2, 149.4, 147.6, 122.1, 118.3 (s, bpy, 10 C), 124.5 (127.8–121.1, q, -CF₃, ¹J_CF = 280.4 Hz, 2C), 71.7 (s, bpy-CH₂, 2C), 67.1 (q, -CH₂CF₃, ²J_CF = 32.3, 2C); GC/MS (M/e): M⁺ = 380, (M - C₂H₃F₃O)⁺ = 281, [M - (C₂H₃F₃O)₂]⁺ = 182, (M - C₂H₃F₃OC₆H₅N)⁺ = 91; FTIR (cm^-1): 1603, 1560, 1440 (bpy-ring, m), 1178, 1153 (CF₂ stretch, s).

S3. Refinement

The current P2₁/n unit cell has parameters a = 12.9726 (9), b = 4.7485 (3) and c = 14.1782 (10) Å, with β = 116.309 (3)°. Changing the setting to the standard space group P2₁/c results in a larger β angle.

All H atoms were generated geometrically, with C—H = 0.93 Å for CH on bpy and 0.97 Å for CH₂ on side chains, and refined using a riding model, with U_iso(H) = 1.2U_eq(C).

Computing details

Data collection: SMART (Bruker, 2001); cell refinement: SMART (Bruker, 2001); data reduction: SAINT (Bruker, 2001); program(s) used to solve structure: SIR92 (Altomare et al., 1993); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: WinGX (Farrugia, 1999).

Figures

	Fig. 1. The molecular structure of (I), showing (a) the atom-numbering scheme and the weak intramolecular C—H···O and C—H···N hydrogen-bonding interactions (dashed lines), and (b) an ellipsoid plot, with displacement ellipsoids drawn at the 35% probability level and H atoms shown as small spheres of arbitrary radii. [Symmetry code: (i) 1 - x, 1 - y, 1 - z.]
	Fig. 2. Weak C—H···N and C—H···F interactions involving methylene H atoms; other H atoms have been omitted for clarity. Additional molecules are drawn to highlight the nature of the π–π stacking. Distances are given in angstroms (Å).
	Fig. 3. The orthogonal stacking of the bpy molecular planes.

4,4'-Bis(2,2,2-trifluoroethoxymethyl)-2,2'-bipyridine

Crystal data

C16H14F6N2O2	Dx = 1.613 Mg m^−3 Dm = 1.56 Mg m^−3 Dm measured by w/v
Mr = 380.29	Melting point: 356 K
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71073 Å
a = 12.9726 (9) Å	Cell parameters from 2952 reflections
b = 4.7485 (3) Å	θ = 3.2–26.4°
c = 14.1782 (10) Å	µ = 0.16 mm^−1
β = 116.309 (3)°	T = 100 K
V = 782.91 (9) Å^3	Prism, colourless
Z = 2	0.28 × 0.2 × 0.18 mm
F(000) = 388

Data collection

Bruker SMART CCD area-detector diffractometer	1587 independent reflections
Radiation source: fine-focus sealed tube	1332 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.029
ϕ and ω scans	θmax = 26.4°, θmin = 2.9°
Absorption correction: multi-scan (SADABS; Sheldrick, 1996)	h = −10→16
Tmin = 0.692, Tmax = 0.745	k = −5→5
5975 measured reflections	l = −17→17

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.031	H-atom parameters constrained
wR(F^2) = 0.082	w = 1/[σ^2(Fo^2) + (0.037P)^2 + 0.3154P] where P = (Fo^2 + 2Fc^2)/3
S = 1.04	(Δ/σ)max < 0.001
1587 reflections	Δρmax = 0.33 e Å^−3
119 parameters	Δρmin = −0.23 e Å^−3
0 restraints	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.013 (3)

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 of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
F1	0.40106 (8)	0.1330 (2)	0.87634 (6)	0.0361 (3)
F2	0.53391 (7)	−0.13672 (19)	0.87744 (7)	0.0337 (3)
F3	0.50836 (8)	0.2721 (2)	0.80648 (7)	0.0357 (3)
O8	0.28749 (8)	0.1244 (2)	0.66037 (7)	0.0175 (2)
N1	0.43093 (9)	0.2740 (2)	0.38652 (8)	0.0168 (3)
C2	0.45226 (11)	0.3956 (3)	0.47902 (10)	0.0146 (3)
C3	0.38952 (11)	0.3315 (3)	0.53421 (10)	0.0159 (3)
H3	0.4072	0.4183	0.5984	0.019*
C4	0.30065 (11)	0.1381 (3)	0.49333 (10)	0.0161 (3)
C5	0.27783 (11)	0.0133 (3)	0.39743 (10)	0.0177 (3)
H5	0.2186	−0.1168	0.3670	0.021*
C6	0.34524 (11)	0.0869 (3)	0.34837 (10)	0.0186 (3)
H6	0.3300	0.0006	0.2846	0.022*
C7	0.22974 (11)	0.0644 (3)	0.55056 (10)	0.0206 (3)
H7A	0.1582	0.1693	0.5196	0.025*
H7B	0.2109	−0.1345	0.5410	0.025*
C9	0.37255 (11)	−0.0799 (3)	0.71625 (10)	0.0174 (3)
H9A	0.4139	−0.1328	0.6763	0.021*
H9B	0.3370	−0.2472	0.7281	0.021*
C10	0.45296 (11)	0.0479 (3)	0.81867 (11)	0.0199 (3)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
F1	0.0327 (5)	0.0543 (6)	0.0216 (5)	0.0114 (5)	0.0123 (4)	−0.0098 (4)
F2	0.0295 (5)	0.0343 (5)	0.0257 (5)	0.0145 (4)	0.0017 (4)	0.0009 (4)
F3	0.0292 (5)	0.0322 (5)	0.0340 (5)	−0.0135 (4)	0.0034 (4)	−0.0040 (4)
O8	0.0156 (5)	0.0215 (5)	0.0152 (5)	0.0011 (4)	0.0065 (4)	0.0004 (4)
N1	0.0172 (5)	0.0163 (6)	0.0175 (6)	−0.0010 (5)	0.0082 (5)	0.0002 (4)
C2	0.0148 (6)	0.0121 (6)	0.0163 (6)	0.0028 (5)	0.0063 (5)	0.0036 (5)
C3	0.0162 (6)	0.0157 (6)	0.0155 (6)	0.0008 (5)	0.0069 (5)	0.0004 (5)
C4	0.0140 (6)	0.0173 (7)	0.0160 (7)	0.0017 (5)	0.0058 (5)	0.0036 (5)
C5	0.0141 (6)	0.0178 (7)	0.0182 (7)	−0.0030 (5)	0.0045 (5)	0.0000 (5)
C6	0.0202 (7)	0.0195 (7)	0.0157 (6)	−0.0011 (6)	0.0074 (6)	−0.0016 (5)
C7	0.0157 (6)	0.0299 (8)	0.0153 (7)	−0.0049 (6)	0.0062 (6)	0.0000 (6)
C9	0.0177 (6)	0.0171 (7)	0.0196 (7)	0.0002 (5)	0.0103 (6)	0.0002 (5)
C10	0.0177 (7)	0.0222 (7)	0.0200 (7)	0.0050 (6)	0.0085 (6)	−0.0001 (6)

Geometric parameters (Å, º)

F1—C10	1.3317 (15)	C4—C5	1.3896 (19)
F2—C10	1.3378 (16)	C4—C7	1.5129 (17)
F3—C10	1.3381 (17)	C5—C6	1.3821 (19)
O8—C9	1.4183 (16)	C5—H5	0.9300
O8—C7	1.4256 (15)	C6—H6	0.9300
N1—C6	1.3359 (17)	C7—H7A	0.9700
N1—C2	1.3446 (17)	C7—H7B	0.9700
C2—C3	1.3899 (18)	C9—C10	1.4905 (19)
C2—C2^i	1.490 (3)	C9—H9A	0.9700
C3—C4	1.3842 (19)	C9—H9B	0.9700
C3—H3	0.9300
C9—O8—C7	112.06 (10)	O8—C7—H7A	109.0
C6—N1—C2	117.06 (11)	C4—C7—H7A	109.0
N1—C2—C3	122.45 (12)	O8—C7—H7B	109.0
N1—C2—C2^i	116.78 (14)	C4—C7—H7B	109.0
C3—C2—C2^i	120.77 (14)	H7A—C7—H7B	107.8
C4—C3—C2	119.76 (12)	O8—C9—C10	107.53 (11)
C4—C3—H3	120.1	O8—C9—H9A	110.2
C2—C3—H3	120.1	C10—C9—H9A	110.2
C3—C4—C5	117.99 (12)	O8—C9—H9B	110.2
C3—C4—C7	121.37 (12)	C10—C9—H9B	110.2
C5—C4—C7	120.64 (12)	H9A—C9—H9B	108.5
C6—C5—C4	118.47 (12)	F1—C10—F2	106.69 (11)
C6—C5—H5	120.8	F1—C10—F3	106.52 (12)
C4—C5—H5	120.8	F2—C10—F3	106.48 (11)
N1—C6—C5	124.27 (12)	F1—C10—C9	113.31 (11)
N1—C6—H6	117.9	F2—C10—C9	110.94 (11)
C5—C6—H6	117.9	F3—C10—C9	112.46 (11)
O8—C7—C4	112.91 (10)

Symmetry code: (i) −x+1, −y+1, −z+1.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
C9—H9A···N1^ii	0.97	2.62	3.573 (2)	166
C7—H7B···F1^iii	0.97	2.49	3.116 (2)	122
C7—H7B···F2^iv	0.97	2.67	3.328 (2)	126

Symmetry codes: (ii) −x+1, −y, −z+1; (iii) −x+1/2, y−1/2, −z+3/2; (iv) x−1/2, −y−1/2, z−1/2.