4′-[2-(Trifluoro­meth­yl)phen­yl]-2,2′:6′,2′′-terpyridine

Peter Ledwaba; Orde Q Munro; Kirsty Stewart

doi:10.1107/S1600536809002384

. 2009 Jan 23;65(Pt 2):o376–o377. doi: 10.1107/S1600536809002384

4′-[2-(Trifluoromethyl)phenyl]-2,2′:6′,2′′-terpyridine

Peter Ledwaba ^a, Orde Q Munro ^a, Kirsty Stewart ^a,^*

PMCID: PMC2968217 PMID: 21581974

Abstract

The title compound, C₂₂H₁₄F₃N₃, is a versatile tridentate N-donor ligand consisting of a terpyridyl (terpy) molecule substituted in the 4′-position by a phenyl group, itself substituted in an ortho-position by a bulky trifluoromethyl group. The phenyl ring is twisted as a result of steric interactions involving the bulky trifluoromethyl substituent. This is reflected in the dihedral angle between the mean plane through the C atoms of the phenyl ring and the terpyridyl unit being 69.2 (1)°. The crystal structure contains no short van der Waals contacts. However, the terpy units stack in a head-to-tail orientation perpendicular to the c axis. The structure is is loosely stabilized by π–π interactions between the terminal pyridine rings of adjacent molecules along the stack. The perpendicular distance between the mean planes through the terpy moieties of adjacent molecules is 3.4 (1) Å.

Related literature

For related structures, see: Bessel et al. (1992 ▶); Brandt et al. (1954 ▶); Dwyer & Mellor (1964 ▶); Field et al. (2002 ▶); Gillard (1983 ▶); Lindoy & Livingstone (1967 ▶); Morgan & Burstall (1932 ▶, 1934 ▶, 1938 ▶); Serpone et al. (1983 ▶); Storrier et al. (1997 ▶). For background, see Constable et al. (1990 ▶, 1992 ▶); Hunter & Sanders (1990 ▶); Kröhnke (1976 ▶); Thummel & Jahng (1985 ▶).

Experimental

Crystal data

C₂₂H₁₄F₃N₃
M _r = 377.36
Triclinic,
a = 7.767 (5) Å
b = 10.923 (3) Å
c = 11.748 (3) Å
α = 75.64 (2)°
β = 74.03 (4)°
γ = 72.93 (4)°
V = 900.8 (7) Å³
Z = 2
Mo Kα radiation
μ = 0.11 mm⁻¹
T = 293 (2) K
0.60 × 0.30 × 0.30 mm

Data collection

Oxford Diffraction Xcalibur2 CCD diffractometer
Absorption correction: multi-scan (CrysAlis RED; Oxford Diffraction, 2003 ▶) T _min = 0.930, T _max = 0.969
3953 measured reflections
3155 independent reflections
2840 reflections with I > 2σ(I)
R _int = 0.032

Refinement

R[F ² > 2σ(F ²)] = 0.052
wR(F ²) = 0.148
S = 1.06
3155 reflections
254 parameters
H-atom parameters constrained
Δρ_max = 0.43 e Å⁻³
Δρ_min = −0.32 e Å⁻³

Data collection: CrysAlis CCD (Oxford Diffraction, 2003 ▶); cell refinement: CrysAlis RED (Oxford Diffraction, 2003 ▶); data reduction: CrysAlis RED; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ▶); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 ▶); molecular graphics: ORTEP-3 (Farrugia, 1997 ▶); software used to prepare material for publication: WinGX (Farrugia, 1999 ▶).

Supplementary Material

Crystal structure: contains datablocks I, global. DOI: 10.1107/S1600536809002384/hg2469sup1.cif

e-65-0o376-sup1.cif^{(20.2KB, cif)}

Structure factors: contains datablocks I. DOI: 10.1107/S1600536809002384/hg2469Isup2.hkl

e-65-0o376-Isup2.hkl^{(151.7KB, hkl)}

Additional supplementary materials: crystallographic information; 3D view; checkCIF report

Acknowledgments

The authors acknowledge financial support from the South African National Research Foundation and the Department of Labour. We also extend our appreciation to Professor John Field for helpful discussions and guidance.

supplementary crystallographic information

Comment

The tridentate coordinating ligand 2,2':6',2''-terpyridine (terpy) was first isolated by Morgan & Burstall (1932, 1934, 1938) as one of the numerous products from the reaction of pyridine with iron(III) chloride.

Since the 1930s, numerous groups have examined terpy, prompted by the use of related ligands 2,2'-bipyridine (bipy) and 1,10-phenanthroline (phen), in photochemical and photophysical processes (Brandt et al., 1954; Dwyer & Mellor, 1964; Gillard, 1983; Lindoy & Livingstone, 1967; Serpone et al., 1983).

Reported here is the crystal structure of the tridentate terpyridyl ligand substituted in the 4'-position by a phenyl group, itself substituted in an ortho-position by a bulky trifluoromethyl group. Ortho-substitution of the 4'-phenyl ring was chosen since steric interactions between the bulky group and the 3'(5')-proton on the central pyridine ring are expected to force the 4'-substituent to rotate around the interannular bond i.e. the ligand will become non-planar.

In the crystal structure of 4'-(2'''-trifluoromethylphenyl)-2, 2':6',2''-terpyridine, the three pyridyl rings of the terpyridyl moiety are essentially co-planar as is preferred for maximum conjugative interaction (Thummel & Jahng, 1985). This is reflected by torsion angles between the two outer rings and the central ring of -6.5 (2)° and 9.9 (2)° for N1—C1—C6—C7 and C9—C10—C11—N3 respectively.

The terminal pyridine rings adopt a trans–trans conformation about the interannular bonds C1—C6 and C10—C11. Several derivatized terpy ligands have been found to adopt this trans–trans geometry by X-ray crystal analysis (Constable et al., 1990) which is more energetically favourable when compared to other conformations as a result of the minimal nitrogen lone pair repulsions (Thummel & Jahng, 1985).

The interannular bond distances C1—C6 and C10—C11 are 1.493 (2) Å and 1.484 (2) Å respectively; these distances are comparable with the averaged values of 1.49 (1) Å and 1.49 (1) Å measured for the terpy (Bessel et al., 1992) and 4'-(Ph)-terpy (Constable et al., 1990) ligands respectively.

As previously postulated, the o-tolyl moiety is twisted about the interannular bond C8—C16, as reflected in a dihedral angle between the mean plane through the carbon atoms of the 4'-substituted and the terpyridyl moiety of 69.2 (1)°. This angle may be compared with those adopted by terpyridyl ligands containing similar substituents in the 4'-position of the terpy moiety in molecules such as the free 4'-phenyl-terpyridine (10.9°) (Constable et al., 1990), 6,6''-dibromo-4'-phenyl-terpyridine (35.1°) (Constable et al, 1992) and 4'-(4-anilino)-terpyridine (27.2°) (Storrier et al., 1997). The larger angle witnessed in the title compound is consistent with the bulky nature of the trifluoro group and the fact that it substitutes the ortho-position of the phenyl moiety. Clearly, substitution of a trifluoro group in the ortho-position of the 4'-phenyl group causes a larger rotation about the interannular bond because of steric interactions between the CF₃ group and a hydrogen atom of the central pyridine ring that is also ortho with respect to the interannular bond.

There are no short van der Waals contacts less than the sum of the van der Waals radii in this system. However it is worth noting, that the terpy units stack in a head to tail orientation perpendicular to the [c]-axis, presumably as a result of minimizing steric interactions between the bulky trifluoromethyl substituents on adjacent molecules. However it is clear that this arrangement is not entirely successful and that poor packing does result from the presence of these bulky substituents reflected in the large solvent accessible void of 31 Å³. This packing orientation allows for π–π interactions between the terminal pyridine rings of adjacent molecules along the stack. The perpendicular distance between the mean planes through the terpy moieties of adjacent molecules is 3.4 (1)Å which is short enough to support π–π interactions being well within the upper distance limit of 3.8 Å for π–π interactions between organic molecules (Hunter & Sanders, 1990).

The stucture of the title compound is shown in Fig. 1. Fig. 2 shows a view perpendicular to the mean plane through the atoms comprising the terpyridyl (terpy) moiety of two adjacent terpy units in the crystals of the 4'-(2'''-trifluoromethylphenyl)-2, 2':6', 2''-terpyridine ligand. Note that the successive molecules are related by a centre of inversion.

Experimental

4'-(2'''-trifluoromethylphenyl)-2,2':6',2''-terpyridine was synthesized by the method of Kröhnke (Field et al., 2002; Kröhnke, 1976).

N-{1-(2'-pyridyl)-1-oxo-2-ethyl}pyridinium iodide (0.68 g, 2.2 mmol) and ammonium acetate (10 g, excess) were added to a suspension of 2-R-{3-(2-pyridyl)-3-oxopropenyl}benzene (2.0 mmol) in absolute ethanol (8 ml) and the mixture heated at reflux for 40 min. An off-white solid precipitated on cooling. This was collected by filtration, washed with 50% aqueous ethanol and dried in vacuo. Recrystallization from ethanol afforded colourless crystals of the desired ligands.

Yield: (0.41 g, 54%). m.p. (148 °C). Anal. (Calcd. For C₂₂H₁₄F₃N₃: C 70.0; H 3.7; N 11.1. Found: C 69.9; H 3.9; N 11.0%). MS(EI) m/z: 377, M⁺). ¹H NMR (CDCl₃): [δ 8.72 (m, 2H, H_6,6''); 8.70 (m, 2H, H_3,3''); 8.54 (s, 2H, H_3',5'); 7.84 (m, 2H, H_4,4''); 7.54 (m, 4H, C₆H₄); 7.35 (m, 2 H, H_5,5'')]. UV/vis (CH₃CN): λ_max/nm (ε/M^-1cm^-1): [303 (sh, 1.3 × 10⁴); 277 (2.9 × 10⁴); 239 (3.4 × 10⁴); 208 (3.6 × 10⁴)].