Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270108010354/sk3224sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108270108010354/sk3224Isup2.hkl |
CCDC reference: 690199
The title compound was prepared from 4'-amino-2,2':6',2''-terpyridine (Fallahpour et al., 1999b) according to the standard procedure of Coudret (1996) (yield 70%, m.p. 399–400 K). The spectroscopic data have been reported by Sauer et al. (1999). Crystals of (I) were grown by slow evaporation of a warm dichloromethane solution of the compound.
H atoms were placed in geometrically idealized positions and constrained to ride on their parent atoms [C—H = 0.95 Å and Uiso(H) = 1.2Ueq(C)].
Data collection: COLLECT (Nonius, 2000); cell refinement: DENZO-SMN (Otwinowski & Minor, 1997); data reduction: DENZO-SMN and SCALEPACK (Otwinowski & Minor, 1997); program(s) used to solve structure: SIR92 (Altomare et al., 1994); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEPII (Johnson, 1976); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008) and PLATON (Spek, 2003).
C15H10IN3 | F(000) = 696 |
Mr = 359.17 | Dx = 1.844 Mg m−3 |
Orthorhombic, Pna21 | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: P 2c -2n | Cell parameters from 15271 reflections |
a = 7.4518 (2) Å | θ = 2.0–30.0° |
b = 17.2432 (3) Å | µ = 2.46 mm−1 |
c = 10.0691 (2) Å | T = 160 K |
V = 1293.81 (5) Å3 | Prism, colourless |
Z = 4 | 0.30 × 0.28 × 0.25 mm |
Nonius KappaCCD area-detector diffractometer | 3713 independent reflections |
Radiation source: Nonius FR590 sealed tube generator | 3524 reflections with I > 2σ(I) |
Horizontally mounted graphite crystal monochromator | Rint = 0.055 |
Detector resolution: 9 pixels mm-1 | θmax = 30.0°, θmin = 2.3° |
ω scans with κ offsets | h = −10→10 |
Absorption correction: multi-scan (Blessing, 1995) | k = −24→24 |
Tmin = 0.448, Tmax = 0.544 | l = −14→14 |
19500 measured reflections |
Refinement on F2 | Secondary atom site location: difference Fourier map |
Least-squares matrix: full | Hydrogen site location: inferred from neighbouring sites |
R[F2 > 2σ(F2)] = 0.040 | H-atom parameters constrained |
wR(F2) = 0.096 | w = 1/[σ2(Fo2) + (0.0449P)2 + 5.4428P] where P = (Fo2 + 2Fc2)/3 |
S = 1.05 | (Δ/σ)max = 0.001 |
3712 reflections | Δρmax = 0.79 e Å−3 |
172 parameters | Δρmin = −1.87 e Å−3 |
1 restraint | Absolute structure: Flack & Bernardinelli (1999, 2000), 1738 Friedel pairs |
Primary atom site location: structure-invariant direct methods | Absolute structure parameter: 0.03 (3) |
C15H10IN3 | V = 1293.81 (5) Å3 |
Mr = 359.17 | Z = 4 |
Orthorhombic, Pna21 | Mo Kα radiation |
a = 7.4518 (2) Å | µ = 2.46 mm−1 |
b = 17.2432 (3) Å | T = 160 K |
c = 10.0691 (2) Å | 0.30 × 0.28 × 0.25 mm |
Nonius KappaCCD area-detector diffractometer | 3713 independent reflections |
Absorption correction: multi-scan (Blessing, 1995) | 3524 reflections with I > 2σ(I) |
Tmin = 0.448, Tmax = 0.544 | Rint = 0.055 |
19500 measured reflections |
R[F2 > 2σ(F2)] = 0.040 | H-atom parameters constrained |
wR(F2) = 0.096 | Δρmax = 0.79 e Å−3 |
S = 1.05 | Δρmin = −1.87 e Å−3 |
3712 reflections | Absolute structure: Flack & Bernardinelli (1999, 2000), 1738 Friedel pairs |
172 parameters | Absolute structure parameter: 0.03 (3) |
1 restraint |
Experimental. Solvent used: dichloromethane Cooling Device: Oxford Cryosystems Cryostream 700 Crystal mount: glued on a glass fibre Mosaicity (°.): 0.547 (1) Frames collected: 260 Seconds exposure per frame: 13 Degrees rotation per frame: 1.9 Crystal-Detector distance (mm): 33.8 |
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s 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. |
x | y | z | Uiso*/Ueq | ||
I1 | 0.34425 (3) | 0.559690 (13) | 0.39444 (6) | 0.01972 (9) | |
N1 | 0.4181 (5) | 0.7922 (2) | 0.6643 (4) | 0.0135 (6) | |
N8 | 0.3567 (6) | 0.8850 (3) | 0.3472 (5) | 0.0174 (10) | |
N14 | 0.5254 (5) | 0.6454 (2) | 0.9086 (5) | 0.0198 (8) | |
C2 | 0.3750 (6) | 0.7977 (3) | 0.5350 (4) | 0.0142 (8) | |
C3 | 0.3506 (6) | 0.7337 (3) | 0.4524 (5) | 0.0171 (8) | |
H3 | 0.3195 | 0.7396 | 0.3615 | 0.021* | |
C4 | 0.3740 (6) | 0.6607 (2) | 0.5088 (4) | 0.0135 (7) | |
C5 | 0.4212 (6) | 0.6536 (2) | 0.6412 (4) | 0.0156 (8) | |
H5 | 0.4404 | 0.6041 | 0.6801 | 0.019* | |
C6 | 0.4397 (6) | 0.7210 (2) | 0.7153 (4) | 0.0140 (7) | |
C7 | 0.3595 (7) | 0.8778 (4) | 0.4813 (5) | 0.0170 (11) | |
C9 | 0.3459 (6) | 0.9560 (3) | 0.2967 (6) | 0.0208 (10) | |
H9 | 0.3452 | 0.9612 | 0.2028 | 0.025* | |
C10 | 0.3356 (6) | 1.0230 (3) | 0.3725 (5) | 0.0197 (12) | |
H10 | 0.3314 | 1.0726 | 0.3315 | 0.024* | |
C11 | 0.3315 (7) | 1.0156 (3) | 0.5119 (5) | 0.0213 (9) | |
H11 | 0.3213 | 1.0600 | 0.5674 | 0.026* | |
C12 | 0.3426 (7) | 0.9417 (3) | 0.5658 (6) | 0.0217 (10) | |
H12 | 0.3389 | 0.9346 | 0.6593 | 0.026* | |
C13 | 0.4838 (6) | 0.7162 (2) | 0.8607 (4) | 0.0142 (8) | |
C15 | 0.5594 (7) | 0.6399 (3) | 1.0357 (5) | 0.0231 (10) | |
H15 | 0.5885 | 0.5902 | 1.0702 | 0.028* | |
C16 | 0.5559 (7) | 0.7025 (3) | 1.1246 (4) | 0.0206 (9) | |
H16 | 0.5833 | 0.6954 | 1.2159 | 0.025* | |
C17 | 0.5116 (7) | 0.7748 (3) | 1.0754 (5) | 0.0211 (9) | |
H17 | 0.5068 | 0.8187 | 1.1324 | 0.025* | |
C18 | 0.4738 (6) | 0.7819 (3) | 0.9398 (5) | 0.0188 (8) | |
H18 | 0.4421 | 0.8307 | 0.9027 | 0.023* |
U11 | U22 | U33 | U12 | U13 | U23 | |
I1 | 0.02336 (13) | 0.01637 (12) | 0.01942 (13) | −0.00224 (8) | −0.0028 (2) | −0.00483 (16) |
N1 | 0.0160 (16) | 0.0137 (16) | 0.0108 (15) | −0.0010 (13) | 0.0029 (13) | 0.0015 (12) |
N8 | 0.020 (2) | 0.0143 (19) | 0.018 (2) | −0.0098 (15) | −0.0003 (15) | −0.0005 (17) |
N14 | 0.0304 (17) | 0.0203 (14) | 0.009 (2) | −0.0005 (13) | −0.0020 (17) | −0.0015 (17) |
C2 | 0.0095 (16) | 0.0147 (18) | 0.018 (2) | −0.0005 (14) | 0.0018 (14) | 0.0019 (15) |
C3 | 0.023 (2) | 0.0137 (19) | 0.0144 (19) | 0.0030 (16) | −0.0002 (16) | 0.0015 (15) |
C4 | 0.0120 (17) | 0.0113 (17) | 0.0171 (19) | −0.0011 (14) | 0.0001 (15) | −0.0007 (15) |
C5 | 0.0162 (19) | 0.0131 (17) | 0.0175 (19) | −0.0009 (15) | −0.0007 (15) | 0.0026 (15) |
C6 | 0.0138 (18) | 0.0144 (18) | 0.0138 (18) | −0.0023 (15) | −0.0033 (15) | 0.0001 (14) |
C7 | 0.014 (2) | 0.021 (2) | 0.015 (2) | 0.0002 (17) | 0.0020 (17) | 0.0086 (19) |
C9 | 0.019 (2) | 0.028 (3) | 0.016 (2) | 0.0014 (18) | −0.0114 (17) | 0.007 (2) |
C10 | 0.0216 (19) | 0.0189 (18) | 0.018 (3) | −0.0003 (14) | −0.0044 (16) | 0.0039 (17) |
C11 | 0.026 (2) | 0.015 (2) | 0.023 (2) | 0.0004 (17) | −0.0033 (18) | −0.0019 (17) |
C12 | 0.026 (3) | 0.020 (2) | 0.019 (3) | −0.0005 (17) | −0.0072 (18) | −0.0004 (18) |
C13 | 0.0117 (16) | 0.0185 (18) | 0.012 (2) | 0.0022 (14) | −0.0020 (12) | 0.0013 (13) |
C15 | 0.025 (2) | 0.020 (2) | 0.023 (2) | 0.0012 (18) | −0.0082 (19) | −0.0008 (18) |
C16 | 0.027 (2) | 0.024 (2) | 0.0113 (17) | −0.0015 (18) | −0.0006 (16) | −0.0001 (16) |
C17 | 0.030 (3) | 0.022 (2) | 0.0115 (18) | 0.001 (2) | 0.0003 (18) | −0.0018 (16) |
C18 | 0.021 (2) | 0.0202 (19) | 0.0151 (17) | 0.0057 (17) | 0.0000 (16) | 0.0021 (15) |
I1—C4 | 2.100 (4) | C9—C10 | 1.386 (8) |
N1—C6 | 1.340 (5) | C9—H9 | 0.9500 |
N1—C2 | 1.344 (6) | C10—C11 | 1.410 (7) |
N8—C9 | 1.328 (7) | C10—H10 | 0.9500 |
N8—C7 | 1.356 (7) | C11—C12 | 1.388 (7) |
N14—C15 | 1.308 (7) | C11—H11 | 0.9500 |
N14—C13 | 1.349 (5) | C12—H12 | 0.9500 |
C2—C3 | 1.393 (6) | C13—C18 | 1.387 (6) |
C2—C7 | 1.487 (7) | C15—C16 | 1.402 (7) |
C3—C4 | 1.391 (6) | C15—H15 | 0.9500 |
C3—H3 | 0.9500 | C16—C17 | 1.382 (7) |
C4—C5 | 1.384 (6) | C16—H16 | 0.9500 |
C5—C6 | 1.388 (6) | C17—C18 | 1.400 (6) |
C5—H5 | 0.9500 | C17—H17 | 0.9500 |
C6—C13 | 1.503 (5) | C18—H18 | 0.9500 |
C7—C12 | 1.398 (8) | ||
C6—N1—C2 | 117.7 (4) | C9—C10—C11 | 118.3 (4) |
C9—N8—C7 | 117.8 (6) | C9—C10—H10 | 120.9 |
C15—N14—C13 | 117.3 (4) | C11—C10—H10 | 120.9 |
N1—C2—C3 | 123.6 (4) | C12—C11—C10 | 118.1 (5) |
N1—C2—C7 | 115.8 (4) | C12—C11—H11 | 121.0 |
C3—C2—C7 | 120.5 (4) | C10—C11—H11 | 121.0 |
C4—C3—C2 | 117.1 (4) | C11—C12—C7 | 119.4 (5) |
C4—C3—H3 | 121.4 | C11—C12—H12 | 120.3 |
C2—C3—H3 | 121.4 | C7—C12—H12 | 120.3 |
C5—C4—C3 | 120.3 (4) | N14—C13—C18 | 123.2 (4) |
C5—C4—I1 | 118.8 (3) | N14—C13—C6 | 116.7 (4) |
C3—C4—I1 | 120.9 (3) | C18—C13—C6 | 120.1 (4) |
C4—C5—C6 | 117.9 (4) | N14—C15—C16 | 124.5 (5) |
C4—C5—H5 | 121.0 | N14—C15—H15 | 117.8 |
C6—C5—H5 | 121.0 | C16—C15—H15 | 117.8 |
N1—C6—C5 | 123.3 (4) | C17—C16—C15 | 118.0 (4) |
N1—C6—C13 | 116.8 (4) | C17—C16—H16 | 121.0 |
C5—C6—C13 | 119.9 (4) | C15—C16—H16 | 121.0 |
N8—C7—C12 | 122.2 (6) | C16—C17—C18 | 118.5 (4) |
N8—C7—C2 | 116.6 (6) | C16—C17—H17 | 120.8 |
C12—C7—C2 | 121.2 (5) | C18—C17—H17 | 120.8 |
N8—C9—C10 | 124.1 (5) | C13—C18—C17 | 118.5 (4) |
N8—C9—H9 | 117.9 | C13—C18—H18 | 120.8 |
C10—C9—H9 | 117.9 | C17—C18—H18 | 120.8 |
C6—N1—C2—C3 | 0.5 (6) | C7—N8—C9—C10 | −0.7 (8) |
C6—N1—C2—C7 | −177.6 (4) | N8—C9—C10—C11 | −1.8 (7) |
N1—C2—C3—C4 | −0.4 (7) | C9—C10—C11—C12 | 1.6 (7) |
C7—C2—C3—C4 | 177.6 (4) | C10—C11—C12—C7 | 0.7 (7) |
C2—C3—C4—C5 | −0.7 (6) | N8—C7—C12—C11 | −3.3 (8) |
C2—C3—C4—I1 | −179.3 (3) | C2—C7—C12—C11 | 179.0 (5) |
C3—C4—C5—C6 | 1.6 (6) | C15—N14—C13—C18 | −0.7 (7) |
I1—C4—C5—C6 | −179.8 (3) | C15—N14—C13—C6 | −178.2 (4) |
C2—N1—C6—C5 | 0.5 (6) | N1—C6—C13—N14 | −172.7 (4) |
C2—N1—C6—C13 | −178.7 (4) | C5—C6—C13—N14 | 8.1 (6) |
C4—C5—C6—N1 | −1.5 (7) | N1—C6—C13—C18 | 9.7 (6) |
C4—C5—C6—C13 | 177.6 (4) | C5—C6—C13—C18 | −169.5 (4) |
C9—N8—C7—C12 | 3.2 (9) | C13—N14—C15—C16 | −0.2 (8) |
C9—N8—C7—C2 | −178.9 (4) | N14—C15—C16—C17 | 0.8 (8) |
N1—C2—C7—N8 | 165.6 (5) | C15—C16—C17—C18 | −0.5 (8) |
C3—C2—C7—N8 | −12.6 (7) | N14—C13—C18—C17 | 1.0 (7) |
N1—C2—C7—C12 | −16.6 (7) | C6—C13—C18—C17 | 178.5 (4) |
C3—C2—C7—C12 | 165.3 (5) | C16—C17—C18—C13 | −0.4 (7) |
D—H···A | D—H | H···A | D···A | D—H···A |
C15—H15···I1i | 0.95 | 3.17 | 3.793 (5) | 125 |
C17—H17···N8ii | 0.95 | 2.69 | 3.526 (6) | 147 |
Symmetry codes: (i) −x+1, −y+1, z+1/2; (ii) x, y, z+1. |
Experimental details
Crystal data | |
Chemical formula | C15H10IN3 |
Mr | 359.17 |
Crystal system, space group | Orthorhombic, Pna21 |
Temperature (K) | 160 |
a, b, c (Å) | 7.4518 (2), 17.2432 (3), 10.0691 (2) |
V (Å3) | 1293.81 (5) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 2.46 |
Crystal size (mm) | 0.30 × 0.28 × 0.25 |
Data collection | |
Diffractometer | Nonius KappaCCD area-detector diffractometer |
Absorption correction | Multi-scan (Blessing, 1995) |
Tmin, Tmax | 0.448, 0.544 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 19500, 3713, 3524 |
Rint | 0.055 |
(sin θ/λ)max (Å−1) | 0.704 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.040, 0.096, 1.05 |
No. of reflections | 3712 |
No. of parameters | 172 |
No. of restraints | 1 |
H-atom treatment | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 0.79, −1.87 |
Absolute structure | Flack & Bernardinelli (1999, 2000), 1738 Friedel pairs |
Absolute structure parameter | 0.03 (3) |
Computer programs: COLLECT (Nonius, 2000), DENZO-SMN (Otwinowski & Minor, 1997), DENZO-SMN and SCALEPACK (Otwinowski & Minor, 1997), SIR92 (Altomare et al., 1994), ORTEPII (Johnson, 1976), SHELXL97 (Sheldrick, 2008) and PLATON (Spek, 2003).
D—H···A | D—H | H···A | D···A | D—H···A |
C15—H15···I1i | 0.95 | 3.17 | 3.793 (5) | 125 |
C17—H17···N8ii | 0.95 | 2.69 | 3.526 (6) | 147 |
Symmetry codes: (i) −x+1, −y+1, z+1/2; (ii) x, y, z+1. |
Much effort has been devoted in recent years to the design and synthesis of terpyridines and their metal complexes, which display binding properties conducive to supramolecular chemistry (Hoogenboom & Schubert, 2006). Such materials also have applications in the conversion of solar energy in dye-sensitized solar cells (Kalyanasundaram, 2006). Derivatives of 2,2':6',2''-terpyridines (tpy) can be linked together by spacers. Metal-coordinated tpy ligands with spacers at the 4'-position provide a means of influencing directionality and linear communication, meaning that electronic communication can occur along the coordination axis. The functionalization of tpy at this position is thus of particular importance. For that purpose, the title 4'-iodo derivative of tpy, (I), may play a crucial role since it can be used as a precursor for the preparation of stannyl or boronic acid compounds, which are used in Stille or Suzuki cross-coupling reactions, respectively. This paper describes the crystal structure of (I) and compares it with the structures of related 4'-substituted tpy derivatives.
In terpyridine derivatives, the three pyridine rings are usually close to being coplanar. For example, the interplanar angles between the terminal rings and the central ring in 4'-ethoxy-5,5''-dimethyl-2,2':6',2''-terpyridine are 4.32 (13) and 11.38 (11)° (Fallahpour et al., 1999a). In 4'-amino-2,2':6',2''-terpyridine, however, the interplanar angles between the two terminal rings and the central ring are 11.24 (15) and 20.71 (11)°, as a result of hydrogen-bond formation (Fallahpour et al., 1999b). In contrast, in 2,2':6',2''-terpyridine 1,1'-dioxide, the two pyridine N-oxide ring planes are almost perpendicular to one another, with an interplanar angle of 87.68 (10)°, while the angle between the plane of the central pyridine N-oxide ring and that of the terminal pyridine ring is 38.46 (10)°. The interplanar angle between the terminal pyridine N-oxide ring and the terminal pyridine ring is 70.98 (11)° (Fallahpour & Neuburger, 2001).
The molecule of (I) (Fig. 1), with unremarkable bond lengths and angles, has the expected nearly planar transoid conformation of the three pyridine rings and pseudo-Cs symmetry, with an r.m.s. fit of the atoms of the two halves of the molecule of 0.16 Å. The angles between the planes of the terminal pyridine rings containing atoms N8 and N14 and the plane of the central ring are 15.1 (2) and 9.4 (2)°, respectively. The angle between the planes of the terminal rings is 12.8 (2)°.
The mean plane of the molecule is nearly parallel to the (100) plane and, in the crystal packing, the molecules are distributed in slightly corrugated layers, which lie parallel to the same plane. There are only two intermolecular distances that are shorter than the sum of the van der Waals radii of any atom pair, although these are only marginally shorter than this limit (<0.06 Å). A very weak intermolecular C—H···N interaction (Table 1) links molecules related by a translation of one unit cell into extended chains, which run parallel to the [001] direction and which can be described by a graph-set motif of C(9) (Bernstein et al., 1995). Within the (100) plane, pairs of parallel chains are crosslinked by weak C—H···I interactions to give a ladder-like arrangement, where the C—H···I interactions are the rungs of the ladder and the C—H···N interactions form the ladder uprights (Fig. 2). Within each segment of the ladder structure, an asymmetric ring motif can be discerned, made up of one C—H···N and two C—H···I interactions. This ring motif can be described by a binary graph set of R33(19). Essentially, one terminal pyridine ring acts as a single acceptor, the other terminal ring acts as a donor for both types of interaction, and the central ring is involved in one acceptor interaction via the I atom. There are no significant intermolecular interactions between adjacent ladders in the same plane.
The molecules are stacked perpendicular to the (100) plane such that the planes of overlapping rings are almost parallel, but adjacent molecules in the stack appear as if they are rotated alternately about an axis almost perpendicular to the plane of the central pyridine ring by approximately 120° with respect to one another [Fig. 3; in reality this is a glide operation, which yields a reflection in a plane parallel to (010)]. This means that the central pyridine rings stack with potential π–π interactions, the centroid–centroid distance being 3.813 (3) Å, while one of the terminal pyridine rings in each molecule also partakes in weak π–π stacking, with a centroid–centroid distance of 3.972 (3) Å, although adjacent ring planes are inclined at an angle of about 21°. In contrast, the other terminal pyridine ring stacks in an alternating fashion with the I atom of the molecule below it, to give centroid···I distances of 3.738 (2) and 3.754 (2) Å. The combination of the ladder-generating C—H···X (X = N or I) interactions with the π–π and I···π interactions generates a three-dimensional framework; adjacent ladders in the π–π stacks are offset from one another, thereby creating a brickwork stacking pattern of ladders.
The crystal structures of the 4'-bromo and 4'-chloro analogues of (I) are known (Clegg & Scott, 2005; Beves et al., 2006). The molecules are slightly flatter than that of (I), the angles between the terminal pyridine ring planes and that of the central pyridine ring being 8.0 (1) and 5.4 (1)° for the chloro derivative and 5.1 (1) and 5.6 (1)° for the bromo derivative. These two structures are isomorphous, but although the structure of (I) has the same space group, the unit-cell dimensions differ markedly. Thus, (I) is not isomorphous with the chloro and bromo analogues, which is manifested in the quite different packing arrangements of the molecules. Unlike in (I), the molecules of the chloro and bromo analogues stack exactly on top of one another without a 120° rotation, as they are related by one unit-cell translation along the short b axis. According to Beves et al. (2006), the molecules are π-stacked, although the planes of the molecules are tilted somewhat with respect to the stacking axis direction. In addition, these two analogues do not exhibit any C—H···X (X = N or halogen) or halogen–π interactions, so the intermolecular interactions only form one-dimensional motifs. The lack of isomorphism between the structure of (I) and those of the other 4'-halo–tpy derivatives must be related to the influence of the bulkier I atom on the packing efficiency of the molecules.
The stacking motifs of adjacent molecules in (I) and related derivatives show four main patterns. Infinite stacks with alternating 120° rotation of adjacent molecules within the stack are formed in (I) and 4'-azido-2,2':6',2''-terpyridine (Fallahpour et al., 1999c). Infinite stacks with exact superposition of the molecules are present in the 4'-bromo and 4'-chloro analogues of (I), as described above, as well as in the orthorhombic polymorph of unsubstituted 2,2':6',2''-terpyridine itself (Bessel et al., 1992). The third motif involves just pairs of molecules, perfectly superimposed but not propagating beyond the pair. This motif is seen in the monoclinic polymorph of unsubstituted 2,2':6',2''-terpyridine (Boves et al., 2005). The fourth motif also involves just pairs of molecules, but this time they are related by a centre of inversion, so that all pyridine rings are sufficiently offset so as to preclude any possibility of π–π interactions. This is much like superimposing a `Y' on an upside-down `Y'. This motif is displayed by 4'-amino-2,2':6',2''-terpyridine (Fallahpour et al., 1999b) and 4'-dimethylamino-2,2':6',2''-terpyridine (Constable et al., 1992).