Fig. 2 A portion of the X-ray structure of (1·6)n hydrogen bonded network showing the formation of a quintuple helical arrangement comprised of five right
handed helical strands represented in different colours for clarity. H atoms, O(CH2)2Me groups and MeNO2 molecules are not shown (for distances and angles
see text).
nitration,14 gave the tetra-nitro compound 4 which was reduced
Notes and references
to the amino compound 5 using SnCl2 in EtOH.10 Finally,
‡
Crystallographic data: for
1
(yellow crystals, 173 K):
treatment of 5 with 4.2 equiv. of 4-pyridylcarbaldehyde in
EtOH at 80 °C in the presence of a catalytic amount of AcOH
for 12 h afforded compound 1 in 52% yield after crystallisation
from CHCl3–MeOH.
C64H64N8O4·4CHCl3, M = 1486.79, tetragonal, space group I41/a, a =
17.3435(6), b = 17.3435(6), c = 24.104(1) Å, U = 7250.4(9) Å3, Z = 4,
Dc = 1.36 g cm23, Mo-Ka graphite monochromated radiation, kappaCCD,
m = 0.510 mm21, 2474 data with I > 3s(I), R = 0.067, Rw = 0.079.
For (1·6)n (colourless crystals, 173 K): C64H64N8O4·C12H10O2·CH3NO2,
M = 1256.53, tetragonal, space group P41 or P43, a = 17.208(1), c =
23.775(1) Å, U = 7039(1) Å3, Z = 4, Dc = 1.19 g cm23, Mo-Ka graphite
monochromated radiation, KappaCCD, m = 0.078 mm21. The diffraction
power of the crystal was rather poor, only 2057 reflections out of 10545 had
I > 3s(I). Therefore, all atoms were kept with isotropic temperature factors
and 2724 data with I > 2s(I) were used for refinements. Attempts to
determine the absolute configuration using the Friedel’s pair method failed.
The structure of 1 was confirmed by X-ray diffraction on a
single-crystal obtained upon cooling from 80 to 25 °C a mixture
of MeNO2 (2 ml) and CHCl3 (1 ml) containing compound 1 (2
mg).‡ As expected, compound 1 adopts a slightly pinched
1,3-alternate conformation [Fig. 1(a)]. The imine groups are in
E configuration with an average CNN distance of 1.256 Å and
C–NNC and C–NNC–C dihedral angles of 117.5 and 172.2° and
2172.2° respectively. Interestingly all four pyridine units are
hydrogen bonded to a CHCl3 molecule with C…N and N…H
distances of 3.233 and 2.338 Å respectively and an NHC angle
of 156.8° demonstrating the H-bond acceptor feature of the
pyridine units.
Upon slow cooling of a MeNO2 (3 ml) solution of 1 (2 mg)
and 6 (1.6 mg) from 80 °C to room temp., single crystals were
obtained. X-Ray diffraction‡ revealed the formation of a
network composed of 1, 6 and MeNO2 molecules in 1+1+1
ratio. The H-bond acceptor 1 and donor 6 form by mutual
bridging a single stranded helical network which crystallises in
the non-centric space group P41 or P43 [Fig. 1(b)]. The reason
for the spontaneous chiral separation remains unclear. The pitch
of the helix which extends along the z axis is composed of four
1, four 6 and four MeNO2. For each calix unit, among the four
pyridines present, two of them, located on opposite sides of the
backbone, participate in the formation of a helical arrangement
through strong H-bonds with N…O distances of 2.703 and
2.776 Å. For the remaining two pyridines, each is located in
close proximity to one MeNO2 molecule with C…N distances
of 3.290 and 3.312 Å. The observed helical structure is derived
from the primary structure of ligand 1 which possess a S4 screw
axis. Rather interestingly, probably for best compacting rea-
sons, five helical strands of the same handedness associate
laterally in a helical fashion leading to a quintuple helical braid
(Fig. 2). The formation of the braided arrangement is probably
due to the uncharged nature of 1 and 6 as well as edge-to-face
interactions between the pyridine units of 1 and the phenyl
groups of 6 which operate in conjunction with van der Waals
interactions. Finally, the quintuple helices associate laterally to
form the crystal.
R
suppdata/cc/1999/xxxx/ for crystallogrpahic files in .cif format.
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In conclusion, using a tetra-H-bond acceptor possessing an S4
screw axis and a divergently oriented di-H-bond donor, a
quintuple helical arrangement was obtained in the crystalline
phase. The formation of the supramolecular assembly may be
described as the result of molecular organisation at three
different levels. The formation of a single stranded H-bonded
helical network (first level), the formation of the quintuple
helical braided network (second level) and the lateral associa-
tion of braided networks into the compacted solid (third level).
The role of the H-bond donor length and the thickness of H-
bond acceptor on the pitch as well as the number of braided
strands is currently under investigation.
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Communication 9/06728K
2314
Chem. Commun., 1999, 2313–2314