Non-centrosymmetric packing of 1-D coordination networks based on chirality

Article abstract of DOI:10.1039/b205556m

Using CoCl₂ and a chiral tecton possessing C₂ chirality and based on two coordination poles composed of a pyridine unit connected at the 4-position to a pyridine bearing at the 2 and 6 positions two optically active oxazoline moieties, a polar solid is obtained. The latter results from the acentric packing of directional 1-D coordination networks.

Full text of DOI:10.1039/b205556m

Non-centrosymmetric packing of 1-D coordination networks based on
chirality
Abdelaziz Jouaiti,a Mir Wais Hosseini*a and Nathalie Kyritsakasb
a Université Louis Pasteur, Laboratoire de Chimie de Coordination Organique, Institut Le Bel, 4, rue Blaise
Pascal, F-67000 Strasbourg, France. E-mail: hosseini@chimie.u-strasbg.fr
b Université Louis Pasteur, Service Commun des Rayons X, Institut Le Bel, 4, rue Blaise Pascal, F-67000
Strasbourg, France
Received (in Cambridge, UK) 7th June 2002, Accepted 10th July 2002
First published as an Advance Article on the web 26th July 2002
Using CoCl2 and a chiral tecton possessing C2 chirality and
based on two coordination poles composed of a pyridine unit
connected at the 4-position to a pyridine bearing at the 2 and
6 positions two optically active oxazoline moieties, a polar
solid is obtained. The latter results from the acentric packing
of directional 1-D coordination networks.
chiral organic tecton and a transition metal complex to generate
non-centrosymmetric crystals.
The strategy leading to the formation of directional 1-D
networks based on the use of acentric organic tectons was
demonstrated by us both for inclusion12 and coordination
networks.10,11 In the latter case, we have demonstrated the
possibility of using octahedral transition metals such as Co(II).
When associated with two coordinating chloride anions occupy-
ing the two apical positions on metal cation (CoCl2), a
metallatecton offering four free coordination sites in a square
planar geometry is obtained. One may use this topological
simplification, i.e. passage from a hexacoordination in an
octahedral mode to a tetracoordination in square geometry, for
the design of directional 1-D coordination networks. Indeed,
considering the four available coordination sites as a (1,3) set,
one may design an organic tecton based on two coordination
poles, one of monodentate and the other of tridentate type (Fig.
2).10,11 Following this strategy, the new organic tecton 6
possessing a C2 chirality and based on two coordination poles
composed of a pyridine unit connected at the 4-position to a
pyridine bearing at the 2 and 6 positions two optically active
oxazoline moieties was designed and synthesised (Scheme 1).
In the crystalline state, the exploitation of tensorial physical
properties such as piezo- and pyro-electricity and SHG requires
non-centrosymmetric crystals. In the latter case, polarity is an
additional desirable property. However, the design of such
solids remains a difficult challenge.1–9 A possible strategy to
solve this rather important problem may be based on the design
of 1-D directional and chiral coordination networks i.e. infinite
structures formed upon interconnection of chiral organic tectons
possessing available sites for metal coordination. The basic
concept of this strategy is to avoid, through the use of chirality,
the centrosymmetric packing of directional 1-D networks which
would generate a non polar crystal. Dealing with 1-D coordina-
tion networks, one may distinguish three possibilities based on
the nature of tectons used: (1) a combination of centric organic
and metallic tectons would lead to non-directional networks,
and thus the solid obtained upon packing of such networks will
be non-polar by nature (Fig. 1a); (2) a combination of proper
acentric tectons will generate directional 1-D networks; how-
ever, mainly due to minimisation of dipolar interactions, the
centrosymmetric packing of such directional networks would
lead again to the formation of non-polar crystals (Fig. 1b); (3)
finally, a combination of acentric and chiral tectons would
generate chiral and directional 1-D networks which may either
pack in parallel or antiparallel fashion depending on the
interplay of subtle forces (Fig. 1c). We have been investigating
this issue for some time now.10–12 Whereas many examples
corresponding to the first case mentioned above have been
published,13 we have proposed a strategy dealing with the
second case.10,11 In this communication we report our strategy
dealing with the third possibility, i.e. the use of an acentric and
Fig. 2 Representation of a portion of the directional 1-D network formed
between the organic chiral tecton 6 and CoCl2.
Fig. 1 Schematic representations of parallel packing of 1-D coordination
networks formed between a metal and (a) centric achiral tecton, (b) acentric
achiral tecton and (c) acentric and chiral tecton.
Scheme 1
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The strategy for the synthesis of 6 was based on the Suzuki
coupling between 314 and 815 which were prepared according to
published procedures. The starting material for the synthesis of
3 was chelidamic acid 1 which upon treatment with SOCl2 in the
presence of a drop of DMF afforded the acylchloride derivative
2. The chirality of the final ligand 6 was fixed upon treatment of
2 with (S)-valinol 7 (0 °C to rt.) in THF in the presence of Et3N
affording the compound 3 in 79% yield. The Suzuki coupling
between 3 and 8 in the presence of Pd(PPh3)4 in DMF and in the
presence of Cs2CO3 afforded the 4,4A-bipyridine derivative 4 in
89% yield. The latter was further treated with SOCl2 under
reflux affording the chloro derivative 5 in 95% yield. The
desired compound 6 was obtained in 79% yield upon treatment
of 5 by NaH in THF.
Upon slow diffusion at room temperature of an EtOH
solution containing CoCl2·6H2O (5 mg) into a CHCl3 solution
of 6 (4 mg), brownish crystals were obtained after ca. 48 h. An
X-ray diffraction study on a single crystal† revealed, as
expected, the presence of directional 1-D networks (Fig. 3). The
crystal (monoclinic, non-centrosymmetric space group C2) was
composed of 6, Co2+ cation, 2 Cl2 and 3 CHCl3 solvent
molecules which were found to be disordered. The latter are H-
bonded to the Cl atoms coordinated to cobalt centres with C(H)–
Cl distance and angle varying between 3.38–3.61 Å and
134–171° respectively. As expected, the two Co cations lying
on a 2-fold crystallographic symmetry axes, adopt a distorted
octahedral coordination geometry with the two Cl2 anions
occupying the two apical positions (dCo–Cl = 2.44 Å) in an
almost linear fashion (ClCoCl angle of 178.7°). The remaining
four coordination sites on the Co2+ centres are occupied by four
N atoms belonging to two consecutive tectons 6, one acting as
a tripodal ligand and the other as a monodentate site. Thus, the
strongly deformed square base of the octahedron is composed of
two pyridine derivatives (dCo–N = 2.09 Å) and two imines
belonging to the oxazoline moieties (dCo–N = 2.23 Å). Whereas
the two pyridines are disposed in a linear fashion (NCoN angle
of 180.0°), for the two imine N atoms, the NCoN angle of
148.8° is far from linearity. Consequently, the N(pyridine)Co-
N(imine) angle is 74.4°. Both oxazoline moieties are almost
coplanar with the pyridine ring (torsion angle of 2.0°).
network is formed. Due to the unsymmetrical and chiral nature
of 6, the 1-D coordination network is chiral and directional.
Although, as stated above, for achiral directional 1-D networks,
one may expect centric packing,10,11 owing to the C2 chirality of
6 and consequently, the chiral nature of the 1-D coordination
network formed with CoCl2, a non-centrosymmetric packing is
achieved. Significantly, the alignment of the 1-D chains occurs
in a parallel, rather than antiparallel fashion.
In conclusion, it has been demonstrated that by using the
neutral acentric and chiral tecton 6 and CoCl2, a chiral and
directional 1-D coordination network may be obtained in the
crystalline phase. The directionality of the network results from
the acentric nature of the organic tecton 6. At this point, it is not
possible to ascertain the reasons for the formation of the parallel
(polar) versus antiparallel (comparatively non polar) packing of
the directional 1-D networks. We are currently investigating the
generality of this result. Finally, probably owing to the weak
dipolar nature of the assembling core, no significant NLO
properties could be measured for the polar crystals thus
obtained. However, the rational design strategy reported here
remains of interest and will be further pursued with more
suitable chromophores as assembling nodes of the network.
We thank B. Foxman (Brandies University) and J. Hulliger
(University of Bern) for comments and helpful discussions.
Notes and references
† Crystallographic data for 1-CoCl2 (brown crystals, 173 K):
C22H26Cl2CoN4O2·3CHCl3, M = 866.45, monoclinic, a = 26.1974(8), b =
11.2679(4) c = 16.6366(5) Å, b = 125.960(5), U = 3975.1(2) Å3, space
group C 2, Z = 4, Dc = 1.45 g cm23, Kappa CCD, Mo-Ka graphite
monochromated, 3404 data with I > 3s(I), R = 0.078, Rw = 0.114. Data
were collected on a Nonius Kappa CCD and structural determination was
achieved using the Nonius OpenMolEn package.16 CCDC 187538. See
http://www.rsc.org/suppdata/cc/b2/b205556m/ for crystallographic data in
.cif or other electronic format.
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