Supramolecular assembly of two two-folded helical structures based on 2-substituted 8-hydroxyquinoline complexes

Article abstract of DOI:10.1016/j.inoche.2013.03.036

Based on two 2-substituted 8-hydroxyquinoline ligands containing NO ₂ and Cl, two complexes [Co(L₂)₂(pyridine) ₂] (1) and [Cu(L₃)₂] (2) were synthesized and characterized with single-crysta

Full text of DOI:10.1016/j.inoche.2013.03.036

Inorganic Chemistry Communications 33 (2013) 19–24
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Supramolecular assembly of two two-folded helical structures based on
2-substituted 8-hydroxyquinoline complexes
⁎
⁎
Guozan Yuan , Lulu Rong, Caibo Yue, Xianwen Wei
School of Chemistry and Chemical Engineering, Anhui University of Technology, Maanshan 243002, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Based on two 2-substituted 8-hydroxyquinoline ligands containing \NO2 and \Cl, two complexes
[Co(L2)2(pyridine)2] (1) and [Cu(L3)2] (2) were synthesized and characterized with single-crystal
X-ray diffractions, powder X-ray diffractions (PXRD), thermal analyses (TGA) and elemental analyses
(EA). In the solid state, the supramolecular structures of 1 and 2 feature two different two-folded helical
chains, which were constructed by non-covalent interactions, such as π⋯π stacking, C\H⋯π, C\H⋯O,
C\Cl⋯π and Cl⋯Cl interactions. The present research holds great promise in the development of novel
functional helical structure, and may contribute to the understanding of helical organized systems in
biology.
Received 4 February 2013
Accepted 25 March 2013
Available online 11 April 2013
Keywords:
Assembly
Non-covalent interactions
Helical
© 2013 Elsevier B.V. All rights reserved.
8-Hydroxyquinoline
Over the past several decades, supramolecular self-assembly
entities have attracted a lot of research attention because of their
intriguing structure and unusual properties, as well as their poten-
tial applications in molecular recognition, separation, molecular
catalysis, molecular transport, and so on [1,2]. Various organic
building blocks and metal ions have been used to self-assemble
a variety of one-, two-, and three-dimensional (1D, 2D, 3D)
networks through or mediated by classical non-covalent inter-
actions, such as hydrogen bonds, van der Waals interactions,
eletrostatic, π⋯π stacking, or metal coordination [3,4]. Recently,
halogen-related interactions, including halogen bonding as well
as related halogen⋯halogen and halogen⋯π intermolecular inter-
actions have been another attractive interactions in crystal engi-
neering [5,6]. However, the presence of halogen bonding in the
building of supramolecular architectures especially helical structures
remains largely unexplored and needs more elaborate and systematic
studies.
Helical structures are universal in nature, and are integral
to multifarious biological functions [7]. Natural biopolymers
such as proteins and DNA have acquired a definite helical sense
(e.g., right handed α-helix) [8]. These helical biopolymers in
turn can associate to form discrete helical superstructures, for
instance coiled-coils in proteins or supercoils in DNA plasmids,
via hierarchical self-assembly processes by which architectures of
increasing complexity are built up in distinct steps. Additionally,
these biological helical superstructures are of key importance for
their elaborate functions, which include for instance replication,
recognition and selective catalytic activity [9]. Inspired by the fascinating
helical structures found in nature, chemists all over the word have made
great efforts to introduce helicity into artificial systems and various
helical complexes have been synthesized successfully [10].
Of particular interest to us are works on the elegant design and
synthesis of supramolecular solid materials where directionality,
reversibility, and the possibility of controlling the strength of
the interactions are influenced by alternating the number of hydro-
gen bonds and other noncovalent interactions [11]. It is therefore
important to identify new supramolecular synthons that involve
different substituents able to form different intermolecular inter-
actions in the solid state. Following in this way, in this work, we
have synthesized two 2-substituted-8-hydroxyquinoline ligands
[12] containing different substituents (\NO2 or \Cl) to fabricate
hydrogen and halogen bondings. Complexes 1 and 2 were obtained
via solvothermal syntheses [13,14] and characterized by elemental
analysis, IR spectroscopy, thermogravimetric analysis (TGA), and
powder and single-crystal X-ray diffraction. In the solid state, two
supramolecular architectures exhibit two-folded helical structures
fabricated by combination of coordination bonds and supramolecu-
lar interactions (such as π⋯π stacking, C\H⋯π, C\H⋯O, C\Cl⋯π
and Cl⋯Cl interactions). The present result represents a significant
example showing that non-covalent interaction is a useful strategy
to regulate the secondary structure of linear helical chain and
thereby 3D supramolecular structure.
Bidentate ligands HL1 and HL2 were prepared by a similar pro-
cedure from the cheap commercial available 8-hydroxyquinaldine
(Scheme 1). Complexes 1 and 2 were obtained via solvothermal
syntheses. Heating CoCl2·6H2O and HL1 in DMF/MeOH/H2O/
pyridine solution afforded red, block-like crystals [Co(L1)2(pyridine)2]
⁎
Corresponding authors. Tel.: +86 555 2311807; fax: +86 555 2311552.
E-mail addresses: guozan@ahut.edu.cn (G. Yuan), xwwei@ahut.edu.cn (X. Wei).
1387-7003/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.inoche.2013.03.036

20
G. Yuan et al. / Inorganic Chemistry Communications 33 (2013) 19–24
Scheme 1. Synthesis of complexes 1 and 2.
(1). While single crystals of [Cu(L2)2] (2) were readily obtained
in good yields by heating Cu(NO3)2·3H2O and HL2 in a mixture of
DMF, MeOH and water. The formulations of 1 and 2 were supported
by microanalysis, IR and single-crystal X-ray diffraction [15].
The reactions were originally accomplished in a 1:2 molar ratio
of Co(II) (or Cu(II))and HL1 (or HL2), but the products were not
significantly affected by a change of the molar ratio. Moreover,
when ethanol or isopropanol was used as a solvent instead of
methanol, the same products were obtained. The products 1 and
2 are thus favorable species irrespective of the reactant molar
ratios and concentrations and the organic solvents used.
Complex 1 crystallizes in the monoclinic space group P21/c, with
one half of a formula unit in the asymmetric unit, that is, 1/2
Co(II) atom, one ligand L1, and one coordinated pyridine molecule.
The Co(II) centers adopt a octahedral geometry with the equatorial
plane occupied by the N2O2 donors of one L1 ligand and the
apical position by two pyridine molecules (Fig. 1). The bond
lengths around Co(II) are 2.0265(15) Å for Co\O, 2.1962(17)
and 2.2419(17) Å for Co\N, respectively. The quinoline ring is
practically coplanar with the basal CoN2O2 plane (dihedral angles
of 3.78°). There are weak nonclassical C\H⋯O intramolecular
hydrogen bonds (Fig. 1) in 1 between the phenolato oxygen
and the C\H groups of quinoline or pyridine ring (C⋯O = 3.048(3)–
3.100(3) Å, C\H⋯O = 118–157°), which play a significant role in the
construction of mononuclear Co(II) units.
adjacent ligand, C\H⋯π: 3.838(3) Å; between C\H group of pyridine
and nitrophenyl ring of adjacent ligand, C\H⋯π: 3.761(3) Å)(Fig. 2).
Two intertwined helical chains (one is right-handed, the other is
right-handed) are thus generated around the crystallographic 2(1)
axis with a pitch of 16.712(4) Å, which is equal to the b axis length.
Each helix is linked by nonclassical C\H⋯O intermolecular hydrogen
bonds along the a axis to form a 2D architecture. Moreover, it is notable
that each unit of 1 involves abundant intermolecular π⋯π stacking inter-
actions. As shown in Fig. 3b, each ligand is parallel to the one in the side
of the neighboring unit along the c axis, and π⋯π interaction exists
between quinoline and 4-nitrophenyl rings with a face-to-face distance
of 3.726(2) Å. By the coactions of these noncovalent interactions,
the structure extends to a 3D network.
An X-ray single-crystal diffraction study reveals that 2 crystallizes
in a monoclinic space group P21/n and displays a 3D supramolecular
structure. With one half of a formula unit in the asymmetric unit
of 2, there exist 1/2 Cu(II) center, one ligands L2. The Cu(II) center
adopts a four-coordinated square-planar geometry (CuO2N2) by coor-
dinating to two oxygen donors from two L2 anions with the Cu\O
distances of 1.8915(17) Å and two nitrogen atoms with the Cu\N
distances of 2.1008(3) Å. The two crystallographically unique ligands
have dihedral angles (ca. 51.4° for 1) between the quinoline and
2,6-dichlorophenyl moieties which indicate that the ligands are
non-coplanar in 2 (Fig. 4). There are weak nonclassical C\H⋯O and
C\H⋯Cl intramolecular hydrogen bonds in 2 between the phenolato
oxygen (or 2,6-dichlorophenyl chlorine) atoms and the C\H group
of ethenyl (C⋯O = 2.903(3) Å, C\H⋯O = 139°; C⋯Cl = 3.145(3) Å,
C\H⋯Cl = 108°), which play a significant role in the construction
of mononuclear Cu(II) unit (Fig. 4). Interestingly, a kind of meso-
helical chains (P + M) along the b axis constructed via C\Cl⋯π
interaction between C\Cl group of dichlorophenyl and phenolato
ring of adjacent ligand (3.9402 Å) is observed in the solid state of
this compound (Fig. 5a). As shown in Fig. 5b, these meso-helical
chains are interlinked by intermolecular cis Cl⋯Cl interaction (Cl⋯Cl:
3.831(2) Å). This observation indicates that the meso-helical chain
is stabilized not only by C\Cl⋯π but also by Cl⋯Cl interactions.
To the best of our knowledge, such helical chains constructed by
C\X⋯π interactions (X = halogen) is very rare. Additionally, strong
intermolecular π⋯π interactions direct the packing of the helicates
into 3D structure.
The mononuclear Co(II) units are linked into a helical chain
along the b axis via two types of strong intermolecular C\H⋯π inter-
actions (between C\H group of quinoline units and phenolato ring of
The study of the folding of linear oligomers into well-defined
conformations has received a lot of attention [16]. For example,
Moore and Wolynes have systematically studied helical folding of
linear oligophenylacetylenes in solution [17]. The resulting structures
Fig. 1. Perspective view of the coordination geometries of Co(II) atoms and intramolecular
hydrogen bonding in 1; H atoms omitted for clarity.

G. Yuan et al. / Inorganic Chemistry Communications 33 (2013) 19–24
21
Fig. 2. View of 1D intertwined supramolecular helical polymeric chains in 1 along the b-axis via two types of strong intermolecular C\H⋯π interactions.
can be stabilized by supramolecular interactions such as hydrogen
bonding, π, π-stacking interactions, electrostatic, solvophobic effect,
and/or metal coordination. They reasoned that such nonspecific
secondary interactions provide the energetic driving force for
folding, while directional interactions play a structure-defining
role. Analogous folding behavior has been observed in the assembly
of two-folded helical structures 1 and 2 in the solid state. The present
results hold great promise in the development of novel helical
structures fabricated by non-covalent interactions (such as halogen-
related interactions, hydrogen bonding, and π⋯π stacking).
2-substituted-8-hydroxyquinoline ligands via solvothermal syn-
theses. In the solid state, the supramolecular structures of 1 and
2 fabricated by intermolecular interaction, such as C\Cl⋯Cl\C,
and C\H⋯O interactions and π⋯π stacking, feature two different
two-folded helical chains and thereby 3D networks, respectively.
This unique self-assembly behavior via non-covalent interactions
is important for future synthesis of functional helical structures,
and may also contribute to the understanding of helical organized
systems in biology [18].
To prove that the crystal structures of complexes 1 and 2 are truly
representative of their bulk materials, powder X-ray diffraction
(PXRD) experiments were carried out on the as-synthesized samples.
As we can see in Fig. 6, the PXRD experimental patterns of 1 and 2 are in
good agreement with the simulated patterns. The thermal stability of
1 and 2 was investigated through thermogravimetric analysis (TGA)
experiments in the temperature range of 40–700 °C under a flow of
nitrogen with heating rate of 10 °C·min−1. There is no obvious weight
loss until 200 °C in 1. On further heating, the complexes 1 decompose
rapidly until about 225 °C (weight loss for 1: 19.82%), which well
corresponds to the loss of two coordinated pyridine molecules (calcd:
19.77%). The organic groups start to decompose gradually after the
temperature increases to 410 °C. As shown in Fig. 7, the backbone of
complex 2 starts to decompose around 370 °C. From TGA results, the
two complexes are found to have formed stable five-membered chelate
rings, which may be attributed to the fact that the M\N and M\O
bonds are highly polarized.
Acknowledgments
This work was supported by the National Natural Science Foundation
of China (Nos. 21201002 and 21271006), Anhui Provincial Natural
Science Foundation (1308085QB22), and Natural Science Foundation
from Bureau of Education of Anhui Province (KJ2013Z028).
Appendix A. Supplementary material
Crystallographic data for the structural analysis have been
deposited with the Cambridge Crystallographic Data Center, CCDC
No. 915958 for compound 1, CCDC No. 915959 for compound 2.
The data can be obtained free of charge via bhttp://www.ccdc.cam.ac.
uk/conts/retrieving.html>, or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-
336-033; or e-mail: deposit@ccdc.cam.ac.uk. Supplementary data
to this article can be found online at http://dx.doi.org/10.1016/j.
inoche.2013.03.036.
In the present work, two complexes [Co(L1)2(pyridine)2] (1) and
[Cu(L2)2] (2) were fabricated by self-assembly of M(II) ions with two

22
G. Yuan et al. / Inorganic Chemistry Communications 33 (2013) 19–24
Fig. 3. a) 2D supramolecular structure of 1 mediated by C\H⋯O hydrogen bonding along the a axis (red dashed lines); b) π⋯π interaction exists between quinoline and
4-nitrophenyl rings along the c axis (red dashed lines).
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