Unexpected helicity control and helix inversion: Homochiral helical nanotubes consisting of an achiral ligand

Article abstract of DOI:10.1039/c3cc43898h

The ligand tppda has been designed and synthesized as molecular leverage for helicity control when reacted with Cd²⁺ ions. The guests MeOH or DMF preferentially stabilize the P-helical isomer, while the guest H ₂O causes a helix inversion to give the M-helical isomer as the major isomer without any chiral auxiliary.

Full text of DOI:10.1039/c3cc43898h

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Unexpected helicity control and helix inversion:
homochiral helical nanotubes consisting of an
achiral ligand†
Kaiju Wei,*ab Jia Ni,c Yuanzeng Min,b Siming Chenb and Yangzhong Liuab
Cite this: Chem. Commun., 2013,
49, 8220
Received 31st May 2013,
Accepted 25th July 2013
DOI: 10.1039/c3cc43898h
www.rsc.org/chemcomm
The ligand tppda has been designed and synthesized as molecular generates conglomerates.8 A conglomerate is a mechanical and
leverage for helicity control when reacted with Cd2+ ions. The racemic mixture of chiral crystals, in which enantiopure crystals
guests MeOH or DMF preferentially stabilize the P-helical isomer, are formed in a chiral space group.9 However, the phenomenon
while the guest H2O causes a helix inversion to give the M-helical of achiral components resolving to yield chiral crystals is
isomer as the major isomer without any chiral auxiliary.
relatively rare compared to the analogous process with chiral
components or auxiliaries.8c,10 Since the formation of enantiopure
Since the discovery of carbon nanotubes (CNTs) in 1991,1 crystals is not predictable, to the best of our knowledge, there have
discrete tubular structures have experienced a revolutionary been no reports of the two types of enantiopure helical tubular
development from inorganic to organic nanotubes.2,3 Among single crystals (P and M columns) selectively crystallized without
all synthetic nanotubes, metal–organic nanotubes (MONTs) any chiral auxiliary.
could be the most potent materials possessing functional activity
2,20-Dipyridylamine is a high-performance unit combining
since these types of compounds contain both metal ions and flexibility and a strong chelation ability for transition metal
organic ligands as functional groups.3 However, the develop- ions.11 It can be anticipated that introducing angular ditopic
ment of MONTs is still behind in comparison with other dipyridylamine derivatives should enhance the possibility
synthetic nanotubes, probably due to the largely unpredictable of forming a helix by linking appropriate transition-metal
assembly structure.4,5
ions. In this study, a series of achiral ditopic ligands with a
1D coordination helices have been proved to be ideal 1201 angle have been designed and synthesized in order to create
candidates for the hierarchical assembly of nanotubular a novel helix system (Scheme 1). Two periodically ordered
structures,6 in which opposite binding sites in special angular homochiral six-fold helical tubes of [Cd(tppda)(NO3)2]nꢀsolvent
ligands could induce the formation of helical structures when (solvent = 0.5nMeOH or nDMF for P helix; solvent = xH2O for
alternately linked by metal ions. Generally, such helical struc- M helix) with P and M columns have been obtained under
tures generate the alternative P and M columns or layers with different reaction conditions (see ESI†). They consist of a large
D or L mirror enantiomers, giving racemic crystals if achiral single walled MONT of [Cd(tppda)(NO3)2]n with an exterior wall
ligands are employed in the constructs. Chiral compounds are diameter of up to 2.2 nm and an interior channel diameter
usually obtained either by the use of chiral ligands or chiral of 0.75 nm.
auxiliaries, or by the spontaneous resolution upon crystallization
without any chiral auxiliary. The former approach typically
yields enantiopure samples,7 while the latter method usually
a Nano Science and Technology Institute, University of Science and Technology of
China, Suzhou 215123, P. R. China. E-mail: kjwei@ustc.edu.cn;
Fax: +86-512-87161359; Tel: +86-512-87161349
b Department of Chemistry, CAS Key Laboratory of Soft Matter Chemistry,
University of Science and Technology of China, Hefei 230026, P. R. China
c School of Pharmacy & Pharmaceutical Sciences,
Anhui University of Traditional Chinese Medicine, Hefei 230038, P. R. China
† Electronic supplementary information (ESI) available: Experimental procedures,
structural figures, PRXD, as well as crystallographic data. CCDC 712881, 712882,
712884 and 712885. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3cc43898h
Scheme 1 Schematic representation of the two types of six-fold helices (P and M)
for the 1201 angular ligands and the metal ‘‘nodes’’.
c
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Yellowish dodecahedral crystals of [Cd(tppda)(NO3)2]nꢀsolvent
(1-P) (solvent = nDMF for 1-P-a and 0.5nMeOH for 1-P-b)
were obtained from an organic solution containing tppda and
Cd(NO3)2ꢀ4H2O in a 1 : 1 molar ratio by slow diffusion of ether
vapour, or from a MeOH solution by the solvothermal method.
Single crystal X-ray diffraction analyses revealed that 1-P crystal-
lized in the chiral hexagonal space group P6(1)22. Each Cd2ꢁ+
ion is coordinated by two oxygen atoms from a pair of NO3
anions and four bridging nitrogen atoms from two dpa groups
of different tppda ligands, forming a distorted octahedral
geometry (Fig. 1a). The crystallographically equivalent Cd2+
ions are bridged by tppda spacers to form an infinite helical
chain along the c axis. The Cd(1) atom lies on a crystallographic
two-fold axis and atoms N(4) and C(13)–H(13) of the tppda
ligand lie on another two-fold axis. The right-handed helix is
constructed around the crystallographic 6(1) axis with a pitch of
B21 Å, which is stabilized by alternating pꢀ ꢀ ꢀp stacking inter-
actions of pyridyl groups from ligands in the intra-helix (Fig. 1b
and c). The most intriguing feature of 1-P is the formation of a
large single-walled chiral MONT, which is constructed from a
six-connected Cd2+ node and an angular organic linker tppda
(Fig. 2). The top view of the open-ended hollow nanotube
indicates that the undulated hexanuclear {Cd6(tppda)6(NO3)12}
metallamacrocycle forms a very large 60-membered ring con-
sisting of six Cd2+ atoms and six tppda ligands (6Cd, 24C and
30N). Each Cd–Cd separation of B9.2 Å is bridged by the tppda
Fig. 2 (a) Coordination mode of tppda in 1-P and 1-M; (b) top view of 1-P and
1-M; (c) intertwist of right-helix into a chiral nanotube in 1-P (Cd: green, C: gray,
O: red, N: blue); (d) left-helix structure of 1-M (crystal structure can not be fully
resolved).
ꢁ
ligand. We have previously investigated the Cu+, Zn2+, Cd2+ and Evidently, the non-bridging anion NO3 plays an auxiliary role
Hg2+ complexes of tppda, which are potential luminescent and in the helical tubular assemblies of 1-P.
molecular recognition materials.11 These complexes exhibit chain
The helical nanotubes are held together by alternating pꢀ ꢀ ꢀp
arrangements based on M–X–M bridging modes (X = Cl, Br, I, SCN). stacking (d = 3.55–3.65 Å) interactions between the helical
columns to form 3D arrays. In this structure, the adjacent
helical columns align in parallel directions and interweave the
central pyridine rings of the dpa groups in a one-over/one-under
fashion (see ESI†). This construction generates a gearwheel-type
role, in which each center helix is in contact with six neighboring
helices of the same handedness via p-stacking interactions.
This gearwheel-type role defines a homochiral supramolecular
affinity,8c which constrains a handedness of helical columns
under the internal pressure of a multiscrew-fastening self-
organization. Such a homodromous gearwheel-type arrangement
along a helical hypersurface is more stable than a heterodromous
one using p-stacking interactions, which may constitute the
driving force of the homochiral crystal phase cohesion. As a
result, there are preferential and extended homochiral inter-
actions between neighboring chiral columns, and the chirality
would be able to extend to higher dimensionalities and hence,
spontaneous resolution would be more likely to occur. In 1-P,
the chirally discriminative interactions arise from the coordi-
nation bonds and pꢀ ꢀ ꢀp stacking interactions, which are sub-
stantially strong, selective, and directional.
The overall crystallization effect is a constitutional self-sorting
process in which a unique enantiopure single crystal of 1-P forms.
The chiral amplification experiment was performed by randomly
collecting 10 crystals/samples from two distinct crystallization
experiments based on single crystal XRD analysis. The results
Fig. 1 (a) Single crystal X-ray structure of the helix chain formed by Cd2+ ions
and tppda in 1-P, showing the coordination of the Cd2+ cation; (b and c) two
face-to-face pꢀ ꢀ ꢀp stacking interactions in the helix chain (1-P-a: dcentroid–centroid
a = b = 3.7413(1) Å and dihedral angle a = b = 13.561; 1-P-b:dcentroid–centroid
demonstrate a handedness preference towards P-crystals: CEE =
a = b = 3.8623(1) Å and dihedral angle a = b = 14.871). Cd: green, C: gray, O: red,
N: blue. Hydrogen atoms are omitted for clarity.
100%. In order to measure the overall chirality of the complex,
c
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high steric hindrance (ESI†).11b Thus, these building blocks can
not achieve spontaneous assembly of the helix structure with
metal ions.
In conclusion, this work demonstrates that the construction
of helical-structures are highly dependent on the presence
of a N atom on the X site of the ligand and the flexibility of
the ‘‘dpa-arm’’ spacers. The chiral tubes in single crystals result
from the self-assembly of achiral components. The homo-
chirality in the crystalline state is promoted by the constitu-
tional chiral affinity of supramolecular helices of the same
handedness, interacting via their van der Waals hypersurfaces.
The internal robustness of the helical columns is mainly
responsible for the transmission of the supramolecular homo-
chiral order. The coordination modes of Cd–tppda depend on
the reaction solvent in this system. The present results provide
knowledge on the rational design of helical nanotubes.
This work was supported by the Applied Basic Research
Projects (Industry) of Suzhou City (No. SYG201253), NSFC
(No. 50903081, 21171156), 973 Program 2012CB932502, CPSFP
(No. 20100480683, T201250545), NSFJSP (No. BK2008579).
Fig. 3 (a) Optic image of complex 1-P; (b) optic image of complex 1-M; (c) X-ray
powder diffraction of 1-P and 1-M; (d) solid-state CD spectra of 1-P (black) and
1-M (red) diluted with a KBr matrix.
the solid-state CD was measured on a bulk crystalline KBr pellet
(Fig. 3). The result shows a positive peak at 425 nm, which
proves the presence of the 1-P structure. Although these data
can not completely rule out the possibility of M-helix enantiomers
based on the analysis on a limited number of crystals, the results
in this work indicate that the P structure is at least dominant in
the [Cd(tppda)(NO3)2]n crystal.
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8222 Chem. Commun., 2013, 49, 8220--8222
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