Helical nonracemic tubular coordination polymer gelators from simple achiral molecules

Article abstract of DOI:10.1039/b813375a

A novel class of helical nonracemic tubular coordination polymers, which can immobilize a wide range of solvents at very low concentrations, have been synthesized for the first time in the absence of chiral influences and appended groups. The Royal Societ

Full text of DOI:10.1039/b813375a

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Helical nonracemic tubular coordination polymer gelators from simple
achiral moleculesw
Shiyong Zhang, Shengyong Yang, Jingbo Lan, Shuaijun Yang and Jingsong You*
Received (in Cambridge, UK) 1st August 2008, Accepted 12th September 2008
First published as an Advance Article on the web 16th October 2008
DOI: 10.1039/b813375a
A novel class of helical nonracemic tubular coordination poly-
mers, which can immobilize a wide range of solvents at very low
concentrations, have been synthesized for the first time in the
absence of chiral influences and appended groups.
Coordination chemistry as a rational design route toward gels is
currently gaining intensive attention.1 Surprisingly, among the
examples describing metal coordination to assist molecular
gelation, the direct use of the concept of coordination polymers,
infinitely extending metal–ligand assemblies with bridging
organic ligands, has been less explored and remains elusive
despite its extensive application in crystal engineering.2 In these
limited reports with regard to coordination polymer gels,3 the
bridging organic units generally require appended moieties
(e.g., hydroxy, long hydrocarbons, ether chains, etc.) to stabilize
aggregates in the gel state through extra noncovalent bonds
such as hydrogen bonds and van der Waals forces. The main
challenge in this area is maintaining the metal–organic poly-
meric structure in the gel rather than in the preferential crystal-
line state, especially in the absence of these auxiliary moieties.
Helical and nonracemic architectures play a fundamental role
in many natural systems, perhaps most prominently represented
by biological macromolecular organizations. In recent years,
coordination polymers have been frequently used to mimic
these architectures.4 One general approach to nonracemic
helicity of coordination polymers depends on the coexistence
of chiral influences (e.g., chiral starting material, template, etc.)
transferring their chirality to the helical sense of the polymers.
In addition, there is also the possibility of the spontaneous
formation of chiral helixes from achiral components (chiral
symmetry breaking). However, only very few examples are
known and the control or predetermination of chirality still
remains a challenge.5
Scheme 1 Chemical structures of BIMMTMB and BMIMMB.
of stacking to form one-dimensional (1-D) nanosized channels
in the crystalline solid state (Scheme 1).7 Here we report a
novel class of coordination polymer gelators which only stem
from the coordination of Ag(I) and the imidazole derivative 2
or 5 (Scheme 2). The bridging organic ligands feature achiral,
structural simplicity and a conformationally restricted bent
shape. Our results demonstrate that the aggregates can be
stabilized in the gel state, avoiding the bulky crystalline solids
out of solution, even in the absence of side chain functionality
fused together into the scaffold, which constitutes the structu-
rally simplest coordination polymer gels in terms of the
secondary building units (SBUs) reported so far. To our
knowledge, the self-assemblies with helical, nonracemic and
nanotubular architectures have been achieved for the first time
from achiral components in the gel state (Fig. 1).
Initial results have shown that optically transparent gels are
formed spontaneously upon simple addition of a methanol
solution of the ditopic monomer 2 or 5 to an aqueous solution
of silver nitrate at a 1 : 1 ratio of ligand : AgNO3 at room
temperature in a few minutes. In contrast, the mixture of Ag(I)
and 1, 3 or 4 could not undergo gelation but were immediately
precipitated out of solution. To our surprise, further investiga-
tion demonstrated that the gelation driven by {[Ag-2]NO3}n
could tolerate a wide scope of solvents involving dimethyl-
formamide (DMF), glycol or mixtures of water and organic
Imidazoles as building blocks are currently attracting spe-
cial interest in the area of supramolecular chemistry due to
their importance in biology and biochemistry.6 Recent studies
of self-assemblies of the conformationally flexible 1,3-bis-
(imidazol-1-ylmethyl)-2,4,6-trimethylbenzene (BIMMTMB) with
CuCl2Á2H2O or 1,4-bis(2-methylimidazol-1-ylmethyl)-benzene
(BMIMMB) with AgBF4 have revealed the formation of
discrete dinuclear metallocyclic complexes that are capable
Scheme 2 Chemical structures of ligands 1–5.
Key Laboratory of Green Chemistry and Technology of Ministry of
Education, College of Chemistry, and State Key Laboratory of
Biotherapy, West China Hospital, Sichuan University, Chengdu
610064, People’s Republic of China. E-mail: jsyou@scu.edu.cn
w Electronic supplementary information (ESI) available: Synthetic
procedures and characterization of 1–5; gelation test; TEM and
SEM; theoretical calculation of the helical tube. See DOI:
10.1039/b813375a
Fig. 1 A schematic representation of the self-assembly processes of
the coordination polymer gels.
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solvents (e.g., methanol, ethanol, i-propanol, acetonitrile, even
chloroform, dichloromethane and toluene) (see ESIw, Table S1).
It is noteworthy that the critical gel concentration (CGC) is as
low as 0.3 wt% of the {[Ag-2]NO3}n gelator in CH3OH and
H2O (3 : 2 wt/wt, see Fig. S1w for optimization of the mixed
solvents’ ratio). These gel materials maintained stability for a
period of several months at ambient temperature. Interestingly,
the gels could survive in a broad range of acidic conditions
(pH = 1–7) although the 3-N position of the imidazolyl unit is
protonated easily. More interestingly, the coordination polymer
{[Ag-2]NO3}n gelator could even gelatinize the mixture of
CH3COOH and H2O. In addition, we found that the counter-
anions of Ag(I) would influence the gelation. For instance,
AgOSO2CF3 drives the formation of the gel whereas AgBF4
and AgSbF6 immediately afford the precipitates, which are
assumed to arise from rapid polymerization to hinder trapping
of solvent molecules into the networks’ interstitial spaces.3j
Transmission electron microscopy (TEM) was first conducted
to investigate the supramolecular structures derived from
AgNO3 and 2. A micrograph obtained in CH3OH–H2O shows
the formation of cylindrical aggregates with a uniform diameter
of ca. 9 nm and lengths up to several micrometres as the sample
is negatively stained with an aqueous solution of uranyl acetate
(Fig. 2a, also see Fig. S2 and S3w for the images unstained or
negatively stained with phosphotungstic acid). The obvious
contrast between the inner and periphery in the cylindrical
structures is characteristic of the projection images of hollow
tubular aggregates rather than tapes. Furthermore, from this
image one can find that these tubules appear to feature a
helical motif.
with a well-grown helical arrangement along its 1-D aggregate
(Fig. 2c), suggesting that thicker fibers consist of a bundle of
helical tubes. Further analysis of these tubes reveals an average
helical pitch of ca. 8.0 nm (Fig. S4w), and a width of ca. 9 nm after
accounting for the tip-broadening factor,8 which is consistent with
the TEM images. In addition, a SEM image obtained from the
freeze-dried {[Ag-2]NO3}n gel (xerogel) in CH3OH–H2O also
demonstrates the formation of well-grown three-dimensional
networks of fibrous assemblies, and these ribbon-like bundles
must be formed during the drying process (Fig. 2d).
The aggregation behavior observed by electron microscopy
was further studied by circular dichroism (CD) spectra at the
molecular level. In the absence of Ag(I), no CD Cotton effect for
2 was observed, as expected for an achiral molecule. As shown in
Fig. 3a, in contrast, the CD spectra of the {[Ag-2]NO3}n gel
exhibit a strong exciton-coupled split Cotton effect with the
y = 0 crossing wavelength at 219 nm near the absorption
maximum of the chromophore (214 nm, Fig. 3b). Possible
contributions of linear dichroism (LD) and birefringence to
the true CD signals have been excluded carefully using the
method reported by Spitz et al.9 These experiments are clearly
indicative of the formation of a helical superstructure with a
preferred handedness,10 suggesting chiral symmetry breaking.
Further repeating experiments show that each sample is CD
active. However, the sign of the CD signals is undetermined for
different batches [sometimes positive and sometimes negative,
roughly giving a statistical (1 : 1) distribution]. These results
indicate that the chirality is essentially governed by chance, and
not by any external chiral impurities.5,11–13 It is important to
stress that the intensity of peaks of the gel phase was too strong
to obtain reliable spectra using a conventional optical cell. Thus,
the CD and UV-Vis spectra were measured with a gel sample
sandwiched between two quartz glass plates. Under these con-
ditions, the ordinate is expressed as an arbitrary unit because it is
very difficult to control the thickness of the sample.14 In this
stage, therefore, we could not determine whether the symmetry
breaking occurs throughout the whole sample or if there is
always a significant bias, one way or the other, of say left-
handed fibers in a certain case but with some contamination of
right-handed fibers. It should be noted that the CD study for the
gel formed by {[Ag-5]NO3}n afforded similar experimental
We further performed atomic force microscopy (AFM) experi-
ments with a dried drop-cast solution of {[Ag-2]NO3}n in
CH3OH–H2O on a freshly cleaved mica surface. The AFM image
shows a well-developed network structure composed of fibrous
aggregates (Fig. 2b). More interestingly, the zoomed-in image of
individual objects unambiguously discloses the tubular structure
Fig.
2 The morphology of the {[Ag-2]NO3}n aggregates in
CH3OH–H2O (3 : 2 wt/wt). (a) TEM image, 0.1 wt%, negatively
stained with uranyl acetate aqueous solution. The inset shows a
magnified picture of the area marked. (b) Tapping-mode AFM height
image on drop-cast mica, 0.08 wt%. (c) A zoomed-in image of the area
marked in (b). The arrows point to the helical structure. (d) A SEM
image of xerogel, 0.8 wt%, Au shadowing at 451.
Fig. 3 Spectral analysis of the {[Ag-2]NO3}n gel (0.8 wt%) in
CH3OH–H2O. (a) CD spectra obtained in different batches, positive
(positive to negative on going from longer to shorter wavelengths,
solid line) or negative (dash line). (b) UV-Vis spectrum.
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results. Interestingly, Yuan and Liu recently reported chiral
molecular assemblies from achiral amphiphilic imidazole-based
ligand 2-(heptadecyl) naphtha[2,3]imidazole and AgNO3, in
which the chiral symmetry breaking occurred for the first time
in the monolayer at the air–water interface and in LB films
though interfacial coordination.5b As far as we know, we herein
present the first example describing chiral symmetry breaking in
the gel state.11–13
J.-H. Lee and M. Lee, Angew. Chem., Int. Ed., 2005, 44, 5810;
(g) H.-J. Kim, W.-C. Zin and M. Lee, J. Am. Chem. Soc., 2004,
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N. Kimizuka, J. Am. Chem. Soc., 2004, 126, 2016; (i) O. Roubeau,
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43, 3283; (j) B. Xing, M.-F. Choi and B. Xu, Chem.–Eur. J., 2002,
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4 M. Albrecht, Angew. Chem., Int. Ed., 2005, 44, 6448.
5 Chiral symmetry breaking in coordination polymers, see:
(a) S.-T. Wu, Y.-R. Wu, Q.-Q. Kang, H. Zhang, L.-S. Long,
Z. Zheng, R.-B. Huang and L.-S. Zheng, Angew. Chem., Int. Ed.,
2007, 46, 8475; (b) J. Yuan and M. Liu, J. Am. Chem. Soc., 2003,
125, 5051; (c) T. Ezuhara, K. Endo and Y. Aoyama, J. Am. Chem.
Soc., 1999, 121, 3279.
6 Selected examples, see: (a) W. Wang, P. Xi, X. Su, J. Lan, Z. Mao,
J. You and R. Xie, Cryst. Growth Des., 2007, 7, 741; (b) H. Wu,
W. Zhou and T. Yildirim, J. Am. Chem. Soc., 2007, 129, 5314;
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45, 2542; (d) W.-G. Lu, C.-Y. Su, T.-B. Lu, L. Jiang and
J.-M. Chen, J. Am. Chem. Soc., 2006, 128, 34; (e) X.-C. Huang,
J.-P. Zhang and X.-M. Chen, J. Am. Chem. Soc., 2004, 126, 13218;
(f) M. A. Alam, M. Nethaji and M. Ray, Angew. Chem., Int. Ed.,
2003, 42, 1940.
We rationalize that the helical architectures are generated
from strong directional interactions derived from the complex-
ation between the rigid bent bridging ligands and Ag(I) that
adopts a linear coordination geometry. In the formation of the
helical polymer, the occurrence of statistical fluctuations in the
initial stage of metal–ligand coordination could lead to an
accidental excess of one helical direction.15 Once a certain helical
sense excess exists, the new aggregates would follow to form a
helical secondary structure with the same direction by chiral
autocatalysis, while the competitive generation of the opposite
helical sense is suppressed, thus leading to chiral amplification
and eventual macroscopic chirality.
7 (a) L. Dobrzan
´
ska, G. O. Lloyd, H. G. Raubenheimer and
ska,
L. J. Barbour, J. Am. Chem. Soc., 2006, 128, 698; (b) L. Dobrzan
´
To further shed light on the possible formation mechanism
of the helical tubular polymers, theoretical calculations have
been carried out on the basis of the first principle density
functional theory (DFT) (see ESIw for details).16 The opti-
mized result shows that the diameter of the helical tubule is ca.
9.1 nm and the helical pitch is ca. 7.1 nm, which is approxi-
mately in agreement with the experimental results from the
TEM and AFM studies (Fig. S10w).
G. O. Lloyd, H. G. Raubenheimer and L. J. Barbour, J. Am. Chem.
Soc., 2005, 127, 13134.
8 (a) A. Ajayaghosh, R. Varghese, S. Mahesh and V. K. Praveen,
Angew. Chem. Int. Ed., 2006, 45, 7729; (b) P. Samorı, V. Francke,
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T. Mangel, K. Mullen and J. P. Rabe, Opt. Mater., 1998, 9, 390.
¨
9 In all CD measurements, the birefringence contribution was
avoided by carefully placing the sandwich ensemble perpendicular
to the light path. We also checked that the CD spectra did not
change when they were recorded rotating the ensemble in steps of
101 around the light axis. For detailed measurement methods, see
In summary, we have demonstrated that a simple achiral
molecule could induce a helical nonracemic nanotubular
structure when linked by a linear metal connecting point, which
forms an entangled fibrillar network thereby immobilizing a
wide range of solvents. In addition, the gels present another
appealing feature in their extremely simple preparation at room
temperature. Thus, the gel could be fabricated into various
geometric morphologies in accordance with the vessel in which
it was assembled.17 We foresee that these findings will open a
new avenue for the design of chiral functional soft materials
from structurally simple achiral building blocks.
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11 Chiral symmetry breaking has been mainly reported in the crystal-
line state. Selected examples, see: (a) D. K. Kondepudi,
R. J. Kaufman and N. Singh, Science, 1990, 250, 975;
(b) R. E. Pincock, R. R. Perkins, A. S. Ma and K. R. Wilson,
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12 Selected examples of chiral symmetry breaking in solution, see:
(a) J. M. Ribo, J. Crusats, F. Sagues, J. Claret and R. Rubires,
´ ´
This work was supported by grants from the National
Natural Science Foundation of China (20772086, 20472057).
We thank the Centre of Testing and Analysis, Sichuan
University for SEM, TEM, AFM and CD measurements.
Science, 2001, 292, 2063; (b) U. De Rossi, S. Dahne, S. C.
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J. Meskers and H. P. J. M. Dekkers, Angew. Chem., Int. Ed. Engl.,
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see: (a) D. K. Schwartz, R. Viswanathan and J. A. N. Zasadzinski,
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15 For relevant descriptions of statistical fluctuation in symmetry
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16 G. Parr and W. Yang, Density functional theory of atoms and
molecules, Oxford University Press, New York, 1989.
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process. Such a process is a disadvantage for many industrial
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