Chiral symmetry breaking by chemically manipulating statistical fluctuation in crystallization

Article abstract of DOI:10.1002/anie.200703443

Breaking out: By introducing a relatively stable intermediate to chemically control the nucleation rate, chiral symmetry breaking was achieved in the crystallization of intrinsically chiral helical coordination polymers. The fewer crystal clusters formed, the more pronounced the symmetry breaking (see picture). (Graph Presented)

Full text of DOI:10.1002/anie.200703443

Angewandte
Chemie
DOI: 10.1002/anie.200703443
Chirality
Chiral Symmetry Breaking by Chemically Manipulating Statistical
Fluctuation in Crystallization**
Shu-Ting Wu, Yan-Rong Wu, Qing-Qing Kang, Hui Zhang, La-Sheng Long,* Zhiping Zheng,*
Rong-Bin Huang, and Lan-Sun Zheng
Naturally occurring amino acids and sugars are building
blocks of the biological world. They are distinctly left- and
right-handed, respectively, with all members of one class
fluctuation inherent to the crystallization of helical coordi-
nation polymers; this class of substances has attracted much
recent interest owing to their potentially useful applica-
[
1]
[13,14]
possessing the same sense of chirality. Chemically and
statistically, however, there should be the same number of
such molecules in both forms, because each enantiomer
should be produced with equal probability if no external
tions.
In the absence of any chiral influence, such as a
chiral catalyst, template, or chiral starting materials, these
intrinsically chiral materials are generally obtained as opti-
cally inactive conglomerates because of the stochastic nature
[
2]
[22]
physical field or chiral reagent is introduced. The origin of
this homochirality remains a subject of much debate in
biology, despite extensive research and various hypothe-
of crystallization.
Our approach may be compared to coin flipping. If a coin
is flipped a sufficient number of times, it will come to rest on
either side with equal probability. However, if the flipping is
limited to just a small number of events, from a statistical
point of view, the situation can differ significantly. Under such
circumstances, the chance of getting just one particular side
up becomes much higher than it would be otherwise. In the
extreme case of just one toss, the chance of getting only a head
or only a tail is 100%. When this statistical argument is
applied to the nucleation of a targeted helical coordination
polymer, the otherwise equal probability of forming the left-
and right-handed primary nuclei is significantly skewed
toward one particular enantiomeric form. Once the stochastic
formation of the primary nucleus is limited, the ensuing
generation of secondary crystal nuclei of the same structure
on the surface of the primary nucleus is made possible by
controlling the concentration of the reactants, eventually
leading to a cluster of crystals of the same chirality.
[
1–9]
ses.
The problem of homochirality also represents a grand
challenge with significant ramifications in modern physical
[
10–14]
and materials sciences,
as homochiral materials are useful
for such important applications as enantioselective separa-
[
15,16]
tion, nonlinear optics, catalysis, and sensor technology.
Abiotic mechanisms based upon asymmetry induction by
[
17]
external fields or forces have been investigated.
imentally more conclusive, however, is asymmetric resolution
Exper-
[
18]
by chiral autocatalysis in crystallization. A state of nonzero
enantiomeric excess can arise spontaneously from an achiral
or a racemic state through a chiral symmetry breaking
transition. An elegant example by Kondepudi et al. demon-
strated that secondary crystal nuclei of the same structure as
the parent crystal (the primary nucleus) are rapidly cloned
(
chiral autocatalysis) under stirring, while competitive crys-
tallization of the opposite enantiomer is suppressed, thus
leading to chiral amplification and eventual production of
The key to manipulating such statistical fluctuation lies in
the control of the number of crystallization events and, more
specifically, the number of primary crystal nuclei. We propose
that the crystallization kinetics of helical coordination poly-
mers may be controlled by the presence of a judiciously
chosen reagent that competes with the polymer-forming
ligands for the metal ions. As such, the availability of the
metal ion for polymer formation is controlled by the concen-
tration of the competing reagent, which in turn determines
how facilely the product crystallizes.
[
19]
enantiopure crystals. Although this particular autocatalytic
formation of crystals has subsequently been verified exper-
imentally, secondary nucleation in general is a rather complex
[
20]
process,
and the explanation for the resulting chiral
[
21]
symmetry breaking remains unclear.
Herein, we present a distinctly different approach to
chiral symmetry breaking by manipulating the statistical
The synthesis of the reported [{Cu(succinate)(4,4’-
[
*] S.-T. Wu, Y.-R. Wu, Q.-Q. Kang, Prof. H. Zhang, Prof. Dr. L.-S. Long,
Prof. Dr. Z. Zheng, Prof. R.-B. Huang, Prof. Dr. L.-S. Zheng
State Key Laboratory of Physical Chemistry of Solid Surface and
Department of Chemistry
College of Chemistry and Chemical Engineering
Xiamen University, Xiamen 361005 (China)
Fax: (+86)592-218-3047
[
23]
bipyridine)} ]·(4H O) (1) was used to validate our hypoth-
n
2
n
esis. This coordination polymer has been shown to possess a
three-dimensional network structure in the solid state with
the helical polymeric chains of copper succinate bridged by
4
,4’-bipyridine. Our initial efforts to reproduce 1 according to
the reported procedure yielded an optically inactive con-
glomerate, which can be rationalized in terms of the equally
probable nucleation of both helical forms owing to the fast
kinetics typical of coordination polymer synthesis.
E-mail: lslong@xmu.edu.cn
zhiping@u.arizona.edu
[
**] We thank the NNSFC (Grant Nos. 20471050 and 20423002), the
Ministry of Education Key Project (104201) the 973 project (Grant
2
007CB815304) from MSTC for financial supports.
Ammonia was then added as a competing reagent in a
subsequent, modified synthesis of 1. By forming the tetraam-
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
2
+
mine complex [Cu(NH ) ] , ammonia competes with succi-
3
4
Angew. Chem. Int. Ed. 2007, 46, 8475 –8479
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
8475

Communications
2
+
nate and 4,4’-bipyridine for Cu . The higher the concen-
ent starting pH values were prepared. Furthermore, solutions
of the same initial pH value were allowed to evaporate at
different rates (see the Supporting Information for details).
Our experimental observations were consistent with the
above analysis. Regardless of the evaporation rate, crystal-
lization of 1 from solutions with an initial pH of 8.3 was
always rapid, affording a large number of small crystals
(Figure 1a). Careful crystallographic analysis established the
tration of ammonia, the more likely the formation of [Cu-
NH ) ] and the less likely the crystallization of the polymer
3 4
2
+
(
product. In other words, how easily the coordination polymer
crystallizes can be modulated by the amount of ammonia
present in the reaction mixture.
Formally, these competitive processes can be summarized
into two interdependent equilibria: the formation of the
polymer product 1 [Eq. (1)] and the formation of [Cu-
2
+
(
NH ) ] [Eq. (2)]. In the absence of ammonia, the rate of
3 4
2
þ
Cu þ L þ L Ð ½fCuðL ÞðL Þg Š
ð1Þ
ð2Þ
S
b
S
b
n
2þ
2þ
4
Cu þ 4 NH Ð ½CuðNH Þ Š
3
3
0
ðL ¼ succinate, L ¼ 4,4 -bipyridineÞ
S
b
formation of 1 (r ) is directly proportional to the concen-
f
2
+
2+
trations of Cu ([Cu ]), succinate ([L ]), and 4,4’-bipyridine
s
2
+
(
[L ]), that is, r = k[Cu ][L ][L ]. In the presence of
b f s b
Figure 1. Crystallization using pH 8.3 solutions. a) A rapid crystalliza-
tion of 1 yielding a large number of small crystals. b) The solid-state
CD spectra of bulk samples of 30 different crystallizations.
ammonia, on the other hand, the rate law becomes r =
k[[Cu(NH ) ] ][L ][L ]/K[NH ] , where k is the rate con-
stant for the formation of 1, K = [[Cu(NH ) ] ]/[Cu ][NH ]
f
2
+
4
3
4
s
b
3
2
+
2+
4
3
4
3
is the equilibrium constant of Equation (2), and [NH ] and
3
2+
[
[Cu(NH ) ] ] are the concentrations of ammonia and the
optical purity of individual crystals. Left- and right-handed
helices were found to coexist; their structures are shown in
Figure 2 together with the overall structure in which individ-
ual helices are bridged by 4,4’-bypridine. Investigation using
3
4
tetraammine complex, respectively. Considering [[Cu-
(
centration of Cu , r can be expressed as r = k[Cu ] [L ][L ]/
{
2
+
2+
2+
2+
NH ) ] ] = [Cu ] ꢀ[Cu ], where [Cu ] is the initial con-
3
4
0
0
2
+
2
+
f
f
0
s
b
4
[24]
K[NH ] + 1}, which can be approximated as rf =
circular dichroism (CD) spectroscopy showed that each of
3
2
+
4
4
k[Cu ] [L ][L ]/K[NH ] , because K[NH ] @ 1. The critical
0
s
b
3
3
importance of [NH ] is clear, because for given amounts of
3
2
+
Cu , succinate, and 4,4’-bipyridine, the rate of formation of
the coordination polymer is inversely proportional to [NH ] .
4
3
A high concentration of ammonia greatly impedes the
formation of the polymer, while lowering ammonia concen-
tration does the opposite. It follows that crystallization of the
coordination polymer from a homogeneous solution contain-
2
+
ing the ligands (succinate and 4,4’-bipyridine) and Cu can be
controlled by modulating the concentration of ammonia. A
low ammonia concentration is expected to result in quick
crystallization and a large number of nuclei in the initial stage
of metal–ligand coordination. This situation will ultimately
lead to a racemic mixture or a product of a low enantiomeric
excess, because the left- and right-handed helical structures
are produced with equal probability. In contrast, when the
concentration of ammonia is high, only a limited number of
nuclei may be formed prior to crystal growth. The probability
of establishing one particular isomeric form, left- or right-
handed, in these nuclei is greatly enhanced as a consequence
of the inherent statistical fluctuation discussed above. As a
result of such a significantly skewed distribution of the two
possible helical forms, the chiral symmetry is broken, and a
product with a high enantiomeric excess, possibly a homo-
chiral product, is formed.
The concentration of ammonia may be modulated by
varying the pH value of the initial reaction mixture, as a high
Figure 2. ORTEP plots with thermal ellipsoids set at the 30% proba-
bility level showing a) the right-handed helix of polymeric [{Cu(succi-
nate)} ], b) the left-handed helix of polymeric [{Cu(succinate)} ], and
+
pH values favor NH in the equilibrium NH + H OQNH
+
3
3
2
4
ꢀ
OH . It can also be moderated by control of the evaporation
rate of the solution or by a combination of these two methods.
To verify our conjecture, reaction mixtures with three differ-
n
n
c) the overall structure of [{Cu(succinate)(4,4’-bipyridine)} ]·(4H O)
n
2
n
(1) viewed along the c axis.
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the 30 bulk samples was CD-silent (Figure 1b). Together,
these results indicate that 1 crystallizes as a racemic con-
glomerate—a mechanical mixture of enantiomerically pure
crystals of one enantiomeric form and its opposite.
When the synthesis was carried out using pH 9.2 solutions,
the outcome of crystallization was found to be profoundly
influenced by how rapidly ammonia evaporated. Specifically,
when the solution was more tightly capped to prevent fast
evaporation, only a limited number of clusters of crystals were
produced (Figure 3a). Out of 30 crystallizations investigated,
predicted, as there is no systematic bias in favor of one
particular enantiomer over the other. Because it is difficult to
separate the left-handed crystals from their right-handed
counterparts in a racemic mixture or to calculate the
enantiomeric excess value based on the solid-state CD
spectra, the ability to cultivate individual enantiopure crystals
and to calculate the enantiomeric excess, as in the present
case, becomes significant. Such a feat is possible only when
the number of crystals and their clusters is limited, as
demonstrated in our controlled crystallization. Carefully
measuring the CD spectra for each of the crystal clusters
and weighing the crystals revealed that half of the samples are
homochiral, while an enantiomeric excess of at least 40% was
achieved for each of the remaining half of the 30 crystal-
lizations. In this case, it is possible that the achiral starting
materials can give 100% homochiral materials, unlike when
racemic compounds are crystallized; then, the maximum yield
1
2 samples featured a single cluster of crystals, while the rest
produced no more than five clusters. The CD spectra of these
0 samples reveal that each bulk sample was CD active
Figure 3b), suggesting chiral symmetry breaking. Further
3
(
crystallographic analysis revealed that every single crystal in
one particular cluster is of the same space group (see the
Supporting Information, Table S1). In other words, the
enantiomeric excess in that crystal cluster is 100%, and the
cluster is homochiral. Our results represent a rare experi-
mental verification of a crystallization event during which
every single crystal within the cluster crystallizes in the same
[
22]
is 50% of the total material.
When the pH 9.2 solutions underwent faster evaporation,
a large number of relatively small crystals were produced
(Figure 3c). Out of 30 crystallizations, only a few bulk
samples were CD active (Figure 3d). This result can be
understood as a consequence of an equilibrium shift toward
[
25]
enantiomeric form.
It should be noted that the specific
handedness of the crystals in any of the clusters cannot be
2
+
the dissociation of [Cu(NH ) ] ; a higher concentration of
3
4
2
+
Cu
conditions.
Additional evidence in support of the above analysis is
facilitates crystallization under otherwise identical
provided by analogous experiments using solutions of an
initial pH 8.7. The 30 crystallizations with fast ammonia
evaporation invariably resulted in crystals of 1 displaying very
similar crystallization behavior and CD spectra to those
prepared using the pH 8.3 mixture. Slow evaporation under
otherwise identical conditions led to a number of crystal
clusters (Figure 4a), some of which are CD active in the bulk
(Figure 4b). The crystallization behavior and optical proper-
ties of the bulk material are qualitatively comparable to those
observed for fast-evaporating pH 9.2 solutions.
On the basis of the above observations, it can be
concluded that both the pH value of the reaction mixture
and the ammonia evaporation rate are indeed critical for the
crystallization behavior of 1, with a high initial pH value and
slow evaporation of ammonia favoring products of high
enantiomeric excess values. On the other hand, the absolute
configuration of the crystals in a particular cluster cannot be
Figure 3. Crystallization using pH 9.2 solutions. a) The production of a
small number of crystal clusters of 1 with slow evaporation of the
reaction mixture. b) The solid-state CD spectra of bulk samples from
3
0 different slow crystallizations. The positive and negative values
correspond to the right- and left-handed helices, respectively. c) The
production of a large number of crystals (the majority not being in
clusterfo rm ) of 1 with faster evaporation of the reaction mixture.
d) The solid-state CD spectra of bulk samples from 30 different fast
crystallizations.
Figure 4. Crystallization using pH 8.7 solutions. a) The production of a
relatively small number of crystal clusters of 1 with slow evaporation
of the reaction mixture. b) The solid-state CD spectra of bulk samples
from 30 different crystallizations.
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ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
8477

Communications
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[{Cu(nitrilotriacetate)Na(H O)} ]
(Supporting
2
n
[
27]
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S1c,d,e) in their respective homochiral forms.
To summarize, we have synthesized homochiral coordi-
nation polymers by chemically manipulating the statistical
fluctuation inherent to the formation of enantiomeric pairs of
such materials. This synthesis was accomplished by moderat-
ing the concentration of a competing ligand in the reaction
mixture to control the kinetics of product crystallization. By
limiting the number of crystallization events to just a few, the
probability of attaining only the left- or right-handed helical
structure is significantly skewed, and in an ideal case, only one
enantiomeric form is obtained. Our successful syntheses of
three different coordination polymers in their homochiral
forms suggest the general applicability of this statistically
controlled nucleation and crystallization approach to chiral
symmetry breaking. In a broader perspective, any new
approaches to obtain enantiomerically pure materials are of
fundamental importance to the application of such materials
in chemical and pharmaceutical industries and, more impor-
tantly, may contribute to the ultimate understanding of the
origin of life. Thus, further development and application of
this straightforward strategy to chiral symmetry breaking
warrant an optimistic outlook.
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H20CuN O ] (1) can be
2 8 n
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Angewandte
Chemie
[
[
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n
crystallographic information on both the left- and right-handed
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2
n
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Angew. Chem. Int. Ed. 2007, 46, 8475 –8479
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