A ligand-conformation driving chiral generation and symmetry-breaking crystallization of a zinc(ii) organoarsonate

Article abstract of DOI:10.1039/c1cc12914g

An unusual ligand-conformation driving chiral generation and symmetry-breaking crystallization occurred simultaneously in the formation of a layered zinc(ii) arsonate Zn(Hcapa)(4,4′-bipy) (1P) and its enantiomorph (1M) without any chiral sources, indicati

Full text of DOI:10.1039/c1cc12914g

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COMMUNICATION
A ligand-conformation driving chiral generation and symmetry-breaking
crystallization of a zinc(II) organoarsonatew
Tian-Hua Zhou, Jian Zhang, Hai-Xia Zhang, Rui Feng and Jiang-Gao Mao*
Received 18th May 2011, Accepted 27th June 2011
DOI: 10.1039/c1cc12914g
An unusual ligand-conformation driving chiral generation and
symmetry-breaking crystallization occurred simultaneously in
the formation of a layered zinc(II) arsonate Zn(Hcapa)(4,40-bipy)
(1P) and its enantiomorph (1M) without any chiral sources,
indicating that the asymmetrical crystallization of the coordination
polymer from achiral precursors may be induced by the confor-
mation control of the ligand.
more and more important for the understanding of chirality
and such symmetry breaking phenomenon.
In this work, for the first time we have identified an unusual
phenomenon about the ligand-conformation driving chiral
generation and symmetry-breaking crystallization of a layered
zinc(II) arsonate, namely, Zn(Hcapa)(4,40-bipy) (1P, H3capa =
2-(carboxymethylamino)phenylarsonic acid;9 4,40-bipy
=
4,40-bipyridine) and its enantiomorph (1M). Also for the first
time the chiral metal–organic arsonates have been obtained by
spontaneous asymmetrical crystallization in the absence of any
chiral sources.
Chirality is an essential feature of life, which influences many
fields such as pharmaceutics, the chemical industry, agriculture
and clinical analysis.1 However, chiral generation is still a
mystery. Recently, the construction of homochiral (or enantio-
enriched) metal–organic framework materials has attracted
much attention because of their intriguing potential applications
in enantioselective separation and asymmetric catalysis.2
Three common strategies have been employed to synthesize
homochiral (or enantioenriched) materials. The first choice is
the assembly of enantiopure building units. So far, many
enantiopure organic ligands have been used to construct
homochiral frameworks.3,4 The second and third approaches
are based on the asymmetric crystallization of achiral precursors,
but they depend on the random and controllable symmetry
breaking, respectively. The controllable symmetry breaking
crystallization of chiral solid-state materials was recently
developed by Morris and Bu.5 To achieve homochirality or
enantiomeric excess (ee) from achiral precursors, an additional
enantiopure source is often used as an inductive reagent, such
as a chiral catalyst, template, or solvent.6
Yellowish needle crystals of compound 1P (or 1M) were
hydrothermally synthesized from a mixture of Zn(NO3)2ꢀ6H2O,
H3capa and 4,40-bipy in a 4.5 : 1 : 4 molar ratio in 6 mL of H2O
at 85 1C for 48 h. Compound 1P crystallizes in the chiral space
group P21 with one unique zinc(II) ion, one Hcapa2ꢁ ion, and
one 4,40-bipy ligand in its asymmetric unit (Fig. S1, ESIw). The
Zn2+ ion is five-coordinated in a distorted trigonal-bipyramidal
coordination geometry consisting of two nitrogen atoms from
two 4,40-bipy ligands, two arsonate oxygen atoms from two
different Hcapa2ꢁ ions, and one carboxylate oxygen atom
from the third Hcapa2ꢁ ion. The Hcapa2ꢁ ion is tridentate
and bridges with three Zn(II) ions. The Zn–O and Zn–N bond
lengths fall in the ranges of 1.960(4)–1.997(3) and 2.243(3)–
2.245(3) A, respectively, which are comparable to those
reported for the other zinc(II) complexes reported previously.
It is notable that each Zn(II) ion, ligated by two twisted
bidentate 4,40-bipy and three asymmetric Hcapa2ꢁ ligands in
a twisted conformation, becomes an asymmetric center. The
interconnection of neighboring Zn(II) ions by the arsonate
groups of the Hcapa2ꢁ ligands resulted in a right-handed 21
helical inorganic chain along the b-axis (Fig. 1a). Such neighboring
chains with the same handedness are further cross-linked by
4,40-bipy into a 2D layer (Fig. 1b).
Comparatively, random symmetry breaking crystallization
without any chiral sources is particularly rare, because the
spontaneous resolution process usually resulted in a mixture of
crystals with opposite handedness or racemic twins (referred to
be a racemic conglomerate).7
Previous research indicated that some supramolecular inter-
actions, such as hydrogen bond or p–p interactions, may be
the driving force and contribute to the occurrence of symmetry
breaking.8 Thus, clarifying that the driving force becomes
State Key Laboratory of Structural Chemistry, Fujian Institute of
Research on the Structure of Matter, Chinese Academy of Sciences,
Fuzhou 350002, PR China. E-mail: mjg@fjirsm.ac.cn
w Electronic supplementary information (ESI) available: Synthesis,
characterizations and additional structural data. CCDC 825906 and
825907. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c1cc12914g.
In order to ascertain the spontaneous resolution process of
this compound, 11 crystals from the same reaction were randomly
picked and refined using single-crystal X-ray diffraction data
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8862 Chem. Commun., 2011, 47, 8862–8864
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Fig. 3 Two mirror-related conformations of the Hcapa ligand: form
A in 1M (a, c) and form B in 1P (b, d).
Fig. 1 (a) A space-filling view of the right-handed helical chains of
–O–As–O–Zn–O– along the b-axis. (b) The connection of helical
chains by 4,40-bipyridine ligands generates a layered structure of 1P
along the c-axis. As, C, O, N and Zn atoms are represented by pink,
black, red, blue and cyan circles, respectively.
A combination of structure analyses and solid-state CD
spectra reveals that spontaneous chiral symmetry breaking
occurred during the course of crystallization. The resulting
enantiomeric excess of compound 1 represents an unusual
example of spontaneous resolution. Obviously, the generation
of chirality is associated with the flexible asymmetric ligand,
the distorted coordination geometry and the twisted 4,40-bipy.
The cooperative effect of the dual-ligands could contribute to
such an unusual symmetry breaking phenomenon.
with the same set of atomic coordinates. The Flack parameter
from each refinement indicates that all of them belong to the
P21 space group, the Flack parameters of 8 crystals are close
to zero whereas those of other 3 are close to one (Table S1,
ESIw). The results demonstrate that the product is enantio-
enriched with crystals containing right-handed helical chains
(1P > 1M).
Remarkably, the conformation of the Hcapa ligand plays a
crucial role in the chiral generation and symmetry breaking.
The coordination of the Hcapa ligand to the Zn(II) centers
makes two mirror-related conformations as shown in Fig. 3.
An intramolecular N–Hꢀ ꢀ ꢀO hydrogen bond between the
amine group and the arsonate oxygen atom contributed
to the formation of such conformations. Interestingly, two
conformations of the Hcapa ligand have been spontaneously
resolved in two independent crystals (1P with form B and 1M
with form A), respectively. Each conformation of Hcapa
displayed the molecular chirality and acted as an enantiopure
building unit, which led to the chiral generation in each single
crystal. The absolute conformation of Hcapa in the structure
also controlled the helicity of –O–As–O–Zn–O-type helices. It
is obvious that 1P with form A has parallel right-handed
21 helices, whereas 1M with form B has parallel left-handed
21 helices. Thus, the ligand-conformation is the primary driving
force for the chiral generation.
Another proof for the enantiomeric excess (ee) arises from
the solid-state circular dichroism (CD) measurements of nine
batches of the bulk sample obtained from repetitive syntheses.
A mixture of 0.7–1.3 mg sample and 35–65 mg dried KCl
powder was well-ground and then pressed into a disk for the
CD measurement with a MOS-450 spectropolarimeter. As
shown in Fig. 2, the results gave the expected Cotton effect,
positive (a) or negative (b), revealing the formation of either
1P or 1M enantiomeric excess (ee) in the bulk sample. An
enantiomeric excess of ca. 40% was evaluated for the bulk
sample with a positive CD signal. Furthermore, all of them
show a significant CD signal (8 positive and 1 negative) (Fig. S3,
ESIw). Thus, the Cotton effect displayed in the CD spectra
further demonstrates that the bulk sample is enantioenriched.
After answering the question about the chiral generation,
the next step is to find the reason for the observed enantio-
meric excess in the bulk sample. We have checked all starting
materials, and found that none of them has optical activity.
That meant no symmetry breaking at the starting stage and the
equal occurrence ratio for each conformation of the Hcapa
ligand. However, the first nucleation might break the balance
between two conformations. In the secondary nucleation, a
randomly generated parent crystal should clone a number of
secondary crystals that have the same crystal structure as the
parent crystal, as previously reported.10 Thus, the particular
handedness of the bulk depends on which handedness
is preferentially formed. Finally, unusual symmetry breaking
Fig. 2 Solid-state circular dichroism spectra of compound 1P (a) and
1M (b).
c
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temperature range of 230–490 1C, which corresponds to the
loss of 4,40-bipy and arsonate ligands, and a total weight loss
of ca. 69% at 667 1C (Fig. S6, ESIw).
In summary, we have successfully isolated two conformations
of a polydentate H3capa ligand in two enantiomorphic crystals
(1P and 1M), respectively. It is demonstrated that a ligand-
conformation controlled both the chiral generation and
asymmetrical crystallization, which allows us to further under-
stand the nature of chirality. This work also provides us with
useful clues to further explore the chiral metal–organic arsonates.
This work was supported by National Natural Science
Foundation of China (Nos. 20973170, 20825104 and
20821061), the major project from FJIRSM (SZD09001) and
973 Program (2011CB932504).
Fig. 4 Emission spectrum of compound 1 in the solid-state at room
temperature.
Notes and references
phenomenon occurs. Since the first nucleation may randomly
choose form A or B of the Hcapa ligand, either 1P or 1M
enantiomeric excess (ee) in the bulk sample is observed.
Anyway, this asymmetric crystallization is dependent on the
kind of conformation of Hcapa in the first nucleation, so the
ligand-conformation also becomes the main driving force for
symmetry breaking.
1 (a) E. L. Eliel and S. H. Wilen, Stereochemistry of Organic Compounds,
Wiley-Interscience, Weinheim, 1994; (b) A. N. Collins, G. N. Sheldrake
and J. Crosby, Chirality in Industry, vol. I and II, J. Wiley & Sons, New
York, 1992 and 1997.
2 (a) D. Bradshaw, T. J. Prior, E. J. Cussen, J. B. Claridge and
M. J. Rosseinsky, J. Am. Chem. Soc., 2004, 126, 6106–6114;
(b) Y. Liu, W. Xuan and Y. Cui, Adv. Mater., 2010, 22,
4112–4135; (c) L. Ma, C. Abney and W. Lin, Chem. Soc. Rev.,
2009, 38, 1248–1256; (d) W. Lin, MRS Bull., 2007, 32, 544–548;
(e) J. S. Seo, D. Whang, H. Lee, S. I. Jun, J. Oh, Y. J. Jeon and
K. Kim, Nature, 2000, 404, 982–986; (f) G. Ferey, Chem. Soc. Rev.,
2008, 37, 191–214.
3 (a) R. Vaidhyanathan, D. Bradshaw, J.-N. Rebilly, J. P. Barrio,
J. A. Gould, N. G. Berry and M. J. Rosseinsky, Angew. Chem., Int.
Ed., 2006, 45, 6495–6499; (b) E. V. Anokhina, Y. B. Go, Y. Lee,
T. Vogt and A. J. Jacobson, J. Am. Chem. Soc., 2006, 128,
9957–9962; (c) D. N. Dybtsev, A. L. Nuzhdin, H. Chun,
K. P. Bryliakov, E. P. Talsi, V. P. Fedin and K. Kim, Angew.
Chem., Int. Ed., 2006, 45, 916–920.
4 (a) O. R. Evans, H. L. Ngo and W. Lin, J. Am. Chem. Soc., 2001,
123, 10395–10396; (b) X. Lin, A. J. Blake, C. Wilson, X. Z. Sun,
N. R. Champness, M. W. George, P. Hubberstey, R. Mokaya and
Considering the excellent luminescent properties of Zn(II)
complexes, the solid state photoluminescent properties of this
compound have been investigated at room temperature. As
shown in Fig. 4, it exhibits an intense blue-green fluorescent
emission band with a peak maximum at l = 510 nm upon
excitation at l = 378 nm. The lifetime of the fluorescence was
measured to be B7.4 ns. To understand the nature of the
emission band, the photoluminescence properties of the free
H3capa and bipy ligands were also investigated. The free
H3capa ligand shows two fluorescent emission bands at
l = 438 and 483 nm upon excitation at 378 nm (Fig. S4,
ESIw), whereas the 4,40-bipy ligand exhibits a fluorescent
emission band at l = 437 nm (excited at 350 nm).11 Therefore,
the emission spectrum of compound 1 may be attributed to an
intraligand emission state.11 The enhancement of luminescence
may be attributed to the coordination of the bipy ligands and
H3capa to zinc ions, which effectively increases the rigidity of
the ligand and reduces the loss of energy via radiationless
decay of the intraligand emission excited state.11 The adoption
of a chiral space group by the compound prompts us to
investigate its second harmonic generation (SHG) properties.
SHG measurements on the powder samples sieved in the size
range of 80–100 mesh indicate that it exhibits SHG response
about 0.5 times that of the commercially used potassium
dihydrogen phosphate (KDP).12 The bulk purity of the sample
was confirmed by comparing its experimental powder X-ray
diffraction (PXRD) pattern to that calculated based on the
single crystal structure (Fig. S5, ESIw). Thermogravimetric
analysis indicates that the first weight loss occurred in the
M. Schroder, J. Am. Chem. Soc., 2006, 128, 10745–10753.
¨
5 R. E. Morris and X. Bu, Nat. Chem., 2010, 2, 353–361.
6 (a) Z. Lin, A. M. Z. Slawin and R. E. Morris, J. Am. Chem. Soc.,
2007, 129, 4880–4881; (b) J. Zhang, S. Chen, R. A. Nieto, T. Wu,
P. Feng and X. Bu, Angew. Chem., Int. Ed., 2010, 49, 1267–1270;
(c) J. Zhang, S. Chen, T. Wu, P. Feng and X. Bu, J. Am.
Chem. Soc., 2008, 130, 12882–12883; (d) Y. Kang, S. Chen,
F. Wang, J. Zhang and X. Bu, Chem. Commun., 2011, 47,
4950–4952.
7 (a) S. 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, 8475–8479; (b) T. Ezuhara, K. Endo and Y. Aoyama,
J. Am. Chem. Soc., 1999, 121, 3279–3283; (c) L. Perez-Garcia and
D. B. Amabilino, Chem. Soc. Rev., 2002, 31, 342–356.
8 L. Perez-Garcia and D. B. Amabilino, Chem. Soc. Rev., 2007, 36,
941–967.
9 R. E. Etzelmiller and C. S. Hamilton, J. Am. Chem. Soc., 1931, 53,
3085–3091.
10 D. K. Kondepudi, R. J. Kaufman and N. Singh, Science, 1990,
250, 975–976.
11 J.-L. Song, H.-H. Zhao, J.-G. Mao and K. R. Dunbar, Chem.
Mater., 2004, 16, 1884–1889.
12 S. K. Kurtz and T. T. Perry, J. Appl. Phys., 1968, 39, 3798.
c
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This journal is The Royal Society of Chemistry 2011