Particular Handedness Excess through Symmetry-Breaking Crystallization of a 3D Cobalt Phosphonate

Article abstract of DOI:10.1021/acs.inorgchem.5b02187

A 3D chiral cobalt phosphonate has been obtained from achiral precursors in the absence of chiral inducers. Remarkably, the bulk sample is largely enantio-enriched with particular handedness through symmetry-breaking crystallization in spite of multiple r

Full text of DOI:10.1021/acs.inorgchem.5b02187

Communication
pubs.acs.org/IC
Particular Handedness Excess through Symmetry-Breaking
Crystallization of a 3D Cobalt Phosphonate
Chao-Ying Gao,†,‡ Fei Wang,§ Hong-Rui Tian,† Lei-Jiao Li,† Jian Zhang,§ and Zhong-Ming Sun*,†,§
†State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,
5625 Renmin Street, Changchun, Jilin 130022, China
§State Key Laboratory of Structure Chemistry, Fujian Institute of Research on the Structure of Matter, CAS, Fuzhou, Fujian 350002,
China
‡University of Chinese Academy of Sciences, Beijing 100049, P. R. China
S
* Supporting Information
enantiopure phosphonate ligands,11 while examples character-
ABSTRACT: A 3D chiral cobalt phosphonate has been
obtained from achiral precursors in the absence of chiral
inducers. Remarkably, the bulk sample is largely enantio-
enriched with particular handedness through symmetry-
breaking crystallization in spite of multiple repeated
experiments. Moreover, protonation of this chiral material
introduces Brønsted acid sites, the structure of which is
unique to the heterogeneous phase for the ring opening of
epoxies.
ized as enantio-enriched through symmetry-breaking crystal-
lization are relative rare. As far as we know, Zheng et al. did
elegant work on the study of metal phosphonates12 and reported
the only two notable cases with enantiomeric excess accom-
plished by using asymmetry achiral ligands,12d,e the structures of
which feature chiral layering.
In this Communication, we identify an unusual phenomenon
that the bulk sample is largely enantio-enriched with particular
handedness through symmetry-breaking crystallization in spite
of multiple repeated experiments. The resulting compound
Co(H2L)(4,4′-bipy) ·H2O [1; H4L = thiophene-2-phosphonic
acid (Scheme S1) and 4,4′-bipy = 4,4′-bipyridine] presents a 3D
chiral metal phosphonate framework built from achiral
precursors in the absence of chiral sources. In addition,
compound 1 displays excellent catalytic activity due to
protonation of the phosphonate ligands.
A mixture of Co(NO3)2·H2O, H4L, and 4,4′-bipy in a 2:1:2
molar ratio in aqueous solution at 100 °C gives pink blocky
crystals of compound 1M (or 1P). The purity of as-synthesized 1
was confirmed by similarities between the simulated and
experimental powder X-ray diffraction (PXRD) patterns (Figure
S1). A single-crystal X-ray diffraction study showed that 1M
crystallizes in the chiral space group P21 with a Flack parameter of
0.021(1), indicating enantiomeric purity of the single crystals
despite the use of achiral reagents. Each asymmetric unit includes
one crystallographically independent cobalt(II) cation, one
protonated {H2L}2− anion, one 4,4′-bipy ligand, and one water
molecule (Figure S2). Each cobalt(II) center is five-coordinated
in a distorted trigonal-bipyramidal geometry environment with
two nitrogen atoms from two 4,4′-bipy ligands and three oxygen
atoms from three different phosphonic ligands in a monodentate
fashion. One phosphonate group of each {H2L}2− linker bonds
two cobalt atoms in a dimonodentate fashion, and the other
phosphonate group coordinates to one cobalt center in a
monodentate fashion. Such a connection results in an infinite 3D
(3,5)-connected hms net (Figures 1 and S3).13 It is worth noting
that each cobalt(II) ion, ligated by two nonplanar bidentate 4,4′-
bipy and three conformationally changed {H2L}2− ligands,
becomes an asymmetric center, and the interconnection of
he current increasing interest in chiral inorganic−organic
T
hybrid materials mainly stems from their enormous
potentials in asymmetric catalysis, nonlinear optics, and chiral
recognition.1 Despite its great importance, the generation of
chirality remains a great challenge. Great efforts have been made
to achieve chirality in solid materials. The straightforward way is
to employ enantiopure bridge2,3 or auxiliary ligands,4 during the
process of which the inherent chirality of the enantiopure ligand
can be imparted to the resultant sample. The other effective
strategy is to fall back on the help of an additional chiral source as
a chiral catalyst, a template, or a solvent,5,6 wherein the existence
of the chiral auxiliary ligand can affect the growth of nucleation in
the reaction system. However, both of the methodologies suffer
from limited availability and the very high cost of the enantiopure
ligands. Therefore, there is a highly desirable requirement to
obtain chiral materials from achiral precursors. Chirality can be
generated from achiral precursors,5a,7 deriving from the spatial
organization (e.g., helix) of achiral building blocks. It is necessary
to stress that the generation of chirality is not the purpose. The
greatest challenge is the creation of an enantiomeric bias to favor
one-handedness because the resultant sample obtained through
spontaneous resolution tends to be a conglomerate, defined as an
equal mixture of crystals with opposite handedness or racemic
twins.8 As mentioned above, realizing asymmetric chiral
amplification from achiral precursors through symmetry break-
ing still reamins one of the most challenging tasks, although there
have been several such examples.9
On the other hand, the chemistry of metal phosphonates has
been expanding rapidly in recent years because of their versatile
structural features and potential applications.10 Although a
variety of metal phosphonates have been reported, those with
chirality are very few, and most of them are built from
Received: September 23, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.5b02187
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Inorganic Chemistry
Communication
Figure 2. Solid-state CD spectra of 10 randomly selected crystals as well
as the bulk sample (labeled in bold red). Inset: solid-state UV−vis
spectra of compound 1 in BaSO4 plates at room temperature.
bulk sample of 1 is enantio-enriched with left-handed helical
chains (1M ≫ 1P).
Figure 1. (a) One set of the 3D framework in 1M along the b axis. A
space-filling view of the left-handed helical −Co−O−P−O−Co− chains
(b) and the −O−P−O···H−O−H···O−P−O− chains (c) as well as
their corresponding highlighted pitch. Hydrogen atoms bonded to
carbon atoms are omitted for clarity in part a.
As established in the former structure analyses, the origin of
the chirality of the single crystal is particular interesting because
there are no driving forces, such as enantiopure catalysts. In
addition, the unique feature of this work is that the bulk sample is
largely enantio-enriched with particular handedness in spite of
multiple repeated experiments. Considering that the impact of
hydrogen bonding has been discussed in previous reports,14 CD
measurement was performed on several desolvated crystals
(Figure S6) to gain further insight into the role of intermolecular
hydrogen-bonding interaction. The appearance of CD signals
from the desolvated samples indicates that hydrogen bonding is
not the key factor for the formation of chirality in this work.
Meanwhile, it is worth noting that the phosphonate ligand H4L
itself is rigid and symmetrical, while it becomes asymmetric due
to a tiny distortion angle upon bonding to a cobalt(II) ion
(Figure S10). Then the mirror between the inherent right- and
left-handed helices is broken once the conformation is twisted,
with the chirality emerging. Thus, the twisted conformation of
the {H2L}2− linker is the conclusive factor for the symmetry-
breaking phenomenon. The enantiomeric excess of particular
handedness in this work is suspected of being associated with the
cooperative effect from the dual ligands, the contribution of
which to symmetry-breaking crystallization has been observed
previously.7b The coordination bonds are strong, selective, and
directional for chirally discriminative interactions.8,9d,15 The
{H2L}2− linker might preferentially take the conformation
adopted in 1M, and then the seed effects the formation of left-
handedness of bulk crystals over right-handedness, leading to an
enantiomeric excess of particular handedness. Such an unusual
phenomenon is similar to that of homochirally biological helices
to a certain extent, such as a DNA molecule, amino acid, glucose,
and so on, the origin of which is a result of the combined action of
many factors and is still unknown to humans up to now. The
following work will be focused on exploration of the external
forces that trigger the conformational changes.
neighboring cobalt(II) ions results in a left-handed 21 helical
inorganic chain viewed along the (1,0,1) axis (Figure 1b). Lattice
water molecules are located among adjacent phosphonate groups
from different {H2L}2− linkers through strong hydrogen-
bonding interactions (Figure 1a). It is worth noting that the
hydrogen bonds exist not only between phosphonate oxygen
atoms and water molecules but also between neighboring
protonated and nonprotonated phosphonate groups, with O−
H···O lengths ranging from 2.582 to 3.024 Å (Figure S4). It is
especially interesting that the arrangement of lattice water
molecules produces a left-handed −O−P−O···H−O−H···O−
P−O− helical chain (Figure 1c).
In order to investigate the absolute configurations of
compound 1, 26 crystals were randomly picked, and the
corresponding crystal structures were refined by single-crystal
X-ray diffraction data with the same set of atomic coordinates.
The results reveal that all of them crystallize in space group P21,
and the Flack absolute structure parameter from each refinement
of 25 crystals is close to 0, whereas the other one is close to 1
(Table S2). It must be highlighted that 10 of the 26 crystals are
from one batch, 5 of them from another one, 3 of them from a
third one, and the other 8 crystals are from different reactors; it is
possible that the bulk sample is largely enantio-enriched with
particular handedness in spite of multiple repeated experiments.
In order to get a better insight into the chirality of 1, ultraviolet−
visible diffuse-reflectance (UV−vis DRS) and solid-state circular
dichroism (CD) spectra on 10 single crystals picked from
different reactor as well as a bulk sample from another
hydrothermal synthesis were measured. As shown in Figure 2,
the maximum absorption frequency (λmax = 260 nm) observed in
the solid-state UV experiment is parallel to the dichroic signals
(λmax = 260−270 nm) in the CD spectra, indicating that the bulk
sample of 1 is enantio-enriched. Remarkably, all of the results of
the CD spectra show a significant signal with nine negative and
one positive from 10 single crystals and one negative from the
bulk sample. Both the Cotton effect displayed in the CD spectra
and the structural refinements of 26 crystals demonstrate that the
Considering the protonation of the two phosphonate groups
per {H2L}2− anion, there is a possibility of the introduction of
Brønsted acid sites, and the catalytic performance of 1 was
investigated in the ring-opening reaction of styrene oxide with
methanol at 60 °C. The only reaction product, 2-methoxy-2-
phenylethanol, was obtained with conversion and selectivity over
99% after 2.5 h, indicating that compound 1 possesses excellent
B
DOI: 10.1021/acs.inorgchem.5b02187
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
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the absence of the catalyst under similar conditions, and such
values of the activity and selectivity are high among the metal−
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In conclusion, herein we demonstrate an unusual phenomen-
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particular handedness through symmetry-breaking crystallization
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solid-state CD spectroscopy. Moreover, protonation of this
chiral material introduces Brønsted acid sites, the structure of
which is unique to the heterogeneous phase for the ring opening
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.5b02187.
Experimental details, additional structural figures, crys-
tallographic refinement details, photographs of crystals,
PXRD, catalytic details, thermogravimetric analysis curve,
and IR spectra (PDF)
CCDC 1418906 (CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: szm@ciac.ac.cn. Web site: http://zhongmingsun.
weebly.com/.
■
(12) (a) Zeng, D.; Ren, M.; Bao, S.-S.; Feng, J.-S.; Li, L.; Zheng, L.-M.
Chem. Commun. 2015, 51, 2649−2652. (b) Liu, X.-G.; Bao, S.-S.;
Huang, J.; Otsubo, K.; Feng, J.-S.; Ren, M.; Hu, F.-C.; Sun, Z.; Zheng, L.-
M.; Wei, S.; Kitagawa, H. Chem. Commun. 2015, 51, 15141−15144.
(c) Nie, W.-X.; Bao, S.-S.; Zeng, D.; Guo, L.-R.; Zheng, L.-M. Chem.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors are grateful for financial aid from the National
Natural Science Foundation of China (Grants 21571171,
U1407101, and 51402286).
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Soc. 2004, 126, 1419−1429.
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DOI: 10.1021/acs.inorgchem.5b02187
Inorg. Chem. XXXX, XXX, XXX−XXX