A homochiral luminescent 2D porous coordination polymer with collagen-type triple helices showing selective guest inclusion

Article abstract of DOI:10.1021/ic2025389

A homochiral luminescent porous coordination polymer, [Cd(L)(H ₂O)]3H₂O, with interconnected collagen like triple-helical chains has been synthesized solvothermally by using cadmium(II) salt and a newly designed d-isosorbide-based, e

Full text of DOI:10.1021/ic2025389

Article
pubs.acs.org/IC
A Homochiral Luminescent 2D Porous Coordination Polymer with
Collagen-Type Triple Helices Showing Selective Guest Inclusion
Biplab Joarder, Abhijeet K. Chaudhari, and Sujit K. Ghosh*
Department of Chemistry, Indian Institute of Science Education and Research (IISER), Pashan, Pune, Maharashtra 411021, India
S
* Supporting Information
ABSTRACT: A homochiral luminescent porous coordination polymer,
[Cd(L)(H2O)]·3H2O, with interconnected collagen like triple-helical
chains has been synthesized solvothermally by using cadmium(II) salt and
a newly designed D-isosorbide-based, enantiomerically pure chiral ligand.
The framework is a 2D porous material and forms a 1D channel along the
a axis, with the channel dimensions ∼6.2 × 4.4 Å2. The compound has
high selectivity in the uptake of water and methanol over other solvents
(e.g., tetrahydrofuran, ethanol, benzene, and cyclohexane) inside the
channels.
homochiral porous materials, where the use of enantiopure
ligands leads to generation of a single enantiomeric product.11
D-Isosorbide is an accessible enantiopure small molecule
derived from glucose, and it and its different derivatives have
therapeutic uses. Like many other moieties containing hydroxyl
groups, modifying the hydroxyl groups of D-isosorbide by
organic moieties with proper functional groups can be one of
the most effective ways to obtain enantiopure organic ligands.
Herein, by attaching a benzoic acid group in a para position
with the hydroxyl groups of D-isosorbide, we have synthesized a
dicarboxylic acid-based ligand LH2. We obtained a porous two-
dimensional (2D) homochiral compound [Cd(L)-
(H2O)]·3H2O (1) by the self-assembly of LH2 with cadmium-
(II) by a solvothermal method. To the best of our knowledge,
D-isosorbide-based enantiopure ligands have not been used to
synthesize coordination polymers.
INTRODUCTION
■
Homochirality and helicity are ubiquitous in nature and are
essential to various biological functions. Helices are also the
central structural motifs in biological molecules, like α-helical
protein, DNA, and collagens are with single-, double-, and
triple-helix structures, respectively. All of these helical
biopolymers are composed of either the D or L form of basic
units and always acquire either left- or right-handedness.
Structural studies of helicate compounds with chiral building
blocks may help to understand such kinds of unique structural
behavior of natural chiral helical biopolymers.1,2
Among different types of chiral compounds, in recent years,
homochiral coordination polymers or metal−organic frame-
works (MOFs)3 have attracted much attention because of their
potential applications in enantioselective catalysis and separa-
tion4−6 and the importance of chirality in biological processes.2
Because of the lack of chiral porous zeolites, homochiral porous
coordination polymers (PCPs) are particularly very attractive
materials for industrial applications for the separation of chiral
mixtures and as heterogeneous asymmetric catalysts for the
economical production of optically active organic compounds.
Although numerous examples of porous, achiral PCPs with
extremely high surface areas are reported,7,8 the rational design
and synthesis of homochiral porous materials still remains a
challenge. Such materials are generally prepared by self-
assembly process using three synthetic strategies: (1)
spontaneous resolution, (2) chiral induction, and (3) use of
enantiopure ligands. In the first method, achiral or racemic
ligands form homochiral materials by spontaneous resolution
during the self-assembly process, but mostly bulk racemates
result with only a few exceptions.9 In the second method, an
auxiliary chiral ligand induces homochirality into the framework
without direct involvement in structure formation.3b,10 These
two methods are quite unpredictable because their mechanisms
are not yet very clear. The use of enantiopure ligands, on the
other hand, is a direct and effective method for the synthesis of
EXPERIMENTAL SECTION
■
Materials and Measurements. All of the reagents and solvents
were commercially available and used without further purification.
Fourier transform infrared (FTIR) spectra (Figure S8 in the
Supporting Information) were recorded on a Nicolet 6700 FTIR
spectrophotometer using KBr pellets. Powder X-ray diffraction
(PXRD) patterns were measured on a Bruker D8 Advanced X-ray
diffractometer at room temperature (RT) using Cu Kα radiation (λ =
1.5406 Å). Thermogravimetric analysis (TGA) was recorded on a
Perkin-Elmer STA 6000 TGA analyzer under a N2 atmosphere with a
heating rate of 10 °C/min.
Synthesis of Ligand L′. D-Isosorbide (10 g, 0.0684 mol) and NaH
(3.45 g, 0.1436 mol) were mixed in N,N-dimethylformamide (DMF;
120 mL) solvent under a N2 atmosphere. Then the reaction mixture
was stirred at 75 °C for 10 min. After that, 4-(fluoroethyl)benzoate
(23.01 g, 0.1368 mol) was added dropwise. The reaction was
continued for 3 h. After cooling to RT, excess NaH was quenched with
ice and ethyl acetate. A maximum amount of DMF was removed by
Received: November 24, 2011
Published: March 26, 2012
© 2012 American Chemical Society
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rotary evaporation under reduced pressure, and the mixture was
extracted with EtOAc (2 × 250 mL). The combined organic layer was
washed with 150 mL of brine, dried over anhydrous sodium sulfate,
and concentrated under reduced pressure to get a crude product,
which was purified on silica gel (100 and 200 mesh) column
chromatography.
Table 1. Crystal Data and Structure Refinement for
Compound 1
identification code
empirical formula
fw
compound 1
C20H24CdO12
568.79
Yield: 15.28 g, 50.52%. 1H NMR (400 MHz, CDCl3): δ 1.39 (dt, J =
1.36 and 7.32 Hz, 6H), 4.06 (d, J = 5.48 Hz, 2H), 4.10 (d, J = 10.52
Hz, 1H), 4.20 (dd, J = 4.12 and 10.56 Hz, 1H), 4.35 (q, J = 7.18 Hz,
4H), 4.67 (d, J = 5.04 Hz, 1H), 4.29−4.87 (m, 2H), 5.04 (t, J = 5.26
Hz, 1H), 6.95−7.01 (m, 4H), 8.00−8.03 (m, 4H). HRMS. Calcd for
L′Na ([M + Na]+): 465.46. Found: 465.1525 (Figures S1 and S2 in the
Supporting Information).
temperature (K)
wavelength (Å)
cryst syst
200(2)
0.71073
orthorhombic
C2221
space group
unit cell dimens
a = 7.3160(5) Å
b = 26.4953(19) Å
c = 24.2927(17) Å
4708.9(6)
8
volume (Å3)
Z
Scheme 1
density (calcd) (Mg/m3)
1.605
abs coeff (mm−1
)
0.988
F(000)
cryst size (mm3)
2304
0.15 × 0.14 × 0.10
θ range for data collection (deg) 1.75−28.69
index ranges
−7 ≤ h ≤ 9, −35 ≤ k ≤ 33, −25 ≤ l ≤ 32
13528
reflns collected
indep reflns
5550 [R(int) = 0.0658]
99.9%
completeness to θ = 28.69°
abs correction
semiempirical from equivalents
0.9077 and 0.8660
full-matrix least squares on F2
5550/0/294
max and min transmn
refinement method
data/restraints/param
GOF on F2
1.125
Synthesis of Ligand LH2 (Scheme 1). L′ (15.28 g, 0.035 mol)
was dissolved in 250 mL of methanol (MeOH) and 150 mL of water
(H2O). Thereafter, LiOH·H2O (8.82 g, 0.21 mol) was added at 0 °C.
The reaction mixture was stirred overnight. Excess MeOH was
removed under reduced pressure and acidified with diluted HCl to get
the desired product. The product was filtered, washed with fresh H2O,
and dried under vacuum.
final R indices [I > 2σ(I)]
R indices (all data)
absolute structure param
R1 = 0.0659, wR2 = 0.1695
R1 = 0.0693, wR2 = 0.1715
0.00(6)
largest diff peak and hole (e/Å3) 1.511 and −2.065
Japan). All of the gases used were of 99.999% purity. An as-
synthesized compound was heated at 160 °C under vacuum for 16 h to
get guest-free compound 1D. Prior to adsorption measurement, the
guest-free sample 1D was pretreated at 150 °C under vacuum for 5 h
using BelPrepvacII and purged with N2 upon cooling.
Yield: 9.28 g, 69.8%. 1H NMR (400 MHz, DMSO-d6): δ 3.90−3.93
(m, 2H), 4.005 (dd, J = 4.16 and 11 Hz, 2H), 4.58 (d, J = 4.12 Hz,
1H), 5.01−5.05 (m, 3H), 7.04 (d, J = 8.24 Hz, 2H), 7.09 (d, J = 8.68
Hz, 2H), 7.86 (d, J = 8.68 Hz, 2H), 7.91 (d, J = 8.72 Hz, 2H) (Figure
S2 in the Supporting Information).
RESULTS AND DISCUSSION
■
Synthesis of [Cd(L)(H2O)]·3H2O (1). Single crystals of 1 were
prepared by reacting 0.125 mmol of Cd(NO3)2·4H2O, 0.125 mmol of
ligand (LH2), and 0.2 mmol of NaOMe in 3 mL of H2O and 2 mL of
tetrahydrofuran (THF) by a solvothermal technique, in a Teflon-lined
autoclave. The autoclave was heated under autogenous pressure at 160
°C for 3 days and then cooled to RT for a 24 h period. Upon cooling
to RT, the desired product appeared in ∼45% yield.
X-ray Structural Studies. Single-crystal X-ray data of compound
1 were collected at 200 K on a Bruker Kappa APEX II CCD Duo
diffractometer (operated at 1500 W power, 50 kV, 30 mA) using
graphite-monochromatized Mo Kα radiation (λ = 0.71073 Å). The
crystal was on nylon CryoLoops (Hampton Research) with Paraton-N
(Hampton Research). Data integration and reduction were processed
with SAINT15 software. A multiscan absorption correction was applied
to the collected reflections. The structure was solved by direct method
using SHELXTL16 and was refined on F2 by a full-matrix least-squares
technique using the SHELXL-9717 program package within the
WINGX18 program. All non-H atoms were refined anisotropically.
All H atoms were located in successive difference Fourier maps, and
they were treated as riding atoms using SHELXL default parameters.
The structures were examined using the Adsym subroutine of
PLATON19 to ensure that no additional symmetry could be applied
to the models. Tables 1 and 2 contain crystallographic data for
compound 1.
Compound 1 was synthesized by a solvothermal reaction of
LH2 and Cd(NO3)2·4H2O in a THF/H2O mixture solvent and
characterized by a single-crystal X-ray diffraction (SC-XRD)
technique. The phase purity of as-synthesized compound 1 was
confirmed by PXRD measurement. SC-XRD analysis revealed
that compound 1 crystallizes into a chiral orthorhombic crystal
system, space group C2221 with a Flack parameter of 0.00(6),
indicating enantiomeric purity of the single crystals. Circular
dichroism (CD) measurement (Figure S5 in the Supporting
Information) and NMR of LH2, together with NMR of the
digested MOF (Figures S3 and S4 in the Supporting
Information), show that racemization of the chiral ligand
does not occur under solvothermal conditions. The asymmetric
unit of 1 consists of one L, two CdII ions with half-occupancy,
and four H2O molecules (one-coordinated and three-non-
coordinated). One cadmium (Cd1) has an octahedral
coordination environment with an O6 donor set from two
coordinated H2O molecules and four carboxylate O atoms from
four different ligands. The other cadmium (Cd2) also has an
octahedral geometry but in distorted shape with an O6 donor
set from two monodentate and two bidentate carboxylate
groups from four different ligands. Cd−O distances are in the
ranges of 2.242−2.276 Å for Cd1 and 2.259−2.427 Å for Cd2.
Low-Pressure Gas Sorption Measurements. Low-pressure gas
sorption measurements were performed using BelSorpmax (Bel
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Table 2. Selected Bond Lengths [Å] and Angles [deg] for
a
Compound1
Cd1−O7#2
Cd1−O7#3
Cd1−O1#4
Cd1−O1
Cd1−O9#4
Cd1−O9
2.242(5)
2.242(5)
2.260(7)
2.260(7)
2.276(7)
2.276(7)
Cd2−O2#2
Cd2−O2
Cd2−O8#2
Cd2−O8
2.259(5)
2.259(5)
2.315(6)
2.315(6)
2.427(5)
2.427(5)
2.242(5)
2.242(5)
Cd2−O7
Cd2−O7#2
O7−Cd1#6
O7−Cd1#6
O7#2−Cd1−O7#3
O7#2−Cd1−O1#4
O7#3−Cd1−O1#4
O7#2−Cd1−O1
O7#3−Cd1−O1
O1#4−Cd1−O1
O7#2−Cd1−O9#4
O7#3−Cd1−O9#4
O1#4−Cd1−O9#4
O1−Cd1−O9#4
O7#2−Cd1−O9
O7#3−Cd1−O9
O1#4−Cd1−O9
O1−Cd1−O9
90.7(3)
98.6(2)
82.1(2)
82.1(2)
98.6(2)
178.9(3)
172.0(3)
89.4(2)
89.2(3)
90.0(3)
89.4(2)
172.0(3)
90.0(3)
89.2(3)
91.7(4)
O2#2−Cd2−O2
O2#2−Cd2−O8#2
O2−Cd2−O8#2
O2#2−Cd2−O8
O2−Cd2−O8
O8#2−Cd2−O8
O2#2−Cd2−O7
O2−Cd2−O7
81.7(3)
93.1(2)
163.8(2)
163.8(2)
93.1(2)
95.9(3)
108.4(2)
89.4(2)
O8#2−Cd2−O7
O8−Cd2−O7
106.74(19)
56.02(19)
89.4(2)
Figure 1. (a) Packing diagram of compound 1 along the bc plane. (b)
View of the 1D channel running along the a axis shown. (c)
Coordination environment around the metal centers showing two
different types of CdII in the metal carboxylate chain. (Color code:
cadmium, yellow; carbon, gray; oxygen, red. H atoms and free solvents
are omitted for clarity).
O2#2−Cd2−O7#2
O2−Cd2−O7#2
O8#2−Cd2−O7#2
O8−Cd2−O7#2
O7−Cd2−O7#2
Cd1#6−O7−Cd2
108.4(2)
56.02(19)
106.74(19)
156.7(3)
109.5(2)
O9#4−Cd1−O9
a
Symmetry transformations used to generate equivalent atoms: #1, x
− 3/2, y − 1/2, z; #2, −x, y, −z + 1/2; #3, x + 1, y, z; #4, −x + 1, y, −z +
1/2; #5, x + /2, y + /2, z; #6, x − 1, y, z.
3
1
One of the two O atoms of the bidentate carboxylate groups
coordinated to Cd2 is bridged with the other cadmium (Cd1)
from both sides, thus extending the metal carboxylate chain
along the a axis. Because of the roof-type arrangement of the D-
isosorbide core and the flexibility in the ether moiety, the
dicarboxylate ligand could arrange itself into a V shape. This V-
shaped ligand coordinated to the cadmium ions at both ends
with carboxylate O atoms, and this subsequently resulted in a
2D porous coordination framework (Figures 1 and S6 and S7
in the Supporting Information).
The most interesting feature of the structure is its triple-
helical12 arrangement of the framework, very similar to the
triple-helical structure of a group of biologically important
protein molecules called collagen. Each V-shaped dicarboxylate-
based ligand connected to CdII at both ends has resulted in a
left-handed helical strand with a pitch of 14.632 Å and a width
of ca. 13 Å. It is noteworthy that the pitch of this helix is even
greater than the pitch of most open protein helices, i.e.,
polyproline (9.36 Å).13 Three such identical helical chains are
extended along the a axis to form a left-handed triple-helical
structure (Figure 2). This structure has a very good
resemblance to the structures of collagen types II, III, VII,
VIII, and X, where three identical α chains (homotrimers) form
right-handed triple-helical supramolecular structures.14 These
helices interconnect each other and also by both sides edge
sharing at CdII with two identical triple helices result in the 2D
homochiral framework.
Figure 2. (a) Structure of collagen. (b) Space-filling views of the left-
handed single-helical chain and (c) triple-helical chains of compound
1.
positive Cotton effect at 297.95 and 291.03 nm, respectively,
revealing its homochiral nature. The enantiopure nature of
compound 1 also supports the SC-XRD structure with chiral
space group C2221 and a Flack parameter value of 0.00(6).
These results also indicate that there is no racemization of the
ligand during solvothermal synthesis, and the chiral feature
transfers from the original chiral ligand to the framework.
The resultant 2D framework is a porous sheetlike
architecture and forms a 1D channel along the a axis within
the 2D sheets, with maximum channel dimensions ∼6.2 × 4.4
Å2 (the channel size is measured by considering the van der
Waals radii for constituting atoms). The cavities of the
framework are occupied by disordered H2O molecules.
The results of solid-state CD spectral measurements in the
wavelength range 400−250 nm confirmed the enantiopure
nature of the ligand LH2 and compound 1. As shown in Figure
3, the ligand and bulk crystals of 1 in powder form show a
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but the PXRD pattern (Figure 4c) of the desolvated compound
is little different from that of the as-synthesized compound.
This indicates that the desolvated structure is not exactly the
same as the as-synthesized compound, and most probably the
pore size is squeezing after removal of the guest molecules. It is
well-known in the literature and we also observed recently in
one of our compounds that it is not always only the pore size
but also host−guest interaction that decides the amount of
adsorbate taken by the MOF.21,22 CO2 has a smaller kinetic
diameter and a larger quadrupole moment value than N2, so it
can interact more strongly with the surface of the pore in the
host framework, thus showing little uptake and hysteresis
behavior. Interesting phenomena were observed in solvent
sorption experiments at 298 K. Selective sorption of H2O23−27
and MeOH was observed over many other organic solvents
[ethanol (EtOH), THF, benzene, and cyclohexane; Figures 5
Figure 3. Solid-state CD spectra of (a) compound 1 and (b) ligand
LH2.
PLATON20 analysis revealed that the 2D porous structure
contains voids of 1035.5 Å3 that represent 22% per unit cell
volume.
TGA analysis revealed that the framework is stable up to 400
°C (Figure S9 in the Supporting Information). The desolvated
phase (1D) was generated by heating the compound for 16 h at
160 °C under reduced pressure. TGA data confirm the removal
of guest H2O molecules. Very similar PXRD data of 1D with
the as-synthesized compound 1 suggest that the framework is
quite rigid after removal of coordinated H2O molecules. A
slight flexible behavior of the framework was observed after
removal of the guest H2O molecules, as we can see from slight
variations in the PXRD pattern of 1D. To test the reversibility
of the transformation, we exposed 1D to H2O vapor for 3 days;
the desolvated phase transformed to its original structure,
which was confirmed by PXRD data (Figure 4).
To check the porous properties of the 2D chiral framework
(1D), we carried out gas and solvent sorption measurements. It
shows no uptake of N2 (1.62 cm3/g) but some uptake for CO2
(13.54 cm3/g) (Figure S10 in the Supporting Information).
The pore size of the as-synthesized compound is 6.2 × 4.4 Å2,
Figure 5. Solvent sorption graphs of compound 1D. (Color code:
H2O, wine; MeOH, royal blue; EtOH, pink; THF, gray; cyclohexane,
dark cyan; benzene, orange).
and S11 in the Supporting Information]. Very small amounts of
other organic solvent sorption compared to H2O and MeOH
are desirable for removal of H2O and MeOH from organic
solvents. After removal of coordinated H2O molecules, the
framework now has open metal sites and also the pore is
hydrophilic in nature because of the presence of several O
atoms on the pore surface. Although the pore size of the
desolvated phase may not be big enough to enter guest
molecules into the pore (as we can see from PXRD, the
structure changed and most probably squeezed after removal of
the coordinated guest molecules present in the pore) but
because of the hydrophilic nature of the pore surface and open
metal sites, H2O and MeOH molecules strongly interact with
the surface and so enter into the pore, which is also supported
by the flexible nature of the ligand. Also, because of the strong
interaction of H2O and MeOH with the pore surface,
desorption becomes very difficult and therefore hysteresis
sorption profiles are shown.28
Figure 4. (a) Simulated, (b) as-synthesized, (c) desolvated, and (d)
resolvated PXRD patterns of compound 1.
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Luminescent properties of compound 1 (d10 metal complex)
were examined at RT by solid-state fluorescence spectra
(Figure 6). The intense fluorescence spectrum was observed at
ACKNOWLEDGMENTS
■
We thank IISER Pune and the Director Prof. K. N. Ganesh for
financial support and encouragement. S.K.G. is grateful to DAE
(Project 2011/20/37C/06/BRNS) for financial support.
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363 and 354 nm (upon excitation at 281 nm) for compounds 1
and 1D, respectively. A luminescent spectrum of LH2 was also
recorded under the same conditions. Ligand LH2 exhibits a
fluorescent band at 402 nm (upon excitation on 291 nm). The
shift of the emission spectra from 402 to 363 nm upon
complexation is due to the deprotonation and the chelating
coordination action of the carboxylate anion. The slight shift of
the spectrum for the desolvated compound from the as-
synthesized compound may be due to the change in the
coordination environment.
In conclusion, we have designed one D-isosorbide-based
enantiopure dicarboxylic acid based ligand, and with CdII, a
luminescent homochiral porous framework has been synthe-
sized solvothermally and characterized. The framework consists
of interconnected triple-helical chains, very similar to the triple-
helical structure of a group of biologically important protein
molecules called “collagen”. Enantiopurity of the new ligand
and its cadmium(II) framework has been checked by CD
measurements. The porous framework is very stable even after
removal of the guest molecules, which shows size-selective
inclusion of H2O and MeOH over other solvents (e.g., THF,
ethanol, benzene, and cyclohexane) inside the channels.
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ASSOCIATED CONTENT
■
S
* Supporting Information
HRMS, NMR, and FTIR spectra, TGA, sorption isotherms, and
X-ray crystallographic data of compound 1 in CIF format. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: sghosh@iiserpune.ac.in. Tel.: +91-20-2590 8076. Fax:
+91-20-2590 8186.
Notes
(24) Serre, C.; Millange, F.; Thouvenot, C.; Nogues, M.; Marsolier,
The authors declare no competing financial interest.
G.; Louer, D.; Fer
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ey, G. J. Am. Chem. Soc. 2002, 124, 13519.
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