Two homochiral bimetallic metal-organic frameworks composed of a paramagnetic metalloligand and chiral camphorates: Multifunctional properties of sorption, magnetism, and enantioselective separation

Article abstract of DOI:10.1021/cg501332u

Two porous metal-organic frameworks [Co(Tt)₂][Cu₄(D-cam)₄]·5H₂O·DMF (1; Tt = tris(triazolyl)borate, D-H₂cam = d-(+)-camphoric acid or (1R,3S)-1,2,2-trimethyl-1,3-cyclopentanedicarboxylic acid) and [Co(Tt)₂][Cu₄(L-cam)₄]·5H₂O·2DMF (2; L-H₂cam = l-(-)-camphoric acid or (1S,3R)-1,2,2-trimethyl-1,3-cyclopentanedicarboxylic acid) were prepared by mixing Cu²⁺, Co(Tt), and camphoric acid under solvothermal conditions. The structures of 1 and 2 reveal that the two-dimensional layers composed of chiral ligands and Cu-Cu paddlewheel units are connected through the metalloligands to form three-dimensional networks. It is noted that these solids show multifunctional properties such as gas adsorption onto the pores of the frameworks, antiferromagnetic coupling between spin carriers, and a small enantioselective separation of racemic alcohols.

Full text of DOI:10.1021/cg501332u

Article
pubs.acs.org/crystal
Two Homochiral Bimetallic Metal−Organic Frameworks Composed
of a Paramagnetic Metalloligand and Chiral Camphorates:
Multifunctional Properties of Sorption, Magnetism, and
Enantioselective Separation
Dae Won Ryu, Woo Ram Lee, Kwang Soo Lim, Won Ju Phang, and Chang Seop Hong*
Department of Chemistry, Research Institute for Natural Sciences, Korea University, Seoul 136-713, Korea
S
* Supporting Information
ABSTRACT: Two porous metal−organic frameworks [Co-
(Tt)2][Cu4(D-cam)4]·5H2O·DMF (1; Tt = tris(triazolyl)-
borate, D-H2cam = D-(+)-camphoric acid or (1R,3S)-1,2,2-
trimethyl-1,3-cyclopentanedicarboxylic acid) and [Co(Tt)2]-
[Cu4(L-cam)4]·5H2O·2DMF (2; L-H2cam = L-(−)-camphoric
acid or (1S,3R)-1,2,2-trimethyl-1,3-cyclopentanedicarboxylic
acid) were prepared by mixing Cu2+, Co(Tt), and camphoric
acid under solvothermal conditions. The structures of 1 and 2
reveal that the two-dimensional layers composed of chiral
ligands and Cu-Cu paddlewheel units are connected through the metalloligands to form three-dimensional networks. It is noted
that these solids show multifunctional properties such as gas adsorption onto the pores of the frameworks, antiferromagnetic
coupling between spin carriers, and a small enantioselective separation of racemic alcohols.
For multifunctional MOFs, it is advantageous to employ
functionally different building components that provide multi-
ple properties in a framework at once. With this strategy in
mind, we chose the paramagnetic metalloligand Co(Tt)2 (Tt =
tris(triazolyl)borate) and chiral camphoric acid to generate a
porous magnetic MOF possessing a homochiral structure. The
metalloligand Co(Tt)2 possesses exodentate sites that enable
multiple coordination to thereby construct a 3D porous
MOF.12,31,32 Herein, we report the syntheses, crystal structures,
and physical properties of two homochiral frameworks
[Co(Tt)2][Cu4(D-cam)4]·5H2O·DMF (1; D-H2cam = D-
(+)-camphoric acid or (1R,3S)-1,2,2-trimethyl-1,3-cyclo-
pentanedicarboxylic acid) and [Co(Tt)2][Cu4(L-cam)4]·
5H2O·2DMF (2; L-H2cam = L-(−)-camphoric acid or
(1S,3R)-1,2,2-trimethyl-1,3-cyclopentanedicarboxylic acid)
(Scheme 1). It is noted that multiple functionalities of sorption,
magnetism, and chiral separation coexist in a single material
INTRODUCTION
■
Over the past decade, metal−organic frameworks (MOFs) have
attracted great attention not only because of their intriguing
molecular topologies but also because of potential applications
as functional materials for gas storage, molecular sensors,
magnetism, luminescence, and so on.1−9 Recently, multifunc-
tional MOFs have been a subject of intense research in
materials science due to the possible creation of novel
functional systems.10−16 The properties of the metal centers
and linkers usually determine the function of a target material,
such as its porosity and other physical properties.
Among the variety of multifunctional MOFs, porous
magnetic materials are a challenging subject because the
materials require a combination of opposing attributes.
Generally, effective magnetic coupling is highly dependent on
the short bridging ligands between the spin centers, while long
organic spacers are commonly used to produce MOFs with
large pores.3,7,17−19 Thus, ligand design is crucial to efficiently
transmit exchange interactions between the metal ions while
maintaining porosity.
Scheme 1. Molecular Drawing of Chiral Camphoric Acids
Enantioselective separation is important in the chemical and
pharmaceutical industries.20−22 Chiral MOFs are appealing
because they provide stable and tunable chiral architectures.
Although some homochiral MOFs have been reported in the
past decade, the preparation of homochiral MOFs still remains
a challenge.23−25 Currently, the effective and general method
for constructing chiral MOFs uses organic enantiopure chiral
ligands as building blocks.16 Specifically, the synthesis of chiral
MOFs has achieved huge success by using natural organic
ligands with polycarboxylate groups.26−30
Received: September 5, 2014
Revised: October 29, 2014
© XXXX American Chemical Society
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Figure 1. (a) Molecular structure of [Co(Tt)2]. (b) Side view of the [Co(Tt)2][Cu2(D-cam)2]4 unit. Extended 2D layer in (c) 1 and (d) 2, showing
the paddlewheel units with four connections. The dotted circles indicate the different methyl positions of the camphoric acids; the methyl group in 1
is located below the cyclopentyl ring, while the methyl group in 2 is situated above the ring.
Thermo Nicolet 380 spectrometer. Thermogravimetric analyses were
carried out at a ramp rate of 10 °C/min in a N2 flow using a Scinco
TGA N-1000 instrument. Powder X-ray diffraction (PXRD) data were
recorded using Cu Kα (λ = 1.5406 Å) radiation on a Rigaku Ultima III
diffractometer with a scan rate of 2°/min and a step size of 0.02°.
Circular dichroism (CD) spectra were recorded using a Jasco J-810
spectropolarimeter. Magnetic susceptibilities for 1 and 2 were carried
out using a Quantum Design SQUID susceptometer. Diamagnetic
corrections of all samples were estimated from Pascal’s Tables.
Gas Sorption Measurements. For gas sorption, 1 and 2 were
immersed in CHCl3 for 3 days to exchange the guest solvents with
chloroform. After the resultant mixtures were filtered and washed with
ether, desolvated samples of 1′ and 2′ were collected by heating at 100
°C under vacuum for 4 h. Gas sorption isotherms were measured using
a BEL Belsorp mini II gas adsorption instrument with up to 1 atm of
gas pressure. Highly pure N2 (99.999%), H2 (99.999%), and CO2
(99.999%) were used in the sorption experiments. N2 and H2 gas
isotherms were measured at 77 K, and CO2 isotherms were measured
at 195, 273, and 298 K.
Crystallographic Structure Determination. Crystals of 1 and 2
were mounted on a cryoloop under a cooling stream of nitrogen gas.
Diffraction data were collected using synchrotron radiation by a 6B
MX-I ADSC Quantum-210 detector with a silicon (111) double
crystal monochromator at the Pohang Accelerator Laboratory. The
ADSC Quantum-210 ADX program (Ver. 1.92) was used for data
collection, and HKL2000 (Ver. 0.98.699) was used for cell refinement,
data reduction, and absorption corrections. The structure was solved
by direct methods and refined by full-matrix least-squares analysis
because magnetic and chiral building blocks play a role in the
construction of the porous framework.
EXPERIMENTAL SECTION
■
Reagent. Co(Tt)2 (Tt = tris(triazolyl)borate) was prepared
according to the literature procedure.33 All other chemicals and
solvents used in the synthesis were of reagent grade and used as
received.
[Co(Tt)2][Cu4(D-cam)4]·5H2O·DMF (1). Co(Tt)2 (0.04 mmol),
CuCl2·2H2O (0.16 mmol), and D-H2cam (0.16 mmol) were dissolved
in 5 mL of a mixed solvent (DMF:H2O:isopropyl alcohol = 3:3:1, v/v)
and stirred for 10 min. The solution was filtered and transferred to a
vial. The vial was capped and placed in an oven at 100 °C for 2 days.
Green crystals were obtained after filtration and dried in air. Yield:
40%. Anal. Calcd for C55H87B2CoCu4N19O22: C, 38.83; H, 5.15; N,
15.64. Found: C, 39.12; H, 4.92; N, 15.74.
[Co(Tt)2][Cu4(L-cam)4]·5H2O·2DMF (2). Co(Tt)2 (0.04 mmol),
CuCl2·2H2O (0.16 mmol), and L-H2cam (0.16 mmol) were dissolved
in 5 mL of a mixed solvent (DMF:H2O:isopropyl alcohol = 3:3:1, v/
v). The clear mixture was stirred for 10 min, and the filtered solution
was transferred to a vial. The sealed vial was placed in an oven at 100
°C for 2 days. Green crystals were filtered and dried in air to produce
2. Yield: 40%. Anal. Calcd for C58H94B2CoCu4N20O23: C, 39.03; H,
5.68; N, 15.65. Found: C, 39.26; H, 5.34; N, 15.79.
Physical Measurements. Elemental analyses for C, H, and N
were performed at the Elemental Analysis Service Center of Sogang
University. Infrared spectra were obtained from KBr pellets with a
B
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Figure 2. (a) Extended structure of 1 in the ab plane where 2D sheets are connected by [Co(Tt)2]. (b) Connolly surface of 1 with a probe radius of
1.4 Å showing the 1D channels.
to a Jahn−Teller elongation.32,34,36 A pair of Cu(II) centers is
using anisotropic thermal parameters for non-hydrogen atoms with the
SHELXTL program.35 Some carbon atoms were disordered over two
bridged by four carboxylates, forming a paddlewheel-shaped
sites with occupancies of 0.5 and isotropically refined. Lattice water
Cu2(CO2)4 cluster unit. The Cu−Cu distances within the
and DMF molecules in 1 and 2 were significantly disordered and could
dimeric Cu2 subunits are 2.6464(12) Å for Cu1−Cu2 and
not be modeled properly; thus, the program SQUEEZE, a part of the
PLATON37 package of crystallographic software, was used to calculate
the solvent disorder area and remove its contribution to the overall
intensity data. All hydrogen atoms were calculated at idealized
positions and refined using a riding model. Crystal data of
2.6313(13) Å for Cu3−Cu4. Furthermore, each Cu2(CO2)4
paddlewheel cluster is connected to four other paddlewheel
clusters by camphorates in the equatorial direction, resulting in
a 2D sheet (Figure 1c). The existence of the chiral D-cam in 1
causes the methyl group to be positioned below the cyclopentyl
ring, compared with L-cam in 2 where the methyl group lies
above the ring (Figure 1c,d).
1(squeezed): empirical formula = C52H70B2CoCu4N18O16, Mr
1537.97, monoclinic, space group C2, a = 26.676(5) Å, b = 18.145(4)
Å, c = 18.954(4) Å, β = 118.54(3)°, V = 8059(3) Å3, Z = 4, Dcalc
=
=
1.268 g cm−3, μ = 1.303 mm−1, Flack parameter = −0.028(15), 32 597
reflections collected, 22 650 unique (Rint = 0.0550), R1 = 0.0726, wR2
= 0.2032 [I > 2σ(I)]. Crystal data of 2(squeezed): empirical formula =
C52H70B2CoCu4N18O16, Mr = 1537.97, monoclinic, space group C2, a
= 26.604(5) Å, b = 18.113(4) Å, c = 18.925(4) Å, β = 118.24(3)°, V =
8034(3) Å3, Z = 4, Dcalc = 1.272 g cm−3, μ = 1.307 mm−1, Flack
parameter = 0.03(2), 21 577 reflections collected, 14 012 unique (Rint
= 0.0669), R1 = 0.0811, wR2 = 0.2290 [I > 2σ(I)].
The Co(Tt)2 metalloligand acts as a pillar to cross-link the
2D sheets into noninterpenetrating 3D architectures (Figure
2a). Notably, 1D porous channels are visible along the
crystallographic c axis (Figures 2b and S1, Supporting
Information). The window size of the 1D channel is
approximated as 5.0 × 10.5 Å, after taking into account the
van der Waals surface of the backbone. The accessible void
volume per unit cell volume is calculated to be 30.8% using
PLATON.37 To simplify the network structure, the midpoints
of the Cu dimers and positions of the Co atoms are represented
by joints, which are linked by triazoles and camphorates
expressed as sticks (Figure S2, Supporting Information). From
a topological analysis, 1 constitutes a 3D 2-nodal framework.
The 4,6-c net with a (4-c)(6-c)2 stoichiometry forms the point
symbol {32.44.54.64.7}2{32.64}, leading to a topology type of
sqc124.38 Other camphorate-based chiral MOFs were also
reported, which demonstrate interesting structural fea-
tures.39−41
Thermal and Structural Stabilities. We recorded
thermogravimetric (TG) data over a temperature range of
25−700 °C to examine the thermal stabilities of 1 and 2
(Figure S3, Supporting Information). The TG curve of 1
suggests that guest molecules (approximately 7 H2O and 1
DMF) are released from the framework in the temperature
range of 30−200 °C (expt. 11.4 wt %, calcd. 11.5 wt %). To
exchange the lattice solvent molecules, the as-prepared sample
was soaked in chloroform for 2 days. The guest molecules in
the pore were successfully replaced with chloroform, as
confirmed by TG data. To obtain the desolvated sample, the
CHCl3-exchanged sample was heated at 100 °C under vacuum
for 3 h. Although the desolvated framework rapidly adsorbed
RESULTS AND DISCUSSION
■
Description of Crystal Structures. Complexes 1 and 2
were synthesized in the same reaction conditions using D-
(+)-camphoric acid and L-(−)-camphoric acid, respectively.
Single-crystal X-ray diffraction analysis reveals that complexes 1
and 2 crystallize in the monoclinic system with the chiral space
group C2. As the overall crystal structure of 1 is similar to that
of 2, we describe the detailed geometric features of 1 only. In
the molecular view of 1, the metalloligand Co(Tt)2 is
sandwiched between two tripodal Tt groups (Figure 1a). The
Co−N distances in the octahedral environment range from
2.082(5) to 2.200(5) Å. Among the six exodentate N sites, four
coordinate to the axial positions of neighboring Cu atoms
(Figure 1b). The binding mode of exodentate N donors in
Co(Tt)2 is similar to that in a porous Cu(Tt)2−Mn complex32
but different from those in other 3D M(Tt)-based bimetallic
systems.12,31 Each Cu(II) ion adopts a square-pyramidal
coordination geometry consisting of four equatorial O atoms
from the four bridging carboxylates of the four independent D-
cam moieties along with one apical N atom from the Co(Tt)2
ligand. The axial Cu−N(triazole) bond distances span from
2.106(6) to 2.196(6) Å, which are larger than the equatorial
Cu−O lengths (1.909(5)−2.028(6) Å), which can be attributed
C
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water from the atmosphere, it was stable up to 260 °C (Figure
S3a). We measured PXRD data of the as-prepared, CHCl3-
exchanged, and desolvated samples (Figure S4, Supporting
Information). All major peaks of the experimental PXRD
profiles of 1 match well with the simulated pattern, indicating
that the original phase is maintained during the solvation and
activation processes. The temperature-dependent PXRD data
of 1 show that sharp and intense peaks are retained until 250
°C, suggesting the structural stability of the 3D compound in
the given temperature range (Figure S5, Supporting Informa-
tion). The TG graph of 2 also reveals that the thermal stability
of 1 is analogous to that of 2, which originates from the
structural similarity (Figure S3b). The guest molecules
(approximately 5 H2O and 2 DMF) in 2 are eliminated in
the temperature range of 30−200 °C (expt. 13.1 wt %, calcd.
13.3 wt %). After desolvation, the framework of 2 remains
intact, as corroborated by the PXRD data (Figure S4).
Supporting Information). Interestingly, a significant hysteresis
is manifest in the adsorption isotherm at 195 K. The large
hysteresis may pertain to the flexibility of the frame-
works.18,43−45 Using the virial equation fitting parameters
obtained from the adsorption data measured at 273 and 298 K,
the isosteric heats of CO2 adsorption at zero coverage are
estimated to be −21.5 kJ mol−1 for 1 and −24.3 kJ mol−1 for 2
(Figures S13−S16, Supporting Information), which are similar
to those of other MOFs.46
Magnetic Properties. Magnetic data for 1 and 2 were
collected as a function of temperature and field (Figure 4). The
Gas Sorption. N2 gas adsorption and desorption isotherms
of 1 and 2 were measured at 77 K (Figure 3). The Brunauer
Figure 4. Plot of χmT versus T for (a) 1 and (b) 2 at 1000 G. Plot of
M versus H for (c) 1 and (d) 2 at 2 K.
χmT values of 1 and 2 at 300 K are 2.67 and 2.68 cm3 K mol−1,
respectively, which are much smaller than the theoretical value
(3.375 cm3 K mol−1) calculated from one Co(II) (SCo = 3/2)
and four Cu(II) (SCu = 1/2). This feature is a combined
consequence of strong antiferromagnetic interactions between
Cu ions within the Cu2(CO2)4 paddlewheel cluster and the
unquenched orbital angular moment of the octahedral high-
spin Co(II) center. The antiferromagnetic coupling in the Cu2
dimer results from the magnetic orbitals of Cu dx −y that are
2
2
subject to superexchange interactions through the carboxylate
bridges. The gradual decrease in χmT with decreasing
temperature is associated not only with the strong anti-
ferromagnetic contribution of the Cu−Cu pair but also with the
spin−orbit coupling of the Co(II) ion. Further cooling of the
sample below 9 K causes the χmT value to decrease more
sharply. Since, at such low temperature, the strong anti-
ferromagnetic exchange coupling forces Cu spins in the dimeric
subunit to be paired up,47 the drastic reduction in χmT can be
explained by the fact that adjacent Co(II) spins below 9 K are
coupled in an antiferromagnetic manner, albeit weakly. In the
magnetization curves, the saturation magnetizations at 7 T
amount to 1.60 Nβ for 1 and 1.62 Nβ for 2. Considering that
Co(II) is saturated in the range of 2.3−2.4 Nβ,48 the observed
value is much smaller than expected. This disparity verifies the
existence of antiferromagnetic interactions between Co(II) ions
throughout the lattice, which is also consistent with the decline
of the χmT product below 9 K.
Figure 3. (a) N2 and (b) H2 isotherms for 1′ and 2′ at 77 K.
Emmett−Teller (BET) surface areas of 1 and 2 are similar,
which are estimated as 472 m2 g−1 for 1 and 488 m2 g−1 for 2
from the N2 adsorption isotherms. The H2 sorption capacities
of 1 and 2 reach 103 cm3 g−1 (0.92 wt %) and 110 cm3 g−1
(0.99 wt %), respectively, at 1 bar and 77 K (Figure S6,
Supporting Information). In the H2 sorption isotherms, no
hysteresis upon adsorption or desorption is detected. An
additional hydrogen isotherm at 87 K was collected to calculate
the isosteric heat of adsorption by fitting with a virial-type
equation (Figures S7 and S8, Supporting Information). The
adsorption enthalpy is −9.6 kJ mol−1 for 1 and −9.9 kJ mol−1
for 2 (Figures S9 and S10, Supporting Information), which are
in the range of some carboxylate-bridged MOFs.42 The CO2
adsorption isotherms of 1 and 2 were measured at different
temperatures of 195, 273, and 298 K (Figures S11 and S12,
Chiral Separation. The CD curve of 1 is a mirror image of
that of 2, which identifies their enantiomeric nature (Figure
S17, Supporting Information). To investigate the chiral
separation capabilities, 1-phenylethanol was selected as an
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analyte. Compound 1′ was immersed in racemic 1-phenyl-
ethanol at room temperature for 3 days. Thereafter, the solid
was filtered and washed several times with dry diethyl ether to
remove any alcohols adsorbed on the solid surface. We then
extracted the remaining alcohol from the pore by soaking the
solid in dry MeOH for 3 days. The filtered liquid was analyzed
by HPLC to inspect the released alcohol. A trace of the
encapsulated 1-phenylethanol in the methanol solution was
analyzed on an OD-H column with a flow rate of 0.5 mL min−1
using an eluent of 95:5 hexane:isopropanol. By this method, 1
shows a small chiral separation for 1-phenylethanol with an ee
of 13.8% (Figure 5a,b). This feature should be associated with
alities such as sorption properties, antiferromagnetic couplings,
and a small enantioselective separation ability of racemic
alcohols.
ASSOCIATED CONTENT
* Supporting Information
■
S
X-ray crystallographic files in CIF format and additional
experimental data for the complexes. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: cshong@korea.ac.kr (C.S.H.).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Korea CCS R&D Center
(KCRC) grant funded by the Korea government (The Ministry
of Science, ICT & Future Planning (MSIP)) (NRF-
2014M1A8A1049253), the Basic Science Research Program
(NRF-2012R1A1A2007141), and the Priority Research Centers
Program (NRF20100020209). We thank Prof. C.-H. Cheon in
Korea University and Dr. E. K. Koh in KBSI for allowing us to
have access to HPLC and SQUID instruments, respectively.
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́
■
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Two porous frameworks, 1 and 2, were prepared by reacting
the paramagnetic metalloligand Co(Tt)2 and chiral camphoric
acid in the presence of Cu(II) metal ions under solvothermal
conditions. In the crystal structures of 1 and 2, the two-
dimensional layers are constructed by chiral ligands and a Cu−
Cu paddlewheel unit, leading to the formation of a three-
dimensional network with the assistance of metalloligand
bridges. Notably, these compounds demonstrate multifunction-
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E
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