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Author Manuscript: Published in Crystal Growth & Design, 2012, 12 (9), pp 4633–4640,
DOI: 10.1021/cg3008443
Variable Water Adsorption in Amino Acid Derivative based
Homochiral Metal Organic Frameworks Tanay Kundu, Subash Chandra Sahoo, and Rahul Banerjee*
Physical/Materials Chemistry Division, National Chemical Laboratory, Dr. Homi Bhabha Road,
Pune 411008, India
E-mail: [email protected] Fax: + 91-20-25902636; Tel: + 91-20-25902535
Abstract: Six new Cd-containing homochiral metal–organic framework materials [{Cd(L1)
(Cl)}(H2O)]∞ (1a), [Cd(L1)(Br)]∞ (1b), [Cd(L2)(Cl)](H2O)]∞ (2a), [{Cd2(L2)2(Br)2}(H2O)3]∞
(2b) [{Cd(L3)(Cl)}(H2O)2]∞ (3a), and [{Cd(L3)(Br)}(H2O)2]∞ (3b) [L1 = 2-((pyridin-4yl)methylamino)-4-methylpentanoic
acid],
[L2=
2-(pyridin-4-yl)methylamino)-3-
hydroxypropanoic acid] and [L3= 2-((pyridin-4-yl)methylamino)-3-hydroxybutanoic acid] have
been synthesized using pyridine functionalized amino acid (L-leucine, L-serine and L-threonine)
homochiral links and Cd(CH3COO)2.2H2O and characterized via single crystal XRD, Powder
XRD, variable temperature PXRD, TGA and water adsorption experiments. Side chains in
different amino acid derivatives and anions (Clˉ, Brˉ) have been explored to play an important
role in structural diversity (from porous to non-porous) as well as physical properties. These
MOFs exhibit distinct water adsorption nature and capacity e.g. high adsorption at low partial
pressure based on architectural diversities.
1
Introduction
Chirality is an essential aspect to many biological functions shown by proteins and nucleic
acids.1 Researchers have made several attempts to induce chirality in organic and inorganic
materials for their possible application in enantioselective catalysis,2 separation,3 and most
importantly to understand the complex biological processes4. Like other materials, induction of
chirality in Metal Organic Frameworks (MOFs) is an interesting phenomena due to their
relevance in chiral separation, enantioselective sensing,5 and nonlinear optical applications.6
MOFs are one class of porous material with diverse topological architectures suitable for
potential applications like gas sorption,7 catalysis,8 magnetism,9 and conductivity.10 Although
chirality is abundant in nature, implementation of chirality in MOF backbone for functional
application still remains challenging. Construction of chiral MOFs has been achieved by
introducing enantiopure ligand as backbone or by using racemic ligands which resolve to
homochiral system during MOF formation. Similarly, introduction of a chiral amino acid moiety
can also induce chirality in MOF structures. Rosseinsky et al. reported diverse three-dimensional
MOFs using pure amino acids and 4, 4-bipyridine-based ancillary ligands.11 However, chiral
MOFs from amino acid-derived links that possess 3D architecture as well as permanent porosity
are rare12, as pure natural amino acids prefer to chelate metal centers utilizing the amino and
carboxylate groups to form mostly zero-/one-dimensional complexes.13 Recently, researchers
have attempted to use amino acid derived link to overcome this problem, still the resulting MOF
adopts a 2D rather than 3D architecture.14 As a result, these MOFs lacks in higher thermal
stability, which is necessary for high temperature and high pressure catalytic application.
Herein, we report six new homochiral Metal-Organic Framework isomers, [{Cd(L1)(Cl)}
(H2O)]∞ (1a), [Cd(L1)(Br)]∞ (1b), [Cd(L2)(Cl)](H2O)]∞ (2a), [{Cd2(L2)2(Br)2}(H2O)3]∞ (2b)
[{Cd(L3)(Cl)}(H2O)2]∞
(3a),
and
yl)methylamino)-4-methylpentanoic
[{Cd(L3)(Br)}(H2O)2]∞
acid],
[L2=
(3b)
[L1
=
2-((pyridin-4-
2-(pyridin-4-yl)methylamino)-3-
hydroxypropanoic acid] and [L3= 2-((pyridin-4-yl)methylamino)-3-hydroxybutanoic acid].
These MOFs have been synthesized using pyridine derivatives of L-leucine, L-serine and Lthreonine, respectively as organic link and Cd(CH3COO)2.2H2O as metal precursor under
hydrothermal condition. The additional pyridyl group together with the carboxylate group of the
2
ligand can bridge transition metal ions to form three dimensional networks, while the remaining
sidearms can act as a functional group for special applications. MOFs 1a/1b, 2a/2b and 3a/3b
are structural isomers with different anions (Clˉ or Brˉ) coordinated to the metal atoms. MOFs
1a/2a and MOF 3a/3b are isostructural, while MOF 2a and 2b posses structural diversity due to
the bulkiness of coordinated halogen atoms. MOF 1a/1b and 2a/2b adopt 3D architecture while
MOF 3a and 3b are two dimensional in nature. This amino acid derivative based MOFs have
higher thermal stability which suits their choice over the conventional and less stable pure amino
acid based MOFs.
Experimental Section
Materials and methods: All reagents were commercially available and used as received.
Powder X-ray diffraction (PXRD) patterns were recorded on a Phillips PANalytical
diffractometer for Cu K radiation ( = 1.5406 Å), with a scan speed of 2° min -1 and a step size
of 0.02° in 2. Fourier transform infrared (FT-IR) spectra were taken on a Bruker Optics
ALPHA-E spectrometer with a universal Zn-Se ATR (attenuated total reflection) accessory in the
600-4000 cm–1 region or using a Diamond ATR (Golden Gate). Thermo-gravimetric analyses
(TGA) were carried out on a TG50 analyzer (Mettler-Toledo) or a SDT Q600 TG-DTA analyzer
under N2 atmosphere at a heating rate of 10 ºC min–1 within a temperature range of 20-800 °C.
Water adsorption studies were performed using BELSORP aqua-3 volumetric adsorption
instrument from BEL, Japan. A known weight (100-125 mg) of the as-synthesized sample was
introduced in the sample cell. Then the sample was dried under high vacuum at 423 K for 12h to
remove the solvent water molecules. The adsorbate was charged into the sample tube, and then
the change of the pressure was monitored and the degree of adsorption was determined by the
decrease in pressure at the equilibrium state. All operations were computer-controlled and
automatic.
Synthesis:
2-((pyridin-4-yl)methylamino)-4-methylpentanoic acid.HCl [L1Cl]: To an aqueous solution
(10 mL) of L-leucine (2 g, 15 mmol) and Na 2CO3 (0.78 g, 7.5 mmol), 4-pyridinecarboxaldehyde
(1.60 g, 15 mmol) in MeOH (10 mL) was added slowly. The solution was stirred for 1 h and
3
cooled in an ice bath. NaBH4 (0.76 g, 20.4 mmol) in 10 mL of H 2O was added. The mixture was
stirred for 1 h, and 3 N HCl was used to adjust the pH to 5−6. The solution was stirred further for
2 h and then evaporated to dryness. The solid was extracted in hot and dry MeOH (150 mL3),
and the filtrate was evaporated to get the ligand as a hydrochloride salt in form of a white
powder. Yield: 2.7 g, 70%. 1H NMR (D2O, ppm): -CH3 (0.90, d, 6H), -CH (1.66, m, 1H), -CH2
(2.19, dd, 2H), -HN-CH (3.55, d, 1H), -HN (3.69, m, 1H), -CH2 (4.19, s, 2H), py-H (7.49, d, 2H),
py-H (8.56, d, 2H).
2-((pyridin-4-yl)methylamino)-4-methylpentanoic acid. HBr [L1Br]: The ligand (L1Br) was
prepared exactly as L1Cl, except HBr was used instead of HCl for pH adjustment. Yield: 3.0 g,
66%. 1H NMR (D2O, ppm): -CH3 (0.92, d, 6H), -CH (1.65, m, 1H), -CH2 (2.19, dd, 2H), -HN-CH
(3.57, d, 1H), -HN (3.71, m, 1H) -CH2 (4.17, s, 2H), py-H (7.48, d, 2H), py-H (8.46, d, 2H).
2-((pyridin-4-yl)methylamino)-3-hydroxypropanoic acid.HCl [L2Cl]: To an aqueous solution
(10 mL) of L-serine (2 g, 19 mmol) and Na 2CO3 (0.99 g, 9.5 mmol), 4-pyridinecarboxaldehyde
(2.03 g, 19 mmol) in MeOH (10 mL) was added slowly. The solution was stirred for 1 h and
cooled in an ice bath. NaBH4 (0.76 g, 20.4 mmol) in 10 mL of H 2O was added. The mixture was
stirred for 1 h, and 3 N HCl was used to adjust the pH to 5−6. The solution was stirred further for
2 h and then evaporated to dryness. The solid was extracted in hot and dry MeOH (150 mL3),
and the filtrate was evaporated to get the corresponding ligand as hydrochloride salt in form of a
white powder. Yield: 2.6 g, 60%. 1H NMR (D2O, ppm): -CH2 (3.65, dd, 2H), -HN-CH (3.58, m,
1H), -CH2 (4.19, s, 2H), py-H (7.45, d, 2H), py-H (8.52, d, 2H).
2-((pyridin-4-yl)methylamino)-3-hydroxypropanoic acid.HBr [L2Br]: The ligand (L2Br) was
prepared exactly as L2Cl, except HBr was used instead of HCl for pH adjustment. Yield: 3.3 g,
63%. 1H NMR (D2O, ppm): -CH2 (3.61, dd, 2H), -HN-CH (3.53, m, 1H), -CH2 (4.22, s, 2H), pyH (7.47, d, 2H), py-H (8.52, d, 2H).
2-((pyridin-4-yl)methylamino)-3-hydroxybutanoic acid.HCl [L3Cl]: To an aqueous solution
(10 mL) of L-threonine (2 g, 16 mmol) and Na2CO3 (0.84 g, 8.0 mmol), 4pyridinecarboxaldehyde (1.71 g, 16 mmol) in MeOH (10 mL) was added slowly. The solution
was stirred for 1 h and cooled in an ice bath. NaBH 4 (0.75 g, 20.0 mmol) in 10 mL of H2O was
added. The mixture was stirred for 1 h, and 3 N HCl was used to adjust the pH to 5−6. The
solution was stirred further for 2 h and then evaporated to dryness. The solid was extracted in hot
and dry MeOH (150 mL3), and the filtrate was evaporated to get the corresponding ligand as
4
hydrochloride salt in form of a white powder. Yield: 2.7 g, 70% yield. 1H NMR (D2O, ppm):
-CH3 (1.18, dd, 2H), -CH (3.72, dd, 1H), -HN-CH (3.55, m, 1H), -CH2 (4.20, s, 2H), py-H (7.46,
d, 2H), py-H (8.50, d, 2H).
2-((pyridin-4-yl)methylamino)-3-hydroxybutanoic acid.HBr [L3Br]: The ligand (L3Br) was
prepared exactly as L3Cl, except HBr was used instead of HCl for pH adjustment. Yield: 3.4 g,
75% yield. 1H NMR (D2O, ppm): -CH3 (1.21, dd, 2H), -CH (3.74, dd, 1H), -HN-CH (3.53, m,
1H), -CH2 (4.21, s, 2H), py-H (7.44, d, 2H), py-H (8.49, d, 2H).
[{Cd(LlCl)(Cl)}(H2O)]∞ (1a): To an aqueous solution (2 mL) of L1Cl (0.05 g, 0.2 mmol),
Cd(CH3COO)2.2H2O (0.027 g, 0.1 mmol) was added and sonicated for 10 min. The clear solution
was kept in a tightly capped 5 mL vial for 24 h at 90 ˚C to produce rod-shaped transparent
crystals. Yield: 0.025 g, 65%. Elemental analysis: calcd C (37.22%), H (4.94%), N (7.23%);
found C (38.20%), H (4.88%), N (7.25%).
[Cd(LlBr)(Br)]∞ (1b): Similar synthetic protocol was adopted like MOF 1a, only 0.06g L1Br was
used instead of L1Cl. Yield: 0.024 g, 60%. Elemental analysis: calcd C (34.84%), H (4.14%), N
(6.77%); found C (34.81%), H (4.11%), N (6.75%).
[{Cd(L2Cl)(Cl)}(H2O)]∞ (2a): To an aqueous solution (2 mL) of L2Cl (0.046 g, 0.2 mmol),
Cd(CH3COO)2.2H2O (0.027 g, 0.1 mmol) was added and sonicated for 10 min. The clear solution
was kept in a tightly capped 5 mL vial for 24 h at 90 ˚C to produce rod-shaped transparent
crystals. Yield: 0.023 g, 62%. Elemental analysis: calcd C (32.19%), H (3.51%), N (7.50%);
found C (32.2%), H (3.48%), N (7.47%).
[{Cd2(L2Br)2(Br)2} (H2O)3]∞ (2b): MOF 2b was prepared by the same procedure as described
for MOF 2a, only 0.055g L2Br was taken instead of L1Cl. Yield: 0.024 g, 58%. Elemental
analysis: calcd C (26.07%), H (3.40%), N (6.75%); found C (26.0%), H (3.41%), N (6.69%).
[{Cd(L3Cl)(Cl)}(H2O)2]∞ (3a): To an aqueous solution (2 mL) of L3 Cl (0.05 g, 0.2 mmol),
Cd(CH3COO)2.2H2O (0.027 g, 0.1 mmol) was added and sonicated for 10 min. The clear solution
was kept in a tightly capped 5 mL vial for 24 h at 90 ˚C to produce rod-shaped transparent
crystals. Yield: 0.027 g, 70%. Elemental analysis: calcd C (30.55%), H (4.35%), N (7.12%);
found C (30.52%), H (4.36%), N (7.14%).
[{Cd(L3Br)(Br)}(H2O)2] (3b): MOF 3b was synthesized by similar procedure like MOF 3a, only
L3Br (0.058 g, 0.2 mmol) was used instead of L3 Cl. Yield: 0.028 g, 65%. Elemental analysis:
calcd C (27.44%), H (3.91%), N (6.40%); found C (27.41%), H (3.92%), N (6.41%).
5
Results and discussion
Crystal Structure of MOF 1a and 1b: MOF 1a (C12H19N2O3Cd1Cl1) crystallizes in the P212121
space group, comprising one Cd(II), one L1 ligand, one Clˉ ion and one lattice H 2O molecule in
the asymmetric unit. The Cd(II) center adopts a slightly distorted square pyramidal geometry (τ
=0.025), chelated by monodentate carboxylate [(Cd1−O1 2.323(4) Å)], one amino
functionality[(Cd1−N1 2.352(4) Å)] of the L1 link, one pyridyl functionality and one carboxylate
oxygen atom of the L1 ligand coordinate to the axial and equatorial positions of the Cd(II)
coordination sphere, respectively. One Clˉ ion coordinates to the equatorial site (Figure 2a).
Noticeably, the amine group upon coordination to the Cd(II) creates one further homochiral
center. All adjacent Cd(II) nodes are bridged by pyridyl groups to form a helical chain along the
crystallographic a-axis. Two carboxylate oxygens stay axial to each other. Four sets of such diCd-centers are bridged together by carboxylate oxygen to form an oval shaped helical channel
along a-axis while pyridyl units extend the lattice along bc-plane. These channels are filled with
H2O molecules despite the hydrophobic isopropyl groups and two metal bound bulky Clˉ ions
are exposed towards the pore (Figure 2c). These channels have a cross sectional area of 9.8 × 9.8
Å. In MOF 1a H2O molecules are hydrogen bonded to the carboxylate oxygen [O2···O3,
3.364(1) Å]. There are no further intermolecular O __H···O hydrogen bonds along the pore as well
as between the neighboring H2O molecules due to large separation (~ 7.12Å). MOF 1b is
isostructural to MOF 1a with increased pore aperture (~ 10.4 × 10.0 Å) (Figure S15).
Interestingly, unlike MOF 1a, there are no lattice H2O molecules present inside the pores of
MOF 1b. We believe that the insertion of Br-atom makes the pore much more hydrophobic as
well as less porous than MOF 1a and hence lattice H2O molecules cannot be accommodated
inside the 1D channels.
Crystal Structure of MOF 2a and 2b: We synthesized a serine derived link (L2) with an
expectation that replacement of hydrophobic isopropyl group [_CH(CH3)2] with much more
hydrophilic hydroxyl functionality (_OH) in the side arm may play a pivotal role in molecular
arrangement via different weak intermolecular interactions. In MOF 2a (C9H11 N2O4Cd1Cl1), the
OH-group provides a new binding site to metal centre and coordinates to metal in its protonated
form. The asymmetric unit of MOF 2a contains one Cd(II) ion , one L2 ligand, one Clˉ ion and
6
one lattice H2O molecule. Like MOF 1a/1b, The Cd(II) centre adopts a distorted octahedral
geometry where one amino functionality, and OH-group of the side arm of the first L2 link and
one pyridyl unit and one carboxylate of second link constitutes the in-plane coordination
geometry (Figure 3a). The monodentate carboxylate oxygen and Clˉ ion coordinates at the axial
positions. In MOF 2a the In-plane Cd–Namine and axial Cd–Ocarboxylate bond lengths are longer
[∼2.360(1) – 2.500(1) Å] compared to the corresponding amino acid complexes (~1.96 Å ). 15
Carboxylate groups of two different ligands are bridged to form a dimeric Cd-center which
repeats along the bc-plane. Each dimeric unit is connected via pyridyl functionality in an anticlockwise fashion to form a one dimensional pore along a-axis (Figure 3b). Two Cl-atoms of
each dimeric unit are exposed towards the pore and hydrogen bonded to one lattice H2O
molecule.16 The cross-sectional area of this one dimensional pore is 12.0 × 8.2 Å. The 3D
architecture shows a hydrophobic and hydrophilic arrangement along the ac-plane induced by
pyridyl
units
and
carboxylate
functionality
(Figure
3c).
We
prepared
MOF
2b
(C18H27N4O9Cd2Br2), a Br-analog of MOF 2a (C9H13N2O4Cd1Cl1) to observe the effect of anion.
Surprisingly, introduction of bulky Br-atom made a dramatic change in lattice arrangement. The
asymmetric unit of MOF 2b posses two different Cd(II) centers, and carboxylate oxygens of L2
ligand bridged to two Cd(II) centers and bound in a monodentate and bidentate fashion. These
dimeric units are arranged as a triangular repeating manner and propagate along the bc-plane.
These arrangements results in almost non-porous structure. Viewing along a-axis show a sheet
like architecture where carboxylate bridged molecular chains (vertically) are linked by pyridyl
units (horizontally) (Figure S17). It is noteworthy that the substitution of halogen has a huge
impact on structural arrangement of MOF 2a and 2b. In MOF 2a, there is only one H2O
molecule per Cd(II) center included in the crystal lattice. These H2O molecules stay close to the
pyridyl ring and there is no interaction among the H2O molecules along the pore due to the large
separation. But in 2b, there are three hydrogen-bonded H2O molecules per asymmetric unit. All
three H2O molecules are clustered by multiple interactions and form a complex hydrogen bonded
network [O8O7, 2.730(3) Å; O7O9, 2.667(6) Å]. This hydrogen bonded network is further
stabilized by the interaction with the metal bound Br-atoms (O8Br1, 3.336(4); O9Br2,
3.367(1) Å) and carboxylate groups [O7O6, 2.714(8) Å; O8O5, 3.003(6) Å].
Crystal Structure of 3a and 3b: We have synthesized ligand L3, a derivate of threonine, with
slight variation in the side arm keeping all binding sites same. The ligand L3 has an extra methyl
7
(-CH3) group in the side arm compared to L2. Surprisingly the resulting MOFs 3a
(C10H13N2O5Cd1Cl1) and 3b (C10H13N2O5Cd1Br1) adopts a 2D arrangement rather than the 3D
network found in MOFs 1a/1b and 2a/2b. MOF 3a and 3b have the halogen atoms (Clˉ/Brˉ) that
stay in axial positions opposite to the –OH group of the side arm. These halogen atoms prevent
this particular site to growth further. As a result these MOFs (3a and 3b) are forced to adopt a 2D
architecture (Figure 5, S19). MOF 3a and 3b posses an inter-layer distance of 6.8 Å. These 2D
layers propagate along bc-plane where carboxylate bound molecular chains are linked by pyridyl
units. In 3a, two H2O molecules reside inside the crystal lattice and are strongly hydrogen
bonded between each other [O19O20, 2.741(5) Å] followed by additional weak interactions
with metal bound Clˉ atoms [ClO19, 3.231(7); ClO20, 3.174(3) Å]. Similarly in 3b, two
lattice H2O molecules are hydrogen bonded with hydroxyl oxygens [O3O4, 3.007(3); O3O5,
2.690(4) Å] and halogen atoms [Br1O4, 3.371(1); Br1O5, 3.290(8) Å].
Comparing the molecular structures of MOF 1a, 2a, and 3a (Figure 6), it is clear that ligand L2
contains an additional coordination site at the sidearm (-CH 2-OH), compared to L1. This plays a
key role for the structural modification of MOF 2a. Similarly comparing 2a with 3a we could
find that the lattice arrangement in MOF 3a is 2D rather than a 3D structure, even though the
binding mode of L2 and L3 are similar. This observation indicates that changing the bulkiness of
the sidearm [from -CH2-OH to –CH(OH)CH3] create steric crowding near the carboxylate
oxygen co-ordination site, which restrict the in-plane rotation of the carboxylate moiety. As a
result the molecular system adopts a 2D architecture in MOF 3a. Moreover, architectural
diversity observed from MOF 2a to 2b can be anticipated by the increased bulkiness of the
halogen atom (from Clˉ to Brˉ) and different bridging modes of the carboxylate moieties, which
results in two kinds of Cd(II) centers (in MOF 2b) rather than only one observed in the former
case. Hence, it is evident that those halogen atoms, along with the side arms, play a pivotal role
for structural diversity among the aforementioned MOFs.
Thermogravimetric (TGA) and X-ray powder diffraction analysis: Thermal
gravimetric analysis (TGA) performed on as-synthesized MOFs revealed that these compounds
have high thermal stability for all data regarding guest mobility and thermal stability. The TGA
trace for MOF 1a showed a gradual weight-loss step of 4.3% [1 H 2O; calcd. 4.6%] (100–120
8
C), corresponding to the escape of all H2O molecules trapped in the pores followed by a plateau
(120–280 C) indicating its high thermal stability in absence of guest molecules (Figure S13).
Further weight loss (~35%) indicates the decomposition of the framework. Similar trend has
been observed in case of MOF 1b. Analysis of TGA trace for MOF 2a reveals 4.4% weight loss
[1 H2O; calcd. 4.9%] (100–140 C), and the dehydrated framework is stable upto 280 0C, as
evident from the plateau region in the TGA traces. MOF 2b shows weight loss step of 6.2% [3
H2O; calcd. 6.5%] (100–140 C) which corresponds to the dehydration phenomena. MOF 3a
exhibit 5% weight loss [2 H2O; calcd. 5.1%] corresponds to escape of occluded H 2O molecules.
MOF 3b follows the similar trend like MOF 3a. It is noteworthy that MOFs 3a and 3b posses
lower decomposition temperature (200 °C) than MOFs 1a/1b and 2a/2b (280 °C) due to lower
degree of dimensionality. In order to confirm the phase purity of the bulk materials, powder Xray diffraction (PXRD) experiments were carried out on these MOFs (1a/1b, 2a/2b and 3a/3b).
Experimental and simulated PXRD patterns are shown in the Figure S1-6 in the supporting
information. All the major peaks of experimental PXRD of the six Cd-MOFs match quite well
with those of simulated PXRD, indicating their reasonable crystalline phase purity in the bulk
scale. The variable temperature PXRD measurements (Figure S17-19) were performed to explore
the thermal stability, framework rigidity as well as crystallinity of these Cd-MOFs at elevated
temperature ranges (upto 200 0C). All these VTPXRD patterns are well matched with the
simulated one, showing the thermal stability as well as crystallinity of these materials at elevated
temperatures. However, peak splitting occurs (at 2 = 16.5 for MOF 3b) at higher temperatures
in some cases, which might be attributed to structural flexibility of the frameworks upon
desolvation as well as thermal vibration of the heavy atoms. The high thermal stability as well as
degree of crystallinity, as evident from VTPXRD and TGA profile, puts these materials above
0D/1D amino acid based complexes from practical application point of view.
Water Adsorption Study: Hydrolytic stability of MOFs is one among the most important
issues for practical application. However, most of the MOFs reported till date are highly
susceptible towards humidity including some well known MOFs like MOF-177, MOF-5 etc. All
six MOFs reported herein exhibit excellent H2O sustainability verified by well matched
simulated and humidified Powder XRD pattern. We have explored the H2O sorption isotherm of
the MOF 1a, 2a, 2b and 3a (Figure 7), which reveals its possible application as a desiccant
9
material like molecular sieve and silica gel. All these MOFs show sorption hysteresis, confirming
the hydrophilic interaction between the polar pore surface and the solvent water molecules.
Among these MOFs, MOF 1a posses the most hydrophobic pore with dangling isopropyl chain
poking within the molecular cavity. It justifies the low water uptake (~90 cm 3(STP)g-1) even upto
P/P0 value of 0.8. However, at higher P/P 0 values (0.8-1.0), the pore condensation of the water
molecules occur on the surface of the MOF 1a which is a common phenomenon for porous
hydrophobic materials like BPL carbon.17 On the contrary, MOF 2a and 3a also shows good
sorption capacity of 120 and 140 cm3/g at STP, respectively. Enrichment of highly hydrophilic
chlorine, oxygen and nitrogen molecules within the framework cavity is chiefly responsible for
high H2O uptake and hysteresis of water sorption isotherm of MOF 2a and 3a. In case of MOF
3a, highly hydrophilic interlayer void plays a pivotal role as H2O molecules connect the 2D
layers via strong interlayer hydrogen bonding. MOF 2b is almost non-porous due to the presence
of bulky bromine atom within the pore aperture, which is evident from its very low H2O uptake
(40 cm3/g at STP). Interestingly, MOF 2a and 3a shows very high H2O uptake at lower P/P0
values (0.0-0.2). This is due to the fact that open framework type architecture (MOF 2a) as well
as strong interlayer hydrogen bonding (MOF 3a) are responsible for high H2O affinity of these
MOFs, showing its capability to work as a suitable material as desiccant for packaging
application.
Conclusion
Six new Cd-containing homochiral metal–organic framework materials have been synthesized
hydrothermally using amino acid derived homochiral links with Cd(CH3COO)2.2H2O. Size of the
sidearm in different amino acid derivatives and anions (Clˉ/Brˉ) play an important role in the
structural diversity among these MOFs. MOFs 1a and 1b show a 3D architecture with a 1D
channel. On the other hand, effect of anions and sidearm dramatically produce diverse structural
motifs in MOFs 2a and 2b. A large open spaced cavity was observed in MOF 2a but it decreased
subsequently after Br-substitution in MOF 2b due to the structural modification. In MOF 3a and
3b, a large structural variation was clearly observed after slight variation in side arm where the
10
resulting MOFs adopts a 2D lattice arrangement rather than 3D as compared to MOF 2a and 2b.
It is noteworthy that there are very few chiral MOFs reported based on amino acids or its
derivatives. Among the limited reports on amino acid based MOFs, owing to high substrate
binding ability of amino acids, in most cases the resulting molecules are 0D/1D orientation13 with
very few higher order12 of molecular arrangements. So introduction of amino acids/amino acid
derivatives in chiral MOF synthesis will be a good roadmap for development of promising chiral
catalyst or chiral separation material. These MOFs exhibit thermal stability as well as retention
of crystallinity upto 200 °C. These MOFs show selective H2O sorption capability. The above
rationalization for synthesis of chiral MOFs may provide potential ways of constructing chiral
MOFs for diverse application like asymmetric catalysis and desiccant material for packaging
purpose.
Acknowledgment
T. K. acknowledges CSIR, New Delhi, India, for a SRF. S.C.S. acknowledges CSIR, New Delhi,
India, for a CSIR-Nehru science postdoctoral research fellowship. R.B. acknowledges CSIR’s
XIth Five Year Plan Project (NWP0022-H and NWP0021-A) for funding. Financial assistance
from the BRNS (2011/37C/44/BRNS) is acknowledged. We gratefully acknowledge Mr. Arpan
Hazra, Mr. Ritesh Haldar and Dr. Tapas K. Maji at JNCASR, India for the water adsorption
study.
11
Figure 1. Schematic representation of linkers L1, L2 and L3 that react with Cd(II) to produce
corresponding MOF architectures (MOF 1a/1b, 2a/2b and 3a/3b). The magenta colored
polyhedra represents Cd(II) centers. These MOFs have structural diversity owing to the linker
variety (steric crowding by the sidearm results in lower dimensionality from MOF 2a to 3a) as
well as due to the coordinated halogen atoms (increased bulkiness from Cl to Br results in
structural diversity from MOF 2a to 2b).
12
Figure 2. (a) SBU representation of the MOF 1a. (b) Polyhedral representation of lattice 1a
viewed down the a-axis. Magenta colored polyhedra represent Cd(II) centers, and chlorine atoms
are shown as green balls. (c) 3D spacefill arrangement of MOF 1a, showing a 1D pore along the
a-axis. (d) Red colored ball represents molecular arrangement of the lattice H2O molecules
viewed along b-axis.
13
Figure 3. (a) SBU representation of the MOF 2a. (b) Polyhedral representation of lattice 2a
viewed down the a-axis. Magenta colored polyhedra represent Cd(II) centers, and chlorine atoms
are shown as green balls. (c) 3D Lattice arrangement of MOF 2a, showing a large 1D pore along
the a-axis. Solvents are removed from the pores for clarity. (d) Lattice H2O molecules are
arranged along b-axis with a separation of 5.87 Å.
14
Figure 4: SBU representation of the MOF 2b. (b) Polyhedral representation of lattice 2b viewed
down the c-axis. Magenta polyhedra represent Cd(II) centers, and bromine atoms are shown as
yellow balls. (c) 3D Lattice arrangement of 2b, along the a-axis. Solvents are removed from the
pores for clarity. (d) Lattice H2O molecules are arranged along b-axis with a complex H-bonding
architecture.
15
Figure 5. (a) SBU representation of the MOF 3a. (b) Polyhedral representation of lattice 3a
viewed down the c-axis. Magenta colored polyhedra represent Cd(II) centers, and chlorine atoms
are shown as green balls. (c) 2D Lattice arrangement of 3a, along the a-axis. Solvents are
removed from the pores for clarity. (d) Lattice H2O molecules are arranged viewed along b-axis
with a separation of 4.15 Å.
16
Figure 6. (a) Comparison of MOFs 1a, 2a, and 3a with their links, SBUs and topological
simplification model showing change in lattice arrangement. The change in lattice
dimensionality from 3D (MOF 1a/1b) to 2D (MOF 3a/3b) may be attributed by steric crowding
between sidearm (methyl group) and carboxylate binding site in MOF 3a/3b, which is relaxed by
long separation between sidearm (isopropyl group) and carboxylate binding site in MOF 1a/1b.
Again, van der Waal repulsion between coordinated bulky bromide ion and different binding
mode of carboxylate oxygen results in architectural diversity in case of MOF 2b than MOF 2a.
17
Figure 7. a) H2O sorption of MOF 1a showing pore condensation behavior along with sorption
hysteresis. b) H2O sorption curve for MOF 2a having ~120 cm3/g H2O sorption capability and
high H2O affinity at lower humidity ( P/P0=0.0-0.2) region. c) H2O sorption isotherm for MOF
2b shows very low H2O uptake (~40 cm3/g) due to congestion of the pore by bulky bromine
atom. d) H2O sorption profile of MOF 3a, which exhibit comparable uptake (~140 cm3/g) and
high H2O affinity at lower P/P0 values.
18
Table 1. Crystal Data and Structure Refinement for the MOFs (1a-3b) in this study.
1a
1b
2a
2b
3a
3b
C12 H17 Cl N2 O3
Cd
C12 H17 Br N2 O2
Cd
C9 H11 Cl N2 O4
Cd
C18 H21 Br2 N4
C10 H13 Cl N2
O9Cd2
O5Cd
C10 H13 Br N2
O5Cd
Mr
385.14
413.59
359.06
822.01
389.08
432.53
CCDC No.
851353
851354
851355
851356
851357
851358
Temperature
296(2)K
296(2)K
296(2)K
296(2)K
296(2)K
296(2)K
Orthorhombic
Orthorhombic
Orthorhombic
Monoclinic
Monoclinic
Monoclinic
Space group
P 212121
P 212121
P 212121
P 21
P 21
P 21
a (Å )
7.123(3)
7.2398(6)
5.8751(17)
9.4628(12)
7.9060(7)
b (Å )
13.896(5)
13.7041(11)
15.067(4)
15.1900(19)
10.7338(10)
c (Å )
15.893(6)
16.5286(13)
16.416(5)
10.0319(12)
8.9647(8)
110.630(2)
110.5280(10)
110.444(13)
Formula
Crystal system
 ()
V[Å3]
7.9689(12)
10.7406(15)
8.9501(13)
1573.1(11) Å3
1639.9(2)Å3
1453.1(7) Å3
1349.5(3) Å3
712.45(11) Å3
717.80(18) Å3
4
4
4
2
2
2
/g cm-1
1.626
1.675
1.641
2.020
1.814
2.001
/mm-1
1.563
3.779
1.689
4.587
1.736
4.320
F(000)
768
808
704
790
384
418
Flack
parameter
0.01(5)
0.0009(2)
0.0009(1)
0.055(14)
-0.02(2)
0.11(3)
Reflections
collected
3677
3871
3420
6139
2583
2245
Independent
reflections
3445
3568
3389
5890
2571
2197
GOF
1.033
1.066
1.097
1.067
1.021
1.081
wR2[I > 2σ(I)]
R1 = 0.0434,
wR2 = 0.1059
R1 = 0.0421,
wR2 = 0.1300
R1 = 0.0336,
wR2 = 0.0952
R1 = 0.0485, wR2
= 0.1325
R1 = 0.0189,
wR2 = 0.0501
R1 = 0.0625,
wR2 = 0.1446
R indices (all
data)
R1 = 0.0475,
wR2 = 0.1079
R1 = 0.0461,
wR2 = 0.1342
R1 = 0.0341,
wR2 = 0.0956
R1 = 0.0506, wR2
= 0.1344
R1 = 0.0190,
wR2 = 0.0501
R1 = 0.0630,
wR2 = 0.1450
Z
Final R1,
References:
19
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21
FOR TABLE OF CONTENT USE ONLY
Six new Cd-containing homochiral metal–organic framework materials have been synthesized
using pyridine functionalized amino acid (L-leucine, L-serine and L-threonine) homochiral links
and Cd(CH3COO)2.2H2O. These MOFs are characterized via single crystal XRD, Powder XRD,
variable temperature PXRD, TGA and water adsorption experiments. Side chains in different
amino acid derivative and anions (Cl, Br) have been explored to play an important role in
structural diversity as well as physical properties (from porous to non-porous). These MOFs
exhibit distinct water adsorption nature and capacity e.g. high adsorption at low partial pressure
based on architectural diversities.
22