Single-crystal-to-single-crystal transformation of a novel 2-fold interpenetrated cadmium-organic framework with trimesate and 1,2-bis(4-pyridyl)ethane into the thermally desolvated form which exhibits liquid and gas sorption properties

Article abstract of DOI:10.1021/cg301760a

A novel 2-fold interpenetrated, pillared, cadmium metal-organic framework, namely, [Cd(HBTC)BPE]_n·nDMF, has been synthesized using 1,3,5-benzenetricarboxylic acid and 1,2-bis(4-pyridyl)ethane (BPE). This compound has been desolvated and subjected to various liquids and gases for sorption studies. Structures of the as-synthesized (1), desolvated (2), and resolvated in benzene (3) have been determined by single-crystal X-ray diffraction analysis and further characterized by elemental analysis, IR spectra, and thermogravimetric/differential scanning calorimetry analysis. Single crystal X-ray analysis revealed a 2-fold interpenetrated, three-dimensional (3D) framework which exhibits a 3,5-connected network with the Schlaefli symbol of [(6³)(6⁹.8) and hms topology. Compound 1 exhibits a temperature-induced single-crystal-to-single-crystal (SC-SC) transformation upon the release of N,N′-dimethylformamide molecules forming compound 2 (stable up to 300 C). SC-SC transformation is also observed when it is immersed in benzene, chloroform, 1,4-dioxane, and tetrahydrofuran. The uptake of different solvent molecules was analyzed, and desolvated samples selectively adsorb benzene, chloroform, 1,4-dioxane, and THF molecules over other selected polar solvents. Gas (N₂, CO ₂, and N₂O) sorption experiments were also performed and the structure showed 2.5% N₂, 4.5% CO₂, and 3.4% N ₂O absorption by mass at room temperature and moderate gas pressures (～10 bar).

Full text of DOI:10.1021/cg301760a

Article
pubs.acs.org/crystal
Single-Crystal-to-Single-Crystal Transformation of a Novel 2‑Fold
Interpenetrated Cadmium-Organic Framework with Trimesate and
1,2-Bis(4-pyridyl)ethane into the Thermally Desolvated Form Which
Exhibits Liquid and Gas Sorption Properties
†
‡
†
§
,†
̈
Ahmad Husain, Mario Ellwart, Susan A. Bourne, Lars Ohrstrom, and Clive L. Oliver*
̈
†Centre for Supramolecular Chemistry Research, Department of Chemistry, University of Cape Town, Rondebosch, 7701, South
Africa
‡Department of Chemistry, Ludwig-Maximilians-Universitat, Munchen, Germany
̈
̈
§Chemical and Biological Engineering, Physical Chemistry, Chalmers University of Technology, Kemivagen 10, Goteborg, Sweden
̈
̈
S
* Supporting Information
ABSTRACT: A novel 2-fold interpenetrated, pillared, cad-
mium metal−organic framework, namely, [Cd(HBTC)-
BPE]n·nDMF, has been synthesized using 1,3,5-benzenetricar-
boxylic acid and 1,2-bis(4-pyridyl)ethane (BPE). This
compound has been desolvated and subjected to various
liquids and gases for sorption studies. Structures of the as-
synthesized (1), desolvated (2), and resolvated in benzene (3)
have been determined by single-crystal X-ray diffraction
analysis and further characterized by elemental analysis, IR
spectra, and thermogravimetric/differential scanning calorim-
etry analysis. Single crystal X-ray analysis revealed a 2-fold interpenetrated, three-dimensional (3D) framework which exhibits a
3,5-connected network with the Schlafli symbol of [(63)(69.8) and hms topology. Compound 1 exhibits a temperature-induced
̈
single-crystal-to-single-crystal (SC−SC) transformation upon the release of N,N′-dimethylformamide molecules forming
compound 2 (stable up to 300 °C). SC−SC transformation is also observed when it is immersed in benzene, chloroform, 1,4-
dioxane, and tetrahydrofuran. The uptake of different solvent molecules was analyzed, and desolvated samples selectively adsorb
benzene, chloroform, 1,4-dioxane, and THF molecules over other selected polar solvents. Gas (N2, CO2, and N2O) sorption
experiments were also performed and the structure showed 2.5% N2, 4.5% CO2, and 3.4% N2O absorption by mass at room
temperature and moderate gas pressures (∼10 bar).
with mixed ligands on loss of included solvent molecules. The
high degree of porosity within MOFs can provide the necessary
space to take up gas molecules, while the tunable pore sizes/
curvatures and functionalized pore surfaces can be utilized to
direct strong interactions with gas molecules.3 Single-crystal-to-
single-crystal (SC−SC) transformations in MOFs have
attracted considerable attention as an interesting solid-state
phenomenon4 and are usually induced by various stimuli such
as heat,5 light,6 solvent molecules,7 or redox reagents.8 Kitagawa
and co-workers described a series of highly flexible MOFs using
tris(2-carboxyethyl)isocyanurate (tci), which included {[Ce-
(tci)(H2O)2]·2H2O}n, {[Pr(tci)(H2O)2]·2H2O}n, and
{[Cu2(tci)(OH)(H2O)3]·1.5H2O}n that exhibited reversible
SC−SC transformation from 2D to 3D frameworks via thermal
dehydration/rehydration processes9 which involve cleavage and
generation of coordination bonds. Perles et al. reported Sc(III)-
INTRODUCTION
■
Metal−organic frameworks (MOFs) have extended structures
and typically contain either single or polynuclear metal centers
interconnected by bridging organic ligands into one-, two-, or
three-dimensional (1D, 2D, or 3D) networks. Owing to their
high specific surface areas, tunable structures and function-
alities, uniform pore size, and high degree of porosity, they have
emerged as a promising class of materials for gas storage and
separation, particularly for hydrogen, methane, and carbon
dioxide.1 As a building block to construct MOFs, 1,3,5-
benzenetricarboxylic acid (H3BTC) ligand exhibits a variety of
coordination modes and has the capability of generating
coordination architectures of diverse size and shape. There have
been many reports on infinite 1D, 2D, and 3D coordination
polymers using H3BTC as a starting material in various states of
deprotonation.2 MOFs constructed by mixed ligands of pyridyl
groups and carboxylate groups not only incorporate interesting
properties of different functional groups but also lend
themselves to a great variety of structures. The recent upsurge
of interest in these materials is due to the robustness of MOFs
Received: November 30, 2012
Revised: February 22, 2013
Published: February 25, 2013
© 2013 American Chemical Society
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Table 1. Crystal Data and Structure Refinement for 1, 2, and 3
1
2
3
empirical formula
formula weight
temperature (K)
λ (Å)
C24H23Cd1N3O7
577.85
C21H16CdN2O6
504.77
C21H16CdN2O6
504.77
173(2)
173(2)
173(2)
0.71073
0.71073
0.71073
crystal system
space group
orthorhombic
Pbcm
orthorhombic
Pbca
orthorhombic
Pbcm
unit cell dimensions (Å)
a = 10.4621(10)
b = 16.6080(15)
c = 13.9436(13)
2422.8(4)
4
a = 16.811(3)
b = 13.891(3)
c = 20.743(4)
4843.9(17)
8
a = 10.3913(8)
b = 16.7981(13)
c = 13.8612(11)
2419.5(3)
4
volume (Å3)
Z
calc density (g cm−3
)
1.587
1.387
1.386
μ(mm−1
)
0.951
0.936
0.937
F000
1172
2024
1008
crystal size (mm3)
0.20 × 0.15 × 0.10
0.21 × 0.18 × 0.14
2.83−25.00
0.20 × 0.15 × 0.10
1.96−24.99
θ range for data collection (°) 1.95−25.00
Miller index ranges
−12 ≤ h ≤ 8, −19 ≤ k ≤ 11, −16 ≤ l ≤ −19 ≤ h ≤ 19, −16 ≤ k ≤ 16, −24 ≤ l ≤ −12 ≤ h ≤ 11, −19 ≤ k ≤ 19, −16 ≤ l ≤
16
24
16
reflections collected
10409
77879
9491
independent reflections
completeness to θmax (%)
max and min transmission
refinement method
2224 [Rint = 0.0516]
99.3
4255 [Rint = 0.0622]
99.7
2204 [Rint = 0.0298]
98.6
0.91 and 0.83
full-matrix least-squares on F2
0.88 and 0.83
full-matrix least-squares on F2
0.91 and 0.84
full-matrix least-squares on F2
data/restraints/parameters
goodness-of-fit on F2
2224/7/238
4255/0/272
2204/18/211
1.255
1.042
1.129
final R indices [I > 2σ (I)]
R indices (all data)
R1 = 0.0510, wR2 = 0.1557
R1 = 0.0651, wR2 = 0.1639
1.089 and −0.877
R1 = 0.0300, wR2 = 0.0745
R1 = 0.0437, wR2 = 0.0808
1.076 and −0.459
R1 = 0.0499, wR2 = 0.1283
R1 = 0.0601, wR2 = 0.1344
1.160 and −0.884
largest diff peak and hole (e
Å−3
)
based MOFs {[Sc2(OH)(O2CC2H4CO2)2.5]}n, {[Sc2(1,5-
O3SC10H6SO3)(OH)4]}n, and {[Sc2(2,6-O3SC10H6SO3)-
(OH)4]}n with exceptional thermal stability up to 350−500
°C but did not show any SC−SC transformation.10 Song et al.
reported11 the first SC−SC transformation observed in 4f-
b a s e d M O F s , n a m e l y , t h e d e h y d r a t i o n o f
[Dy2 (phen)2 (L)6 ]· 2H 2 O to an hy dro us ph ase
[Dy2(phen)2(L)6] (where L is β-naphthoic acid and phen is
phenanthroline) by calcinations under a vacuum at 160 °C for
24 h. Bernini et al. showed12 a reversible temperature induced
SC−SC transformation in [Yb(C4H4O4)1.5] which occurred at
130 °C. Cao et al. reported the MOFs [M(OBPT)2]·0.6H2O
(where OBPT = 4,6-bis(4-pyridyl)-1,3,5-triazin-2-ol and M =
Co, Ni) which exhibits reversible SC−SC transformation driven
by thermal treatment involving dehydration and rehydration
processes.13 Xue et al. reported14 [Cd(μ-OH2)(bct)] (where
bct is bis(carboxymethylmercapto)-1,3,4-thiadiazole acid) to
exhibit SC−SC transformation upon heating at 180 °C under a
vacuum into dehydrated [Cd(bct)].
Previous work on the reaction between Cd(II) and H3BTC
has yielded a number of MOF structures based on either
H3BTC or mixed ligand systems, with the majority of these
formed under hydrothermal conditions. Paz et al. reported15
hydrothermal reaction between Cd(NO3)2, trimesic acid
(H3BTC), and 1,2-bis(4-pyridyl)ethane (BPE) in the presence
of triethylamine to yield [Cd1.5(BTC)(BPE)(H2O)2]·(H2O).
The authors reported that the absence of the base triethylamine
instead yielded layered structures. Herein we report that when
we used the same starting materials with a binary water and
N,N′-dimethylformamide (DMF) solvent system, and in the
absence of triethylamine, a porous, 3D MOF [Cd(HBTC)-
BPE]n·nDMF was synthesized. The synthesized MOF exhibits
SC−SC transformation upon desolvation and subsequent
sorption of liquid and gaseous solvent molecules. Liquid
sorption selectivity was toward nonpolar solvents such as
benzene, toluene, THF, CHCl3, and 1,4-dioxane, while gas
sorption selectivity favored CO2, N2, and N2O over H2.
EXPERIMENTAL SECTION
■
Materials and Physical Measurements. All chemicals were of
reagent grade, purchased from commercial sources, and used without
further purification.
Elemental analysis was performed on a Thermo Flash EA-1112
CHNS-O elemental analyzer. The IR spectra were obtained from KBr
pellets in the range 4000−400 cm−1, using a Perkin-Elmer Spectrum
100 FT-IR spectrometer.
Hot-stage microscopy was performed on a Nikon SMZ-10
stereoscopic microscope fitted with a Linkam THMS600 hot stage
and a Linkam TP92 control unit. Samples were placed under silicone
oil on a coverslip and heated at 10 °C min−1. Thermal events were
monitored with a Sony Digital Hyper HAD color video camera and
captured using the Soft Imaging System program analysis.16
Thermogravimetric (TG)/differential thermogravimetric (DTG)
measurements were performed at a heating rate of 10 °C min−1 in the
temperature range 25−600 °C, under a dry nitrogen flow of 60 mL
min−1 on a TGA Q500 instrument. Approximately 2−5 mg of sample
was placed in an open aluminum crucible.
Differential scanning calorimetry (DSC) measurements were
performed at a heating rate of 10 °C min−1 in the temperature
range 25−400 °C, under a dry nitrogen flow of 50 mL min−1 on a
DSC Q200 instrument. Approximately 1−2 mg of sample was placed
in an aluminum pan with a lid. A sealed and empty pan was used as a
reference.
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Powder X-ray diffraction (XRD) analysis was performed on a
HUBER-Guinier 670 Imaging Plate X-ray powder diffractometer using
Cu Kα radiation (λ = 1.5405 Å) generated by a Philips X-ray generator
at 40 kV and 20 mA. A sample was placed in a 1 mm capillary tube,
and a diffraction pattern was acquired under ambient conditions in
transmission mode while the sample rotation axis was perpendicular to
the beam.
Synthesis of 1. Trimesic acid (20 mg, 0.095 mM) and 1,2-bis(4-
pyridyl)ethane (18 mg, 0.095 mM) were dissolved in 4.8 mL of DMF,
whereas Cd(NO3)2·4H2O (29 mg, 0.095 mM) was dissolved in 3.6
mL of H2O and acidified with H2SO4 (1 M, 0.2 mL). Both the
solutions were heated, and hot clear solutions were combined in a
sealed vial and kept in a Dewar flask. The solution was slowly cooled in
the Dewar flask from 80 °C to room temperature over a period of 2
days, which yielded good quality colorless crystals (17 mg, 35%). Anal.
Calc: for C24H24N3O7Cd1 (FW = 578.88): C, 49.75; H, 4.14; N, 7.25.
Found: C, 49.48; H, 4.00; N, 7.45. IR (KBr, cm−1): 3361, 2936, 1612,
1576, 1504, 1434, 1369, 1224, 1072, 1016, 831, 756, 691, 548, 487.
Synthesis of 2. Heating the colorless crystals of 1 at 300 °C for 5 h
in an oven under a vacuum results in the formation of 2. The crystals
were stable and only the solvent was removed.
equipment) in order to remove any residual solvent. After the system
was cooled down to 25 °C, approximately 20 bar of pressure was
released from the gas cylinder into chamber A. The valve between the
chambers was then opened and closed rapidly, allowing a certain
amount of gas into chamber B (volumes of both A and B were
known). The pressure in chamber A remained constant, and any
decrease in the pressure in chamber B indicated that gas was being
sorbed by the sample. Gas pressures in chambers A and B were
monitored using the program Absorbance.28 From the measurement of
the decrease in pressure over time, the percentage sorption by mass of
the material was calculated.
RESULTS AND DISCUSSION
■
The reaction between Cd(NO3)2·4H2O, trimesic acid
(H3BTC), and 1,2-bis(4-pyridyl)ethane (BPE) gives rise to a
highly crystalline product formulated as [Cd(HBTC)-
BPE]n·nDMF (1) (Scheme 1), in which infinite 2D sheets
Scheme 1. Synthesis of 1
Synthesis of 3, 4, and 5. Crystals of 2 were soaked in benzene (3),
chloroform (4), and THF (5) and checked for their respective unit
cells. It was observed that the unit cell values were similar to solvated
1. Data were collected for 3, 4, and 5, but only in the case of 3 could
the host framework be satisfactorily refined. In all cases, the guest
molecules were too disordered to be assigned despite their presence
being confirmed by TG results.
Single Crystal X-ray Diffraction Analysis and Structure
Determination. Suitable single crystals of 1, 2, 3, 4, and 5 were
selected and coated with oil before being mounted in air onto a loop.
The data collection for 1 and 3, 4, and 5 was carried out with a Bruker
DUO APEX II CCD diffractometer at 173(2) K using an Oxford
Cryostream-700. Data reduction and cell refinement were performed
using SAINT-Plus,17 and the space group was determined from
systematic absences by XPREP18 and confirmed using the program
Layer.19
The data collection for 2 was carried out with a Nonius Kappa CCD
diffractometer at 173(2) K using an Oxford Cryostream-600. Data
reduction and cell refinement were performed using DENZO,20 and
the space group was determined from systematic absences by
XPREP.18
Graphite monochromated Mo Kα (λ = 0.71073 Å) radiation was
used in both cases. The X-ray diffraction data have been corrected for
Lorentz-polarization factor and scaled for absorption effects by
multiscan using SADABS.21 The structures were solved by direct
methods, implemented in SHELXS-97.22 Refinement procedure by
full-matrix least-squares methods based on F2 values against all
reflections have been performed by SHELXL-97,22 including
anisotropic displacement parameters for all non-H atoms.
Calculations concerning the molecular geometry, the affirmation of
chosen space groups, and the analysis of hydrogen bonds were
performed with PLATON.23 The molecular graphics were rendered
with ORTEP-324 and MERCURY (Version 3.0).25 The calculated
PXRD patterns of 1, 2, and 3 were generated by Lazy Pulverix.26 The
crystal parameters, data collection, and refinement results for 1−3 are
summarized in Table 1.
CCDC 908241−908243 contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge via www.
ccdc.cam.ac.uk/data_request/cif [or from the Cambridge Crystallo-
graphic Data Centre (CCDC), 12 Union Road, Cambridge CB2 1EZ,
U.K.; fax: +44(0)1223-336033; e-mail: deposit@ccdc.cam.ac.uk].
Gas Sorption Analysis. The procedure in the gas sorption analysis
was followed according to the method described by Atwood et al.27
The apparatus consisted of two chambers A and B, which were
connected to pressure sensors Pa and Pb. The chambers were
connected to a gas line, while the gas-flow into and between the
chambers was regulated by two valves. Chamber A is left empty as a
control, while chamber B contains the sample of known mass, and the
whole system was evacuated under heat (65 °C, maximum of the
pillared by flexible bipyridyl ligands to form a 3D framework.
Interestingly, heating the single crystals of 1 at 300 °C for 5 h
under a vacuum resulted in a SC−SC transformation of 1 into
the desolvated form 2 formulated as [Cd(HBTC)BPE]n with
double the unit cell volume of 1.
Structure of 1. X-ray single-crystal analysis reveals that the
structure contains a distinct repeating structural motif
containing a unique Cd2+ cation in a distorted octahedral
coordination geometry (Figure 1). Crystallographic data,
selected bond lengths and bond angles are listed in Tables 1,
2, and S1. The asymmetric unit of 1 consists of 0.5 units of
Cd(II), HBTC2−, BPE, and DMF. Cd(1) is coordinated to four
oxygen donor atoms from three HBTC2− anions in the
equatorial plane and to two nitrogen donor atoms from two
1,2-bis-(4-pyridyl)ethane ligands occupying the apical positions
(Figure 1). The Cd−O and Cd−N bond distances are 2.331 Å
(average) and 2.319(6) Å (Tables 2 and S1) respectively. Each
trimesate anion bridges three metal centers through its three
functional groups in two different coordination modes (Scheme
1 and Figure 1) to form 2D parallelogram-like sheets
[Cd(HBTC)]n parallel to the ab plane. Adjacent sheets are
connected by BPE ligands to form a 3D layered framework.
The first carboxylate group (C1/O1/O2) is bonded to Cd2+ in
a symmetrical bidentate chelating mode with the angle
subtended at the metal atom being 55.5(2)°, while the two
carboxylic acid groups bond via the carbonyl oxygen atoms (O3
and O5) in a monodentate fashion (Scheme 1). The
carboxylate and carboxylic acid groups are coplanar with
respect to the phenyl ring. The Cd---Cd distances across the
parallelogram-like sheets [Cd(HBTC)]n are 10.462 and 9.364
Å along the a- and b-axes, respectively.
The bidentate BPE ligands bridge translated metal centers
along the [001] direction, thus connecting adjacent [Cd-
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Figure 1. ORTEP plot of 1 showing the coordination environment of Cd(II) (thermal ellipsoids are drawn at the 30% probability level). Disordered
atoms and hydrogen atoms have been omitted for clarity. Symmetry equivalent positions are (i) x, y, z; (ii) 1 − x, 1/2 + y, 1/2 − z; (iii) 1 + x, y, z;
(iv) x, y, 1/2 − z; (v) x, y, z − 1; (vi) x, y, 3/2 − z.
(HBTC)]n layers to afford a 3D open framework (Figure 2).
The BPE ligand adopts an anticonformation (C−CH2−CH2−
C torsion angle of 180°); however, the pyridyl rings are nearly
perpendicular to each other (interplanar angle being 72.48°).
The Cd---Cd separation across the BPE ligand is 13.863 Å. It
may be regarded as a pillar of the [Cd(HBTC)]n layers through
trans-coordination.
(Figure 2). A short, strong hydrogen bond reinforces the
monodentate coordination mode of the two carboxylic groups
[O(4)···O(6) 2.389(14) Å, H(4a)···O(6) 1.69 Å, O(4)−
H(4a)···O(6) 139°, (−x, 1/2 + y, −1/2 − z)] forming an
eight-membered ring involving Cd1. The interpenetrated
frameworks interact through C−H···O interactions
[C(15)···O(1) 3.29(2) Å, H(15B)···O(1) 2.46 Å, C(15)−
H(15B)···O(1) 141°, (1 − x, −y, 1/2 + z)] and [C(16)···O(4)
3.395(18) Å, H(16B)···O(4) 2.43 Å, C(16)−H(16B)···O(4)
164°, (1 + x, 1/2 − y, −z)] (Figure 4a) between the BPE
ethylene hydrogen atoms and the oxygen atoms of the HBTC2−
anions, respectively.
Structure of 2. When single crystals of 1 were heated at
300 °C under a vacuum for 5 h, SC−SC transformation of 1
into the desolvated form 2 was observed. Structure elucidation
by single crystal X-ray diffraction of 2 confirms that the
framework structure and packing mode of 1 are retained and
that the space previously occupied by DMF molecules becomes
devoid of any appreciable electron density (Figure S1 and
Figure 3). This was further confirmed by TG analysis with no
appreciable mass loss occurring for 2 over the temperature
range of 25−300 °C. Crystallographic data, selected bond
lengths, and bond angles are listed in Tables 1, 2, and S1. The
transformation from 1 to 2 involves changes in space group
symmetry (from Pbcm to Pbca) but in effect amounts to similar
lattice parameters (although axis ∼10 Å doubles to ∼20 Å) and
The non-nitrogen atoms of the BPE ligands are disordered
over two positions with site occupancy factors of 50% as
required by a crystallographic mirror plane across which the
molecule is located (the mirror plane is perpendicular to the
long axis of the molecule). The DMF molecule is
uncoordinated and disordered over two positions with site
occupancy factors of 50%. All bond distances of the DMF
molecule were restrained to ideal values as unrestrained
refinement lead to unacceptable bond distances and angles. A
2-fold rotation axis (x, 1/2 − y, −z) with direction [100] at [x,
1/4, 0] passes through the center of the DMF molecule which
relates one disordered position to the other. The structural
analysis revealed the [Cd(HBTC)]n layers are positioned on
mirror planes perpendicular to [001]. The cadmium and the
trimesate ligand act as 3-connecting linkers in the ab-plane, so
overall adopts a 2D sheet structure which further extends in the
third dimension via the Cd2+ ions using BPE linkers as shown
in Figure 2. The structure comprises a 2-fold interpenetrated
network with cavities containing one DMF molecule each
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Table 2. Bond Lengths [Å] for 1, 2, and 3
(1)
2.297(6)
(2)
(3)
2.391(5)
Cd(1)−
N(1)−
2.290(3) Cd(1)−
O(1)
Cd(1)
O(1)
Cd(1)−
O(3)#1
2.298(7)
N(2)−
Cd(1)
#2
2.294(3) Cd(1)−
2.346(6)
O(2)
Cd(1)−
O(5)#2
2.315(6)
2.319(7)
O(1)−
2.265(2) Cd(1)−
O(4)#1
2.364(2) Cd(1)−
O(6)#4
2.289(7)
2.374(6)
Cd(1)
Cd(1)−
O(3)−
Cd(1)
#1
N(1)
Cd(1)−
2.417(6)
O(4)−
Cd(1)
#1
2.384(2) Cd(1)−
2.293(5)
1.255(9)
O(2)
N(10)
O(1)−
1.258(10) O(5)−
2.400(2) O(1)−
C(1)
Cd(1)
#3
C(1)
C(1)−
1.242(10) C(1)−
1.247(4) C(1)−
1.266(9)
O(2)
O(1)
O(2)
O(3)−
1.201(10) C(1)−
1.256(4) O(3)−
1.283(15)
1.209(12)
1.288(12)
1.177(11)
C(5)
O(2)
C(9)
O(4)−
1.255(12) C(8)−
1.252(4) O(4)−
C(5)
O(4)
C(9)
O(5)−
1.196(10) C(8)−
1.265(3) O(5)−
C(8)
O(3)
C(8)
O(6)−
1.274(12) C(9)−
1.217(4) O(6)−
C(8)
O(5)
C(8)
C(9)−
1.293(4)
O(6)
Symmetry transformations used to generate equivalent atoms:
#1 −x + 1, y + 1/2, z
#1 x, −y + 3/2, z − 1/2 #1 x + 1, y, −z + 1/2
; #2 x, y −1, z
Figure 2. View down the a-axis showing the 2-fold interpenetrated
pillared network (individual frameworks are depicted in red and green
respectively) of 1.
#2 x + 1, y, z
#3 x + 1/2, −y + 3/2, −z #4 −x + 2, y − 1/2, z
molecular stacking arrangements. Despite the close similarity in
unit cell parameters of 1 and 2, the structures are indeed
different as unequivocally demonstrated by the diffraction data;
i.e., it is not possible to index 1 with the same unit cell as 2.
The asymmetric unit of 2 consists of one Cd(II), one
HBTC2− anion, and one BPE molecule. The coordination
environment of Cd(1) and the connectivity are the same as in
the parent molecule (Figure 1). The average Cd−O bond
distance of 2.354 Å is longer than the parent molecule (2.331
Å), while the Cd−N bond distance of 2.292 Å is smaller than
that of the parent molecule (2.319 Å) (Table 2 and S1). The
carboxylate groups deviate slightly from the plane of the
corresponding linking phenyl ring. The Cd---Cd separation
across the parallelogram-like sheets [Cd(HBTC)]n being
10.379 (10.462 Å in 1) and 9.386 Å (9.364 Å in 1) along
the c- and a-axis, respectively. As in the case of 1, strong
hydrogen bonds reinforce the monodentate coordination mode
of the carboxylic group [O(2)−H(2)---O(6)] forming an eight-
membered ring each involving Cd1 (Table 3).
Figure 3. Views of the 2-fold-interpenetrated 3D topological network
for 2 constructed from two identical hexagonal nets viewed down the
(a) a-axis, (b) b-axis, (c) c-axis. (d) hms topology network displaying
hexagonal packing.
The following description of the network topology of 2 is
applicable to that of 1 as well (crystallographic axes labels are
different in 1 and 2). Each Cd(II) center serves as a 5-
connected node by linking to three HBTC2− anions with the
vertex symbol of (6.6.6.62.62.62.62.62.62.86), and each HBTC2−
anion represents a 3-connected node by linking to three Cd(II)
with the vertex symbol of (63.63.63) to generate sheets parallel
to the ac-plane, which in turn are connected in the third
dimension by BPE ligands. The sheets feature a honeycomb-
like layer with topological representation of 63. Thus, a 3-D
[6(3).6(3).6(3)] is generated with doubly interpenetrated
(3,5)-connected hms topology29 as depicted in Figure 3. As
in the case of 1, the interpenetrated frameworks interact
through C−H---O interactions (Figure 4b) between the
ethylene H15b and H16b to O3 and O6 and phenyl C−H
(H18 and H21) to O4 and O2 of carboxylic groups,
respectively, which may be responsible for the structural
stability of 2.
Two-fold interpenetration leads to the disappearance of the
big channels of the individual frameworks, and to the
subsequent formation of small cavities formed by the
HBTC2− anion and BPE ligands of both frameworks. In 1
̂
(3,5)-connected network with the Schlafli symbol of {63}
̈
̂
{69.8}-VS[6.6.6.6(2).6(2).6(2).6(2).6(2).6(2).*]-
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Table 3. Hydrogen Bonds in Compound 2
donor---H····acceptor
symmetry equivalent positions
H····A
D····A
D−H····A
O(2)−H(2)····O(6)
[1/2 + x, 1/2 − y, −z]
[1 − x, −y, −z]
[1/2 − x, −1/2 + y, z]
[1/2 − x, −y, 1/2 + z]
[1 − x, −y, −z]
2.15
2.36
2.47
2.58
2.59
2.468
103
161
150
156
166
C(15)−H(15B)····O(3)
C(16)−H(16B)····O(6)
C(18)−H(18)····O(4)
C(21)−H(21)····O(2)
3.3145
3.3639
3.4723
3.5217
Figure 4. (a) A DMF molecule (displayed in space-filling mode) situated in a cavity of 1. Non-nitrogen atoms of the pyridyl rings are disordered
over two positions with site occupancy factors of 50%. (b) Cavity formed in 2 by the evacuation of DMF in 1. Two-fold interpenetrated networks
(3,5-connected hms topology) are shown in red and green for both 1 and 2, and C−H···O and O−H···O interactions between the frameworks are
shown for 2 (also present in 1).
Figure 5. PXRD pattern at variable temperature and compared with the calculated patterns of 1, 2, and 3.
these cavities are isolated, have a size of 158 Å3, and are
occupied by a single DMF molecule (Figure 4a). The
desolvation of 1 upon heating must thus involve cooperative
conformational changes in the frameworks in order to allow for
the evacuation of the DMF molecules with the retention of
monocrystallinity. Interestingly, in 2 the cavity size effectively
doubles to 328 Å3 (Figure 4b) as a result of two neighboring
pyridyl groups from different frameworks which “opens” and
allows two neighboring cavities to “connect” as when compared
to 1. The disorder in structure 1 prohibits this scenario of two
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Crystal Growth & Design
Article
parallel pyridyl rings as the other disordered component (as
required by the mirror plane in 1) will put the neighboring
pyridyl groups in too close a proximity. This illustrates that
neighboring pyridyl rings in 2 are rotated differently with
respect to one another when compared to both disordered
situations in 1, which further supports that the frameworks of 1
and 2 indeed have subtle differences, aiding our argument that
the desolvation process may be described as a SC−SC
transformation.30 The total potential solvent area volumes of
1311 Å3 represent 27.1% of the unit cell volume for structure 2
(volumes were determined using PLATON with 1.2 Å probe
radius).23
Structure of 3. When single crystals of 2 were soaked in
benzene overnight, SC−SC transformation of 2 to solvated
compound 3 (2·benzene) was observed. Single crystal X-ray
diffraction of 3 confirms that the framework structure and
packing mode are retained but the unit cell values are similar to
1. Crystallographic data, selected bond lengths, and bond
angles are listed in Tables 1, 2, and S1.
Due to disorder within the structure, we were only able to
observe some electron density within the pores of 3 by single
crystal X-ray diffraction but were not able to assign specific
atom positions for the benzene molecule,31 despite the
presence of the benzene molecule (1:1 ratio) being confirmed
by TG.
Single Crystal−Single Crystal Transformation Trig-
gered by Thermal Treatment. PXRD was used to check the
structural identity and possible phase transition of 1 into 2. Due
to the close similarity of the crystal structures of 1 and 2, it is
not surprising that their calculated powder patterns are virtually
identical. However significant differences do exist as illustrated
in Figure 5.
The room temperature PXRD pattern of 1 closely resembles
that of the calculated pattern; however the three peaks at 11−
13° 2θ resemble that of the calculated pattern of 2, which
perhaps indicates that some of the sample may already have
desolvated when the analysis commenced. Despite repeated
attempts, we could not obtain an experimental pattern of
different samples of 1 which resembles that of the calculated
PXRD pattern more closely. We do however observe a general
trend of the experimental PXRD pattern of 1 to resemble that
of the calculated pattern of 2 with increasing temperature. For
example, the shoulder at 2θ = 17° starts disappearing, the peaks
at 2θ = 10−13° increase in relative intensity to the peak at 2θ =
17°, while the peaks at 2θ = 25.6−27.5° separate. More
importantly, PXRD measurements at variable temperatures
illustrated the robustness of the framework upon removal of
DMF molecules and up to 300 °C. The patterns calculated
from the crystal structure of 3 and calculated from that of 1
(DMF coordinates omitted) are also included in Figure 5 for
reference.
TGA. The TGA data of complex 1 indicate two mass losses
(Figure S2). The first mass loss 115−350 °C (13.2%)
corresponds to the loss of half a DMF molecule per asymmetric
unit (calculated: 12.6%). An abrupt mass loss in the range 360−
480 °C (66.2%) corresponds to structure decomposition. DSC
shows only one broad endotherm peak which should be due to
the loss of DMF (Figure S2). Hot stage microscopy indicates
that visual loss of DMF occurs for 1 in the temperature range
270−280 °C (Figure 6). Visual observation of chloroform and
benzene release (Figure 6) was also recorded for the crystals of
3 and 4, respectively, under the hot stage microscope by
immersing the sample in silicone oil and heating to 300 °C.
Figure 6. Hot stage microscope photographs of compounds 1,
2·benzene (3), and 2·CHCl3 (4) showing the release of solvents at
various temperatures.
Bubbles of chloroform and benzene were released in the range
90−110 °C and 160−170 °C, respectively. The TGA analysis
for 3, however, revealed a clear mass loss (12.3%) between
160−170 °C (Figure S3).
Sorption of Organic Solvents. Due to the great stability
of framework the uptake of different solvent molecules was
analyzed. Therefore few crystals of 2 were immersed into the
different solvents (DMF, ethanol, methanol, mesitylene, THF,
C6H6, CHCl3, CH2Cl2, toluene, and 1,4-dioxane) overnight and
afterward thermogravimetric analyses were performed (Figure
S3 and Table 3). Compound 2·solvent showed a prolonged
weight loss in the range of 60−250 °C for benzene, 30−250 °C
for CHCl3, 30−100 °C THF and 30−215 °C for 1,4-dioxane
that amounted to 11.63, 16.18, 12.34, and 16.79%, respectively
(Table 4). The different profiles of the TGA suggest that the
uptakes were not simply due to different amounts of water
absorbed in the event that the solvents used were wet.
Table 4. Results of the Solvent Uptake Experiment
a
entry
2·solvent
% mass loss in TGA
% calculated mass loss
1
DMF
3.02
0.1
12.6
8.3
2
EtOH
3
MeOH
mesitylene
benzene
chloroform
toluene
DCM
0.6
5.9
4
2.5
19.2
13.3
19.1
15.4
14.3
14.8
12.4
5
11.63
16.18
6.86
10.44
16.79
12.34
6
7
8
9
1,4-dioxane
THF
10
a
Calculated % mass loss for the case that every cavity is occupied by
one solvent molecule.
As shown in Table 4 there was a high selectivity for the
benzene, chloroform, 1,4-dioxane, and THF, whereas it was not
possible to resorb the DMF again to the same degree as for
these solvents. Due to the higher affinity of the compound to
some solvents than to DMF, it was assumed that a guest
exchange might also be possible. Hence some crystals of the as-
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Article
synthesized, DMF containing compound 1 were immersed in
chloroform, H2O, ethanol, methanol, benzene, dichloro-
methane, tetrahydrofuran, and 1,4-dioxane for overnight and
analyzed by TGA afterward. However, the TGA spectrum only
showed a mass loss (the same value as original) at the
temperature range of 200−300 °C which is specific for the loss
of the DMF and not for other solvents, which should occur at
lower temperature. It was assumed that there was no guest
exchange between the two solvents, which is remarkable
because the structure seems to be having a higher affinity for
nonpolar solvents than for DMF. This could be due to slight
conformational difference in 1 and 2 change which allows
passage of molecules in 2 whereas the release of DMF
molecules in 1 requires heating to higher temperatures. In all
cases, the sorption of the various organic solvents seemed to be
SC−SC transformations as judged from visual observations as
well as the ability to in each case determine unit cells using
single crystal X-ray diffraction.
Gas Sorption. Due to the high total potential solvent area
of 1311 Å3 per unit cell volume or 27.1% for 2 gas sorption
experiments were performed. The structure showed compara-
ble sorption of 2.5% N2, 4.5% CO2 and 3.4% N2O absorption
by mass at a pressure of 9−12 bar. The graphs of the gas
sorption are shown in Figure 7, the sorption of N2, CO2, and
N2O occur rapidly within the first few minutes of the analysis.
Hydrogen sorption by 2 was negligible.
CONCLUSION
■
In conclusion, a new 3D MOF has been synthesized based on
Cd(NO3)2, trimesic acid, and 1,2-bis(4-pyridyl)ethane, which
exhibits temperature-induced SC−SC transformation involving
the release of DMF molecules. The resulting desolvated form is
highly stable up to 350 °C and exhibits single-crystal-to-single-
crystal transformation in selected solvents (benzene, chloro-
form, 1,4-dioxane, and THF molecules over other polar
solvents). The desolvation and solvation processes must involve
cooperative conformational changes in the structures in order
to retain monocrystallinity. In addition, the solvated and
desolvated states of the framework reveal structural differences
which further supports describing the desolvation/solvation
processes as SC−SC transformations. The solvent-free porous
framework shows a total potential solvent area of 27.1% of the
structures’ volume. The structure showed 2.5% N2, 4.5% CO2,
and 3.4% N2O absorption by mass. Studies are underway in our
laboratory to further explore the robustness of this MOF
system and to explain the selectivity of the sorption of the
investigated solvents and gases.
ASSOCIATED CONTENT
* Supporting Information
Figures S1−S4, Table S1 and X-ray crystallographic informa-
tion files (CIF) are available for compounds 1−3. This material
is available free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: Clive.Oliver@uct.ac.za.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
A.H. and C.L.O. thank the South African National Research
Foundation and the University of Cape Town for financial
support.
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