A CdSO4-type 3D metal-organic framework showing coordination dynamics on Cu2+ axial sites: Vapochromic response and guest sorption selectivity

Article abstract of DOI:10.1021/cg3017563

A unique 2-fold interpenetrated CdSO₄ coordination network of the formula {[Cu₂(4-pmpmd)₂(CH₃OH) ₄(opd)₂]·2H₂O}_n [4-pmpmd = N,N′-bis(4-pyridylmethyl)phenyldiimide; opd = o-phthalic acid] has been synthesized and characterized by IR spectra, thermogravimetric (TG) analyses, elemental analyses, and single crystal and powder X-ray diffraction methods. The metal-organic framework (MOF) exhibits reversible dehydration and rehydration in a single-crystal-to-single-crystal (SC-SC) process. Moreover, the dehydrated material, having coordinatively unsaturated Cu²⁺ sites, can encapsulate CH₃OH molecules with a color change, again in a reversible SC-SC fashion, and shows selective adsorption of CO₂ over N₂ and H₂. This feature of obvious color variation induced by the presence of small hydroxylic molecules is highly promising for detecting hydroxylic molecules through a simple sensing mechanism. In addition, the MOF selectively interacts with hydroxylic guests and shows sorption selectivity for water, methanol, ethanol, and n-propanol over benzene guests. Notably, this compound shows complete selectivity in adsorption for n-propanol over 2-propanol owing to the effect of shape exclusion.

Full text of DOI:10.1021/cg3017563

Article
pubs.acs.org/crystal
A CdSO4‑Type 3D Metal−Organic Framework Showing Coordination
Dynamics on Cu2+ Axial Sites: Vapochromic Response and Guest
Sorption Selectivity
Guo-Bi Li, Lei Li, Jun-Min Liu,* Tao Yang, and Cheng-Yong Su*
KLGHEI of Environment and Energy Chemistry, MOE Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of
Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou,
510275, China
S
* Supporting Information
ABSTRACT: A unique 2-fold interpenetrated CdSO4 coordination network of the formula {[Cu2(4-
pmpmd)2(CH3OH)4(opd)2]·2H2O}n [4-pmpmd = N,N′-bis(4-pyridylmethyl)phenyldiimide; opd = o-phthalic acid] has been
synthesized and characterized by IR spectra, thermogravimetric (TG) analyses, elemental analyses, and single crystal and powder
X-ray diffraction methods. The metal−organic framework (MOF) exhibits reversible dehydration and rehydration in a single-
crystal-to-single-crystal (SC−SC) process. Moreover, the dehydrated material, having coordinatively unsaturated Cu2+ sites, can
encapsulate CH3OH molecules with a color change, again in a reversible SC−SC fashion, and shows selective adsorption of CO2
over N2 and H2. This feature of obvious color variation induced by the presence of small hydroxylic molecules is highly promising
for detecting hydroxylic molecules through a simple sensing mechanism. In addition, the MOF selectively interacts with
hydroxylic guests and shows sorption selectivity for water, methanol, ethanol, and n-propanol over benzene guests. Notably, this
compound shows complete selectivity in adsorption for n-propanol over 2-propanol owing to the effect of shape exclusion.
formation is eagerly desirous, since it allows direct visualization
of how the crystal structure is changed during the trans-
formation process. These direct insights further the under-
standing of the subtle interaction between the guest molecules
and the framework, which may opens new ways to study gas
storage, separation, sensing, and catalysis. So far, the
exploration of selective adsorption and sensing for small
molecules, such as water and methanol, via SC−SC trans-
formations is rare.
Some intriguing examples of well-designed porous structures
with remarkable sensing selectivity have been reported, and
most of the complexes reported up to now were dependent on
the special pore sizes or open metal sites.4 To achieve these
targets, two types of strategies are used: incorporation of
coordinatively unsaturated metal centers and introduction of
INTRODUCTION
■
Metal−organic frameworks (MOFs) have attracted great
attention due to their diverse architectures and potential
applications in adsorption separations, sensing, gas storage, and
catalysis.1 MOFs with switchable or tunable properties are of
great current interest for their highly sensitive response to small
molecules and potential applications in chemical sensors.2,3
Flexible frameworks are very sensitive to the guest molecules
and undergo remarkable variations in structures and physical
properties upon removal and uptake of guest molecules by the
shrinkage/expansion or sliding motions. The available free
volume in channels of the flexible frameworks is usually
occupied by solvent and/or guest molecules, which must be
exchanged or removed for exploiting the porous features. But in
most cases, the single crystallinity of MOFs is not maintained
after guest molecules are exchanged or removed or some other
physical stimuli occur, since the simultaneous shrinking or
expansion of the overall framework may lead to its breakdown.
Therefore, the single-crystal-to-single-crystal (SC−SC) trans-
Received: November 30, 2012
Revised: March 9, 2013
© XXXX American Chemical Society
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maintained at this temperature for 1 day, and then allowed to cool
slowly to room temperature. Blue crystals were obtained together with
a large amount of blue crystalline solids. The yield is estimated at
about 35% on the basis of the original ligand. IR (KBr, cm−1): 3477s,
3125w, 3032w, 2956w, 2113s, 1774s, 1715s, 1621s, 1430m, 1393s,
1368s, 1336m, 1314w, 1262m, 1180w, 1128m, 1072w, 1034m, 964w,
939m, 789s, 733s, 645m, 598w, 485m. Anal. Calcd for
C64H56O22N8Cu2: C, 54.28; H, 3.99; N, 7.91. Found: C, 54.37; H,
4.06; N, 7.95. Phase purity was verified by powder XRD.
Hydrated [Cu(4-pmpmd)(H2O)2(opd)·2H2O}]n, 3. . The single
crystal 1 was heated at 110 °C under an air atmosphere for 2 h and
exposed to air for several days, resulting in complete rehydration and
formation of hydrated crystal 3. IR (KBr, cm−1): 3430s, 1773w, 1716s,
1620m, 1565m, 1427w, 1393s, 1364m, 1157w, 1121w, 1090w, 942m,
754w, 727m, 631w, 587w. Anal. Calcd for C30H26O12N4Cu: C, 51.62;
H, 3.75; N, 8.03. Found: C, 51.35; H, 4.06; N, 7.97.
X-ray Data Collection and Structure Refinement. The
diffraction data were collected on a Oxford Gemini S Ultra
diffractometer equipped with Cu Kα radiation (λ = 1.541 78 Å) for
complexes 1, 1′, 2, 3, and 3′ by using φ and ω scans. Analytical
adsorption corrections were applied for complexes 2 and 3. Multiscan
adsorption corrections were applied for all others. The structures were
solved by the direct methods (SHELXS) and refined by the full matrix
least-squares method against Fo2 using the SHELXTL software.12 The
coordinates of the non-hydrogen atoms were refined anisotropically.
Most of hydrogen atoms were introduced in calculated positions and
refined with fixed geometry with respect to their carrier atoms, and the
hydrogen atoms of water have been not added. Details of the crystal
parameters, data collections, and refinement for the five complexes 1,
1′, 2, 3, and 3′ are summarized in Tables 1 and 2. Further details are
organic groups to provide guest-accessible functional organic
sites.5 However, the chance of finding a MOF that contains
coordinatively unsaturated metal centers and yet exhibits
permanent porosity is rare, as only a few MOFs are stable
upon thermal liberation of volatile coordinated ligands.6
However, it is of great current interest due to their highly
sensitive response to small molecules and potential applications
in chemical sensors.7 In particular, coordinatively unsaturated
metal complexes could display an absorption in the visible
region of the spectrum that can be tuned by binding of guest
species to the metal center, thus offering the possibility of a
selective colorimetric detection of guest molecules even by the
naked eye.8 On the other hand, successful immobilization of
guest-accessible functional organic sites onto the MOF pore
surface is still difficult because organic groups tend to
coordinate metal ions in a self-assembly process to give a
framework in which functional organic sites are completely
blocked.9 Therefore, introducing a functional part into open-
framework materials that tunes framework flexibility and
structural dynamism is an important challenge for crystal
engineers and in the fabrication of new materials.10
Here we report the structure and properties of a 2-fold
interpenetrating CdSO4 network topology 1, synthesized from
a bidentate ligand with flexible arms (−CH2−) and a second
building unit with functional groups (−CO), which shows a
dynamic behavior, directly visualized by SC−SC trans-
formations. The process involves an unusual change in the
coordination number of the copper center owing to the
reversible removal of methanol molecules coordinated to Cu2+.
The desolvated phase 2 containing coordinatively unsaturated
metal centers shows a highly selective sensing and sorption
behavior toward some specific adsorbates, and the selectivity
mainly arises from the interactions between the adsorbates and
the coordinatively unsaturated Cu2+ metal sites or/and
hydroxyl groups on the surface of the adsorbent.
a
Table 1. Crystallographic Data for 1−3
1
2
3
empirical
formula
C64 H58 Cu2 N8
O23
C30 H18 Cu N4
O8
C30 H18 Cu N4
O13
formula weight 1434.26
626.02
Ibca
706.02
Ibca
space group
crystal system
a (Å)
Ibca
orthorhombic
15.2841(10)
16.7279(10)
27.5547(15)
90
orthorhombic
14.6232(10)
16.9144(10)
27.9419(18)
90
orthorhombic
14.9179(19)
16.766(2)
27.650(3)
90
EXPERIMENTAL SECTION
■
b (Å)
Physical Methods. All materials were reagent grade, obtained
from commercial sources, and used without further purification.
Solvents were dried by standard procedures. The ligand was prepared
according to previous literature methods.11 Elemental analyses (C, H,
N) were carried out with a Perkin-Elmer 240C elemental analyzer. FT-
IR spectra were recorded from KBr pellets in the range of 4000−400
cm−1 on a VECTOR 22 spectrometer. The powder X-ray diffraction
(PRXD) was recorded on a Bruker D8 ADVANCE diffractometer (Cu
Kα, 1.5418 Å) at 40 kV and 40 mA. Thermal analyses were performed
on a TGA V5.1A Dupont 2100 instrument from room temperature to
800 °C with a heating rate of 10 °C min−1 in the air, and the data are
consistent with the structures. The adsorption isotherms of CO2 (at
195 K) and N2 and H2 (at 77 K) were measured by using BELmax
00027 adsorption equipment (BEL Japan). The methanol (298 K),
alcohol (298 K), and toluene (298 K) vapor were measured with a
BELSORP-max automatic volumetric sorption apparatus. An exactly
measured amount of the sample was introduced into the gas sorption
instrument after the sample was predesolvated in a Schlenk tube at 120
°C under vacuum for 24 h. The adsorbate was placed into the sample
tube, and then the change of the pressure was monitored and the
degree of adsorption was determined by the decrease of the pressure at
the equilibrium state. The sorption properties were analyzed using
Autosorb 1 for Windows 1.24 software.
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
90
90
90
90
90
90
7044.9(7)
4
6911.2(8)
8
6915.6(14)
8
ρcalcd (g cm−3
)
1.352
1.203
1.356
T (K)
150(2)
1.426
383(2)
1.308
150(2)
1.483
μ (mm−1
GOF
)
1.087
1.013
1.044
Rint
0.0634
0.0764
0.1999
0.0620
0.0524
0.1447
0.0886
0.0841
0.2337
R1 [I > 2σ(I)]
wR2 (all data)
a
Crystallographic data for the structures reported in this table have
been deposited with the Cambridge Crystallographic Data Centre as
supplementary publication no. CCDC 858869, 858871, and 858872.
Copies of the data can be obtained free of charge from www.ccdc.cam.
ac.uk/conts/retrieving.html.
Preparation of {[Cu2(4-pmpmd)2(CH3OH)4(opd)2]·2H2O}n, 1.
A mixture of CuCl2·4H2O (21 mg, 0.1 mmol), 2,6-bis(4-
pyridinylmethyl)benzo[1,2-c:4,5-c′]dipyrrole-1,3,5,7(2H,6H)-tetrone
(4-pmpmd) (40 mg, 0.1 mmol), disodium phthalate (21 mg, 0.1
mmol), CH3OH (6.0 mL), and CHCl3 (12.0 mL) was heated in a 20
mL Teflon-lined autoclave for 7 days at 110 °C, cooled to 80 °C,
provided in the Supporting Information. Crystallographic data for the
structures reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplementary publication
no. CCDC 858869, 858870, 858871, 858872, 858873 for the five new
compounds 1, 1′, 2, 3, and 3′, respectively.
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N,N′-bis(4-pyridylmethyl)phenyldiimide; opd = o-phthalic
acid)] were obtained from 4-pmpmd, CuCl2·4H2O, and
disodium phthalate in a mixture of methanol and chloroform
by solvothermal synthesis. The crystal structure was determined
using X-ray crystallography, and the building unit with the
atom-labeling scheme is shown in Figure 1a. In 1, each
octahedral Cu2+ ion sits in a special position attached to two
nitrogen atoms (N1 and N1′) via the two 4-pmpmd ligands,
two oxygen atoms (O5 and O5′) via the two opd ligands, and
two apical oxygen atoms (O3 and O3′) via methanol
molecules. The distances of Cu−N and Cu−O bond
coordinated with two ligands are 2.024(4) and 1.973(3) Å,
respectively. However, the other Cu−O bond length is 2.453 Å,
implying that the interaction of Cu2+ ion and methanol is a
weak coordination. Cu2+ ions are bridged by pyridyl groups in
N1−Cu−N1′ (180°) angle to form a 1D zigzag chain that
extends along the b- and c-directions, and the intrachain
Cu···Cu distance is 17.837 Å (Figure 1b). The opd ligand,
acting as a bisconnector, links Cu to form the 1D zigzag chain
complexes running along the a-axis with the Cu···Cu distance
of 7.642 Å (Figure 1c). These three 1D chains are
interconnected with each other in the crystal packing, and
this continuous network forms the 2-fold interpenetrating 3D
CdSO4 net structure (Figure 1d). Each opd ligand bridges two
Cu2+ anions, every 4-pmpmd ligand connects two Cu2+ anions,
one of the 4-pmpmd ligands points down, and one of 4-pmpmd
ligands points up, to form a tetragonal cage with a volume of
1018 Å3 (Figure 1e). The resultant 3D framework contains 1D
circular channels (6.48 Å, Vvoid = 23.4% of the total crystal
volume) along the crystallographic a-axis, occupied by water
guest molecules, which are H-bonded with the carbonyl groups
of the ligand and the coordinated methanol molecules.
Table 2. Crystallographic Data for 1′ and 3′ Measured at
a
Different Temperature
1′
3′
empirical formula
formula weight
space group
crystal system
a (Å)
C64 H58 Cu2 N8 O23
1434.26
Ibca
C30 H18 Cu N4 O9
642.02
Ibca
orthorhombic
15.3107(15)
16.7284(19)
27.412(5)
90
orthorhombic
14.7932(8)
16.7885(7)
27.7725(16)
90
b (Å)
c (Å)
α (deg)
β (deg)
90
90
γ (deg)
90
90
V (Å3)
7020.8(16)
4
6897.5(6)
8
Z
ρcalcd (g cm−3
)
1.357
1.237
T (K)
150(2)
1.430
150(2)
1.346
μ (mm−1
GOF
)
1.081
1.002
Rint
0.0415
0.0363
R1 [I > 2σ(I)]
wR2 (all data)
0.0668
0.0463
0.1911
0.1460
a
Crystallographic data for the structures reported in this table have
been deposited with the Cambridge Crystallographic Data Centre as
supplementary publication no. CCDC 858870 and 858873. Copies of
the data can be obtained free of charge from www.ccdc.cam.ac.uk/
conts/retrieving.html.
RESULTS AND DISCUSSION
Preparation and X-ray Structure of 1. Blue crystals of 1
[{[Cu2(4-pmpmd)2(CH3OH)4(opd)2]·2H2O}n (4-pmpmd =
■
Figure 1. (a) The coordination environments of Cu2+ ion in 1. (b) The 1D zigzag chain formed by Cu2+ ions bridging 4-pmpmd ligands. (c) The 1D
zigzag chain formed by Cu2+ ions bridging opd ligands. (d) The 2-fold interpenetrating 3D CdSO4 net structure constructed by three
interconnecting 1D chains. (e) The 3D framework containing 1D zigzag channels occupied by water molecules. Hydrogen atoms are omitted for
clarity. Symmetry codes: (a) 0.5 − x, −y, 0.5 + z; (b) −x, 0.5 + y, 0.5 − z; (c) 0.5 + x, 0.5 − y, −z.
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A Single-Crystal-to-Single-Crystal Transformation
among 1, 2, and 3. Guest solvent molecules occupied in
cavities or channels can often be removed without causing
framework collapse and, furthermore, can sometimes also be
reinserted.13 To explore this possibility, blue single crystal 1
was heated and then analyzed. Temperature-dependent XRPD
measurements of 1 were first carried out, the result of which
shows that the framework is robust and is maintained even
without any solvent molecules being present at 200 °C (Figure
S1, Supporting Information). Subsequently, the desolvated
crystal [Cu(4-pmpmd)(opd)]n (2) was obtained by heating the
single crystal 1 at 110 °C under an air atmosphere for a period
of 2 h. The X-ray structure of 2 indicates that the porous
structure with a 3D interpenetrating framework is retained after
removal of the coordinated methanol and guest water
molecules (Figure 2). Although the crystal system and space
Figure 4. The 3D void framework of 2 upon removal of guest
molecules.
blue color of the crystal 1′. Single-crystal cell measurement
reveals that 1′ has reconstituted the original structure of 1, that
is, the dynamic structural transformation is reversible.
Interestingly, brief immersion of single crystal 2 into water or
exposure to air for several days results in complete rehydration
and formation of hydrated crystal [Cu(4-pmpmd)-
(H2O)2(opd)·2H2O}]n (3), including two coordinated water
molecules and three guest water molecules. Thus, the
rehydration process is accompanied by return of the original
blue color. The single-crystal X-ray structure determination of 3
revealed that the crystal system and space group remain the
same as those of 2, but the copper center has re-form from a
square planar (as in 2) to an octahedral coordination
environment, owing to the introduction of coordinated water
molecules. It is worth mentioning that on rehydration the two
closely packed interpenetrating networks undergo stress relax,
which is due to the expansion of the channel size (5.72 Å) to
provide a void space composing 31.3% of the total crystal
volume. The TG profile of 3 also exhibits a stepwise release and
shows that all the coordinated and uncoordinated water
molecules are lost at 80 °C (calcd, 10.0%; found, 9.8%)
(Figure S2, Supporting Information). Thus, heating 3 to 110
°C for 2 h completely reverses the structural change, and the
original material 2 is reconstituted after the removal of all the
water molecules. After determining the structure, the simulated
powder X-ray diffraction (XRPD) was found to match that of
as-synthesized compound 1.
Figure 2. The coordination environments of Cu2+ ion in 1−3.
group remain unchanged, the 3D framework is significantly
changed from that of 1 in that the copper center in 2 is now
only four-coordinate and has changed from an octahedral (as in
1) to a square planar coordination environment. This is
accompanied by a sharp change in color from blue to violet
(Figure 3). Moreover, a mismatch between the number of
To investigate the dynamic structural transformation
between 1 and 3, single crystal 1 was immersed into water
for several days and analyzed by single-crystal X-ray diffraction.
Surprisingly, the methanol molecular coordinated to copper in
1 was completely replaced by water molecule, and crystal 3′
was obtained, whose structure is the same as 3. However,
immersion of 3 into methanol heated to 75 °C for several days
did not result in a structural change (Figure 5). This may be
due to the fact that the binding of Cu2+ ion with water is
stronger than with methanol.
Thermal Stability. The stepwise thermogravimetric curve
of 1 indicates that the release of the guest water molecules and
coordinated methanol molecules occurred at 77 °C to give the
desolvated form, [Cu(4-pmpmd)(opd)]n (2), which is stable
up to 210 °C (calcd, 14.0%; found, 14.4%). Therefore, the
desolvation can be accomplished by heating at the temperature
range between 77 and 210 °C. On further heating, one 4-
pmpmd molecule is lost at a temperature of 400 °C, and then
the compound decomposes to form unidentified products
(Figure S2a, Supporting Information). For 2, the first step
occurs in the range of 25−250 °C, corresponding to no escape
Figure 3. The color changes of crystal 1 at different temperatures.
coordination sites occupied by methanol in 1 and the number
of coordination sites required for complete coordination of
ligands indicates that there are coordinatively unsaturated metal
sites in 2. Due to these open metal sites and permanent
porosity, 2 is expected to be useful for attaining high gas take.
The single-crystal X-ray structure of violet 2 reveals that
although the cross section of the 1D circular channel shrinks
from 6.48 to 5.66 Å, the void space in 2, which may be
generated after removal of the coordinating methanol
molecules, increases to 31.0% as opposed to 23.4% in 1
(Figure 4). It is noteworthy that the framework can easily make
some adjustments with available free space to absorb some
specific materials, due to the flexible −CH2− parts of the
ligand. When a single crystal of 2 was immersed in methanol for
a period of more than 1 d, the violet color of 2 returned to the
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the air upon cooling to room temperature. These procedures
were repeated for several cycles to establish the reversibility of
the process (Figure 6).
Figure 5. Reversible single-crystal-to-single-crystal transformations
among 1, 2, and 3.
Figure 6. TGA measurements of cyclic water desorption and
absorption processes for 3 were repeated. The variation of weight
loss with time is shown as a black line. The variation of temperature
with time is shown as a red line.
of guest molecules, which proves 2 to be the void framework.
The complete decomposition of the complex finishes at about
500 °C (Figure S2b, Supporting Information). The TG profile
of 3 also exhibits a stepwise release and shows that all the
coordinated and uncoordinated water molecules are lost at 80
°C (calcd, 10.0%; found, 9.8%) (Figure S2c, Supporting
Information).
Sorption Properties of 2. To study the effect of structural
transformation and verify the porosity of 2, which contains
coordinatively unsaturated Cu2+ ions, the gas sorption proper-
ties were measured for N2 (kinetic diameter = 3.64 Å), H2 (2.8
Å), and CO2 (3.3 Å) gases. All adsorption isotherms of 2 show
type I behavior, as expected for materials with micropores (∼5
Å). (Figure 7a). Interestingly, 2 can absorb a significant amount
of CO2 at 1 atm and 77 K [84.6 cm3 g−1 (STP)], exhibiting
highly selective sorption of CO2 over N2 [12.4 cm3 g−1 (STP)]
and H2 [4.4 cm3 g−1 (STP)]. It was also observed that there is
some sort of hysteresis between CO2 sorption/desorption
curves, which might originate from the strong interactions of
CO2 molecules with the host solid 2 and/or a dynamic
structural transformation from desolvated to guest occupied
structure. The different adsorption capabilities for gases can be
ascribed to the existence of a larger quadrupolar moment for
CO2 (quadrupolar moment = 4.30 × 1026 esu cm2) than N2
(1.52 × 1026 esu cm2) and H2 (0.662 × 1026 esu cm2), thereby
providing stronger framework partial charge/quadrupole
interactions for CO2. We reasoned that the open metal sites
in 2 also might enhance the selectivity. The capture and
separation of carbon dioxide is important for the safe storage of
carbon into deep geological formations, and this selectivity can
be used for CO2 separations from synthesis gas and flue gas
mixtures.
It should be noted here that the MOFs with coordinatively
unsaturated metal centers have been anticipated to show high
H2 adsorption because H2 adsorption is regarded as a mixture
of physisorption and chemisorption. However, the present
result indicates that coordinatively unsaturated metal centers in
the MOF do not help the H2 adsorption.
Sorption experiments with different solvents were carried out
at 298 K (Figure 7b). It shows stepwise sorption for water,
some hydroxylic molecules, and benzene. For water vapor
adsorption, an increase in the amount of adsorbed vapor under
low pressure (0 < P/P0 < 0.3) indicates initial adsorption with
one molecule of water per unit pore, and then at 0.3 < P/P0 <
0.4, a rapid increase of adsorption with two molecules of water
per unit pore, and the third stepwise sorption at 0.4 < P/P0 <
1.0 is an adsorption with four molecules of water per unit pore.
It was found that the desorption curve did not trace the
Selective Sensing of Small Hydroxylic Molecules by 2.
Upon immersing the complex 2 into water or methanol for 30
min, it is interesting to find that 2 shows selective sensitivity to
small hydroxylic molecules, with a visual color change from
purple to blue, while other solvents such as ethanol,
chloroform, acetone, benzene, THF, and DMF (2 dissolves
in DMSO) fail to induce a color change that could be discerned
by eyes. The UV−vis spectra of blue 3, which formed by
immersing 2 in water, and purple 2 show a broad d−d
absorption band at 670 and 530 nm, respectively (Figure S3,
Supporting Information). Compared with the spectrum of
purple 2, there is about 140 nm red shift of the absorption band
induced by water molecule coordination in the spectrum of 3,
which can be assigned as the LMCT transition and corresponds
to the excitation wavelength. Therefore, 2 represents a
structurally simple copper(II)-based optical sensor for small
hydroxylic molecules whose response is determined by a shift
of the metal-based d−d absorption upon binding of the
methanol or water to the metal center, with a consequent
complexation of the metal center from the ligand.8,14 On the
other hand, when the complex 2 is exposed to water or
methanol vapor, its color also changes from purple to blue. This
feature of obvious color variation induced by the presence of
small hydroxylic molecules is highly promising for detecting
small guest molecules through a simple sensing mechanism.
The detection limits for water (100 ppm) and methanol vapor
(150 ppm) of complex 2 have been determined by exposing 2
to water and methanol vapor with different concentrations,
respectively. The exposure time is set as 5 min. The complex 2
also exhibits excellent recyclability on sensing. When purple 2 is
exposed to water, its color turned to blue. After evacuation at
150 °C for 6 h, the purple color of 2 can be recovered, which
can be used to detect water molecules again. The UV−vis
spectra reveal that the color change from purple to blue
between 2 and 3 is reversible (Figure S3, Supporting
Information). In particular, the TGA studies indicate that the
guest water molecules in the pores of 3 could be desorbed by
heating it to 150 °C and readsorbed by exposing the material to
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Figure 7. (a) Gas CO2 at 195 K and H2 and N2 at 77 K adsorption/desorption isotherms for 2 after activation at 110 °C for 24 h. (b) H2O, MeOH,
EtOH, n-propanol, 2-propanol, and benzene vapor adsorption/desorption isotherms at 298 K for 2 after degassing for 24 h at 110 °C. (c) Schematic
representation of the vapor adsorption. (d) Schematic representation of the selective adsorption.
adsorption curve any longer, which has a large hysteresis.
According to the interesting phenomenon, we deduced that
one water molecule is first adsorbed in the coordinatively
unsaturated Cu2+ metal sites, the following two molecules of
water enter the tetragonal cages in which one of the two water
molecules is coordinated with unsaturated Cu2+ metal site, and
finally, four molecules of water are adsorbed in the channels.
The water molecules strongly interact with the Cu2+ metal sites
and channels’ surfaces, respectively, and therefore, desorption
becomes difficult and shows a hysteresis profile. The above
assumption is consistent with the results of single crystal X-ray
diffraction analysis, which indicates that 2 could adsorb two
water molecules at the unsaturated Cu2+ metal site in the
framework. Similarly, the methanol (kinetic diameter = 3.6 Å)
vapor adsorption curve shows that after an initial adsorption of
about two molecules of methanol at 0.3 < P/P0 per unit pore,
the adsorption of the other eight molecules of methanol
occurred at 0.3 < P/P0 <1.0. In the case of MeOH, two
methanol molecules could be first adsorbed in the unsaturated
Cu2+ metal sites and the following molecules of methanol are
adsorbed in channels’ surface by H-bonding interactions with
the carbonyl groups of the ligand, which results in a two-step
adsorption profile. The stepwise sorption curves of 2 for water
and methanol show a hysteretic sorption behavior correlating
with the diversity of the binding sites. On the other hand, at
298 K, ethanol (4.5 Å) can interact with the channels’ surfaces
and diffuse into the channels of 2. However, ethanol molecules
cannot enter the tetragonal cages because they are too large and
thus cannot coordinate with the Cu2+ metal sites, as
demonstrated by complex 2 not showing any color change
when immersed in ethanol or ethanol vapor. The amount of
adsorption was calculated using the Dubinin−Radushkevich
equation, which shows that for every Cu2+ atom 7.0 water
molecules, 11 methanol molecules, and 5 ethanol molecules can
be absorbed into the microspores. Importantly, the n-propanol
(4.7 Å) also can be adsorbed in 2, and two-step adsorption
profiles were observed. The amounts of n-propanol adsorbed
corresponded to 1.5 molecules per formula, and we deduced
that the n-propanol molecules are not allowed to enter the
tetragonal cages owing to them, like ethanol, being too large
(Figure 7c). The most interesting feature is that 2 takes up a
much larger amount of n-propanol than 2-propanol molecules
(4.7 Å) due to the effect of shape exclusion, indicating that 2 is
a promising microporous material for the n-propanol/2-
propanol separation at ambient temperature (Figure S3,
Supporting Information). Moreover, the absorption experi-
ments with benzene (5.3 Å) at 298 K show that benzene
molecules cannot enter the channels of 2. The selective
F
dx.doi.org/10.1021/cg3017563 | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
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sorption of water, methanol, and ethanol over benzene in 2 is
due to selective H-bonds with the hydrophilic channels’ surface,
which helps alcohol solvents to enter the channels and small
tetragonal cages (Figure 7d).
CONCLUSION
■
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In summary, we have prepared a 3D open MOF 1 by using a
flexible ligand and a second functional building unit from the
simple solvothermal reaction. Although 1 has no free space
because the methanol molecules coordinating Cu2+ ions and
water molecules occupy the channels, they can be removed to
obtain desolvated crystal 2. Crystal 2, containing accessible
coordination sites on Cu2+ sites, reveals selective sorption
capabilities for CO2 gas and small hydroxylic molecules. The
coordinatively unsaturated Cu2+ metal sites have strong
interactions with some small hydroxylic molecules, and
meanwhile, the other small hydroxylic molecules can be
adsorbed in channels’ surface by H-bonding interactions with
the carbonyl groups of the ligand, which shows that the
adsorbate−adsorbent interaction is very important in the
selective sorption phenomenon. Furthermore, its adsorption
capacity for CO2 gas over N2 and H2 gases may be applied in
the removal of CO2 from synthesis gas and flue gas mixtures.
The MOF selectively interacts with hydroxylic guests and
shows sorption selectivity for water, methanol, ethanol, and n-
propanol guests. Notably, this compound shows complete
selectivity in adsorption for n-propanol over 2-propanol.
Moreover, the coordinatively unsaturated Cu2+ centers
selectively encapsulate hydroxylic guest molecules with the
color changing in a reversible SC−SC fashion. This MOF is an
important example of open hydrogen-bonding MOFs showing
the ability for selective colorimetric detection of hydroxyl
groups by the naked eye.
ASSOCIATED CONTENT
* Supporting Information
■
(4) (a) Wang, Z.; Cohen, S. M. Chem. Soc. Rev. 2009, 38, 1315.
(b) Gadzikwa, T.; Farha, O. K.; Mulfort, K. L.; Hupp, J. T.; Nguyen, S.
T. Chem. Commun. 2009, 3720. (c) Farha, O. K.; Malliakas, C. D.;
Kanatzidis, M. G.; Hupp, J. T. J. Am. Chem. Soc. 2009, 132, 950.
(d) He, H.; Yuan, D.; Ma, H.; Sun, D.; Zhang, G.; Zhou, H. C. Inorg.
Chem. 2010, 49, 7605. (e) Lun, D. J.; Waterhouse, G. I. N.; Telfer, S.
G. J. Am. Chem. Soc. 2011, 133, 5806.
S
Temperature-dependent XRPD patterns of complex 1, TGA
curves of complexes 1−3, UV−vis spectra of purple 2 and blue
3 for four cycles, and sorption isotherms of 2 for n-propanol
and 2-propanol molecules. This material is available free of
charge via the Internet at http://pubs.acs.org.
(5) (a) Li, K.; Olson, D. H.; Lee, J. Y.; Bi, W.; Wu, K.; Yuen, T.; Xu,
́
Q.; Li, J. Adv. Funct. Mater. 2008, 18, 2205. (b) Ferey, G.; Mellot-
AUTHOR INFORMATION
Corresponding Author
*Phone: +86-20-84115178. Fax: +86-20-84115178. E-mail:
liujunm@mail.sysu.edu.cn; cesscy@mail.sysu.edu.cn.
■
Draznieks, C.; Serre, C.; Millange, F. Acc. Chem. Res. 2005, 38, 217.
(c) Kitaura, R.; Seki, K.; Akiyama, G.; Kitagawa, S. Angew. Chem., Int.
Ed. 2003, 42, 428. (d) Hanson, K.; Calin, N.; Bugaris, D.; Scancella,
M.; Sevov, S. C. J. Am. Chem. Soc. 2004, 126, 10502. (e) Dietzel, P. D.
C.; Panella, B.; Hirscher, M.; Blom, R.; Fjellvag, H. Chem. Commun.
̊
Notes
2006, 959. (f) Lee, E. Y.; Jang, S. Y.; Suh, M. P. J. Am. Chem. Soc. 2005,
127, 6374. (g) Beau-vais, L. G.; Shores, M. P.; Long, J. R. J. Am. Chem.
Soc. 2000, 122, 2763. (h) Fang, Q. R.; Zhu, G. S.; Jin, Z.; Ji, Y. Y.; Ye, J.
W.; Xue, M.; Yang, H.; Wang, Y.; Qiu, S. L. Angew. Chem., Int. Ed.
2007, 46, 6638.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the 973 Program of China
(2012CB821701), NSFC Projects (U0934003, 21173272, and
21121061), China Postdoctoral Science Foundation
(20110490950, 2012T50737), Fundamental Research Funds
for the Central Universities, and Technology Planning Project
of Guangdong Province (2011J2200053).
(6) (a) Barbour, L. J. Chem. Commun. 2006, 1163. (b) Murray, L. J.;
Dinca, M.; Long, J. R. Chem. Soc. Rev. 2009, 38, 1294. (c) Biradha, K.;
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Ramanan, A.; Vittal, J. J. Cryst. Growth Des. 2009, 9, 2969.
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