Tuning the flexibility in MOFs by SBU functionalization

Article abstract of DOI:10.1039/c5dt03504j

A new approach for the fine tuning of flexibility in MOFs, involving functionalization of the secondary building unit, is presented. The "gate pressure" MOF [Zn₃(bpydc)₂(HCOO)₂] was used as a model material and SBU functio

Full text of DOI:10.1039/c5dt03504j

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ARTICLE
Tuning the flexibility in MOFs by SBU functionalization
Volodymyr Bon,a Negar Kavoosi,a Irena Senkovska,a Philipp Müller,a Jana Schaber,b Dirk
Wallacher,c Daniel M. Többens,d Uwe Mueller,e and Stefan Kaskela*
Received 00th January 20xx,
Accepted 00th January 20xx
A new approach for the fine tuning of flexibility in MOFs, involving functionalization of the secondary building unit, is
presented. The “gate pressure” MOF [Zn3(bpydc)2(HCOO)2] was used as a model material and SBU functionalization was
performed by using monocarboxylic acids such as acetic, benzoic, or cinnamic acids instead of formic acid in the synthesis.
The crystal structures of resulting materials are isomorphous to that of [Zn3(bpydc)2(HCOO)2] in the “as made” form, but
show different structural dynamics during the guests removal. The activated materials have entirely different properties in
the nitrogen physisorption experiments clearly showing the tunability of the gate pressure, at which the structural
transformation occurs, by using monocarboxylic acids with varying backbone structure in the synthesis. Thus, increasing
number of carbon atoms in the backobone leads to the decreasing gate pressure required to initiate the structural
transition. Moreover, in situ adsorption/PXRD data suggests differences in the mechanism of the structural
transformations: from “gate openning” in the case of the formic acid to “breathing” if benzoic acid is used.
DOI: 10.1039/x0xx00000x
www.rsc.org/
Introduction
Flexible Metal-Organic Frameworks (MOFs) or so called Soft Porous
of NO molecules in the flexible Ru- and Rh-based coordination
polymers could find its application in biomedicine.7
Crystals are a unique class of materials capable to switch between All envisioned applications based on the flexibility require materials
the porous and dense (or less porous) states as a response to with well-defined switching behavior. However, tuning and
external stimuli such as temperature, pressure, electromagnetic designing of flexible MOFs with specific switching properties remain
irradiation or adsorption of guest molecules.1-3 The existence of two very challenging, which limits the application potential of flexible
different states with different physical and textural properties gives crystalline materials. The most often applied strategies to influence
rise to a wide potential application field of such materials. For the flexibility in a soft MOFs8 are: i) introducing of flexible side
example, NH2-MIL-53(Al) is able to transform between the narrow chains into the ligand;9 ii) modulation of host–guest interaction
and large pore phase and therefore could be used as an optical strength by linker functionalization;10 iii) incorporation of different
switch.4 Sensor technology is also one of the prospective metals into the same framework;11 iv) adjustment of the crystal
applications fields for switchable MOFs. Thus the Mg based flexible size.12
MOF with the composition [Mg(H2dhtp)(H2O)2]·DMAc, denoted as
In this contribution we present an switchability tuning approach
AEMOF-1·DMAc (AEMOF-1 - alkaline earth MOF-1; H4dhtp - 2,5-
based on the systematic substitution of monocarboxylates in the
dihydroxy-terepthalic acid; DMAc - N,N‘-dimethylacetamide) was
secondary building unit (SBU) of
a “gate pressure” MOF
used as a highly efficient luminescent sensor for the real time
detection of water traces in various organic solvents.5 The flexibility
was also successfully used to achieve the separation of CO and N2,
which have very similar physical properties.6 The selective trapping
[Zn3(bpydc)2(HCOO)2] (bpydc 2,2′-bipyridyl-5,5′-dicarboxylate),
-
also known as JLU-Liu4 (JLU - JiLin University).13 It contains two
formic acid anions directly coordinated to the metal cluster. The
solvent assisted exchange of monocarboxylic ligand was already
demonstrated for rigid Zr based MOFs.14 We could show that
utilizing monocarboxylic acids with different backbone in the
synthesis (for instance acetic, benzoic, or cinnamic acid) results in
the formation of isomorphous frameworks, showing completely
different adsorption behavior. Crystal structures of the large pore
and narrow pore phases as well as adsorption behavior of the
materials bearing different monocarboxylic ligands were studied to
enlighten the structural transformations in quest.
a.Department of Inorganic Chemistry, Technische Universität Dresden, Bergstrasse
66, D-01062 Dresden, Germany
b.
Department of Bioanalytical Chemistry, Technische Universität Dresden,
Bergstrasse 66, D-01062 Dresden, Germany
c. Department of Sample Environments, Helmholtz-Zentrum Berlin für Materialien
und Energie, Hahn-Meitner-Platz 1, 14109 Berlin, Germany
d.Department of Crystallography, Helmholtz-Zentrum Berlin für Materialien und
Energie, Hahn-Meitner-Platz 1, 14109 Berlin, Germany
e. MX Group, Institute for Soft Matter and Functional Materials, Helmholtz-Zentrum
Berlin für Materialien und Energie, Albert-Einstein-Str. 15, 12489 Berlin, Germany
† Corresponding author: Prof. Stefan Kaskel, E-mail: Stefan.Kaskel@tu-dresden.de
Electronic Supplementary Information (ESI) available: Temperature dependent
PXRD measurement for 1’, TG curves for 2’-4’, FT-IR spectra for 1’-4’, PXRD
patterns for 3’ and 4’. See DOI: 10.1039/x0xx00000x
Experimental
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Journal Name
Single crystal X-ray diffraction
Materials
View Article Online
DOI: 10.1039/C5DT03504J
The crystal structure of compounds 1 - 4 were determined by single
crystal X-ray diffraction. All single crystals were prepared in glass
capillaries with some amount of solvent. The capillaries were sealed
with melted wax. The datasets were collected at BESSY MX BL14.2
beamline of Helmholtz-Zentrum Berlin für Materialien und Energie
(HZB).16 All diffraction experiments were performed at room
temperature using the radiation with energy of 14 keV (λ = 0.88561
Å). The φ-scans with oscillation range of 1° were used for data
collection. The datasets were processed using CCP4 software.17 The
crystal structures were solved by direct methods and refined by full
matrix least-squares on F2 using SHELXTL program package.18 All
non-hydrogen atoms were refined in anisotropic approximation.
Hydrogen atoms were refined in geometrically calculated positions
using “riding model” with Uiso(H) = 1.2Uiso(C) for sp, sp2 hybridized
carbon atoms and with Uiso(H) = 1.5Uiso(C) for sp3 hybridized carbon
atoms. The atoms of phenyl rings in benzoate (compound 3) and
cinnamate (compound 4) anions are disordered and were treated
using AFIX 66 instruction. In the case of compound 4, SIMU and
DELU instructions has been used to restrain the thermal parameters
of disordered fragments. Because of the high symmetry of the
crystal system it was not possible to localize the lattice solvent
molecules in the pores. Therefore, the SQUEEZE procedure was
used to correct reflection intensities, corresponding to disordered
solvent molecules.19 CCDC-1422814, CCDC-1422816, CCDC-
1422818, and CCDC-1422820 contain the supplementary
crystallographic data for compounds 1, 2, 3, and 4, correspondingly.
These data can be obtained free of charge from the Cambridge
All chemicals were purchased from commercial sources and used
without further purification. N,N-dimethylformamide (DMF)
(anhydrous 99.8 %), trans-cinnamic acid (97 %), acetonitrile
(99.8 %), and 5,5´-dimethyl-2,2´-dipyridyl (98 %, dmpby) were
purchased from Sigma Aldrich. 2,2´-bipyridyl-5,5´-dicarboxylic acid
(H2bpydc) was synthesised according to the reported procedure15
(for more details see ESI). Benzoic acid (99.5 %) and formic acid (99
%) were purchased from Grüssing GmbH, Zn(NO3)2·4H2O from
MERCK, acetic acid (100 %) from ROTH Company, heptane (99 %)
and dichloromethane 9.99 % from Fisher, anhydrous toluene from
VWR, and 1-propanol 99.5 % from TCI.
Synthesis
[Zn3(bpydc)2(HCOO)2] JLU-Liu4 (1)
JLU-Liu4 (1) was synthesized using slightly modified procedure.
Zn(NO3)2·4H2O (52.2 mg, 0.2 mmol), H2bypdc (20 mg, 0.082 mmol)
and formic acid (1 mL, 26.5 mmol) were mixed with 6.5 mL DMF
and transferred to the Pyrex© tubes. The mixture was heated for 24
h at 120 °C. The resulting white octahedral crystals were filtered
and washed 3 times with DMF. Filtration was performed under
argon flow. After that, the solvent was exchanged to ethanol over a
period of 3 days. At the end, the sample was activated for 5 hours
at 70 °C using dynamic vacuum. Yield: 32.2 mg (63 %). After
activation, all samples were handled under argon atmosphere.
PXRD patterns measured for “as made” and activated samples
confirmed the phase purity.
[Zn3(bpydc)2(CH3COO)2] (2)
Crystallographic
www.ccdc.cam.ac.uk/data_request/cif.
Data
Centre
The
via
extended
A mixture of Zn(NO3)2·4H2O (52.2 mg, 0.2 mmol), H2bypdc (20 mg,
0.082 mmol), and acetic acid (1 mL, 26.5 mmol) with 6.5 mL DMF
was added to Pyrex© tubes and heated 24 h at 120 °C. Then crystals
were collected and washed several times first with DMF and then
with ethanol. Subsequently, they were immersed in dry ethanol for
3 days. The crystals were filtered in argon flow and evacuated for
12 h at 70 °C. Yield: 17.26 mg (52.5 %).
Elemental analysis (wt.%): calc. C 42.89, N 6.98, H 2.74; found C
41.13, N 7.26, H 1.03.
[Zn3(bpydc)2(C6H5COO)2] (3)
crystallographic data for the structures 1 - 4 are given in Tables S1
of ESI.
Crystal data for [Zn3(bpydc)2(HCOO)2] (1): C26H14N4O12Zn3, M =
770.52 g mol-1, tetragonal, P43212 (Nr. 96), a = 15.320(2) Ǻ, c =
23.240(5) Ǻ, V = 5454.5(18) Å3, Z = 4, ρcalc= 0.938 g cm-3, λ =
0.88561 Å, T = 298 K, ϴmax = 32.1°, reflections collected/unique
12822/5847, Rint = 0.0194, R1 = 0.0287, wR2 = 0.0852, S = 1.089,
largest diff. peak 0.569 e Å-3 and hole -0.543 e Å -3.
Crystal data for [Zn3(bpydc)2(CH3COO)2] (2): C28H18N4O12Zn3, M =
798.57 g mol-1, tetragonal, P43212 (Nr. 96), a = 15.300(2) Ǻ, c =
23.280(5) Ǻ, V = 5449.6(18) Å3, Z = 4, ρcalc= 0.973 g cm-3, λ =
0.88561 Å, T = 298 K, ϴmax = 32.1°, reflections collected/unique
19800/6337, Rint = 0.0275, R1 = 0.0313, wR2 = 0.0960, S = 1.099,
largest diff. peak 0.629 e Å-3 and hole -0.532 e Å -3.
Crystal data for [Zn3(bpydc)2(C6H5COO)2] (3): C38H22N4O12Zn3, M =
922.70 g mol-1, tetragonal, P43212 (Nr. 96), a = 15.160(2) Ǻ, c =
23.030(5) Ǻ, V = 5292.9(18) Å3, Z = 4, ρcalc= 1.158 g cm-3, λ =
0.88561 Å, T = 298 K, ϴmax = 32.1°, reflections collected/unique
40507/6196, Rint = 0.0556, R1 = 0.0515, wR2 = 0.1546, S = 1.079,
largest diff. peak 0.656 e Å-3 and hole -0.764 e Å -3.
Zn(NO3)2·4H2O (52.2 mg, 0.2 mmol), H2bypdc (20 mg, 0.082 mmol),
and benzoic acid (1.4 g, 11.5 mmol) were dissolved in 6.5 mL DMF.
The mixture was placed into the Pyrex© tubes and heated at 120 °C
for 48 h. Washing and activation procedures were the same as
described for compound 2. Yield: 16 mg (42.65 %).
Elemental analysis (wt.%): calc. C 49.23, N 6.04, H 2.8; found C
48.18, N 6.16, H 1.27.
[Zn3(bpydc)2(C6H5CHCHCOO)2] (4)
A mixture of Zn(NO3)2·4H2O (52.2 mg, 0.2 mmol), H2bypdc (20 mg,
0.082 mmol), and cinnamic acid (1.25 g, 8.4 mmol) with 6.5 mL DMF
was placed into the Pyrex© tubes and heated at 80 °C for 72 h.
Washing and activation procedures were the same as described for
compound 2. Yield: 21 mg (53 %).
Crystal
data
for
[Zn3(bpydc)2(C6H5CH=CHCOO)2]
(4):
C42H26N4O12Zn3, M = 974.78 g mol-1, tetragonal, P43212 (Nr. 96), a =
15.270(2) Ǻ, c = 23.050(5) Ǻ, V = 5374.6(18) Å3, Z = 4, ρcalc= 1.205
Elemental analysis (wt.%): calc. C 51.52, N 5.72, H 3.06; found C
49.77, N 5.87, H 1.96.
g cm-3, λ
= 0.88561 Å, T = 298 K, ϴmax = 32.1°, reflections
2 | J. Name., 2012, 00, 1-3
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collected/unique 20740/6207, Rint = 0.0556, R1 = 0.0377, wR2
=
View Article Online
DOI: 10.1039/C5DT03504J
a)
0.1195, S = 1.123, largest diff. peak 0.500 e Å-3 and hole -0.756 e Å -
3
.
Crystal structures of 1’- 4’ determined from PXRD.
The crystal structures for narrow pore phases were solved from the
PXRD patterns that were measured at room temperature on
evacuated samples at the BESSY KMC-2 beamline of HZB. The data
were collected in transmission geometry using 2 scans in the range
of 5 to 50°. The patterns were indexed using the X-Cell program
implemented in Material Studio 5.0 software.20 The most intensive
reflections in the 2ϴ range between 2 and 20° were used for
indexing. The C2221 space group was chosen for 1’, 2’, and 4’
considering group-subgroup relations and extinction rules. The
starting models for the crystal structure refinement were derived
from “as made” 1, 2, and 4 structures, which were converted into
the corresponding space group. After adapting the unit cell
parameters to those obtained during indexing, the structural
models were optimized using geometry optimization tool with
implementation of universal force field. Due to the low
data/parameter ratio, the rigid-body Rietveld refinement with
energy option was used to refine the structures.
b)
Crystallographically independent Zn atoms, carboxylate groups, and
phenyl rings were treated as rigid bodies.
In the case of compound 2’, the minor impurity of open phase 2 is
present after activation. Therefore, reflections at 7.0° and 11.6° 2ϴ,
which belongs to the open phase 2 (Fig. 3), were excluded from the
Rietveld refinement.
c)
Final Rietveld plots are given in Figure 1. CCDC-1422815, CCDC-
1422817, CCDC-1422819, and CCDC-1422821 contain the
supplementary crystallographic data for compounds 1’, 2’, 3’, and
4’, correspondingly. These data can be obtained free of charge from
the
www.ccdc.cam.ac.uk/data_request/cif.
Refinement data for Zn3(bpydc)2(HCOO)2 (1’): C26H14N4O12Zn3, Fw
Cambridge
Crystallographic
Data
Centre
via
=
770.58, Orthorhombic, C2221 (No. 20), a = 27.821(1) Å, b =
10.184(1) Å, c = 21.626(1) Å, V = 6127.4(3) Å3, Z = 8, 12 refined
motion groups, 56 refined DOF, Rwp = 15.68 %, Rp = 12.20 %.
Refinement data for Zn3(bpydc)2(CH3COO)2 (2’): C28H18N4O12Zn3, Fw
= 798.63, Orthorhombic, C2221 (No. 20), a = 27.715(1) Å, b =
10.575(1) Å, c = 21.691(1) Å, V = 6245.4(3) Å3, Z = 8, 14 refined
motion groups, 68 refined DOF, excluded regions 6.60 – 7.30, 11.50
– 11.77, Rwp = 7.60%, Rp = 5.83%.
d)
Refinement data for Zn3(bpydc)2(C6H5COO)2 (3’): C38H22N4O12Zn3, Fw
= 922.77, Tetragonal, P43212 (No. 96), a = 15.019(1) Å, c = 21.905(2)
Å, V = 4941.4(3) Å3, Z = 4, 6 refined motion groups, 28 refined DOF,
Rwp = 17.99 %, Rp = 12.10 %.
Refinement
data
for
Zn3(bpydc)2(C6H5CHCHCOO)2
(4’):
C42H26N4O12Zn3, Fw = 974.84, Orthorhombic, C2221 (No. 20), a =
17.313 (1) Å, b = 24.594(1) Å, c = 22.139(2) Å, V = 9426.6(3) Å3, Z =
8, 18 refined motion groups, 92 refined DOF, Rwp = 11.65%, Rp
=
8.98 %.
Figure 1. Rietveld plots for compounds 1’ (a), 2’ (b), 3’ (c), and 4’ (d).
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In situ monitoring of nitrogen adsorption (77 K) by powder X-ray crystallize in the same space group P43212, and areViieswoAstrtriculectOunrlianle,
DOI: 10.1039/C5DT03504J
diffraction
possessing similar frameworks with ant topology (Fig. 2a). As
expected, the main difference in the structures is the
monocarboxylic ligand included (Fig. 2b-2e). The volume of
monocarboxylic anions (calculated as Connolly solvent excluded
volume using ChemDraw software) and solvent accessible void
volume change during activation are given in Scheme 1. According
to the PLATON report, the solvent accessible void in the as made
crystal structures changes from 59.4 % in 1 to 32.6 % in 4 (Table 1).
Concerted adsorption of N2 at 77 K and X-ray powder diffraction
experiments on compounds 3 and 4 were performed at KMC-2
beamline of HZB using specially designed instrumentation.21 All
adsorption experiments were performed at 195 K in the relative
pressure range between 10-3 and 1 (p0 = 101.3 kPa). All diffraction
measurements were performed in 2ϴ range from 2 to 50° using
monochromatic synchrotron radiation (8048 eV; λ = 1.5406 Å). The
diffraction images from 2D detector were integrated using
Datasqueeze 2.2.9 software.22 Hexagonal boron nitride was used as
external standard.
Table 1. Calculated textural properties of investigated compounds.
Compound
Solvent
accessible void
volume of unit
cell / %
59.4
Calculated
pore volume
(Vp) / cm3 g-1
Calculated
geometrical surface
area / m2 g-1
Physical measurements
1
1’
2
0.70
0.09
0.63
0.09
0.36
0.30
0.30
0.17
2425
76
2142
66
1047
690
747
188
Thermogravimetric analyses were carried out under air using STA
409 PC from NETZSCH Company. Mass analysis was performed
using Aëolos QMS 403 from NETZSCH Company. The heating rate
was set to 5 K/min. FT-IR spectra were measured on VERTEX-70
spectrophotometer (Bruker) using DRIFT technique. Nitrogen
adsorption measurements up to 100 kPa at 77K were performed on
approximately 35 mg of sample using BELSORP-max apparatus. The
nitrogen with the purity of 99.999% was used for the experiments.
Powder X-ray diffraction patterns were measured on STADI P (Stoe)
diffractometer at RT using monochromatic Cu–Kα1 radiation (λ =
1.5406 Å) (step size: 0.05, exposure time: 23 sec/point) was used.
All measurements were performed in transmission geometry. The
samples were prepared in the Ar-filled Glovebox. Temperature
dependent PXRD measurements were performed on Xpert
diffractometer (PANALYTICAL) in reflection geometry using θ/2θ
scans and MRI (Material Research Instruments) sample stage. PXRD
patterns were collected under dynamic vacuum every 25 °C in the
temperature range between 25 and 450 °C . The heating rate was
set to 3 K/min.
17.6
55.7
2’
3
3’
4
4’
15.8
40.5
36.7
32.6
24.8
After removal of solvent molecules, crystallographic transformation
takes place in all compounds, according to the changes in the PXRD
patterns (Fig. 3). The patterns of all activated compounds (further
referred as 1’, 2’, 3’, and 4’, respectively) were successfully indexed
and the crystal structures were solved using molecular simulations
combined with Rietveld refinement.
Activated phases 1’ and 2’ have very similar structures: both
crystallize in the C2221 space group with similar unit cell parameters
(due to this similarity, only structure of 1’ is described in details
below). The C2221 space group is a sub-group of the P43212 (the
space group of the parent compounds 1 and 2).
Due to the rigidity of the linker, the main contribution to the
structural transformation comes from the cluster distortion. The
reduction of the symmetry creates two independent SBUs (SBU 1
and SBU 2) in the 1’ showing differences in distortion with respect
to the linearity of the Zn3 array and coordination environment of
the ligand molecules. In the SBU 1 the angle between three Zn
atoms decreases from 178.65(1)° in 1 to 160.48(1)° in 1’. The
dihedral angle between oppositely located 2,2’-bipyridil rings
changes from 80.31(5)° in 1 to 57.53(5)° in 1’ (Fig. 4a, 4b). A
significant contribution to the structural contraction originates from
bending of carboxylate hinges (dihedral angles between the plane
of carboxylate group and O-Zn-Zn-O plane). These change from
5.75(3)° and 6.07(3)° in 1 to 19.42(3)° and 20.63(3)° in 1’. This is also
often the source of flexibility and structural transitions in other
switchable MOFs .23, 24 The Zn atoms in SBU 2 are arranged in nearly
linear fashion with a Zn…Zn…Zn angle of 176.67(1)°. The formate
anions, which are coordinated on the opposite sides of SBU in 1
(Fig. 4a, 4b) change the position in SBU 1 of 1’ and move to the
same side of the cluster (Fig. 4b).
Results and Discussion
JLU-Liu4 developed by Liu and co-workers consists of the trinuclear
Zn(II) units coordinated by four carboxylic groups, two 2,2’-
bipyridine units, and two formate anions. The structure comprises
of a 3D framework (ant topology) and displays channels 4.9 × 5.1 Å
in dimensions.13 3D framework of JLU-Liu4 undergoes structural
transformation upon removal of guest molecules from the pores to
give a MOF with gate opening behavior. This is indicated by shifting
and intensity changing of the peaks in the PXRD pattern. However,
a structure of the desolvated compound was not reported yet.
To study the influence of monocarboxylic ligand nature on the
structure formation of JLU-Liu4 (1), acetic (HAc), benzoic (HBz), or
cinnamic (HCn) acids were used in the synthesis instead of originally
reported formic acid.
In all cases crystalline products with the composition
Zn3(bpydc)2(Ac)2 (2), Zn3(bpydc)2(Bz)2 (3), and Zn3(bpydc)2(Cn)2 (4)
could be obtained. Since all new materials could be obtained as
crystals suitable for single crystal X-ray analysis, the crystal
structures of all new materials were determined. All compounds
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Figure 3. PXRD patterns for compounds 1 - 4 and 1’ - 4’.
The comparison of the calculated pore volumes for 3 and 3’ shows
only minor changes from 0.36 cm3 g-1 to 0.30 cm3 g-1.
The thermal stability of the investigated materials was studied by
temperature dependent PXRD and thermogravimetric (TG) analysis
(Fig. S2 – S8 ESI). Investigation of the 1’ by thermo-XRD in argon
atmosphere confirms retention of crystallinity up to 300 °C (see ESI,
Fig. S1) confirming the TG data reported earlier.13 According to TG
analysis data, materials 2’, 3’, and 4’ are stable up to 300 °C, 330 °C,
and 320 °C, respectively (see ESI, Fig. S6 – S8). Therefore, the
samples were activated at 150 °C in vacuum prior to the adsorption
experiments.
Figure 2. a) View on the structure of the JLU-LIU4 framework along
c axis (green spheres represent monocarboxylate anions). b-e)
Structure of SBUs in compounds 1 - 4. Colour code: turquoise – Zn
atoms, grey – carbon atoms, red – oxygen atoms, blue – nitrogen
atoms.
On the framework level, the structural contraction in 1 occurs along
[110] direction (Fig. 4c), corresponding to [010] direction in 1’ (Fig.
4d).
The textural properties of 1’ calculated using Poreblazer 3.0.2
software25 show, that the compound has nearly no accessible pore
volume (0.09 cm3 g-1) as well as very small pore limiting diameter of
2.11 Å. This completely supports the experimental results, since 1’
shows no uptake of nitrogen up to reaching “gate opening
pressure” of 0.47.
The crystal structure of compound 3’ was refined in the same space
group as the structure of 3 (Fig. 1) with different lattice cell
constants. Obviously, the compound 3 cannot undergo complete
transformation because of bulky phenyl rings of the benzoate. It
hinders the cluster transformation as it occurs in 1. Namely, two
coordinated benzoate anions, located on the opposite sides of the
Zn cluster in 3 cannot regroup in the same way as in 1, and are
located on the same side of the SBU plane, because of the small
aperture between the ligand molecules, coordinated to the cluster.
Figure 4. a) SBU 1 of 1; b) SBU 1 and SBU 2 of 1’; c) crystal structure
of 1; d) crystal structure of 1’. (Colour code in (c) and (d): blue
spheres – SBU units, red spheres – bpydc2- linkers).
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compounds 3’ and 4’ in semilogarithmic scale. BlacVkiewcirAcrtlieclse O–nl1in’e,
DOI: 10.1039/C5DT03504J
blue triangles – 2’, red squares – 3’, green diamonds – 4’. Close
symbols – adsorption, open symbols – desorption.
Compound 1’ shows classical gate pressure behaviour during
physisorption of nitrogen at 77K, with almost no uptake up to p/p0
= 0.47. The total pore volume calculated at p/p0 = 0.95 amounts to
0.55 cm3 g-1 which is slightly lower than the theoretical value (Table
1). As expected, the nitrogen adsorption isotherm of 2’ shows a
slightly different trend, reaching the plateau after adsorbing nearly
25 cm3 g-1 nitrogen at very low relative pressures. After reaching
p/p0 = 0.3 very steep increase in uptake takes place. The isotherm
reaches a second plateau at p/p0 = 0.95. The adsorbed amount in
saturation is 295 cm3 g-1 that correspond to the pore volume of
0.456 cm3 g-1. Materials 3’ and 4’ show completely different
behaviour during the adsorption of the nitrogen at 77 K (Fig. 5).
Thus, compound 3’ adsorbs nearly 100 cm3 g-1 of nitrogen already at
p/p0 = 10-4 confirming the incompleteness of crystal structure
closing (narrow pore state) of 3’ discussed above. The second step
in the isotherm, most likely connected to the structural
transformation, occurs in the relative pressure range between 0.03
and 0.07. After that the course of the isotherm is characterized by
the continuous increase of the uptake reaching 227 cm3g-1 at p/p0 =
0.97.
Scheme 1. Schematic representation of the monocarboxylic ligand
influence on the activation of the LiU-4 series. The volume of
monocarboxylic anions was calculated as Connolly solvent excluded
volume using ChemDraw software. The volume of the cell
represents the solvent accessible volume for Z = 4.
Implementation of the larger cinnamate anion into JLU-Liu4 has
further influence on the adsorption properties of the solid. As in the
case of 3’, the hysteresis related to the guest induced structural
changes is shifted to lower pressure range, and is located between
p/p0 = 5∙10-3 and p/p0 = 5∙10-2 . The isotherm reaches saturation at
p/p0 = 0.95 showing the nitrogen uptake of 203 cm3g-1.
While the structural transformations during the adsorption of
nitrogen on 1’ and 2’ is supposed to be one step “gate opening”,
the adsorption behaviour of 3’ and 4’ was studied by parallelized
adsorption and PXRD experiments. PXRD patterns measured on
evacuated material 3’ and at the first adsorption point (Fig. 6, PXRD
No. 1) show the presence of narrow pore phase. Surprisingly,
further five PXRD patterns, measured in the relative pressure range
from 5∙10-4 up to 10-1 show a shift of the main (101) reflection from
2 = 7.19° to 7.44°, indicating the formation of an intermediate
phase (ip) and contraction of the unit cell (Fig. 4, PXRD patterns No.
2
- 6). Unfortunately, the patterns could not be indexed
unambiguously because of the presence of the reflections of more
than one phase. Further increase of the gas pressure leads to the
gradual shift of the (101) reflection back to lower angles, but the
initial position is not completely reached (Fig. 4, PXRDs No 2 and 7).
PXRD patterns, measured in the p/p0 range of 0.1 to 1 are
characterized by intensity increase of the reflection at 2 = 6.98°,
characteristic for the as made phase 3. Although the PXRD patterns
from 2 to 6 could not be indexed, the shape of the isotherm as well
as reflection shift suggest that the transition is similar to breathing
phenomena, observed in MIL-53(Cr) and MIL-53(Al) materials.26
Desorption of nitrogen proceeds without phase transitions showing
the presence of mixture of compounds 3’ and 3 up to low
pressures. Only evacuation of the sample after the experiment at
Figure 5. Nitrogen physisorption isotherms for compounds 1’- 4’ at
77 K: a) linear representation of the isotherms; b) isotherms of
6 | J. Name., 2012, 00, 1-3
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room temperature leads to the structural transformation to the presented. Only at relative pressures lower than 10-2,VtiheweArretivcleerOsniblinlee
DOI: 10.1039/C5DT03504J
transformation from 4 to intermediate phase is observed.
initial narrow pore state.
Nitrogen physisorption at 77 K combined with PXRD analysis on 4’
(Fig. 7) reveals completely different structural transformations
induced by the cinnamate anion. The PXRD pattern of completely
evacuated 4’ shows a reflection at 2θ = 7.50° matching the
intermediate phase, obtained during the adsorption of nitrogen on
3’. Indeed, the direct comparison of above mentioned PXRD
patterns shows good match (Fig. S11, ESI). The indexing of the PXRD
patterns obtained from evacuated 4’ gives several possible unit cells
one of which is an orthorhombic C-centered one. Taking into
account that 1’ crystallizes in the same crystal system, this unit cell
was used for the Rietveld refinement. Pore volume calculated from
the final structural model matches with value estimated from the
adsorption isotherm of the compound.
Figure 7. In situ adsorption of N2 at 77K on 4’ by PXRD.
Figure 6. In situ adsorption of N2 at 77K on 3’ by PXRD.
Although the adsorption isotherms of 3’ and 4’ are similar to some
extent, in situ adsorption/PXRD experiments suggest a distinct
pathway for the structural evolution in 4’. In case of 4’, the most
intense reflection shifts towards lower 2 angles with increasing
pressure, probably indicating the stepwise opening of the crystal
structure. The phase 4’ exists up to the relative pressure of 0.013.
Further adsorption of nitrogen leads to phase transition,
accompanied by the shift of most intense reflection from 7.50° to
7.29° 2. Unfortunately the pure intermediate phase could not be
obtained and therefore PXRD patterns could not be indexed
unambiguously. In the relative pressure range between 0.2 and 1
during the adsorption and desorption almost pure phase 4 is
Scheme 2. Mechanisms of structural transformation during the
adsorption of N2 at 77 K for 1 - 4.
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Conclusions
9.
S. Henke, A. Schneemann, A. Wütscher and R. A. Fischer,
View Article Online
J. Am. Chem. Soc., 2012, 134, 9464D-9O4I:7140..1039/C5DT03504J
P. Horcajada, F. Salles, S. Wuttke, T. Devic, D. Heurtaux, G.
Maurin, A. Vimont, M. Daturi, O. David, E. Magnier, N.
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O. Kozachuk, M. Meilikhov, K. Yusenko, A. Schneemann,
B. Jee, A. V. Kuttatheyil, M. Bertmer, C. Sternemann, A.
Pöppl and R. A. Fischer, Eur. J. Inorg. Chem., 2013, 2013,
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Y. Sakata, S. Furukawa, M. Kondo, K. Hirai, N. Horike, Y.
Takashima, H. Uehara, N. Louvain, M. Meilikhov, T.
Tsuruoka, S. Isoda, W. Kosaka, O. Sakata and S. Kitagawa,
Science, 2013, 339, 193-196.
In summary, the fine tuning of the gate pressure and switchability
in JLU-Liu-4 was possible by modification of the SBU. Compounds
containing coordinated formate, acetate, benzoate or cinnamate
anions on cluster have the same frameworks with ant topology, but
show variations in structural transformations during the adsorption
of nitrogen at 77 K. While the utilisation of small carboxylic acids
during the synthesis (compounds 1 and 2) results in complete
closing of the framework after solvent removal and therefore “gate
opening” behaviour during the adsorption, bulkier carboxylates
lead to incomplete closing of the frameworks after desolvation.
Compounds containing benzoate (3’) and cinnamate (4’) anions
10.
11.
12.
show nearly the same adsorption behaviour but undergo 13.
completely different structural transformation during the
J. Wang, J. Luo, J. Zhao, D.-S. Li, G. Li, Q. Huo and Y. Liu,
Cryst. Growth Des., 2014, 14, 2375-2380.
14.
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I. Gamba, I. Salvadó, G. Rama, M. Bertazzon, M. I.
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adsorption. While the structural changes during adsorption on 3’
could be rather described as classical breathing,27 the changes in 4’
lead to the step-wise increase of the unit cell volume. We believe,
15.
this approach can be further generalized and applied for other
flexible MOFs with coordination vacancies on the metal cluster
16.
17.
(typically Zn3 and Co3 clusters) in order to obtain materials with
targeted adsorption properties.
Acknowledgements
N.K. thanks the “excellence initiative by the German federal
and state government” (Institutional strategy, measure
“support the best”). V.B. thanks the German Federal Ministry
for education and research (Project BMBF No 05K13OD3). The
HZB is gratefully acknowledged for the allocation of
synchrotron radiation beamtime on KMC-2 and MX BL14.2
beamlines and for travel grants.
18.
19.
20.
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