A family of highly stable lanthanide metal-organic frameworks: Structural evolution and catalytic activity

Article abstract of DOI:10.1021/cm100503q

A family of homeotypic porous lanthanide metal-organic frameworks (MOFs), [Ln(btc)(H₂O)]·guest (Nd (1), Sm (2), Eu (3), Gd (4), Tb (5), Ho (6), Er (7), and Yb (8); guest: DMF or H₂O) was synthesized. The structures of the as-synthesi

Full text of DOI:10.1021/cm100503q

3
316 Chem. Mater. 2010, 22, 3316–3322
DOI:10.1021/cm100503q
A Family of Highly Stable Lanthanide Metal-Organic Frameworks:
Structural Evolution and Catalytic Activity
†
,‡
†,§
,†,§
Mikaela Gustafsson, Agnieszka Bartoszewicz, Bel ꢀe n Martı
´
n-Matute,*
Junliang Sun, Jekabs Grins, Tony Zhao, Zhongyue Li, Guangshan Zhu, and
†,‡
†,‡
§
,†,‡
Xiaodong Zou*
†
‡
Berzelii Centre EXSELENT on Porous Materials, Inorganic and Structural Chemistry, Department of
§
Materials and Environmental Chemistry, and Organic Chemistry, Stockholm University, SE-106 91
Stockholm, Sweden, and State Key Laboratory of Inorganic Synthesis and Preparative Chemistry,
College of Chemistry, Jilin University, Changchun 130023, China
Received July 2, 2009. Revised Manuscript Received April 16, 2010
A family of homeotypic porous lanthanide metal-organic frameworks (MOFs), [Ln(btc)(H O)]
2
3
guest (Nd (1), Sm (2), Eu (3), Gd (4), Tb (5), Ho (6), Er (7), and Yb (8); guest: DMF or H O) was
2
synthesized. The structures of the as-synthesized compounds are tetragonal and contain 1D channels
with accessible lanthanide ions. In situ single crystal X-ray diffraction shows that 1 undergoes a
single-crystal to polycrystalline to single-crystal transformation from room temperature to 180 °C.
During the release of DMF and water molecules from the channels by evacuation and subsequent
heating, the structures of 1 and 7 transformed from tetragonal to monoclinic, and then to tetragonal,
while the structure of 8 remained tetragonal. The transformation between the monoclinic and the low
temperature tetragonal phases is reversible. The Ln(btc) MOFs are stable to at least 480 °C and are
among the most thermally stable MOFs. The Ln(btc) MOFs act as efficient Lewis acid catalysts for
the cyanosilylation of aldehydes yielding cyanohydrins in high yields within short reaction times.
1 also catalyzes the cyanosilylation of less reactive substrates, such as ketones at room temperature.
The Ln(btc) MOFs could be recycled and reused without loss of their crystallinity and activity.
Introduction
As a result of the porous nature of MOFs and their
structural flexibility, large efforts have been made toward
imparting catalytic properties to MOFs for heteroge-
Porous solids constitute today an important part in the
developments of functional materials in various disciplines
such as heterogeneous catalysis, gas processing, sensor
technology, and so forth. Metal-organic frameworks
4
neous catalysis. Using metal ions in the framework as
active centers for catalysis is one of the most common
5
ways to generate catalytically active MOFs. Immobiliza-
(
MOFs), or coordination polymers, are constructed by
tion of metal complexes within the MOFs to create
6-8
metal ions/clusters that are linked by organic ligands and
belong to an interesting class of functional micro- and
1
mesoporous materials. The size of periodic pores/chan-
heterogeneous catalysts has also been reported.
Some
(
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1
d,2
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*
E-mail: xzou@mmk.su.se, belen@organ.su.se.
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pubs.acs.org/cm
Published on Web 05/07/2010
r 2010 American Chemical Society

Article
Chem. Mater., Vol. 22, No. 11, 2010 3317
of the reported MOFs that were used as catalysts also
9
homeotypic lanthanide MOFs, [Ln(btc)(H O)] guest
2
3
-11
showed size selectivity.
MOFs based on lanthanide ions have attracted great
interest during the past decade, primarily for their lumi-
(Ln: Nd (1), Sm (2), Eu (3), Gd (4), Tb (5), Ho (6), Er
(7), and Yb (8); guest: DMF (dimethylformamide) or H O).
2
The lanthanide MOFs were characterized by X-ray powder
diffraction (XRPD). The structures of 1 and 8 were
determined in this study by single crystal X-ray diffrac-
tion, and the structures of 3 and 5 were previously
1
2,13
nescence and magnetic properties.
Lanthanide-based
MOFs have great potential to be excellent heterogeneous
catalysts since lanthanide ions have a flexible coordina-
tion sphere and can create coordinatively unsaturated
1
9a,c
reported.
Structural changes of 1, 7 and 8 under
1
4
metal centers. There are some reports where lanthanide-
5-18
evacuation and heating were further studied by in situ
XRPD. The structural transformation of 1 during heating
was also studied by in situ single crystal X-ray diffraction.
Finally the catalytic performance of the lanthanide MOFs
in terms of activity, heterogeneity, and recyclability were
tested in the cyanosilylation of aldehydes and ketones.
1
based MOFs have been used as catalysts.
In search-
ing for new lanthanide-based MOF catalysts, we have
focused our attention on a family of lanthanide MOFs
constructed by seven-coordinated Ln(III) ions linked by
1
,3,5-benzenetricarboxylates (BTCs). Three such iso-
structural MOFs have been reported, [Tb(btc)(H O)]
2
3
19b
1
9a
(
H O)0.5DMF (MOF-76),
2
[Dy(btc)(H O)] DMF,
2
1
and [Eu(btc)(H O)] (H O)1.5. These lanthanide MOFs
3
Experimental Section
9c
2
3
2
All chemicals were purchased from commercial suppliers and
used without further purification.
contain 1D channels and show high thermal stability
(
350-500 °C). The coordinated water molecules and
Syntheses of [Ln(btc)(H
2
O)] guest (Ln: Nd (1), Sm (2), Eu (3),
3
guest species in the channels can be removed by evacuation
and heating to form permanent pores and to give coordi-
natively unsaturated metal centers. Recently, we have
synthesized another lanthanide MOF in the family [Nd-
Gd (4), Tb (5), Ho (6), Er (7), and Yb (8)). A mixture of
-4
Ln(NO3)3 nH O (n=5-6) (0.04 g, 1ꢀ10 mol), 1,3,5-benze-
3
2
-4
netricarboxylic acid (H
formamide (DMF) (10 mL), distilled water (2 mL), cyclohexanol
3
BTC) (0.02 g, 1ꢀ10 mol), dimethyl-
-
1
(
that 1is homeotypic to the three above-mentioned lanthanide
btc)(H O)] (H O) DMF (1). Preliminary results showed
2 2 0.5
3
(2 mL), dibutylamine (two drops), and 2 mol L HNO (three
3
3
drops) was added to a 50 mL glass beaker. The transparent
solution (pH=5) was stirred for 2 h at room temperature and
placed in an oven at 85 °C for 16 h. The products, rod-like
2
0
MOFs but with a different space group. This prompted
us to make a systematic study of the lanthanide MOF family
and their potential as heterogeneous Lewis acid catalysts.
Here we report the synthesis, structural evolution,
characterization, and catalytic activity of a series of new
3
crystals of 10ꢀ10ꢀ1000 μm in size (1: purple; 2-5, 8: colorless;
6
-7: pink), were recovered by filtration, washed with DMF
(
∼15 mL) and methanol (∼10 mL), and finally dried at room
temperature. 1-8 are stable in air, water, and common organic
solvents.
(
(
(
7) Alaerts, L.; Wahlen, J.; Jacobs, P. A.; De Vos, D. E. Chem.
Commun. 2008, 1727–1737.
8) Alkordi, M. H.; Liu, Y.; Larsen, R. W.; Eubank, J. F.; Eddaoudi,
M. J. Am. Chem. Soc. 2008, 130, 12639–12641.
9) (a) Dybtsev, D. N.; Nuzhdin, A. L.; Chun, H.; Bryliakov, K. P.;
Single Crystal X-ray Diffraction. Single crystal X-ray diffrac-
tion data of the as-synthesized 1 (Nd, 1(as)) were collected at
2
0 °C on a Bruker SMART diffractometer equipped with a
CCD camera and using Mo KR (λ=0.71073 A) radiation. In situ
single crystal X-ray diffraction was performed on an XCalibur3
˚
Talsi, E. P.; Fredin, V. P.; Kim, K. Angew. Chem., Int. Ed. 2006, 45,
916. (b) Cho, S.-H.; Ma, B.; Nguyen, S. T.; Hupp, J. T.; Albrechy-
Schmitt, T. E. Chem. Commun. 2006, 2563–2565. (c) Hasegawa, S.;
Horike, S.; Matsuda, R.; Furukawa, S.; Mochizuki, K.; Kinoshita, Y.;
Kitagawa, S. J. Am. Chem. Soc. 2007, 129, 2607. (d) Wu, C.-D.; Lin,
W. Angew. Chem., Int. Ed. 2007, 46, 1075–1078.
diffractometer equipped with a CCD camera and using Mo KR
(λ=0.71073 A) radiation. Single crystal X-ray diffraction data
of the as-synthesized 8 (Yb, 8(as)) were collected on an
˚
(
(
(
(
(
(
(
10) Seo, J. S.; Whang, D.; Lee, H.; Jun, S. I.; Oh, J.; Jeon, Y. J.; Kim, K.
˚
MarCCD at 100 K using synchrotron radiation (λ=0.907 A)
Nature 2000, 404, 982–986.
11) Zecchina, A.; Groppo, E.; Bordiga, S. Chem.;Eur. J. 2007, 2,
at the Beamline I911:5, Max Lab, Lund University, Sweden.
The structures were solved and refined using the SHELX-97
2
440–2460.
12) Li, Z.; Zhu, G.; Guo, X.; Zhao, X.; Jin, Z.; Qiu, S. Inorg. Chem.
007, 46, 5174–5178.
2
program. The 1(as) crystallizes in space group P4 with the
1
3
2
˚
unit cell parameters a=10.4278(4) and c=14.2602(12) A. R1=
0.0314 and wR2 = 0.0572 for all 3121 unique reflections. At
13) Guo, X.; Zhu, G.; Sun, F.; Li, Z.; Zhao, X.; Li, X.; Wang, H.; Qiu,
S. Inorg. Chem. 2006, 45, 2581–2587.
14) Kitagawa, S.; Noro, S.-I.; Nakamura, T. Chem. Commun. 2006,
1
1
80 °C, the space group of 1(180 °C) changed to P4 22, with a=
3
7
01–707.
15) Evans, O. R.; Ngo, H. L.; Lin, W. J. Am. Chem. Soc. 2001, 123,
0395–10396.
16) (a) G ꢀa ndara, F.; Garcı
Guti ꢀe rrez-Puebla, E.; Monge, A. M.; Snejko, N. Inorg. Chem. 2007,
6, 3475–3484. (b) Snejko, N.; Cascales, C.; Gomez-Lor, B.; Guti ꢀe rrez-
˚
0.4520(2) and c = 13.8486(9) A. R1 = 0.0750 and wR2 =
1
0.0567 for all the 1509 unique reflections. 8(as) crystallizes in
˚
space group P4 22 with a=10.2410(3) and c=14.4026(5) A,
´
a-Cort ꢀe s, A.; Cascales, C.; G oꢀ mez-Lor, B.;
1
R1=0.0296 and wR2=0.0795 for all the 1382 unique reflections.
Crystallographic data and structure refinements of 1(as),
1(180 °C) and 8(as) are given in the Supporting Information
Table S1.
4
Puebla, E.; Iglesias, M.; Ruiz-Valero, C.; Monge, M. A. Chem.
Commun. 2002, 1366–1367. (c) G ꢀa ndara, F.; de Andr ꢀe s, A.; G ꢀo mez-
Lor, B.; Guti ꢀe rrez-Puebla, E.; Iglesias, M.; Monge, M. A.; Proserpio,
D. M.; Snejko, N. Cryst. Growth Des. 2008, 8, 378–380.
X-ray Powder Diffraction Analysis. X-ray powder diffraction
(XRPD) was performed on a PANalytical X’Pert PRO diffracto-
(
(
17) Han, J. W.; Hill, C. L. J. Am. Chem. Soc. 2007, 129, 15094–15095.
18) Dewa, T.; Saiki, T.; Aoyama, Y J. Am. Chem. Soc. 2001, 123,
502–503.
meter equipped with a Pixel detector and using Cu KR
radiation (λ=1.5406 A). All samples were ground prior to data
1
(
19) (a) Rosi, N. L.; Kim, J.; Eddaudi, M.; Chen, B.; O’Keeffe, M.;
Yaghi, O. M. J. Am. Chem. Soc. 2005, 127, 1504–1518. (b) Gao,
X. D.; Zhu, G. S.; Li, Z. Y.; Sun, F. X.; Yang, Z. H.; Qiu, S. L. Chem.
Commun. 2006, 3172–3174. (c) Chen, B.; Yang, Y.; Zapata, F.; Lin, G.;
Qian, G.; Lobkovsky, E. B. Adv. Mater. 2007, 19, 1693–1696.
20) Gustafsson, M.; Li, Z.; Zhu, G.; Qiu, S.; Grins, J.; Zou, X. Stud.
Surf. Sci. Catal. 2008, 174, 451–454.
˚
collection and dispersed uniformly on zero-background Si
plates.
(
(21) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112–122.

3
318 Chem. Mater., Vol. 22, No. 11, 2010
Gustafsson et al.
In situ XRPD data were collected from RT to 600 °C in
vacuum on a PANalytical X’Pert PRO MPD diffractometer
equipped with an Anton-Parr XRK900 reaction chamber, using
˚
Cu KR radiation (λ=1.5418 A) and variable slits. The heat-
ing rate was 2 °C/min, and the temperature was equilibrated for
2
min prior to each data collection. Measurements in vacuum
ca. 0.1 mbar) were performed using a closed Macor glass
(
ceramic sample holder. For measurements under a nitrogen
atmosphere, an open Macor glass ceramic sample holder with a
sieve-like bottom (pore size 0.5 mm) was used. The temperature
was controlled by a thermocouple approximately 3 mm from the
sample.
High quality XRPD data of 1 (Nd) were collected with the 2θ
range of 5-90° in vacuum and in situ at 25, 80, 120, and 220 °C
to study the structural changes. A heating rate of 2 °C/min was
used, and the temperature of the sample was equilibrated for 15
min prior to data collection. Data were collected using variable
Figure 1. Crystal structure of 1 illustrating (a) chiral chains of seven-
coordinated Nd clusters linked together with tridentate BTC ligands and
2
˚
(b) the channels (7.0 ꢀ 7.0 A
) in parallel to the c-axis. The coordinated
water molecules (light blue) are pointing toward the channel centers. Nd
atoms are shown in orange, oxygen in red, and carbon in black. The DMF
and uncoordinated water moleculesare disorderedin the channels and are
not shown.
2
slits and 1 cm irradiated sample area with a total measuring
time of 1 h, yielding patterns with maximum peak intensities of
approximately 10 000 counts. The unit cell parameters of 1-8
were refined from the corresponding XRPD patterns using the
and 8(as) (Yb) are chiral, with the space groups P4 and
3
22
FullProf program.
P4 22, respectively. The asymmetric unit consists of one
1
Other Analyses. Thermogravimetric Analysis (TGA) was
performed under a nitrogen atmosphere from 28 to 650 °C with
a heating rate of 1 °C/min using a high-resolution thermogravi-
metric analyzer (Perkin Elmer TGA 7). Elemental analyses of
C, H, and N were performed at the elemental analysis section at
Santiago de Compostela University using Fisons Instruments
unique Ln(III) ion and one unique BTC ligand. The
Ln(III) ion is seven-coordinated and binds to six oxygen
atoms from six different BTC ligands and one water
molecule (Supporting Information Figure S1c). Each
BTC ligand binds to six different Ln(III) ions. The LnO6-
(H O) polyhedra are located around the 4 /4 axis and
2 1 3
1
108. The samples were exposed to a constant He flow before
connected by corner-sharing to form chiral -Ln-O-
C-O-Ln- chains along the c-axis (Figure 1). These
chains are linked together via the BTC ligands in the
a- and b-directions to form a 3D framework, with chan-
nels running along the c-axis. The coordinated water
molecules point toward the channels. Although each
crystal exhibits only one hand, the sample is not in an
enantiopure form. The chemical formulas deduced from
the elemental analysis are [Nd(btc)(H O)] (H O)0.5DMF
analyses. Nitrogen adsorption and desorption isotherm was
measured at 77 K on a Micromeritics ASAP 2020 system. The
sample was degassed at 220 °C for 6 h. FT-IR spectroscopy was
performed on a Varian 670-IR spectrometer to check the coordi-
nation of the lanthanide atoms with water and the aldehyde
substrate.
Catalytic Studies. Cyanosilylation of aldehydes and ketones
was used for studying the catalytic performance of the Ln(btc)
MOFs as Lewis acid catalysts. These studies were performed on
four MOF samples 1 (Nd), 6 (Ho), 7 (Er), and 8 (Yb), which
were grinded and then examined by SEM to ensure that the
samples had similar particle sizes so that the effects of the
particle sizes on the reactions were minimized. Samples from
the same synthesis batch were used for all the catalytic experiments.
A MOF sample (4.5-9.0 mol %) was activated in vacuum
2
3
2
for 1 (observed (wt %): C 31.4, H 2.8, N 3.0; calculated
(wt %): C 31.9, H 2.9, N 3.1) and [Yb(btc)(H O)]
2
3
(H O)0.5(DMF) for 8 (observed: C 27.4%, H 2.2%, N
2 0.6
1
.9%; calculated C 28.8%, H 2.3%, N 1.9%). The DMF
and uncoordinated water molecules are disordered in
the channels. All water and DMF molecules can be re-
moved upon heating. The free diameter of the channel is
(
0.2 mbar) at 120 °C for 4 h to generate unsaturated lanthanide
metal centers and then cooled down to room temperature under
a nitrogen atmosphere. Then a mixture of trimethylsilyl cyanide
2
˚
about 7.0ꢀ7.0 A , if the water and DMF molecules are
(
was added. The reaction mixtures were stirred at room tem-
TMSCN, 140 μL, 1 mmol) and aldehyde (or ketone) (0.5 mmol)
removed.
The unit cell parameters of the different [Ln(btc)(H O)]
1
perature, and aliquots were taken and analyzed by H NMR
2
3
guest MOFs obtained from the XRPD data are compared
3
spectroscopy at given time intervals. Details of the catalytic
experiments and analysis are given in the Supporting Informa-
tion S17).
2
to the ionic radii for the corresponding Ln(III) ions
Figure 2 and Supporting Information Table S2). The
(
a- (and b-) parameters decrease linearly with decreasing
ionic radius of the Ln(III) ion (giving shorter Ln-O
distances), which in turn decreases with increasing atomic
number. However, a similar trend is not observed for
the c-parameter, which increases from Nd to Gd and then
decreases. The structures of 1 (Nd) and 8 (Yb) refined
Results and Discussion
2
4
Structures of the Lanthanide MOFs. All the as-synthe-
sized [Ln(btc)(H O)] guest MOFs (1(as)-8(as)) are homeo-
2
3
typic, as shown by the similarity of their XRPD patterns
Supporting Information Figure S2). Single crystal X-ray
diffraction shows that the structures of both 1(as) (Nd)
(
(23) Shannon, R. D. Acta Crystallogr. 1976, A32, 751–767.
(
24) (a) Gerkin, R. E.; Reppart, W. J. Acta Crystallogr. 1984, C40, 781–
7
86. (b) Chatterjee, A.; Maslen, E. N.; Watson, K. J. Acta Crystallogr.
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2005.

Article
Chem. Mater., Vol. 22, No. 11, 2010 3319
Figure 3. In situ XRPD patterns of 1 heated in vacuum. The peaks were
shifted to high angles, and the intensities of the weak reflections (110),
Figure 2. Unit cell parameters a (2) and c (0) of 1-8 obtained from
XRPD, compared with the ionic radii of Ln(III) ions. The a parameter
decreases linearly with the increasing ionic radius. The c parameter
increases first from 1 (Nd) to 4 (Gd) and then decreases from 4 (Gd) to
(
111), and (112) decreased significantly once the vacuum was applied. In
additionsomepeaks (for example, (011) and(210)) were split into two. No
further changes were observed after the sample had been in vacuum for
2 h. The XRPD pattern was further changed upon heating. The structure
changed from tetragonal (1(as)) to monoclinic (1(25 °C), 1(80 °C), and
8
0
(Yb). The standard deviations of the unit cell parameters are between
˚
.001 and 0.003 A.
1
(120 °C)) and finally back to tetragonal (1(220 °C). The tetragonal phase
was stable up to 560 °C. The pattern at 160 °C is a mixture of 1(220 °C)
main phase) and 1(120 °C) (minor phase).
from single crystal X-ray diffraction data show that
˚
the Yb-O distances (2.26(1) A) are shorter compared
(
˚
with those of the Nd-O distances (2.40(8) A), resulting in
the shorter a- and b-axes. The longer c-axis for 8 com-
up to 80 °C. The XRPD pattern recorded at 120 °C could
be indexed by a monoclinic phase, 1(120 °C), with a larger
pared to that for 1 is due to the larger O-Yb-O angles
˚
(
165.4(4)°, O3-Yb-O3) compared to the correspond-
γ angle (111.0°) and a shorter c-parameter (13.37 A) com-
ing O-Nd-O angles (154.7(2)°, O2-Nd-O4) along the
c-axis.
pared to those of 1(80 °C). The unit cell volume decreased
by 14% from 1(as) to 1(120 °C) (Table 1). The monoclinic
phase 1(120 °C) transformed to a tetragonal phase 1(220 °C)
upon heating from 120 to 220 °C. The transformation was
completed at this temperature. Further heating to 560 °C
only gave a small contraction of the unit cell (Figure 3)
and no significant structural changes were observed.
It is important to know if the phase transformation led
to significant structural changes of the framework since
they may change the coordination environment of the
Ln(III) ions. In situ high quality XRPD data of 1(as),
1(80 °C), 1(120 °C), and 1(220 °C) were collected at 25, 80,
120, and 220 °C, respectively, to identify the structural
changes. The XRPD data allowed us to locate the posi-
tions of the Nd(III) ions, while the positions of the BTC
and the coordinated water molecules could only be deter-
mined approximately. Unfortunately, it was not possible
to perform reliable structure refinements on the XRPD
data due to the large scattering difference between the
Nd(III) and the BTC molecules.
Structural Changes during Evacuation and Heating. In
situ XRPD showed that 1 (Nd) undergoes structural
changes upon removal of the DMF and water molecules,
from tetragonal to monoclinic and then back to tetra-
gonal. The structure of 1(as) started to change prior to the
heating as soon as vacuum (ca. 0.1 mbar) was applied
(
Figure 3). The structure was finally stabilized after 2 h
leading to a second phase with a significant change in the
XRPD pattern (1(25 °C), Figure 3). Subsequent heating
to 80 °C resulted in only minor shifts of the peak positions
(
1(80 °C), Figure 3). At 120 °C, 1(80 °C) transformed
completely to a third phase, 1(120 °C) (Figure 3). A fourth
phase (1(220 °C)) started to form above 120 °C and the
phase transformation finally completed at 220 °C. Further
heating only resulted in some minor shifts of the peak
positions to high angles.
The unit cell parameters refined from the XRPD
patterns at different temperatures (Table 1) show that
1
(as) transformed from tetragonal to monoclinic (1(25 °C))
In situ X-ray diffraction on a single crystal of 1 was then
performed to determine the structural changes. The crys-
tal was heated in air from room temperature to 180 °C,
and the diffraction was monitored simultaneously. To
our surprise, the reflections almost disappeared when the
crystal was heated to 80 °C and then came back at 180 °C.
within 15 min when vacuum was applied. The major
change was the γangle, from 90.0° to 106.5°. Thec-parameter
decreased by 0.44 A, from 14.22 to 13.78 A, while the
˚
a-parameter only decreased slightly (<0.12 A). Only
minor changes were observed when the sample was heated
˚
˚
Table 1. Unit Cell Parameters of 1 in Air, in Vacuum, and at Different Temperatures, from in Situ XRPD Data
3
V [A ]
˚
a [A]
˚
b [A]
˚
c [A]
˚
phases
temperature [°C]
γ [deg]
1
1
1
1
1
1
25 (no vacuum)
25 (vacuum)
80
120
220
560
10.411(2)
10.291(3)
10.2869(5)
10.318(1)
10.473(1)
10.460(1)
10.411(2)
10.327(3)
10.2873(5)
10.3017(8)
10.4735(7)
10.460(1)
14.225(3)
13.777(8)
13.7604(4)
13.3739(8)
13.832(2)
13.851(2)
90
1542
1404
1395
1328
1517
1484
(25 °C)
(80 °C)
(120 °C)
(220 °C)
(560 °C)
106.47(1)
106.644(3)
110.99(2)
90
90

3
320 Chem. Mater., Vol. 22, No. 11, 2010
Gustafsson et al.
After three hours of preheating at 180 °C to ensure a
complete structural transformation, single crystal X-ray
diffraction data were collected at this temperature. The
high-temperature phase 1 (180 °C) has a similar structure
Thermogravimetric Analysis (TGA). Thermogravi-
metric analyses of 1 (Nd) and 8 (Yb) showed weight
losses in several steps from 28 to 600 °C (Supporting
Information Figure S6). The total weight loss of 1 before
280 °C corresponds to the loss of all water and DMF
molecules. 1 started to lose its BTC molecules at 480 °C
and consequently the structure started to collapse. We
noticed that the temperatures at which the different
molecules started to leave 1 are different from those
observed by in situ XRPD. This may be due to the
different heating rate and different atmosphere used in
the two experiments. 8 has a slightly higher thermal
stability than 1, and started to lose the BTC molecules
at 500 °C. The nitrogen adsorption and desorption study
of 1 degassed at 220 °C for 6 h shows that it is a
microporous solid with the Type I isotherm (Supporting
Information Figure S7). FT-IR spectroscopy on a sample
of 1 showed that water molecules were removed after the
MOF was activated at 120 °C for 3 h (Supporting
Information Figure S11).
compared to 1(as) but with a higher symmetry (P4 22
3
instead of P4 ). The BTC ligands within the framework
3
were slightly rotated compared to those in 1(as). The
coordinated water molecules were removed (Supporting
Information Figure S1). Our study shows that 1 under-
went a single-crystal to polycrystalline to single-crystal
transformation upon the release of the DMF and water
molecules from the channels. When the crystal was cooled
down to room temperature, the reflections disappeared,
due to the rehydration of the sample.
PXRD showed that the high temperature structure was
retained when the activated sample was cooled down to
room temperature, as long as the sample is in vacuum.
The sample lost its crystallinity upon rehydration in air in
less than 10 min. Interestingly, the crystallinity was
completely recovered in less than 20 min after a few drops
of DMF were added to the sample (Supporting Information
Figure S3). This implies that the addition of the organic
solvent facilitates the rearrangement of the metal and
BTC molecules and stabilizes the MOF structures.
The major structural changes during evacuation and
heating occurred below 180 °C, which is most probably
due to the rearrangement of the framework associated
with the release of the DMF and water molecules from the
channels. This agrees with that observed in the XRPD
patterns where peak intensities decreased significantly
during this process. The structure was stabilized and
became more ordered after all water and DMF molecules
were removed, so that the intensities of the diffraction
peaks increased and the peaks became sharper. The
diffraction of 1 could be observed by X-ray powder
diffraction but not by single crystal X-ray diffraction
during heating from room temperature to 180 °C. This
is probably associated with that the tetragonal unit cell
can be transformed into the monoclinic unit cells by four
different ways, which may result in small twin domains in
the monoclinic crystal.
Catalytic Tests. To compare the catalytic properties of
our compounds with other MOFs, we choose the cyano-
silylation of aldehydes and ketones (Scheme 1). The Lewis
acid-catalyzed reaction of carbonyl compounds with
cyanide affords the highly versatile cyanohydrin trimethyl-
silyl ethers. Cyanohydrins can be readily converted into
important compounds such as R-hydroxycarboxylic acids
2
5
or β-amino alcohols. The 1 (Nd) catalyst (9.0 mol % for
aldehydes and 4.5 mol % for ketones) was activated at
120 °C under vacuum (0.2 mbar) for 4 h to generate
coordinatively unsaturated Nd(III) centers and subse-
quently cooled down to room temperature under a nitro-
gen atmosphere. A solution of aldehyde (or ketone)
(0.5 mmol) and TMSCN (1 mmol) was then added.
TMSCN acted here both as a reagent and as the solvent;
no other solvent was used. The reaction mixture was
stirred at room temperature under a nitrogen atmosphere
for the time indicated, and aliquots were taken and analy-
1
zed by H NMR spectroscopy (Supporting Information
Figure S15).
The catalyst 1 shows very high activity in the cyanosi-
lylation of benzaldehyde and 88% conversion was
reached in only 1 h (Table 2, entry 1). A high conversion
was also obtained in 2 h (99%) when CH Cl was used as
The thermal behaviors of 7 (Er) and 8 (Yb) (Supporting
Information Figures S4 and S5) were to some extent
different from those of 1 (Nd). 7(as) was transformed
2
2
first to a tetragonal superstructure at 80 °C (a(80 °C) ≈
the solvent. The catalyst 1 showed a higher activity in the
√
2
a(as)), then to a monoclinic phase between 100 and
1
(
60 °C and finally to a tetragonal phase at 180 °C
Supporting Information Table S3). The decrease of the
Scheme 1. Cyanosilylation of Aldehydes (R = H) or Ketones
6¼ H) Catalyzed by 1 (Nd)
(R
unit cell volume from 7(as) to 7(120 °C) was only 4.7%,
much smaller than the corresponding decrease of 1(as)
(
14%). The monoclinic angle of 7(120 °C) is 100.2°, also
much smaller than that of 1(120 °C), 111.0°. The struc-
tural changes of 8 were much less compared to those of 1
and 7; 8 remained tetragonal throughout the heating to at
least 500 °C (Supporting Information Table S4).
The in situ XRPD shows that 1, 7, and 8 were stable to
at least 500-550 °C (Figure 3 and Supporting Informa-
tion Figures S4-S5). They are among the MOFs that
have the highest thermal stability.
(
25) For some examples, see: (a) Matthews, B. R.; Gountzos, H.;
Jackson, W. R.; Watson, K. G. Tetrahedron Lett. 1989, 30, 5157–
5
5
158. (b) Ziegler, T.; H €o rsch, B.; Effenberger, F. Synthesis 1990, 575–
78. (c) Jackson, W. R.; Jacobs, H. A.; Jayatilake, G. S.; Matthews,
B. R.; Watson, K. G. Aust. J. Chem. 1990, 43, 2045–2062. (d) Jackson,
W. R.; Jacobs, H. A.; Matthews, B. R.; Jayatilake, G. S.; Watson, K. G.
Tetrahedron Lett. 1990, 31, 1447–1450. (e) Effenberger, F.; Stelzer, U.
Angew. Chem., Int. Ed. 1991, 30, 873–874. (f) Kim, S. S.; Song, D. H.
Eur. J. Org. Chem. 2005, 1777–1780.

Article
Chem. Mater., Vol. 22, No. 11, 2010 3321
a,b
Table 2. Cyanosilylation of Aldehydes and Ketones Catalyzed by 1 (Nd)
a
1
b
2 2
The yields were determined by H NMR spectroscopy (Figure S15). 99% yield in 2 h with solvent CH Cl .
cyanosilylation of benzaldehyde than that obtained with
5
Table 3. Cyanosilylation of Benzaldehyde Catalyzed by a Variety of
Ln(btc) MOFs
a
a,b,d,j
other MOFs;
the Mn MOF catalyst reported by
5
j
Long et al. afforded 98% conversion of benzaldehyde
after 9 h at room temperature when using 11 mol %
catalyst loading. Removal of 1 by filtration shut down the
reaction completely, and no significant increase of the
conversion of benzaldehyde was observed after 72 h
entry
MOF
yield (%)
time (h)
TON
1
2
1 (Nd)
6 (Ho)
99
76
99
61
57
2
2
3
5
5
22
17
3
4
7 (Er)
8 (Yb)
14
13
(
Supporting Information Table S5), as shown in a control
a
The corresponding aldehyde (0.5 mmol) and TMSCN (1 mmol)
Cl at
room temperature. The reaction mixture was stirred under nitrogen
atmosphere for the time indicated.
experiment using 4.5 mol % of the catalyst 1. This
demonstrates that the homogeneous species was not
involved in the catalysis, and the catalyst 1 did not leach
Nd(III) ions into the solution under the reaction condi-
tions. The catalyst could be recycled and reused for at
least five times without losing the activity (Supporting
Information Table S6), giving rise to a total TON of 111.
FT-IR spectroscopy and XRPD showed that benzalde-
hyde interacted with coordinatively unsaturated Nd(III)
centers (Supporting Information Figures S13 and S14).
An excellent result was also obtained for p-fluorobenzal-
dehyde (73% conversion in 1 h, Table 2, entry 2). When a
larger R-naphthaldehyde was used, a significantly lower
yield (18%) was obtained (Table 2, entry 3).
were added to the activated Ln(btc) MOF (4.5 mol %) in CH
2
2
S9). All the catalysts could be recycled and reused at
least five times without losing their activity (Supporting
Information Table S6) and crystallinity (Supporting In-
formation Figure S10).
Ketones are much less reactive than aldehydes. The
cyanosilylation catalyzed by 1 (4.5 mol %) at room
temperature for 18 h gave excellent yields for acetophe-
none (91%, Table 2, entry 4). This conversion is much
higher than those obtained by other MOFs (the Mn MOF
gave 28% conversion of acetophenone in 24 h with 11 mol %
5
j
The cyanosilylation of benzaldehyde was also carried
out with catalysts 6 (Ho), 7 (Er), and 8 (Yb). As shown in
Table 3, the activity of the Ln(btc) catalysts decreases
with the ionic radius of the Ln(III) ions, in an order of 1, 6,
catalyst loading at room temperature). A high yield was
also obtained for propiophenone (Table 2, entry 5), whereas
a low yield was obtained for a larger ketone (Table 2,
entry 6). 1 is the first example of MOFs that catalyzes the
cyanosilylation of ketones with high yields in a reasonable
reaction time.
7
, and 8. 1 and 6 showed much higher activity, with 99%
and 76% conversions in 2 h, respectively, while 7 and 8
afforded lower yields (61% and 57%, respectively) even
after extended reaction times (5 h, Table 3). This trend is
opposite to that observed for homogeneous lanthanide
Conclusions
We have synthesized
a
Ln(btc)(H O)] guest MOFs. The structure is tetragonal
series of homeotypic
triflate (Ln(OTf) ) catalysts, where the relative Lewis
3
[
2
3
acidities of Ln(III) increases in an order of Nd(III),
6,27
2
and contains 1D channels. The guest and coordinated
water molecules can be easily removed by heating to give
Ln(btc) MOFs with micropores and coordinatively un-
saturated Ln(III) centers. During the evacuation and
subsequent heating, the structures of 1 and 7 transform
from tetragonal to monoclinic, and then to tetragonal,
while 8 remains tetragonal. The transformation between
the monoclinic and the low temperature tetragonal phases
is reversible. 1 undergoes a single-crystal (tetragonal) to
polycrystalline (monoclinic) to single-crystal (tetragonal)
transformation when being heated from room tempera-
ture to 180 °C. The transformation between the mono-
clinic and the low temperature tetragonal phases is
reversible. The Ln(btc) MOFs are stable up to at least
480 °C and are among the most thermally stable MOFs.
Ho(III), Er(III), and Yb(III).
The observed decrease
in reactivity of the Ln(btc) catalysts with the ionic radius
may be due to the increased steric interactions of the
substrates when approaching the coordinatively unsatu-
rated Ln(III) ions in the framework. XRPD shows that
under the catalytic conditions, the structure of 1 was
monoclinic while the structures of 6, 7, and 8 were close
to tetragonal (Supporting Information Figures S8 and
(
26) (a)Imamoto,T.;Nishiura,M.;Yamanoi,Y.;Tsuruta,H.;Yamaguchi,
K. Chem. Lett. 1996, 875–876. (b) Tsuruta, H.; Imamoto, T.; Yamaguchi,
K. Chem. Commun. 1999, 1703–1704. (c) Yamanaka, M.; Nishida, A.;
Nakagawa, M. Org. Lett. 2000, 2, 159–161.
(27) For an investigation of the catalytic effect of Lewis acids in water,
see: Kobayashi, S.; Nagayama, S.; Busujima, T. J. Am. Chem. Soc.
1998, 120, 8287–8288.

3
322 Chem. Mater., Vol. 22, No. 11, 2010
Gustafsson et al.
Our Ln(btc) MOFs catalyze the cyanosilylation of alde-
hydes and ketones in good to excellent yields within short
reaction times. The catalysis is completely heterogeneous
and can occur under solvent-free conditions at room
temperature. The catalysts can be recycled and reused
without loss of their activity and crystallinity. As a result
of their high thermal stability, we believe that new appli-
cations of these materials will be soon developed.
spectra. This project is supported by the Swedish Research
Council (VR) and the Swedish Governmental Agency for
Innovation Systems (VINNOVA) through the Berzelii
Center EXSELENT, the G o€ ran-Gustafsson Foundation and
the Ministry of Education of China through the “111” program.
Supporting Information Available: Crystallographic data and
refinement parameters for 1, 1(180 °C) and 8, the corresponding
cif files, XRPD and in situ XRPD patterns, TGA curves,
nitrogen sorption isotherm, FT-IR spectra, general procedures
1
Acknowledgment. Assoc. Prof. Niklas Hedin is acknowl-
edged for his advise with the in N sorption measurement.
Prof. Janos Mink is acknowledged for evaluating the FT-IR
for the catalytic experiments and tests, and H NMR spectra
(PDF, CIF). This material is available free of charge via the
Internet at http://pubs.acs.org.
2