Cd(II)-MOF-IM: Post-synthesis functionalization of a Cd(II)-MOF as a triphase transfer catalyst

Article abstract of DOI:10.1039/c6cc00576d

A robust and porous Cd(ii)-MOF based on a bent imidazole-bridged ligand was synthesized and post-synthetically functionalized with linear alkyl chains to afford imidazolium salt (IM)-type triphase transfer catalysts for organic transformations. The imidazolium salt decorated Cd(ii)-MOF-IM exhibits typical solid phase transfer catalytic behavior for the azidation and thiolation of bromoalkane between aqueous/organic phases. Moreover, they can be easily recovered and reused under the PTC conditions. Cd(ii)-MOF-IM herein created a versatile family of solid phase transfer catalysts for promoting a broad scope of reactions carried out in a biphasic mixture of two immiscible solvents.

Full text of DOI:10.1039/c6cc00576d

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Cd(II)-MOF-IM: post-synthesis functionalization of Cd(II)-MOF for
triphase transfer catalyst
Received 00th January 20xx,
Accepted 00th January 20xx
Jian-Cheng Wang, Jian-Ping Ma, Qi-Kui Liu, Yu-Hong Hu, Yu-Bin Dong∗
DOI: 10.1039/x0xx00000x
www.rsc.org/
A robust and porous Cd(II)-MOF based on a bent imidazole-
Metal-organic frameworks (MOFs),4 as a new class of solid
bridged ligand was synthesized and post-synthetically materials, provide a variety of attractive features. The organic-
functionalized with linear alkyl chains to afford imidazolium salts inorganic hybrid composition, together with the porous
(IM)-type triphase transfer catalysts for organic transformations. structural feature enables the MOFs colourful physical and
The imidazolium salt decorated Cd(II)-MOF-IM exhibits typical chemical properties. In addition, MOFs can be chemically
solid phase transfer catalytic behavior for the azidation and modified via post-synthetic approach,5 especially on the micro-
thiolation of bromoalkane between aqueous/organic phases. and nanoscale MOFs particles, consequently, endowing MOFs
Moreover, they can be easily recovered and reused under the PTC with new chemical and physical properties. In principle, by
conditions. Cd(II)-MOF-IM herein created a versatile family of solid tuning attached functional groups, the hydrophilic-
phase transfer catalysts for promoting a broad scope of reactions hydrophobic balance of the MOFs could be adjusted, and then
carried out in a biphasic mixture of two immiscible solvents.
they might serve as a carrier to transfer the substrates
between two immiscible solvent phases.
Over the years, phase transfer catalysis (PTC) has been
emerged as a very useful and far reaching methodology for
many common synthetic transformations.1 It is well known
that many advantages are associated with PTC method,
including relative mild reaction conditions, the simplicity of the
reaction procedure, as well as the use of inexpensive and eco-
friendly regents. In traditional PTC reactions, surfactants such
as quaternary ammonium salts2 were used as the catalysts to
enhance the interfacial surface area via emulsification, and
consequently, facilitate the reactions between two immiscible
solvent phases (commonly aqueous vs organic phases).
Despite fruitful advancements in academia and industry, they
cannot be more extensively applied because of inherent
difficulties in separation of the surfactants from the reaction
systems. Compared to traditional phase transfer catalysts,
solid particles would be more easily recoverable and have also
been shown in many cases to stabilize aqueous-organic
emulsions.3 So the development of new type of solid phase
transfer catalysts is very significant.
In this contribution, we report, first of kind, the MOF-based
triphase transfer catalysts which are generated from a new
imidazolium (IM)-based Cd(II)-MOF by post-synthesis
functionalization. The obtained Cd(II)-MOF-IM decorated with
linear alkyl chains. We demonstrate that the Cd(II)-MOF-IM is
an ideal triphase transfer catalyst which can promote the
azidation and thiolation of bromoalkane under PTC conditions.
Scheme 1. i) NaH, THF, 80
pyridylboronic acid, K2CO3, Pd(PPh3)4, EtOH/H2O/C7H8, r.t. 81.0
Cd(OAc)2, MeOH, 120 C, 72 h, 78 % yield.
°
C, 1 h; ii) BrCH2CN, THF, 80
°
C, 12 h, 65.5 % yield; iii) 4-
%
yield; iv)
°
The imidazole-bridged ligand
(HL) was prepared from
dibromoimidazole (A)6 via three-step sequence, as illustrated
in Scheme 1. Cd(II)-MOF was synthesized by solvothermal
method. A mixture of HL and Cd(OAc)2 (L/Cd(OAc)2 = 2 : 1) in
MeOH was heated at 120
yield the neutral CdL2 ) as colorless crystals in 78 % yield (Fig.
1, ESI). The measured and simulated XRPD patterns are well
°C for 72 h in a flame-sealed tube, to
College of Chemistry, Chemical Engineering and Materials Science, Collaborative
Innovation Center of Functionalized Probes for Chemical Imaging in Universities of
Shandong, Key Laboratory of Molecular and Nano Probes, Ministry of Education,
Shandong Normal University, Jinan 250014, P. R. China. E-mail:
(
1
matched to each other, indicating that
phase (ESI).
1 was obtained in pure
yubindong@sdnu.edu.cn.
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: Synthesis and
characterization of 1-3, crystal data, typical catalytic and leaching experiments and
product characterization (GC-MS). See DOI: 10.1039/x0xx00000x
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Fig. 1 a) Coordination sphere of Cd(II) and b) single square like unit in
uncoordinated N donors are highlighted in green). c) Crystal packing of
post-synthesis functionalization of Cd(II)-MOF-IM ( ) and Cd(II)-MOF-IM (
pictures of 1-3 are inserted.
1
(the
and
). The
1
Fig. 2 a) XRPD patterns of 1-3. The SEM images of 1-3 are inserted. b) TGA traces
of
2
3
2 and 3. c) The picture of water/Cd(II)-MOF-IM (2)/1-bromobutane system. It
is seen that the solid Cd(II)-MOF particles decorated with linear alkyl chains
preferentially migrate to the interface.
1
crystallizes in the triclinic P-1 space group (ESI). The Cd(II)
Post-synthesis functionalization of Cd(II)-MOF
performed by treating Cd(II)-MOF ( ) with 1-bromododecane
) and 1-bromopropane ( ) in CH3CN (80 C) via solid-liquid
reactions, respectively. As shown in Fig. 1, the colour of Cd(II)-
MOF ( ) changed from colourless to light yellow after the
reaction. X-ray powder diffraction (XRPD) indicated that the
crystalline and structural feature of Cd(II)-MOF was
retained in both Cd(II)-MOF dodecyl ( ) and Cd(II)-MOF-propyl
) after the post-synthesis functionalization (Fig. 2a). The
(1) was
center lies in a octahedral coordination sphere consisting of
four basal pyridyl N-donors and two apical carboxylate O-
donors from six deprotonated L, respectively. The Cd(II) nodes
1
(
2
3
°
are doubly connected to each other by the bent ligands to
generate a double-edged 2D net containing square-like unit. As
shown in Fig. 1, the 2D nets are extended in the
crystallographic bc plane and further stack together along the
crystallographic a axis to generate square like channels
1
(1)
-
2
(
3
(dimension of ca. 11
× 11 Å), in which the guest molecules are
decoration of HL with linear paraffin 1-bromododecane and 1-
bromopropane was examined before the Cd(II)-MOF synthesis.
The reactions proceeded very smoothly and the expected
imidazolium products were well confirmed by the IR and MS
spectra (ESI).
located. The solvent-accessible void space was calculated to be
41.1 % using PLATON. Thermogravimetric analysis (TGA, Fig. 2)
of
1
showed a solvent content of 15.1 % and the 1H NMR
spectrum (ESI) indicated that the guest species are H2O and
MeOH. The encapsulated solvent molecules can be removed
by heating (70°C) for 2 h without loss of its structural integrity
Ion-chromatography (Cd(II) contents in
10.00 %, respectively, ESI) and inductively coupled plasma (ICP,
based on Br- contents in
and , ESI) measurements indicated
the yields of alkyl-decoration of and are 64.4 and 70.1 %,
2 and 3 are 9.78 and
(ESI). Notably, when the desolvated 1 was allowed to stand at
room temperature for a while (ca. 10 min), water molecules in
the air were encapsulated in the framework again based on
TGA analysis (ESI), indicating that the channels in
basically hydrophilic.
2
3
2
3
respectively. In general, the post-synthetic functionalization
can be achieved on both the interior and exterior of the MOF-
1 are
particles due to their porous nature. Compared to
post-synthetic yield of might be resulted from the larger
sized dodecyl group. Based on their scanning electron
microscope (SEM) images (Fig. 2a), the particle size of are
3, the lower
As mentioned above, one of the two imidazole N-donors on
is uncoordinated and might be decorated with linear paraffin
2
L
such as alkyl bromides to new MOF-based solid imidazolium
salts. Our chief concern is to endow such MOF-based solid
imidazolium salts with the ability to achieve a facile anion
extraction and promote hetero-phase reactions under PTC
conditions.
1
enlarged and more non-uniform somehow after the post-
synthetic modification, which might be resulted from the
entanglement between the long linear paraffin chains in 2 and
2 | J. Name., 2012, 00, 1-3
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3
. In addition, the TGA trace indicated that
and stable to up to 200 C (Fig. 2b). It is well known that the
common quaternary ammonium salts are usually stable
around 100 and would
C.1 The good thermal stability of
2 and 3 are robust
°
°
2
3
make them suitable to promote organic transformations at
relative higher temperature.
As shown above, both 2 and 3 possess the hydrophilic
imidazolium moiety with linear lipophilic chains linked to the
solid MOF particles with hydrophilic cavities. Such particles
should be biphasic and prone to stay at the liquid/liquid
interface to stabilize aqueous-organic emulsions (Fig. 2c).
Fig. 4 Left: leaching test for catalytic azidation based on
of and it after each catalytic run.
2. Right: XRPD patterns
2
As shown above, the post-synthetic yield for imidazolium
moiety of is 64.4 %, so the amount of the imidazolium active
sites included in at 2.6 mol % loading is 1.7 mol %. On the
other hand, azidation of 1-bromobutane catalysed by
2
2
1
without imidazolium was also tested under the same reaction
conditions, the yield is only 50.1%, which is comparable to that
of blank experiment (50.0%). This could be an additional
evidence to demonstrate that this reaction is an imidazolium-
promoted reaction. As a heterogeneous catalyst,
2
2 can be
reused. After each catalytic cycle, was easily recovered by
Fig. 3 CO2 adsorption isotherms for 1-3 at 195K (1 atm) are 136.8, 166.0 and
170.9 cm3/g, respectively (
1: red lines, 2: green lines, 3: blue lines).
centrifugation and filtration, and then directly reused in the
next run under the same PTC conditions. As shown in Table 1,
azidated yield was still obtained in an ideal yield (92 %) after
The permanent porosity of 1-3 was determined by
measuring CO2 adsorption at 195 K.7 They all exhibit
microporous materials feature. As shown in Fig. 3, upon
three catalytic cycles. In addition, 2 was demonstrated to be
highly crystalline from XRPD patterns even after three catalytic
decoration of the imidazolium salts,
improved adsorption capacity, thus indicating that the pores of
are not blocked by the introduced alkyl chains. The
adsorption amount enhancement might be resulted from the
significant CO2-philicity of the imidazolium moieties in and
3 8
With this information in hand, we set out to study the
phase-transfer-catalyzed nucleophilic substitution based on
and
2 and 3 exhibited an
runs (Fig. 4), which confirmed the reusability of
catalyst.
3 as a triphase
1
Table 2. Phase-transfer-catalyzed thioetherfication of 1-bromobutane catalyzed by 2
2
.
2
3
.
conversion c
(%)
selectivity (%) c
TOF [h-1]d
Table 1. Phase-transfer-catalyzed azidation of 1-bromobutane catalyzed by 2a
entry
dibutylsulfane
dibutyldisulfane
1 a
2 a
3 a
4 b
94
92
88
94
90
74
75
97
10
25
26
3
18.8
18.3
17.5
18.8
entry
conversion (%) b
selectivity (%) b
TOF [h-1]c
11.3
1
2
3
95
94
92
100
100
100
11.2
a Reaction condition: Na2S ( 1 mmol ), 1-bromobutane ( 2mmol ), 2 (2.6 mol %),
11.0
b
H2O (4.5 mL), at 70°C for 3 h in air. The reaction was carried out in N2
atmosphere with as-synthesized catalyst of 2. c Determined by GC-MS. d TOF = %
a Reaction condition: NaN3 ( 7.7 mmol ), 1-bromobutane ( 1 mmol ), 2 (2.6 mol %),
H2O (4.5 mL), at 100°C for 5 h in air. b Determined by GC-MS. c TOF = % conversion
(mmol of substrate/mmol of active site h). The reaction was monitored by GC.
conversion (mmol of substrate/mmol of active site h).
Cd(II)-MOF (2) also showed interesting catalytic hehavior for
the thioetherfication of 1-bromobutane under PTC conditions.
A mixture of 4-bromobutane (1 equiv), Na2S (6.4 equiv) in H2O
(4.5 mL) and 2 (2.6 mol %) was heated at 70°C for 3 h
(monitored by GC) in air to generate the expected
dibutylsulfane and also 1, 2-dibutyldisulfane as the by-product
with a good conversion of 94 % (entry 1, Table 2). The
selectivity of this PTC reaction is mainly toward dibutylsulfane
(90 %). The higher selectivity for 1, 2-dibutyldisulfane was
At 2.6 mol % Cd(II)-MOF-IM (2) loading, azidation of 1-
bromobutane with a large excess of NaN3 in aqueous solution
afforded desired n-butyl azide in high conversion (95 %) with
100 % selectivity under the reaction conditions (entry 1, Table
1). In order to gain insight into heterogeneous nature of
leaching test was carried out. As indicated in Fig. 4, no further
2,
reaction took place without
reaction at 3 h (ESI).
2 after initiation of the oxidation
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observed at the second and third catalytic run (up to 25-26 %,
We are grateful for financial support from NSFC (Grant Nos.
entry 2-3, Table 2). We reasoned that the part of n-butyl 21475078 and 21271120), 973 Program (Grant Nos.
mercaptane intermedia was rapidly oxidized by the oxygen to 2012CB821705 and 2013CB933800) and the Taishan Scholar’s
form the dibutyldisulfide when the PTC reactions were carried Construction Project.
out in air.9 Indeed, when above reaction was carried out in
inert atmosphere, the selectivity toward dibutylsulfane was
Notes and references
largely enhanced (up to 97 %, entry 4, Table 2) and only tiny
amount of dibutyldisulfane was detected based on GC-MS
analysis (ESI). Although we distance ourselves from any type of
explanation and say forthright that we do not know why the
selectivity to dibutyldisulfide species in second and third
catalytic runs is enhanced, it might be caused by the catalytic
1
(a) C. M. Starks, J. Am. Chem. Soc., 1971, 93, 195. (b) K.
Maruoka and T. Ooi, Chem. Rev., 2003, 103, 3013. (c) J.
Dupont, R. F. de Souza and P. A. Z. Suarez, Chem. Rev., 2002,
102, 3667. (d) J. Tan and N. Yasuda, Org. Process Res. Dev.,
2015, 19, 1731. (e) K. Maruoka, Org. Process Res. Dev., 2008,
12, 679.
activity decline of
2
2
Again, herein also exhibits
for thioetherfication after the first run.
heterogeneous catalytic
2
(a) M. Uyanik, H. Okamoto, T. Yasui and K. Ishara, Science,
2010, 328, 1376. (b) M. Uyanik, H. Hayashi and K. Ishihaba,
Science, 2014, 345, 291. (c) V. Rauniyar, A. Lackner, G. L.
Hamilton and F. D. Toste, Science, 2011, 334, 1681. (d) T. Ooi,
a
behaviour and can be collected by centrifugation after the
reaction and reused directly in the next catalytic run (ESI).
Notably, all the PTC reactions described herein needed no
additional organic solvents except for the reaction substrates.
Thus as this reaction is easy to manipulated and avoids the use
of toxic and volatile solvents, it could be considered as a clean
catalytic organic synthesis approach.
Y. Uematsu and K. Maruoka, J. Am. Chem. Soc., 2006, 128
,
2548. (e) K. Sharma, P. P. Wolstenhulme, D. Yeo, F. Grande-
Carmona, C. P. Johnston, D. J. Tantillo and M. D. Smith, J. Am.
Chem. Soc., 2015, 137, 13414. (f) S. Shirakawa, K. Liu and K.
Maruoka, J. Am. Chem. Soc., 2012, 134, 916. (g) S. E.
Denmark, R. C. Weintraub and N. D. Gould, J. Am. Chem. Soc.,
2012, 134, 13415.
Table 3. Phase-transfer-catalyzed azidation and thiolation of 1-bromobutane catalyzed
by 3a
3
4
(a) S. Crossley, J. Faria, M. Shen and D. E. Resasco, Science,
2010, 327, 68. (b) A. D. Dinsomre, M. F. Hsu, M. G. Nikolaides,
M. Marquez, A. R. Bausch and D. A. Weitz, Science, 2002, 298
1006. (c) G. M. Whitesides and B. Grzybowski, Science, 2002,
295, 2418.
,
entry
conversion (%) b
83
selectivity (%) b
100
1
(azidation)
(a) M. O’Keeffe and O. M. Yaghi, Chem. Rev., 2012, 112, 675.
(b) J.-R. Li, J. Sculley and H.-C. Zhou, Chem. Rev., 2012, 112,
dibutylsulfane dibutyldisulfane
2
869. (c) Y. Cui, Y. Yue, G. Qian, and B. Chen, Chem. Rev., 2012,
112, 1126. (d) C. Wang, T. Zhang and W. Lin, Chem. Rev.,
2012, 112, 1084. (e) M. Yoon, R. Srirambalaji and K. Kim,
Chem. Rev., 2012, 112, 1196.
81
(thiolation)
81
19
a Reaction conditions are the same as those of 2 except 3 was used instead of 2. b
5
(a) S. M. Cohen, Chem. Rev., 2012, 112, 970. (b) M. F. Lin, R.
Matsuda and S. Kitagawa, Chem. Mater., 2014, 26, 310. (c) C.
He, D. Liu and W. Lin, Chem. Rev., 2015, 115, 11079. (d) P.
Determined by GC-MS.
For further demonstrating above reactions are a PTC
process, Cd(II)-MOF-IM (3) with shorter propyl chains is used
Kaur, J. T. Hupp and S. T. Nguyen, ACS Catal. 2011, 1, 819. (e)
J. D. Evans, C. J. Sumby and C. J. Doonan, Chem. Soc. Rev.,
instead of to perform above reactions under the same
2
2014, 43, 5933.
A. Puratchikody and M. Doble, Bioorg. Med. Chem., 2007, 15,
1083.
A N2 adsorption isotherm was first measured, but it exhibits
negligible uptake by the framework even at low temperature.
Such a phenomenon has been observed for some MOFs. See:
(a) K. Otsubo, Y. Wakabayashi, J. Ohara, S. Yamamoto, H.
Matsuzaki, H. Okamoto, K. Nitta, T. Uruga and H. Kitagawa,
Nat. Mater., 2011, 10, 291. (b) J. Y. Cheng, P. Wang, J.-P. Ma,
Q.-K. Liu and Y.-B. Dong, Chem. Commun., 2014, 50, 13672.
6
7
conditions. In principle, the phase transfer catalysts with
longer paraffin chains are more efficient due to their larger
extraction constant.10 The conversions of bromoalkane
azidation and thiolation catalyzed by
respectively, which are much lower than those of
3
are 83 and 81,
. The
2
selectivity for n-butyl azide is 100 %, while selectivity for
dibutylsulfane and dibutyldisulfane are 81 and 19 %,
respectively (Table 3). So the Cd(II)-MOF-IM with longer
paraffin chains is more efficient for these nucleophilic
reactions.
8
9
S. Soll, Q. Zhao, J. Weber and J. Yuan, Chem. Mater., 2013, 25
3003.
(a) J. P. Tam, C.-R. Wu, W. Liu and J.-W. Zhang, J. Am. Chem.
,
In conclusion, we report the first example of the
imidazolium-based MOFs via post-synthesis functionalization.
The obtained Cd(II)-MOF-IM can be highly active solid phase
transfer catalyst to promote the azidation and thiolation of
bromoalkane under PTC conditions. Our results herein
highlight the preliminary application of MOF-based triphase
transfer catalysts localized at the interface between two liquid
phases. We anticipate that tailoring such emulsion-stabilizing
MOFs solid particles decorated with additional catalytic
functional groups will facilitate a broad range of reactions
between two immiscible solvent phases.
Soc., 1991, 113, 6657. (b) Y. Zheng, L. Zhai, Y. Zhao and C. Wu,
J. Am. Chem. Soc., 2015, 137, 15094.
10 G. E. Boyd and Q. V. Larson, J. Am. Chem. Soc., 1967, 89
6038.
,
4 | J. Name., 2012, 00, 1-3
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The post-synthetically imidazolium decorated Cd(II)-MOF-IM can be a highly active triphase transfer catalyst to promote the
azidation and thiolation of bromoalkanes between aqueous and organic phases.
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