An NHC-CuCl functionalized metal-organic framework for catalyzing β-boration of α,β-unsaturated carbonyl compounds

Article abstract of DOI:10.1039/c9dt00645a

A microporous metal-organic framework functionalized with in situ generated NHC-CuCl units (1) has been successfully synthesized from a novel imidazolium-containing ligand. In particular, the MOF 1 can catalyze β-boration of α,β-unsaturated carbonyl compounds, while the isoreticular version of 1 (1-im) modified with only imidazolium moiety cannot. This work demonstrates for the first time the heterogeneous catalysis of NHC-Cu(i)Cl within a MOF solid.

Full text of DOI:10.1039/c9dt00645a

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An NHC-CuCl functionalized metal–organic
framework for catalyzing β-boration of
Cite this: DOI: 10.1039/c9dt00645a
α,β-unsaturated carbonyl compounds†
Received 13th February 2019,
Accepted 25th March 2019
a
a
a
a
b
b
a
Qingxia Yao,* Xiufang Lu, Kaili Liu, Chao Ma, Jie Su, Cong Lin, Dacheng Li,
a
b
a
DOI: 10.1039/c9dt00645a
Jianmin Dou,
Junliang Sun * and Wenzeng Duan
*
rsc.li/dalton
1
3
A
microporous metal–organic framework functionalized with MOFs. Alternatively, Yaghi and coworkers have shown that
in situ generated NHC-CuCl units (1) has been successfully syn- pre-designed NHC-PdI complexes can be used as framework
2
1
4
thesized from a novel imidazolium-containing ligand. In particular, struts. Furthermore, in situ generated NHC complexes during
9
the MOF 1 can catalyze β-boration of α,β-unsaturated carbonyl MOF synthesis have also been reported. However, MOFs con-
compounds, while the isoreticular version of 1 (1-im) modified taining NHCs and their precursors are still at their infant
1
5
with only imidazolium moiety cannot. This work demonstrates for stage.
the first time the heterogeneous catalysis of NHC-Cu(I)Cl within a
On the other hand, β-boration reaction of α,β-unsaturated
MOF solid.
carbonyl compounds can afford valuable organoborane inter-
mediates, which can be further transformed into other func-
tional groups such as alcohols, amines, and alkenes. For
Metal–organic frameworks (MOFs), or porous coordination
16
polymers have shown many promising applications in gas
example, oxidation of β-boro-carbonyl compounds provides
new access to pharmaceutically active β-hyrdoxy carbonyl com-
1
–4
storage, separation, and catalysis.
Their desired structures
and functions can be tailored by judicious selection of organic
17
pounds as an alternative to the aldol reaction. To date, a
ligands bearing active sites that can be modified pre-, in situ,
number of homogeneous catalysts involving Pt, Pd, Rh, and
5
or post-synthetically. This tailorable feature, together with
18
17,18d,e,19
Cu have been developed and Cu systems
have been
their large porosity, renders MOFs as an ideal platform for
heterogeneous catalysis. In this regard, diverse functional
most applied for β-boration of α,β-unsaturated carbonyl com-
pounds. However, very few heterogeneous catalysts, like
NHC-Cu(I) functionalized MOFs, have been used for β-boration
of α,β-unsaturated carbonyl compounds.
Attracted by the distinct structural features and functional-
ities of NHCs, we have designed a bent ligand bearing central
imidazolium moiety, 1,3-bis(2-methy-4-carboxyphenyl)-imida-
6
organic building ligands are explored, such as salens and por-
7
phyrins. Among them, azolium-containing ligands are
unique. Imidazolium salts are precursors of N-heterocyclic car-
benes (NHC), which are ubiquitous stabilizing ligands for a
8
wide range of catalytic metals. Therefore, the immobilization
of functional imidazolium salts and NHC-metal species into
zolium chloride (H LCl, Scheme 1). The two methyl groups
2
9
–12
porous solids like MOFs attracts considerable interest.
and the bent geometry of the ligand are expected to direct the
metal–ligand orientation for the formation of MOF structure.
Here, we report a novel MOF material, [Cu(L)0.69(L-Cu–
To date, three approaches have been developed to immobi-
lize NHCs into MOFs. Imidazolium-containing ligands have
been directly utilized in MOF synthesis and, in several cases,
the incorporated imidazolium moieties have been post-syn-
thetically modified into NHC-Pd(II) complexes inside the
2 x 3
Cl)0.31(H O)]·(Cl) ·(NO )0.69−x·(solvent) (1), comprised of both
imidazolium-based ligand (L) and in situ generated NHC-
based ligand (L-Cu–Cl). MOF 1 possesses a 2D + 2D → 3D
interlocked structure with 3D channels modified by the
a
School of Chemistry and Chemical Engineering, and Shandong Provincial Key
Laboratory/Collaborative Innovation Center of Chemical Energy Storage and Novel
Cell Technology, Liaocheng University, Liaocheng 252000, PR China.
E-mail: duanwenzeng@163.com, qyaolcu@163.com
b
College of Chemistry and molecular engineering, Peking University, Beijing, 100871,
PR China. E-mail: junliang.sun@pku.edu.cn
†
Electronic supplementary information (ESI) available: Synthesis, characteriz-
ation, and additional structural data. CCDC 1572628. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c9dt00645a
2
Scheme 1 Schematic representation of the H LCl ligand.
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NHC-Cu(I) chloride units. Furthermore, the catalytic experi- dazolium moiety and reduction of Cu(II) to Cu(I) can be sim-
ments show the MOF solid 1 is efficient for β-boration of ultaneously achieved by heating the reactants of imidazolium
9
a
α,β-unsaturated carbonyl compounds under mild conditions.
Solvothermal reaction of H LCl and Cu(NO in DMF at structures containing NHC-Cu(I) units are still quite rare.
20 °C provided high quality greenish-blue crystals of 1. In It is worthy to note that in our MOF structure, NHC-Cu(I)
chloride and copper nitrate in DMF. To date, known MOF
9
2
3 2
)
1
fact, free refinement of the site occupation for in situ generated chloride are only formed for a portion of ligands (not all
NHC-Cu(I) chloride complex (L-Cu–Cl) resulted in only 31% ligands).
occupation and significant more reliable R-factors.‡ Thus, the
chemical composition of 1 is tentatively determined as ligands (L and L-Cu–Cl) to generate sql layers (Fig. 1b). The
Cu(L)0.69(L-Cu–Cl)0.31(H O)]·(Cl) ·(NO 0.69−x·(solvent). It should counter anions could not be located crystallographically. The
The copper paddle-wheel clusters are linked by the mixed
[
2
x
3
)
be noted that the slight variations in chemical composition 2D sql layers parallel to (110) and (−110) crystallographic
are probable due to different amounts of L-Cu–Cl participated planes are intersected with each other to give a 3D porous
in MOF formation and the possible anion exchange between entangled structure (Fig. 1c). Interestingly, the framework con-
the crystals and mother liquid. X-ray structural analysis reveals tains interconnected channels (Fig. 1d–f). The 1D rhombus
that 1 crystallizes in the space group Pcan and has a porous channels along the c-axis have the diagonal length of ca. 6.7 ×
interlocked framework (Fig. 1). There are two types of copper 11.4 Å, considering the van der Waals radii of constituting
species within the framework (Fig. 1a). The first is a copper atoms (Fig. 1d). Meanwhile, these 1D channels are intercon-
paddle-wheel cluster that acts as the secondary building unit nected through another type of channels along the a-axis, with
of the framework. The second copper specie is the in situ gen- the window size of ca. 5.0 × 5.0 Å (Fig. 1f, and Fig. S1, ESI†).
erated NHC-Cu(I) chloride unit tethered on the ligand. It has The total solvent accessible volume estimated by PLATON
been previously demonstrated that the deprotonation of imi- is 51.9%. More importantly, these channels are lined with
Fig. 1 (a) Two types of copper species. Note that the site occupation factor for Cu(I)–Cl is refined as 31% occupation. (b) 2D layer with sql topology;
(
c) 2D + 2D → 3D interpenetration mode. (d) View of 1D rhombus channels along the c-axis. (e) Perspective view of 1D channels along the c-axis in
space-filling representations. Note that the methyl groups on the ligands are omitted to show the crossover of the rhombus channel. (f) View of 1D
channels along the a-axis in stick representations.
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exposed NHC-Cu(I) chloride units, which provide an opportu-
nity to assess its catalytic performance.
Powder X-ray diffraction (PXRD) pattern of the as-syn-
thesized 1 (at 298 K) is slightly different from the simulated
one (at 173 K), indicating the temperature-dependent frame-
2
0
work dynamics, a very typical feature of flexible MOFs
Fig. 2). Thermogravimetric analysis of 1 shows that the com-
(
pound after a solvent weight loss of 30.2% can be thermally
stable up to 280 °C (Fig. S3†). The crystals of 1 undergo struc-
tural collapse upon desolvation, which is also confirmed by
negligible N2 adsorption of desolvated sample at 77 K
(Fig. S4†). Nonetheless, crystals of 1 show stability in common
organic solvents like THF and DMF, etc., which is a prerequi-
site for catalytic reactions conducted in solvents.
Relatively few MOFs containing NHC sites have been tested
for their catalytic performance, especially for NHC-Cu(I). There
is only one MOF material with bis(NHC)-copper units, being
9
c
an active framework-based catalyst for hydroboration of CO
2
.
However, homogeneous NHC-Cu(I) chloride complexes are
Fig. 3 Recycling experiments of β-boration reactions of 2-cyclohexen-
known to catalyze a number of reactions such as the hydrosilyl- 1-one. Reaction time of each run is 24 h.
ation of ketones, click reaction of azide–alkyne, hydroboration
of CO , and β-boration of α,β-unsaturated carbonyl com-
2
2
1
pounds. Accordingly, we set out to investigate the potential
of compound 1 to catalyze β-boration of α,β-unsaturated carbo- as catalyst, the catalytic process should be in principle hetero-
3
nyl compounds. In order to exclude any soluble NHC species geneous. Thus, a simple filtration test is performed. After a
possibly adsorbed on the surface of the material, the crystals mixture of MOF 1, B
of 1 are thoroughly rinsed thrice with fresh DMF (each 10 mL stirred at 25 °C for 24 h and filtered, 2-cyclohexen-1-one with
for 2 days) and washed with dried THF before the catalytic additional Cs CO and MeOH was subsequently added into
studies. MOF 1 can catalyze the β-boration reactions of 2-cyclo- the filtrate. After another 24 h, no product was detected by H
(pin) , Cs CO , MeOH, and THF was
2
2 2 3
2
3
1
hexen-1-one with B
2
(pin)
2
in THF at 25 °C for 24 h in the pres- NMR spectroscopy, demonstrating no leaching of active sites
ence of MeOH and Cs
2
CO
3
in almost quantitative yield (Fig. 3). from the MOF to solution. Furthermore, the Cu content in the
The turnover number of the catalyst is about 54, lower than filtrate was analyzed by ICP-AES and almost no Cu was
1
9d
that of the homogeneous counterpart.
When using a solid detected. MOF 1 can also be readily recovered by filtration and
used for consecutive runs, without deteriorating the catalytic
activity (Fig. 3 and Table S2, ESI†). A PXRD pattern of recovered
1
after the fifth run clearly shows that the structure was
retained during the catalytic process (Fig. 2). Furthermore,
MOF 1 can also catalyze β-boration of other substrates with
product yields from moderate to good (Table S1†). These
results clearly demonstrate the stability and heterogeneous
nature of this MOF catalyst. To our knowledge, this is the first
report on the catalysis of NHC-Cu(I) chloride within a MOF
solid.
In order to further verify that the catalytic activity of MOF 1
is derived from the in situ generated NHC-Cu(I) chloride, not
22
from the Cu(II) or the imidazolium-based ligand, we syn-
thesized an analogue of 1 containing only imidazolium moiety
(
denoted as 1-im hereafter) and explored its catalytic activities.
Lah and Son et al. have noticed in their work that the existence
9
a
of halide was critical to obtain in situ NHC-Cu(I) complexes.
Therefore, the ligand H L(NO ) [1,3-bis(2-methy-4-carboxyphe-
nyl)-imidazolium nitrate] instead of H LCl was employed to
synthesize 1-im. Although large crystals of 1-im were success-
fully obtained after subtle tuning of the synthetic conditions,
we cannot obtain the single-crystal structure of 1-im due to
very weak X-ray diffraction. However, PXRD pattern of 1-im
2
3
2
Fig. 2 PXRD patterns of the simulated of 1, the as-synthesized of 1, the
as-synthesized of 1-im, and the recovered 1 after the 5 run respectively
th
(
from the bottom to top), indicating the structural integrity of the
samples. The PXRD patterns were collected from samples with minor
DMF solvent.
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reveals that 1-im is an isoreticular structure of 1 (Fig. 2). As
expected, 1-im cannot catalyze β-boration reactions of 2-cyclo-
hexen-1-one under the same catalytic reaction conditions. The
above results clearly confirm that the in situ formed NHC-Cu(I)
chloride in MOF 1 is the vital factor correlating with its cata-
lytic activities. A possible mechanism for such NHC-Cu(I) cata-
lyzed β-boration of α,β-unsaturated carbonyl compounds is
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1
7
proposed based on Yun’s model (see S9 in ESI†).
In summary, a new functional azolium-containing ligand
has been assembled with copper nitrate to afford a MOF 1,
which possesses a 2D + 2D → 3D interpenetrated structure
with 1D channels decorated by in situ generated NHC-copper(I)
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(e) S. Wang, Q. Yang, J. Zhang, X. Zhang, C. Zhao, L. Jiang
There are no conflicts to declare.
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‡
Crystal data for 1: orthorhombic, Pcan, a = 13.2144(15), b = 22.3311(11), c =
3
2
3.9725(12) Å, V = 7074.19 Å , T = 173 K, Z = 8. F(000) = 1801.7, GOF = 0.986, a
total of 7920 reflections were collected, of which 6175 were unique (Rint
=
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0.0377). R (wR ) = 0.0587 (0.1754) for 259 parameters and 3534 reflections (I >
2σ(I)). CCDC 1572628.†
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