The use of reduced copper metal-organic frameworks to facilitate CuAAC click chemistry

Article abstract of DOI:10.1039/c6cc06890a

A reduced copper metal-organic framework (rCu-MOF) containing Cu^I ions was prepared by reducing raw MOFs (Cu-BTC). A series of polymer functionalizations and coupling reactions could subsequently be achieved via CuAAC click chemistry, thus demonstrating the high activity, facile recyclability and good structural stability of rCu-MOFs for catalytic applications.

Full text of DOI:10.1039/c6cc06890a

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The use of reduced copper metal-organic frameworks to facilitate
CuAAC click chemistry
Q. Fu,‡ K. Xie,‡ S. Tan, J. M. Ren, Q. Zhao, P. A. Webley, G. G. Qiao*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
reported a facile synthetic strategy for the fabrication of a mixed
valence state Ce-MOF, which exhibited intrinsic oxidase-like
A reduced copper metal-organic framework (rCu-MOF) containing
CuI ions was prepared by reducing raw MOFs (Cu-BTC). A series of
polymer functionalizations and coupling reactions could activity.18 Steiner and Zhang discovered that mixed-valence state
subsequently be achieved via CuAAC click chemitry thus,
demonstrating high activity, facile recyclablity and good structural
stability of rCu-MOFs for catalytic applications.
Cu-MOF had dual pore size distribution and presented superior
water stability compared to HKUST-1.20 Chen and Shustova
investigated the electronic properties and selective adsorbate
binding of the mixed-valence state Cu-BTC and Cu-NIP.21 These
studies provide guidance for the application of MOFs for
heterogeneous catalysis and gas purification.
Metal-organic frameworks (MOFs) are
a class of crystalline
materials consisting of metal ions or clusters coordinated to organic
2
ligands.1,
The large diversity in structures, pore sizes and
The CuI-catalyzed azide-alkyne cycloaddition (CuAAC)22-26 is
arguably the most powerful toolbox for polymer functionalization
and coupling applications in modern polymer chemistry.
Conventional catalysts, such as copper halide (CuX) compounds
have suffered from their inefficient catalytic ability due to their low
solubility (hence low surface area) and low stability under common
reaction conditions (i.e. heating and exposure to oxygen). Due to
this short coming, some improved liganded CuI systems were
studied and shown to significantly increase the solubility of CuI in
both organic and aqueous solvents. This led to enhanced catalytic
abilities.27, 28 However, in these systems, the dissolved CuI ions may
result in potential contamination, which in turn leads to
complicated purification steps. We thus saw this is an opportunity
to develop a new heterogeneous catalyst for CuAAC click chemistry
by employing the reduced Cu-MOF containing CuI ions.
adsorption affinities make MOFs of great interest for energy and
environmental applications, such as hydrogen storage3-5 and gas
separation.6-8 Recently, rapid progress in the development of
heterogeneous catalysts has been achieved in the form of MOFs.
This is attributed to their high surface area, well-defined
coordination nano-space, diversity in metal and functional groups,
as well as, the ability to retain crystallinity and regularity after
catalytic reactions.9 Generally, MOFs are engineered to offer three
attributes for catalytic purposes: (i) metal ions or metal clusters,10
(ii) functional struts11, 12 and, (iii) encapsulated/loaded catalysts.13, 14
In addition, MOFs also present potential to overcome the technical
barrier in current commercially available heterogeneous catalysts,
where the control of active site geometry cannot be achieved due
to the reconstruction and redistribution of the metal atoms.15, 16
This is in contrast to MOFs which exhibit stable structure and
synthetically tuneable active site geometry.
Herein, we report a study on the catalytic properties of a reduced
Cu-MOF (rCu-MOF). As far as we know, this is the first example of
reduced MOF nanoparticles (containing CuI ions) which can
facilitate CuAAC click chemistry. The rCu-MOF was synthesized in
the laboratory and well-characterized by XRD, SEM, XPS and TGA,
etc. The catalytic ability of the rCu-MOFs was then demonstrated by
a series of successfully polymer functionalizations and coupling
reactions. This rCu-MOF catalyst also presents advantages such as,
high hydrothermal stability and facile recyclability in comparison
with conventional catalysts.
MOFs containing mixed valence state ions (notably for Cu, Fe, Ce
and V) are emerging microporous materials which have unique
properties thus allowing them to have significant potential in
various applications.17, 18 For example, Daturi et al. demonstrated
the co-existence of CuII/CuI in the Cu-MOF (HKUST-1).19 Ye et al.
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Scheme 1. a) Schematic illustration for the preparation of reduced copper
metal-organic framework (rCu-MOF): (i) coordination reaction between Cu2+
and benzene 1,3,5-tricarboxylate (BTC) and (ii) reduction of the Cu-BTC in
the presence of hydroquinone at 150 oC. b) Schematic representations for
various polymer functionalizations and coupling reactions utilizing rCu-MOFs
via CuAAC chemistry. c) Chemical structures of the synthesized precursors
for the CuAAC reactions.
The rCu-MOF containing both CuII/CuI ions was prepared via a
two-step process as shown in Scheme 1a. Firstly, the MOF Cu-BTC
(HKUST-1) was synthesized according to a previously reported
procedure.29 The raw MOF was reduced in the presence of
hydroquinone to give a dark colour crystal, rCu-MOF. This ‘green’
synthesis of MOFs employs non-hazardous reactants at relatively
mild synthetic conditions, producing less waste than conventional
procedures. A series of CuAAC reactions for polymer modification
and coupling purposes were then investigated by utilizing the
microporous rCu-MOF as the heterogeneous catalyst (Scheme 1b).
Fig. 1 Digital images of a) Cu-BTC and b) rCu-MOF dispersions in 5 different
solvents immediately after ultrasonication. (Left to right: DMSO, methanol,
DI H2O, THF and CHCl3). c-d) SEM images of the synthesized Cu-BTC and the
rCu-MOF crystals, respectively. The scale bars represent
2 µm. Their
respective DLS data are inserted. e) High resolution Cu 2p3/2 XPS spectra for
the Cu-BTC (top) and the rCu-MOF (bottom) crystals. f) Comparison of the
XRD patterns for the synthesized Cu-BTC (top) and the rCu-MOF (bottom).
studies.20, 21 This result demonstrates the co-existence of CuI and
CuII in the rCu-MOF. The broad XPS spectra of the MOFs are shown
in Fig. S1 (ESI†). The X-ray diffraction (XRD) pattern shows that the
crystalline structure changed after MOF reduction (Fig. 1f). The
porous feature of the rCu-MOF was confirmed by conducting a CO2
adsorption experiment. Surprisingly, the rCu-MOF exhibits an
enhanced CO2 uptake ability (19.5 mmol/g) due to its enlarged
surface area (> 3260 m2/g, BET) compared to the Cu-BTC, where the
CO2 uptake is of 6.3 mmol/g and the BET surface area is of 570 m2/g.
TGA measurements revealed that rCu-MOF contains 36 wt. % of Cu
(Fig.S2, ESI†).
Digital images were taken to display the dispersion quality of the
Cu-BTC and the rCu-MOF crystals in different solvents immediately
after ultrasonication (Fig. 1a and b). Just after ultrasonication, the
Cu-BTC shows poor dispersion in all the solvents, but the rCu-MOF
shows relatively good dispersion in DMSO and methanol. We
characterized the morphology of Cu-BTC and rCu-MOF by SEM
measurements (Fig. 1c and 1d) which indicate successful formation
of the MOF crystals. It should be noted that the rCu-MOF (Fig.1d)
presented an irregular morphology compared to Cu-BTC (Fig. 1c).
This may be attributed to partial structural damage during the
reduction process. The strong aggregation tendency of both Cu-BTC
and rCu-MOF is also suggested by the SEM images. The DLS results
for Cu-BTC and rCu-MOF aqueous suspensions are inserted in Fig.
1c and 1d. The rCu-MOF exhibits a smaller average hydrodynamic
diameter (DH) of ca. 165 nm, while the raw Cu-BTC has a DH value of
ca. 218 nm.
We subsequently investigated the possibility of applying the
rCu-MOF in the CuI-catalyzed azide-alkyne cycloaddition (CuAAC)
chemistry. Firstly, ‘click’ reactions for polymer modification
were achieved. An azide functionlized poly(ethylene glycol)
(PEG-N3, P1) was used to conjugate with propargyl 1-pyrene
butyrate (C1) and propargyl alcohol (C2) in the presence of rCu-
MOF to afford ω-pyrene and ω-OH terminated PEGs. MALDI-
ToF MS of the resulting products revealed a single monomodal
series and a clear shift to higher molecular weights, indicating
a quantitative yield of the coupling products (Fig. 2a-c). This
was also demonstrated by the observation of proton signals
XPS was used to investigate the oxidation states of Cu in the Cu-
BTC and the rCu-MOF crystals. Fig. 1e shows the peaks in the Cu
2p3/2 region. In the top spectrum, the peak at 934.6 eV is attributed
to CuII in the Cu-BTC. In the case of rCu-MOF, the relative
abundance of CuI and CuII can be obtained by a peak fitting
procedure performed on the Cu 2p3/2 signal (bottom spectrum).
Calculation of the areas of the CuI peak and the CuII structure gives
a ratio of 44% CuI to 56% CuII which is in agreement with previous
1
from the characteristic triazole rings by H NMR analysis (Fig.
S3a-b,ESI†). As ‘click’ chemistry is known as a powerful
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Fig. 3 1H NMR spectroscopy was used to trace the reaction mixture
composition throughout the course of the CuAAC reaction in the presence
of rCu-MOF. a) Schematic representations for the CuAAC coupling reaction
between C2 and C3. b). Full 1H NMR spectra obtained at various time
intervals. c) Evolution of the single proton derived from the alkyne group b
and the protons derived from the methylene group j. d) Evolution of the
protons derived from the methylene group a. e) Conversion vs. time plots of
rCu-MOF for 3 cycles compared to CuBr for control experiment. The
Fig. 2 a-c) MALDI-TOF MS of azide functionalized PEG (P1), the resultant ω-
OH and ω-pyrene terminated PEGs. d) Schematic illustration of the
preparation of PMMA-b-PMMA pseudo-block copolymer via CuAAC reaction
using rCu-MOFs. e) GPC evolution of the resultant PMMA-b-PMMA
copolymer.
synthetic strategy for the preparation of block copolymers, we
investigated a ‘click’ coupling reaction between high molecular
weight poly(methyl methacrylate)-azide (P2, Mn = 5.2 kDa,
Mw/Mn = 1.11) and the alkyne-functionalized poly(methyl
methacrylate) (P3, Mn = 12.0 kDa, Mw/Mn = 1.27) under similar
conditions (Fig. 2d). The GPC trace (blue colour) of the
resulting pseudo-block copolymer (PMMA-b-PMMA, Fig. 2e)
shows a clear shift to the higher molecular weight region with
conversion
was
calculated
by
the
following
Eq.
Aj
, where Aj and Aj’ are the integral areas of
Conv.(%) = (1−
) ×100%
Aj + Aj'
peaks j and j’, respectively.
Table 1. Summary of the catalytic performance.
a
small amount of tailing observed. This revealed a high yield of the
coupling product. The successful coupling reaction was also shown
by the 1H NMR analysis (Fig. S3c, ESI†). These combined results
demonstrate the catalytic ability of the rCu-MOF for CuAAC click
chemistry.
Entries
Cycle-1
Cycle-2
Cycle-3
C-1
Catalysta
rCu-MOF
rCu-MOF
rCu-MOF
Cu-BTC
120 min TONb
TOFa (min-1)b
19.4
19.0
18.8
-
19.2
18.7
17.9
-
To further explore the catalytic performance of the rCu-MOF, a
CuAAC reaction between the propargal alcohol (C2) and the 3-
azidopropyl anthrancene-9-carboxylate (C3) was used as a probe
reaction (Fig. 3a) so that the characteristic proton signals display
acceptable peak resolution. Briefly, the reaction mixture (in
CD3OD) was degassed by bubbling N2 for 20 min and
subsequently sampled in a NMR tube. After the addition of ca.
C-2
CuBr
7.6
6.6
aThe rCu-MOF (1.2 mg), Cu-BTC (1.0 mg) and CuBr (1.0 mg) were fed as
catalysts. bTON and TOF are calculated based on the total number of CuI ions.
The mass of CuI ions in rCu-MOF was calculated through the TGA (36 wt.% of
CuI and CuII) and XPS (44% of CuI) analysis.
1.2 mg rCu-MOF, the NMR tube was placed in
a
o
1
60 C oil bath. In situ H NMR spectra was then recorded at
various time intervals to track the concentrations of each
species (Fig. 3b). Characteristic proton signals assigned to (i) the
alkyne group b and methylene group a of C2, (ii) the methylene
group j of C3 and, (iii) the methylene groups a’ and j’ next to the
triazole ring could be easily identified in the enlarged 1H NMR
spectra (Fig. 3c-d). This demonstrates the successful CuAAC click
chemistry.
using the aforementioned method. The proton signal assigned to
the triazole ring b’ is not suitable for calculation as it is overlapped
by the proton signals of aromatic ring. Generally, the CuAAC system
in the presence of rCu-MOF presents a high reaction speed with ca.
90 % conversion of the C3 and C2 within 2 hours. In the case of the
control experiment using CuBr as catalyst, only ca. 80 % conversion
was observed within same time frame.
Furthermore, like other heterogeneous catalysts, rCu-MOF
catalysts allow for easier post-reaction separation, less
contamination and facile recyclability when compared to
homogeneous catalysts. To examine the recyclability of the rCu-
MOF catalyst, CuAAC reactions of C2 and C3 was further repeated
twice (Fig. 3e) in the presence of rCu-MOF recovered from the
The evolution of these proton signals also revealed the reaction
speed of the CuAAC system. As shown in Fig. 3e, the integral ratios
of the peaks j and j’ were used to plot the conversion of C3 over
reaction time. It should be noted that the conversion calculated
from the other reference peak (i.e. from a) is identical to the value
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previous reaction After 2 hours of heating, the repeat reactions
Journal Name
4.
5.
6.
L. J. Murray, M. Dinca and J. R. Long, Chem. Soc. Rev., 2009, 38,
1294-1314.
J. Goldsmith, A. G. Wong-Foy, M. J. Cafarella and D. J. Siegel,
Chem. Mater., 2013, 25, 3373-3382.
R. J. Kuppler, D. J. Timmons, Q.-R. Fanga, J.-R. Li, T. A. Makal, M.
D. Young, D. Yuan, D. Zhao, W. Zhuang and H.-C. Zhou, Coord.
Chem. Rev., 2009, 253, 3042-3066.
J.-R. Li, R. J. Kuppler and H.-C. Zhou, Chem. Soc. Rev., 2009, 38,
1477-1504.
M. Matsumoto and T. Kitaoka, Adv. Mater., 2016, 28, 1765-1769.
J. Liu, L. Chen, H. Cui, J. Zhang, L. Zhang and C.-Y. Su, Chem. Soc.
Rev., 2014, 43, 6011-6061.
I. Luz, F. X. L. i. Xamena and A. Corma, J. Catal., 2012, 285, 285-
291.
S. S.-Y. Chui, S. M.-F. Lo, J. P. H. Charmant, A. G. Orpen and I. D.
Williams, Science, 1999, 283, 1148-1150.
S. Horike, M. Dincǎ, K. Tamaki and J. R. Long, J. Am. Chem. Soc.,
2008, 130, 5854-5855.
.
were successfully carried out as revealed by 1H NMR analysis (Table
S1, ESI†). This indicates the high structural stability of the rCu-MOF.
Zhang et al. also observed this ideal when heating rCu-MOF under
O2 at 200 oC which resulted in only a slight oxidation of CuI to CuII.20
We further characterized the filtrated reaction mixture by ICP-OES
to detect any leached Cu ions. Notably, no trace of copper could be
detected (sensitivity ~10 ppb). Through the calculation of turnover
number (TON) and turnover frequency (TOF), we further identified
the difference between rCu-MOF and CuBr in catalytic activity
(Table 1). Firstly, we can find that the 2 hour TON value dropped ca.
2 % after each cycle. In addition, from TOF for each cycle, we can
see that the CuI ions activity dropped ca. 3 and 5 % for Cycle 2 and
Cycle 3, respectively. This small amount of decay activity from
Cycle-1 to Cycle-3 is likely attributed to both material loss and
oxidation of CuI to CuII.30 Despite the small reduction (<5 %), the
overall activity of the rCu-MOF is still outstanding. In contrast, raw
Cu-BTC (C-1, containing CuII ions only) could not exhibit any catalytic
ability for the CuAAC reaction under the same conditions.
Additionally, the conventional CuBr catalyst presents a much lower
activity with the TON and TOF values of 7.6 and 6.6, respectively (C-
2 in Table 1). All of these results demonstrate excellent catalytic
activity as well as facile recyclability of the rCu-MOFs.
7.
8.
9.
10.
11.
12.
13.
14.
15.
L. Chen, Y. Yang and D. Jiang, J. Am. Chem. Soc., 2010, 132, 9138-
9143.
K. Xie, Q. Fu, Y. He, J. Kim, S. J. Goh, E. Nam, G. G. Qiao and P. A.
Webley, Chem. Commun., 2015, 51, 15566-15569.
F. Tao, M. E. Grass, Y. Zhang, D. R. Butcher, J. R. Renzas, Z. Liu, J.
Y. Chung, B. S. Mun, M. Salmeron and G. A. Somorjai, Science,
2008, 322, 932-934.
F. Tao, M. E. Grass, Y. Zhang, D. R. Butcher, F. Aksoy, S. Aloni, V.
Altoe, S. Alayoglu, J. R. Renzas, C.-K. Tsung, Z. Zhu, Z. Liu, M.
Salmeron and G. A. Somorjai, J. Am. Chem. Soc., 2010, 132, 8697-
8703.
G. Férey, F. Millange, M. Morcrette, C. Serre, M.-L. Doublet, J.-M.
Grenèche and J.-M. Tarascon, Angew. Chem. Int. Ed., 2007, 46,
3259-3263.
Y. Xiong, S. Chen, F. Ye, L. Su, C. Zhang, S. Shen and S. Zhao,
Chem. Commun., 2015, 51, 4635-4638.
16.
17.
For comparison, a homogeneous liganded CuI system for CuAAC
reaction of C2 and C3 was performed. An improved catalytic activity
of the CuI/PMDETA was observed, whereby the conversion achieves
~ 99% within 40 minutes and the calculated TOF is of 27 (Table S2,
ESI†). Although the rCu-MOFs system (TOF is of 19) is less active
than the solution system, it brings a huge advantage of no
requirement for purification steps.
18.
19.
20.
21.
J. Szanyi, M. Daturi, G. Clet, D. R. Baerc and C. H. F. Peden, Phys.
Chem. Chem. Phys., 2012, 14, 4383-4390.
A. Ahmed, C. M. Robertson, A. Steiner, T. Whittles, A. Ho, V.
Dhanak and H. Zhang, RSC Adv., 2016, 6, 8902-8905.
A. S. Duke, E. A. Dolgopolova, R. P. Galhenage, S. C. Ammal, A.
Heyden, M. D. Smith, D. A. Chen and N. B. Shustova, J. Phys.
Chem. C, 2015, 119, 27457-27466.
22.
H. C. Kolb, M. G. Finn and K. B. Sharpless, Angew. Chem. Int. Ed.,
2001, 40, 2004-2021.
23.
24.
M. Meldal and C. W. Tornøe, Chem. Rev., 2008, 108, 2952-3015.
V. K. Tiwari, B. B. Mishra, K. B. Mishra, N. Mishra, A. S. Singh and
X. Chen, Chem. Rev., 2016, 116, 3086-3240.
Conclusions
25.
26.
27.
28.
29.
C. Wang, D. Ikhlef, S. Kahlal, J.-Y. Saillard and D. Astruc, Coord.
Chem. Rev., 2016, 316, 1-20.
D. Ikhlef, C. Wang, S. Kahlal, B. Maouche, D. Astruc and J.-Y.
Saillard, Comp. Theor. Chem., 2015, 1073, 131-138.
C. Deraedt, N. Pinaud and D. Astruc, J. Am. Chem. Soc., 2014, 136,
12092-12098.
C. Wang, D. Wang, S. Yu, T. Cornilleau, J. Ruiz, L. Salmon and D.
Astruc, ACS Catal., 2016, 6, 5424-5431.
R. Ameloot, E. Gobechiya, H. Uji-i, J. A. Martens, J. Hofkens, L.
Alaerts, B. F. Sels and D. E. D. Vos, Adv. Mater., 2010, 22, 2685-
2688.
In summary, a reduced copper metal-organic framework (rCu-MOF)
has been prepared and fully-characterized. Using these rCu-MOFs a
series of ‘Click’ reactions were successfully conducted with
quantitative yield. When compared to the conventional copper
halide catalyst, higher activity and facile recyclability of the rCu-
MOF catalyst was observed. These properties combined with its
ease of synthesis, lack of heavy metal contamination and high
structural stability make the rCu-MOFs attractive catalytic materials
for industrial applications. This study opens new avenues for rCu-
MOFs in the field of polymer chemistry.
30.
J. A. Bennett, I. P. Mikheenko, K. Deplanche, I. J. Shannon, J.
Wood and L. E. Macaskie, Appl Catal B Environ., 2013, 140–141,
700-707.
Notes and references
1.
S. R. Batten, N. R. Champness, X.-M. Chen, J. Garcia-Martinez, S.
Kitagawa, L. Ohrstrom, M. O'Keeffe, M. P. Suh and J. Reedijk,
Cryst. Eng. Comm., 2012, 14, 3001-3004.
2.
3.
H.-C. J. Zhou and S. Kitagawa, Chem. Soc. Rev., 2014, 43, 5415-
5418.
J. L. C. Rowsell and O. M. Yaghi, Angew. Chem. Int. Ed., 2005, 44,
4670-4679.
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