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Accepted Manuscript
A readily available urea based MOF that act as a highly active
heterogeneous catalyst for Friedel-Crafts reaction of indoles and
nitrostryenes
Chengfeng Zhu, Qingqing Mao, De Li, Changda Li, Yiyang Zhou,
Xiang Wu, Yunfei Luo, Yougui Li
PII:
DOI:
Reference:
S1566-7367(17)30423-5
doi:10.1016/j.catcom.2017.10.010
CATCOM 5221
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
26 August 2017
1 October 2017
9 October 2017
Please cite this article as: Chengfeng Zhu, Qingqing Mao, De Li, Changda Li, Yiyang
Zhou, Xiang Wu, Yunfei Luo, Yougui Li , A readily available urea based MOF that
act as a highly active heterogeneous catalyst for Friedel-Crafts reaction of indoles and
nitrostryenes. The address for the corresponding author was captured as affiliation for all
authors. Please check if appropriate. Catcom(2017), doi:10.1016/j.catcom.2017.10.010
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ACCEPTED MANUSCRIPT
A readily available urea based MOF that act as a highly active
heterogeneous catalyst for Friedel-Crafts reaction of indoles and
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nitrostryenes
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Luo* and Yougui Li
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Chengfeng Zhu*, Qingqing Mao, De Li, Changda Li, Yiyang Zhou, Xiang Wu, Yunfei
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Anhui Province Key Laboratory of Advanced Catalytic Materials and Reaction Engineering, School of
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Chemistry and Chemical Engineering, Hefei University of Technology, Hefei, 230009, P. R. China.
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Corresponding author. Chengfeng Zhu, ZhuCF@hfut.edu.cn; Yunfei Luo, yunfluo@hfut.edu.cn.
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Qingqing Mao and De Li are contribute equally to this work.
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Abstract
A new urea-containing metal-organic framework has been rationally synthesized from a V-shaped
dicarboxylate ligand and utilized as hydrogen-bond-donating catalyst, which is highly efficient and
recyclable for Friedel-Crafts reaction. The MOFs catalyst has the advantages of excellent product yields
and a low catalyst loading. Moreover, the catalyst shown a remarkably superior activity compared to its
Heterogeneous
catalysis,
Metal
organic
frameworks
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Keywords:
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homogenous urea counterparts.
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Hydrogen-bond-donating catalyst, Urea.
based
catalysts,
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1. Introduction
Metal- organic fra meworks (MOFs), co mposed of organic linkers and inorganic metal ions or
clusters[1, 2], have recently e merged as pro mising materials in heterogeneous catalys is [3-8].
MOF materials provided an effic iently alternative avenue for the heterogeneous catalysis by
provid ing a precisely determined porous structure with unifor mly installed catalytic sites
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through a judicio us co mbination of functio nalized privileged molecular catalysts and inorganic
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nodes[9-11]. To date, numerous MOFs catalysts constructed fro m metalloporphyr ins [12],
their
ho moge nous
counterparts due
to
their
unique
properties
such
as
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co mparing
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metallosalen[13- 15] and BINOL system[16] have shown highly efficient catalytic perfor mance
unprecedentedly highly dense active sites and unifor m pore structures. Nonetheless, deep
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ins ights into MOF catalysts regarding to take advantage of spatial isolation effects to improve
catalytic activities of functional constituent are still needed [17, 18].
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Urea containing compounds are known as an important class of hydrogen-bond-donating (HBD)
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organocatalysts[19, 20]. However, the activity of HBD catalysts in homogeneous catalysis can be
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suffered by the self-quenching of the catalysts[21]. As a result, a high catalyst loading and long reaction
time are necessary to achieve high yield in the urea catalyzed Friedel-Crafts reaction of indoles and nitro
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olefins[22]. To overcome the self-quenching of urea groups, one general strategy is to immobilize
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urea-based HBD homogeneous catalyst onto the porous solid supports such as mesoporous silica or
porous organic polymers[23, 24]. Unlike the traditional immobilization of catalysts onto inorganic or
organic supports, the structure of MOFs can be precisely determined so the catalytic performance can be
easily evaluated and potentially be promoted by rational investigation of structure-property
relationships[25, 26]. Recently, Hupp and other research groups have successfully constructed a few
urea-containing MOFs through either direct synthesis or post modification and demonstrated their
catalytic activity in Friedel-Crafts (F-C) reaction[27, 28]. However, the results are still unsatisfactory
because either high catalyst loading or toxic additive is necessary to obtain excellent catalytic
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performance [28, 29]. Given the fact that the urea moiety has been separately fixed in the porous MOFs
structure, the activity of the MOFs catalysts have not shown significant increase than their
corresponding free ligands containing urea moiety. We assumed that this discrepancy result might be
attributed to the high complexity of channel structure that in MOF HBD catalysts, which retarded the
diffusion of the reactants and products in the frameworks of solid catalysts [25]. This detrimental effect
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offset the promotion of the activity by fixing the HBD moiety on MOF structures. In this context, we
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attempt to construct simple 2D layered urea-based MOFs to address this phenomenon. Considering in a
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2D MOF structure, the relatively weak non-covalent forces between layers can make the space between
them adjustable in some extent to accommodate the guest molecules. As a result, the active sites in a 2D
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MOF catalyst would be more easily accessible for reactant, so the reactivity will be enhanced
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significantly.
By surveying literature precedents, it was found that a type of simple and cheap V-shaped
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dicarboxylate ligands are ideal for constructing 2D MOFs in high reliability and their pore structure
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could be precisely controlled [30, 31]. The high reliability and controllability in synthesis plus their
simple 2D structure makes this type MOFs very suitable to serve as a highly tunable skeleton of solid
V-shaped
dicarboxylate
ligand
containing
a
pre- installed
urea
group
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a
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catalyst by pre- functionalization of the free ligand. To take this advantage, we designed and synthesized
(1,3-di(4-carboxyphenyl)-5-(3-phenylureido)-benzene) and utilized it as organic connectors to prepared
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2D MOFs. This generically as-synthsized 2D MOF catalyst containing separately fixed urea moiety was
subjected to catalyze a bench mark reaction, the Friedel-Crafts reaction of indoles and nitro olefins, to
evaluate its catalytic performance. The results demonstrated that this new 2D MOF catalyst has shown
remarkable increase in activity.
2. Experimental section
2.1 Synthetic chemistry
Two V-shaped dicarboxylate ligand, L1 -H2 and L2 -H2 , were readily synthesized from 1,
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3-dibromobenzene and 3, 5-di(4- methoxycarbonylphenyl)aniline (see ESI), which were used as organic
strut to synthesize MOFs catalysts (as outlined in Sche me 1). Single crystals of [CuL1 ·DMA]·DMF (1)
was readily obtained in good yield by heating L1 -H2 and Cu(NO 3 )2 ·3H2 O in a DMA/DMF/H2 O mixture
at 80 °C for 12 hours. Blue crystals [CuL2 ·H2 O]·2DMF·H2 O (2) were prepared in similar procedure by
heating a mixture of Cu(ClO 4 )2 /L2 -H2 /DMF/H2 O. The formulations of 1 and 2 were supported by the
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results of single-crystal X-ray diffraction, IR spectroscopy and TGA analysis. These products were
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stable in air and insoluble in water and common organic solvents. Their phase purity of the bulk sample
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was cross checked by comparison of its observed and simulated powder X-ray diffraction (PXRD)
((Insert Scheme 1 here))
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2.2 Catalytic procedure for Friedel-Crafts reactions
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patterns (Fig. S5-6).
The activation of catalysts 1 and 2 were accomplished by exchanging guest molecules with anhydrous
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MeNO2 , MeOH and CH2 Cl2 in turn, respectively, followed by evacuation under vacuum for 2 h at
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100 °C. The activation of 2 was confirmed by FT-IR spectroscopy and thermal gravimetric analysis
(TGA) (Fig. S20 and S22), which shown that the gust molecules in the pores of MOF were removed.
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Typically, 1.5 mol% catalyst (catalyst loading based on L1 ligand or urea moiety) was added to a
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solution of β-nitrostyrene (0.10 mmol) and indole (0.2 mmol) in acetonitrile (0.5 mL), the mixture was
stirred at 60 °C and stirred for 18 hrs. The mixture was centrifuged at 14,000 rpm for 10 min and the
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supernatant was concentrated under vacuum. The yields were determined by 1 H NMR.
3. Results and discussion
3.1 Structural description
In this
work,
we initia lly
sought
to
synthesize 2D
sheet MOF
by
emp loying
1,3-di(4- carboxyphenyl)benzene, a known liga nd with V- shape, as model strut to react with
Cu(NO 3 )2 ·3H2 O via solvother ma l synthesis. We are pleased to found that a neutral 2D
fra mework was obtained in a monoclinic space group C2/c according to structure analys is and
its asymmetr ic unit contains one formula unit. In 1, each Cu(II) ion is square- pyramidally
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coordinated by four carboxylate oxygen atoms fro m L1 ligands at the basal positions and one
oxygen atom fro m DMA mo lecule at the apical position (Fig. S7). The Cu-O bond lengths range
fro m 1.955(4) to 2.158(5) Å. Each pairs of Cu(II) ions is bridged by fo ur carboxylate groups to
generate a paddle- wheel Cu2 (COO)4 SBU. Typ ically, each SBU, acting as a 4-connecting node,
is linked by four L1 ligands and each L1 liga nd is linked to two SBUs to create an und ulating 2D
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layer structure possessing a sql type net with point Schlä fli symbol (44 .62 ) (Fig. S8 and S9).
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Thus, four adjacent SBUs and four L1 ligands linked together to for m a distorted rhomb ic
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channe l with dia gonal distances of ∼ 19.7×27.6 Å (Fig. S8), allowing another identical network
to penetrate it in a nor mal mode. This double interpenetration makes the 2D network to exhib it
effective dimens ion of ca. 3.5×6.8 Å2 and
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two different channels along c direction with
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5.8×7.6 Å2 , respective ly (Fig. S10). Further exa minations reveal that the adjacent 2D networks
with nearest interlayer Cu…Cu distance of ca. 8.8 Å are stacked in a slipped fashion, where π-π
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stacking fro m two neighboring layers is observed (Fig. S 11). PLATON calculatio ns indicate that
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1 has ca. 37.3% of free spaces available for accommodating the guest molecules.
((Insert Fig. 1.here))
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Encouraged by the successful synthes is of 1 with 2D layered structure fro m a V- shaped ligand
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and Cu2 (COO)4 SBU, 1,3- di(4-carboxyp henyl)-5- (3-phenylureido)- benzene, the urea- modified
analogue, was synthesized and used to construct 2D MOF via solvother mal reaction. Under
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similar reaction conditio ns as that in synthesis of 1, blue crystals [CuL2 ·H2 O]·2DMF·H 2 O(2),
was readily obtained in good yield (see Sche me 1). As we expected, 2 adopts layered 2D structure
of sql topology (Figure 1a and Fig. S14). It also crystallized in same monoclinic space group C2/c with
one formula unit in its asymmetric unit. Notably, 2 prefers a non-interpenetrated 2D network with an
open channel of ca. 7.6 × 156 Å along the c-axis (Figure 1b). This might be due to the presence of the
larger size of phenylureido group on the organic strut to prevent its interpenetration. The neighboring
2D networks, having the nearest Cu…Cu distance of ca. 8.4 Å between adjacent layers, are arranged in
a slipped fashion in c direction as 1 (Fig. S16). This stacking mode results in open channels with an
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effective dimension of ca. 10.1 × 3.0 Å2 along the b-axis and round channels with diameter of ca. 7.0 Å
along the c-axis (Fig. S17-18). Calculation using the PLATON program indicates that 2 possess 36.6%
of total volume that is accessible for solvent molecules. TGA revealed that the guest water and DMF
molecules could be readily removed in the temperature range 30-155°C, and the framework is stable up
to ~300°C (Fig. S20). PXRD experiments indicate that the framework and crystallinity of 2 remain
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intact upon complete removal of guest molecules.
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Catalytic performance
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With structural details in hand, we started to evaluate the catalytic perfor mance of
urea-containing 2 by using it in the typical Friedel-Crafts (F-C) reaction of nitrostyrene and
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indo le (Table 1). To our pleasant, the solid catalyst 2 can effectively catalyze the F-C reaction,
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in which only 1.5 mol% catalyst loading can afford full conversion in 18 hours (entry 1, 98%
NMR yie ld or 96% isolated yield). This new MOF catalyst shows re markably higher activity
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than that the free ligand L2 - H2 and its methyl ester L2 - Me2 when they are used as ho mogeneous
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catalysts to run the same reactio n under sa me conditions but a longer reaction time (entries 2-3,
15% and 13% yield, respective ly). In addition, the control experime nt without catalyst, as shown
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in e ntry 4 of Table 1, gave no desired product of F- C reaction eve n after 36 hours. These results
activity.
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confir ming that the urea-containing 2 can effectively catalyze the F- C reaction with a superb
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((Insert Table. 1.here))
Next, to evaluate the role of copper cluster in the catalytic reaction, MOF 1, sharing a similar
structure with 2 but witho ut urea moiety on its skeleton, was used in the same F- C reaction. It
showed somewhat reactivity (entry 5, 28% yield), which could come from the weak Lewis
acid ity of the dicopper SBUs in 1[32, 33]. So it suggested that the copper moiety in 2 could also
contrib ute for the activity in so me extent as well. A contrast experime nt, in w hich copper nitrate
salt was used as catalyst, was carried out and it gave similar conversio n (entry6, 22%).
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Mechanica lly mixed catalysts of copper nitrate salt and free liga nds provided reactivities near to
the sum of ind ividua l activity which was reflected by the yield (entries 7-9). A consistent result
was obtained by mixing 1 and L2 liga nds (entries 10- 11). These results again imp lied that the
superior activity of 2 was not a simp le sum of activity fro m either copper salt or copper cluster
in MOF and free ligands.
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To further understand the relationship of catalytic activity and structure of 2, its detailed structure was
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examined. As shown in figure 1d, the urea groups were separately fixed on the backbone of 2,
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periodically pointing to the channels, to prevent its possible self-quecnching in homogenous system.
This spatial isolation effect provided highly dense naked urea protons to catalyze the reaction efficiently.
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This is a clear contrast to homogenous catalysts of L2 -H2 and L2 -Me2 , in which the effective catalytic
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site could be quenched by the strong intermolecular hydrogen bond interactions between its N-H bonds
and carbonyl. However, according to literature precedent[29, 34], the HBD based MOF catalysts have
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not shown superior activity over their free ligand catalysts despite the structure analysis shown that the
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HBD moieties are fixed on MOF channels in isolated orientation. To explain this, a hypothesis, that the
diffusion rate of reactants and product played an important role in this type of catalysts catalyzed
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reaction, could be validated for this phenomenon. These results clearly demonstrated that to fix a HBD
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functional moiety onto a MOF structure is indeed an effective strategy to promote the activity when a
proper 2D MOF catalyst was synthesized [34, 35].
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((Insert Table. 2.here))
Substrates tolerance of MOF catalyst 2 was examined by vario us indoles and β- nitrostyrenes.
It was found that 2 has a wide substrates tolerance for both indoles and β- nitrostyrenes. A range
of aromatic trans β- nitrostyrene nitroalkenes derivatives bearing both electron-donating and
electron- withdrawing substituents were found to react efficiently with indole to generate
correspond ing products
in excelle nt yields (92 to 99%, entries 1-5). Even for 1- and
2- naphthylnitroalkene, possessing relative large size aromatic rings, also give 86% and 84%
yie lds (entries 6- 7), respective ly. Additionally, indoles scope was exa mined by reacting
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β- nitrostyrene with vario us substituted indoles, which include 1- methyl, 2- methyl, 6- methyl and
6-chloro functiona l groups. All these indoles gave the desired products in good to excelle nt
yie lds (81 to 99%, entries 8- 11). It was noteworthy that either the electronic nature or the
positio n of the substituents at the aromatic ring of nitrostyrene or indo les could not deteriorate
the activity of the reaction. This result suggests that 2 is a highly effic ient HBD catalyst in F-C
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reaction.
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((Insert Table. 3.here))
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Meanwhile, the heterogeneity of the MOF catalyst 2 for the F-C reaction was also exa mined.
The supernatant fro m the F-C reaction of indole and β- nitrostyrene after filtration through a
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regular filter did not afford any additional product. To probe the stability of the urea-containing
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catalysts, we recycled 2 in the F-C reaction of indole and β- nitrostyrene. Upon completion of the
reaction with 18 hours, MOF 2 can be easily recovered in quantitative yie ld fro m the reaction
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mixture by a centrifugation and used repetitively without loss of cata lytic activity for four runs
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(See table 3. ca. 97, 95, 94 and 96% yields for runs 1- 4, respective ly). PXRD characterization
showed that the recycled catalyst 2 rema ined its crystallinity after the 3th run, although its
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structure got slightly distorted after the 4th run (Fig. S6). Inductively coupled plas ma optical
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emission spectro metry analysis of the F-C product 3-(2- nitro- 1-phenylethyl)- 1H- indole
ind icated little loss of the copper ion (~0.13 ppm, see Fig. S24) from the structure, which might
4. Conclusions
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be either molecular species or particles too small to be removed by filtration through Celite.
In summar y, two 2D layered MOFs were rationally designed and synthesized fro m V- shaped
dicarboxylate functionalized organic linkers and dicopper unit. This new 2D urea-containing
MOF could serve as a highly active and recyclab le HBD catalyst for the Friedel- Crafts reaction.
A significant pro motion of the catalytic activity was observed when it was compared with the
ho mogeneous HBD catalysts. This suggest that to fix the HBD catalytic mo iety onto a
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structurally simp le 2D MOF is a successful approach by preventing the catalysts self-quenching
and increasing diffus ion rate of reactants and product at the same time. Further investigation
aiming to elucidation of the significant promotion in reactivity is currently carried out in our lab.
Acknowledgements
This work was supported by the Nationa l Natural Science Foundatio n of China (No.
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21401037, 21672049) and Anhui Provincial Natural Science Foundation (1508085QB26).
data
to
this
article
can
be
found
online
at
http://dx.
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Suppleme ntary
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Appendix A. Supplementary data
doi.org/xx.xxxx/j.catcom.xxxx.xx.xxx.
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Figure Captions:
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Scheme 1. Synthesis of 1 and 2.
Fig. 1. Structures of MOFs 2: a) view of the connectivity between the [Cu2 (COO)4 SBUs and bridging
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ligands in 2 (Atoms labeled gray, carbon; purple, copper; red, oxygen; green, hydrogen.); b) the open
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channels of 2 along c direction (urea group was highlighted).
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Table 1. Catalytic performance of different catalysts in the Friedel-Crafts reaction between indole and
β-nitrostyrenea
NO2
CH3CN, 60 °C
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N
H
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Cat.
+
Cat.loading
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Entry
T
NH
NO2
Time (h)
Yield(%) b
18
98
24
15
24
13
36
trace
18
28
24
22
2 (1.5 mol%)
2
L2 -H2 (1.5 mol%)
3
L2 -Me2 (1.5 mol%)
4
No catalyst
5
1 (1.5 mol%)
6
Cu(NO3 )2 (1.5 mol%)
7
Cu(NO3 )2 (1.5 mol%)+ L1 -H2 (1.5 mol%)
24
25
8
Cu(NO3 )2 (1.5 mol%)+ L2 -H2 (1.5 mol%)
24
45
9
Cu(NO3 )2 (1.5 mol%)+ L2 -Me2 (1.5 mol%)
24
39
10
1 (0.75%)+ L2 -H2 (0.75 mol%)
24
27
11
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M
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1
1 (0.75%)+ L2 -Me2 (0.75 mol%)
24
b
19
1
For reaction details see experimental section. Determined using H NMR.
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a
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Table 2. Results of Friedel-Crafts reactions of indoles with trans-β-nitrostyrenesa.
R'
R'
R
MOF 2 catalyst
+
N
H
CH3CN, 60 °C
NO2
R
Entry
R
R'
Yield(%) c
1
C6 H5
H
98
2
4-Me-C6 H4
H
95
3
4-Cl-C6 H4
H
97
4
4-Me-C6 H4
H
5
2-thienyl
H
6
1-naphtyl
H
7
2-naphtyl
8
T
d
94
86
C6 H5
1-Me
92
9
C6 H5
2-Me
96
10
C6 H5
6-Me
99
11
C6 H5
6-Cl
81
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84
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92
H
For reaction details see experimental section. bCatalyst loading based on urea moiety (1.5 mol%);
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a
NO2
NH
b
AC
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Determined using 1 H NMR. d Isolated yield >99%.
c
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Table 3. Recyclability studya
NH
MOF 2 catalystb
NO2
+
NO2
CH3CN, 60 °C
Reuse
Time (h)
Yield(%) c
1
Cycle Ⅰ
18
97
2
Cycle Ⅱ
18
95
3
Cycle Ⅲ
18
94
4
Cycle Ⅳ
18
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T
Entry
96
For reaction details see experimental section. bCatalyst loading based on urea moiety (1.5 mol%);
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a
N
H
AC
CE
PT
ED
M
AN
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Determined using 1 H NMR.
c
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Graphical abstract
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AN
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A urea-containing 2D metal-organic framework was prepared and employed as a highly active and recyclable
hydrogen-bond-donating catalyst for the Friedel-Crafts reaction.
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Highlights
An easily accessible 2D MOF was synthesized for serving as a HBD catalyst.
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This MOF catalyst demonstrated exceptional reactivity in Friedel-Craft reaction.
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The exceptional reactivity is attributed to its simple layered structure.
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2017, 010, catcom
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