Bottom-Up Assembly of a Highly Efficient Metal-Organic Framework for Cooperative Catalysis

Article abstract of DOI:10.1021/acs.inorgchem.8b02434

In this study, we demonstrate a bottom-up assembly of a monomeric copper complex and a two-dimensional (2-D) heterometallic metal-organic framework (MOF) from a carboxylate-functionalized tridentate Schiff base ligand and metal ions. The obtained 2-D MOF

Full text of DOI:10.1021/acs.inorgchem.8b02434

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Bottom-Up Assembly of a Highly Efficient Metal−Organic
Framework for Cooperative Catalysis
Changda Li,†,# Haitong Tang,†,# Yu Fang,‡ Zhifeng Xiao,‡ Kunyu Wang,‡ Xiang Wu,† Helin Niu,§
Chengfeng Zhu,*,† and Hong-cai Zhou*,‡,∥
†Anhui Province Key Laboratory of Advanced Catalytic Materials and Reaction Engineering, School of Chemistry and Chemical
Engineering, Hefei University of Technology, Hefei, 230009, P. R. China
‡Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255, United States
§School of Chemistry and Chemical Engineering, Anhui University, Hefei, 230039, P. R. China
∥Department of Materials Science and Engineering, Texas A&M University, College Station, Texas 77843-3003, United States
S
* Supporting Information
ABSTRACT: In this study, we demonstrate a bottom-up assembly of a monomeric copper
complex and a two-dimensional (2-D) heterometallic metal−organic framework (MOF) from a
carboxylate-functionalized tridentate Schiff base ligand and metal ions. The obtained 2-D MOF
features a unique bimetallic copper center which is different from its monometallic precursor
and acts as an efficient heterogeneous catalyst for the Friedel−Crafts reaction and Henry
reaction. The MOF catalyst shows a remarkably superior activity compared to its homogeneous
counterparts in a wide range of substrates. It is presumably ascribed to the dual activation of the
substrates by the active bimetallic copper center confined in the MOF network, which is
supported by the significant changes in catalytic activity at low catalyst/substrates ratios when
using the 2-D MOF and its precursor as catalysts, respectively. Moreover, the MOF catalyst also
shows an excellent stability and recyclability. Our work, therefore, provides a stepwise strategy to
design a heterogeneous cooperative catalyst, by taking advantage of the modulated structure of
MOF and tunable functionality of the tridentate Schiff base, with high performance in a variety
of organic synthesis.
cooperative catalysts based on precise structure details are
challenging and gaining more and more interest.
INTRODUCTION
■
Enzyme catalysis is a predominant process in nature due to its
excellent conversion, specificity, efficiency, as well as trans-
formation range, which usually requires cooperation between
metal centers and protein residues to activate substrates.1
Therefore, great efforts have been devoted to the synthesis of
enzyme-mimicking cooperative catalysts with bi- or poly-
nuclear metal centers in the past few decades. Numerous
homogeneous cooperative catalysts, especially bimetallic
catalysts, were first studied due to their facileness in design
and synthesis, while they were limited in stability and
recyclability.2 In order to overcome shortcomings of
homogeneous catalysis, one strategy to design heterogeneous
cooperative catalysts was to load molecular catalysts onto
inorganic or organic supports.3 For example, Li and co-workers
recently developed a series of high-performance solid catalysts
with multiple active sites working cooperatively in asymmetric
catalysis by utilizing the inorganic microporous/mesoporous
materials or porous organic frameworks as supports.4 Even
though such heterogeneous cooperative catalysts have achieved
success, the concise structure of active centers in solid catalysts
is still mysterious. As we know, the configuration and spatial
distance of the active sites greatly affect the efficiency of
catalysts, and thus the rational design and synthesis of
Metal−organic frameworks (MOFs) have emerged as a class
of crystalline materials,5 which offer great potential in
heterogeneous catalysis owing to their high porosity, excellent
stability, composition tunability, and well-defined reaction
microenvironments.6 In particular, structure engineering and
modulated synthesis of MOFs have achieved appreciable
progress in recent years, allowing for the possibility of spatial
control of active centers and making MOFs an ideal platform
to design high-performance cooperative catalysis.7 For
instance, Cui and co-workers recently reported examples of
metallosalen MOF catalysts, which showed high efficiency and
selectivity in asymmetric cyanosilylation of aldehydes and
kinetic resolution of epoxides, respectively, by taking advantage
of bimetallic cooperative catalytic effect.8 In another group, Lin
and co-workers confined Co(III)(porphyrin) pairs in an
interpenetrating MOF to realize high-efficient cooperative
catalysis for hydration of terminal alkynes.9 Despite these
impressive advances, cooperative catalysis based on MOFs is
rarely reported, and the design of novel catalysts with high
efficiency in organic transformations is of ongoing interest. To
Received: August 28, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b02434
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Scheme 1. Synthesis of Tridentate Schiff Base Ligand H3L, Mononuclear Copper Complex 1 and MOF 2
obtain a MOF-based cooperative catalyst with bi- or multiple
active centers, a judicious choice of ligands and metals is
demanding. It is a straightforward strategy to integrate catalytic
centers into ligands that can tolerate the harsh synthetic
environment of the targeting MOF.
EXPERIMENTAL SECTION
■
Materials and General Procedures. All of the chemicals used in
this study were commercially available and used without further
purification. The IR (KBr pellet) spectra were recorded (400−4000
cm−1 region) on a Nicolet Magna 750 FT-IR spectrometer. H and
1
13C NMR data were collected on an Agilent VNMRS-600
spectrometer. Electrospray ionization mass spectra (ES-MS) were
recorded on a Finnigan LCQmass spectrometer using DMSO as
mobile phase. Single-crystal X-ray (XRD) data of 1 were collected on
an D8 VENTURE diffractometer using Cu Kα radiation (λ = 1.54184
Å) at 100 K, while single-crystal XRD data of 2 were collected at 100
K at NFPS (Shanghai) synchrotron radiation on BL17B beamline
using λ = 0.65251 Å. All of the data were indexed, integrated, and
scaled using the APEX3 program. The structures of 1 and 2 were
determined by the direct method and refined on F2 by the full-matrix
least-squares technique using the SHELX-2018 program package.13
All the hydrogen atoms attached to the ligand were placed in
calculated positions and refined using a riding model. Contributions
to scattering due to these highly disordered guest molecules in 2 were
removed using the SQUEEZE subroutine of the PLATON software
package.14 The structure was then refined again using the resulting
new HKL file. Crystals 1 and 2 can be best formulated as [Cu(HL)·
MeOH]·[Cu(HL)·(MeOH)2] and [Cd3(CuL)8·(DMF)6·(H2O)8·
(NMe2)2], respectively, on the basis of single-crystal diffraction, IR
spectra, and thermogravimetric analysis (TGA). Crystal data and
details of the data collection are given in Table S1, while the selected
bond distances and angles are presented in Tables S2 and S3. CCDC
1863032 (for 1) and 1863033 (for 2) contain the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre. Powder
X-ray diffraction (PXRD) experiments were carried out on a
DMAX2500 diffractometer using Cu Kα radiation. The simulated
PXRD spectra were produced using the SHELXTL-XPOW program
based on the single-crystal data. TGA data were performed in an
argon atmosphere with a heating rate of 5 °C/min on a TGA-50
(SHIMADZU) thermogravimetric analyzer.
The tridentate Schiff base is a well-known ligand in
numerous catalytic systems.10 In these catalytic systems, the
tridentate ligands can bind strongly to metal ions via the ONO
coordination pockets and form complexes with multiple metal
centers to afford a synergistic catalytic center.11 To integrate
the Schiff base complex into a MOF, terminal carboxylic
moiety is introduced to transform the complex into a MOF
linker, allowing further extension to an infinite network. Since
the tridentate ONO pocket and the terminal carboxylate
exhibit different binding ability to metals, it is possible to
organize the metal center step by step. The copper(II) ion
usually shows tetra-, penta-, and hexa-connected coordination
geometry through bonding to N or O donors of various
ligands,12 making it promising to form a multinuclear metal
center. Our strategy is (1) obtain a discrete mononuclear Cu
(II) complex based on a tridentate Schiff base ligand bearing a
spare carboxylic group; (2) use the copper complex as a
secondary building unit (SBU) to assemble a MOF with
another metal; (3) perform catalytic reactions by using this
heterogeneous MOF catalyst.
Herein, we report the bottom-up assembly of a porous two-
dimensional (2D) MOF (2) through a reaction between a
strategically installed Schiff-base monomeric copper complex
(1) and two and four connected cadmium clusters. Unlike the
mononuclear precursor, the as-formed 2 has a dinuclear copper
center connected by bridging oxygen atoms. Because of the
suitable distance and geometry of the two metal ions in the
active site of 2, the MOF exhibits efficient cooperative
Friedel−Crafts alkylation of indole with nitrostyrene and
Henry reaction between aromatic aldehydes and nitromethane,
respectively. To the best of our knowledge, complex 2 is the
first example of heterogeneous MOF catalyst based on a
tridentate Schiff base with dinuclear copper center, which
shows high performance cooperative catalytic properties.
Synthesis of H3L. As outlined in Scheme 1, the carboxylate-
functionalized tridentate Schiff base ligand (H3L) was readily
synthesized by the condensation reaction of (1S, 2R)-(−)-cis-1-
amino-2-indanol (0.82 g, 5.5 mmol) with 3′-tert-butyl-5′-formyl-4′-
hydroxybiphenyl-4-carboxylic acid (1.49 g, 5.0 mmol) in methanol
(MeOH) at 60 °C. After the reaction, the ligand was washed several
times with MeOH and characterized by H NMR, 13C NMR, and
1
B
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Figure 1. A view of (a) the single framework of 2 along the a axis and (b) 2-fold interpenetrated structure of 2 (Cd, teal; Cu, green; O, red; N,
blue; C, gray; Cd1O8 and Cd2O7 units are shown as teal and yellow polyhedron, respectively; only the O atom of DMF molecules are shown for
clarity)
mass spectroscopy, respectively (Figures S1−S3). Fourier transform
infrared (FT-IR) data (KBr pellet, cm−1) (Figure S5): 3480(w),
2952(s), 2910(s), 2867(m), 1700(w), 1670(w), 1630(s), 1605(s),
1533(s), 1480(m), 1460(m), 1440(m), 1386(s), 1268(m), 1251(m),
1224(w), 1169(m), 1095(w), 1052(w), 1016(w), 993(w), 952(w),
888(w), 860(m), 790(m), 775(w), 750(s), 712(w), 651(w), 634(w),
620(w), 564(w), 527(m), 500(w), 449(w).
Synthesis of [Cu(HL)·MeOH]·[Cu(HL)·(MeOH)2] (1). A mixture of
Cu(OAc)2·H2O (20.0 mg, 0.1 mmol) and H3L (42.9 mg, 0.1 mmol)
was placed in a glass vial containing N,N′-dimethylformamide (DMF)
(0.5 mL), MeOH (3.5 mL), and water (0.10 mL). The vial was sealed
and heated at 60 °C for 24 h. Blue, needle-like crystals were formed
and washed with MeOH and dried in air at room temperature. The
yield was ∼75.0% based on H3L. FT-IR data (KBr pellet, cm−1)
(Figure S6): 3480(s), 2950(s), 2910(s), 2870(m), 2360(w),
1700(w), 1620(s), 1604(s), 1533(s), 1478(w), 1458(w), 1420(m),
1386(s), 1328(m), 1275(s), 1258(m), 1228(m), 1166(s), 1107(s),
1072(m), 1053(w), 1013(w), 980(w), 950(w), 895(w), 857(m),
805(w), 788(m), 774(m), 751(m), 731(w), 711(w), 680(w), 640(w),
605(w), 548(w), 521(m), 491(w), 448(w).
Synthesis of [Cd3(CuL)8·(DMF)6·(H2O)8·(NMe2)2] (2). A mixture of
CdBr2 (5.44 mg, 0.02 mmol) and complex 1 (21.5 mg, 0.02 mmol)
was placed in a glass vial containing DMF (3.0 mL), MeOH (2.0 mL),
and water (0.2 mL). The vial was sealed and heated at 80 °C for 36 h.
Blue, block-like crystals were collected, washed with MeOH, and
dried in air at room temperature. The yield was ∼60.0% based on
cadmium ion. Notably, complex 2 cannot be prepared by a one-pot
reaction of Cu2+, Cd2+, and H3L. FT-IR data (KBr pellet, cm−1)
(Figure S7): 3450(s), 2951(s), 2908(s), 1660(s), 1620(s), 1603(s),
1529(s), 1478(w), 1457(m), 1393(s), 1324(m), 1277(s), 1257(s),
1230(m), 1199(w), 1163(s), 1099(m), 1010(w), 892(w), 860(s),
806(w), 788(s), 776(s), 752(s), 712(m), 662(m), 607(m), 544(m),
519(w), 464(m).
geometry with Cu−O bond lengths ranging from 1.884(3) to
1.980(3) Å and Cu−N bond length of 1.905(3) Å, whereas the
five-coordinated Cu2 center is loaded in a square-pyramidal
geometry with Cu−O bond lengths ranging from 1.896(3) to
2.451(3) Å and Cu−N bond length of 1.924(3) Å (Table S2).
However, the coordination of the axial methanol molecule with
the Cu2 center (Cu−O distance is 2.451(3) Å) is obviously
weaker than that of the equatorial methanol molecule with the
Cu2 center (the Cu−O distance is 1.943(3) Å). The Cu(HL)
units in 1 are linked via intermolecular hydrogen bonding
interactions (O−H···O distances range from 2.49 to 2.51 Å)
between carboxyl groups and neighboring two hydroxyl groups
from 1-amino-2-indanol and methanol molecule, respectively,
to generate a one-dimensional (1D) chain (Figure S13). After
that, the 1D chains are further assembled into a three-
dimensional (3D) supramolecular structure by edge-to-face
stacking intermolecular π−π interaction (π−π 3.62 Å) between
adjacent benzene rings of indane moieties and CH···π
interaction (C−H···π 3.71 Å) between the butyl group and
adjacent benzene ring from another Cu(HL) unit and CH···π
interaction (C−H···π 3.88 Å) between CH- group of indane
moiety and adjacent benzene ring from another Cu(HL) unit
ligand (Figure S14). It is noteworthy that the carboxyl groups
in 1 are protonated, which is consistent with the protonated
COOH νCO stretch at 1700 cm−1 comparing with the free
H3L ligand.15 Thus, in this study, the Cu(HL) moiety was
utilized as a molecular building block to construct an infinite
network structure as follows.
Gratifyingly, a solvothermal reaction of mononuclear
complex 1 and Cd2+ ion yields a single crystal 2. The crystal
structure determined by single-crystal XRD reveals 2 exhibits a
porous 2D MOF that crystallizes in the orthorhombic space
group P21212 with half of one formula unit in the asymmetric
unit. As shown in Scheme 1, the molecular building blocks of
CuL in 2 possess a similar square-planar coordinated mode as
Cu1(HL) unit in 1, with the Cu−O bond lengths ranging from
1.884(5) to 1.960(6) Å and Cu−N bond lengths ranging from
1.910(6) to 1.924(6) Å, respectively. Different from 1, the
CuL units acting as molecular building units are bridged
through two indane-bound oxygens to generate two cis dimeric
(CuL)2 moieties with a Cu−Cu distance of 2.806 and 2.841 Å,
respectively. With two crystallographically independent Cd2+
ions in 2, the Cd1 center is coordinated to eight oxygen atoms
from four bidentate carboxylate groups, and the Cd2 center is
RESULTS AND DISCUSSION
■
Crystal Structure of 1 and 2. A single-crystal X-ray
diffraction analysis reveals that 1 crystallizes in P21 space
group, and its asymmetric unit contains a [Cu(HL)·MeOH]
and a [Cu(HL)·(MeOH)2] unit. In the structure of 1, the
ligand H3L acts as a tridentate ONO chelator through the
imine group and two hydroxyl groups. As shown in Scheme 1,
the Cu1 center in the [Cu(HL)·MeOH] unit is four-
coordinated by the ONO atoms from tridentate ligand H3L
and one MeOH molecule, while the Cu2 center in [Cu(HL)·
(MeOH)2] unit is five-coordinated by ONO donors from H3L
and two MeOH molecules (Figure S12). As a four-coordinated
copper center, the Cu1 is loaded in a distorted square-planar
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coordinated by one DMF molecule, two water molecules, and
four oxygen atoms from two bidentate carboxylate groups.
Thus, the Cd1 ion is in a distorted bicapped trigonal prismatic
geometry with Cd−O distances ranging from 2.389(6) to
2.447(7) Å, while Cd2 adopts a distorted pentagonal
bipyramidal coordination environment with Cd−O distances
ranging from 2.279(8) to 2.411(1) Å (Table S3), and all of the
Cd−O bond length is in the normal range according to
previously reported Cd(II)-based carboxylates.16 As shown in
Figure 1, the dimeric (CuL)2 units can serve as 2-connected
building blocks and were linked by 4-connected Cd1O8
clusters and 2-connected Cd2O7 clusters, respectively, to
generate a 2D framework, which possesses two open channels
of ∼29.2 × 10.8 Å2 and ∼13.7 × 12.0 Å2, respectively, along
the a axis (Figure. 1a) and an open channel of ∼14.2 × 10.5 Å2
along the c axis (Figure S17). Although the overall structure of
2 adopts a 2-fold interpenetrated framework (Figure 1b) to
stabilize the porous network, an open channel of ∼14.2 × 10.5
Å2 along the c direction still remains after its mutual
interpenetration (Figure S19). Furthermore, the 2D structure
is further stabilized by CH···π interactions (C−H···π 3.44 and
3.58 Å) between the butyl group and adjacent benzene ring
from another Cu(HL) unit and parallel-displaced stacking
intermolecular π−π interactions (π−π 3.33 and 3.36 Å)
between the benzene ring of the indane moiety and the
adjacent benzene ring from another Cu(HL) unit. Calculations
show that the void space in 2 is 41.8% by utilizing the
PLATON software,14 which is available for accommodating
guest molecules. In addition, the permanent porosity of 2 was
confirmed by N2 adsorption isotherms at 77 K and dye
adsorptions in solution. 2 exhibited a Type-I sorption behavior
with a BET surface area of 499.8 m2/g (Figure S20) and could
adsorb ∼1.44 methylene blue (MB, ∼1.25 nm × 0.50 nm ×
0.38 nm in size) per formula unit (Figure S21). TGA reveals
that the guest molecules such as methanol, water, and DMF
molecules can be readily removed in the temperature range of
25−220 °C, and the framework is stable up to ∼315 °C. PXRD
experiments indicate that the framework and crystallinity of 2
remains intact upon complete removal of guest molecules.
Catalytic Performance. Given that Schiff-base copper
complexes are promising Lewis acid catalysts in synthetic
chemistry,17 we initially selected the Friedel−Crafts (F−C)
alkylation of indoles with nitrostyrenes as a model reaction to
evaluate the catalytic performance of 2 because indole and its
derivatives are important intermediates for a number of
biologically active natural and synthetic products.18 For
comparison, the mononuclear copper complex 1, CdBr2, and
a previous reported 2D Cd-MOF (Figure S22) were also
employed as a control group to investigate the potential of the
cooperative catalyst. The F−C reaction was carried out with a
1:1.2 molar ratio of nitrostyrene and indole at room
temperature by using CH2Cl2 as the solvent. Under the
optimized conditions, it was found that a loading of 1 mol %
activated 2 could smoothly catalyze the F−C reaction of indole
with β-nitrostyrene, 4-Me-β-nitrostyrene, and 4-CF3-β-nitro-
styrene, respectively, which afforded corresponding products
with 96%, 92%, and 95% yield (entry 1, 3, 5 in Table 1). In
contrast, when using mononuclear complex 1 as the catalyst
(the same loading of CuL units as 2), the reaction yields
dramatically dropped to 39%, 31%, and 42%, respectively
(entry 2, 4, 6 in Table 1). To assess the role of unsaturated Cd
centers in 2 in the F−C reaction, the 2D Cd-MOF and CdBr2
were used as catalysts, respectively, in the reaction of indole
Table 1. Catalyzed Friedel-Crafts Alkylation of Indoles with
Nitrostyrenes by 1 and 2
a
b
c
entry
cat.
R
R1
R2
R3
yield (%)
1
2
3
4
5
6
7
8
2
1
2
1
2
1
2
2
2
Ph
Ph
H
H
H
H
H
H
H
H
H
H
H
Me
H
H
Me
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Me
Ph
Me
Me
Me
Me
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
95
39
92
31
95
42
96
89
90
4-Me-Ph
4-Me-Ph
4-CF3−Ph
4-CF3−Ph
3-OMe-ph
4-Cl-Ph
4-NO2-Ph
Ph
Ph
Ph
Ph
Ph
9
d
e
10
11
12
13
14
15
16
17
18
19
Cd-MoF
CdBr2
5 /13
d
e
18 /29
2
2
2
2
2
2
2
2
94
97
90
90
93
91
92
89
Ph
Ph
Ph
Ph
5-Me
5-OMe
5-Cl
Ph
6-Me
a
b
For reaction details, see Supporting Information. 1.0 mol % loading
of 2 based on the empirical formula. For monomeric complex 1, the
c
loading of CuL units is same with 2. Determined by 1HNMR
d
spectroscopy of the crude reaction mixture. The loading of cadmium
e
is same with 2. Reaction time is 6 h.
with β-nitrostyrene. It showed somewhat reactivity (within 6
and 12 h, 5% and 13% conversions for Cd-MOF and 18% and
29% conversions for CdBr2, respectively. entry 10 and 11 in
Table 1), which implies that the activity of this F−C reaction
mainly comes from the active copper sides, even though the
weak Lewis acidity of the Cd1O8 and Cd2O7 clusters in 2
could also contribute for the activity in some extent. From the
control experiments, we noted the evident difference in
catalytic performance between 1 and 2, and we presumed
that the difference derives from their distinct metal centers in
the structures. From the X-ray crystal structural analysis, it is
known that the active catalytic center in 1 exists in the form of
monomeric Cu(HL)·MeOH moiety, which was also supported
by electrospray ionization mass spectrometry (the molecular
ion [M + H]+ with an m/z value of 523.1 corresponding to
[Cu(HL)·MeOH+H]+, Figure S4). In contrast to 1, the active
catalytic species in 2 is a bimetallic (CuL)2 moiety bridged
with two indane oxygens. The channel surfaces of 2 are
uniformly lined by saddle-like (CuL)2 dimers with coordina-
tively unsaturated copper sites pointing to the open channels.
What is more, the Cu−Cu separation within the (CuL)2 dimer
is around 2.8 Å; therefore, such a close distance and
appropriate spatial orientation of active CuL catalyst could
provide a possibility that the indoles and nitrostyrenes were
activated by this bimetallic species simultaneously. This might
be the reason why the MOF catalyst 2 shows a remarkably
higher activity than its homogeneous monomeric complex 1.
To ascertain the presence of the synergistic dual activation
of indoles and nitrostyrenes within the channels of MOF
catalyst, we further examined the catalytic activity of 1 and 2
D
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under much lower C/S ratios (the molar ratio of catalyst to
nitrostyrene). As shown in Table 2, it was obviously
To rationalize the observed results above, we propose a
mechanism that involves the cooperative activation of the two
substrates in the F−C reaction, mediated by catalyst 2. We
envision that the indole and trans-β-nitrostyrene are synergisti-
cally activated by weakly coordinating to the adjacent Lewis
acidic copper site in the bimetallic (CuL)2 moieties through N
atom on the indole ring and NO2-group of nitrostyrene,
respectively (Figure 2a).19 Thus, the two organic substrates
Table 2. Catalytic Performance of 1 and 2 at Different
a
Catalyst/Substrate Ratios
1
2
1
2
conv.
c
conv.
c
conv.
c
conv.
c
b
C/S
R
(%)
(%)
R
(%)
(%)
1/100
1/1000
1/10000
Ph
Ph
Ph
39
14
0
95
52
17
4-Me-Ph
4-Me-Ph
4-Me-Ph
31
8
0
92
49
12
a
b
For reaction conditions, see Supporting Information. The molar
ratio of catalyst to nitrostyrene. The same loading of CuL units for 1
Figure 2. (a) A proposed dual activation pathway of F−C alkylation
and Henry reaction, respectively, in the cooperative catalysis; view of
the optimized structures to depict noncovalent interactions in
intermediates of 2 (b) and 1 (c).
c
1
and 2 at different C/S ratios. Determined by HNMR spectroscopy
of the crude reaction mixture.
demonstrated that the catalytic activity of 1 in the F−C
alkylation of indole with β-nitrostyrene sharply decreased from
39% to 14% and 0% as the C/S ratios decreased from 1:100 to
1:1000 and 1:10000, respectively. However, at a lower C/S
ratio of 1:1000 and 1:10000, MOF catalyst 2 is still capable of
affording a 52 and 17% conversion for β-nitrostyrene. The
similar performance was also observed when the substrate was
replaced by 4-Me-β-nitrostyrene under the identical reaction
conditions. The low activity of homogeneous monomeric
copper complex at lower C/S ratios is possibly due to the
longer distance between two CuL species, which impedes the
cooperation of CuL units during the catalytic process. In
contrast to the low activity of 1, the activity enhancement of 2
further confirms the proposed cooperative catalytic effect of
bimetallic (CuL)2 units that confined in the MOF structure.
Substrates tolerance of the cooperative catalyst 2 was further
examined by various β-nitrostyrenes and indoles. In the
presence of 2, a wide range of aromatic trans-β-nitrostyrene
derivatives bearing both electron-donating and electron-
withdrawing substituents were found to react efficiently with
indole and generated corresponding products in excellent
yields of 89−96% (entry 1, 3, 5, 6−9 in Table 1). Additionally,
indoles scope was examined by reacting trans-β-nitrostyrene
with various indoles derivatives, which included mono- or
disubstituted groups. All these indoles gave the desired
products in good to excellent yields of 89−97% (entry 12−
19 in Table 1). These results suggest that 2 containing
bimetallic active centers could be used as a highly efficient
catalyst in a F−C reaction with a wide range of substrates.
The recyclability of catalyst 2 for the F−C reaction of indole
and trans-β-nitrostyrene was also investigated. Upon com-
pletion of the reaction within 12 h, catalyst 2 can be easily
recovered quantitatively from the reaction mixture through
centrifugation and washed by clean solvents. Then, the catalyst
can be used repetitively without loss of catalytic activity for the
following runs (ca. 97, 95, 94, 93, and 93% yields for runs 1−5,
respectively, Figure S23). PXRD characterization of 2 after the
fifth was also performed, and the results showed that the
catalyst still retained high crystallinity, despite a slight
structural distortion (Figure S9). Moreover, it is worth noting
that the reaction will cease immediately if the catalyst was
removed by centrifugation.
were put approximately and furnished the formation of the
alkylated product with higher yield. Density functional theory
(DFT) calculations on the F−C reaction between indole and
trans-β-nitrostyrene by utilizing the DMol3 program20 provide
further insight into the proposed mechanism of cooperative
catalysis. We only chose the active catalytic CuL unit as a
structure fragment to calculate for the complexity and large
size of the primitive crystal structure. As we proposed in the
dual activation mechanism, optimized structures of intermedi-
ates including two substrates and bimetallic (CuL)2 species
indicate that evident interactions between two substrates and
binuclear copper center in 2 could constrain movement of
indole and nitrostyrene (the closest distance of noncovalent
interactions between copper centers and the two substrates are
2.77 and 3.37 Å, respectively. Figure 2b), leading to a higher
reaction probability. Although the interaction between trans-β-
nitrostyrene and monomeric CuL unit is similar that in 2, the
interaction between indole and copper center is getting much
weaker (the closest distance of noncovalent interactions
between copper center and indole nitrogen atom is changed
from 3.37 to 4.21 Å, Figure 2c), as a result, the reaction
probability will drop. Meanwhile, the affinity of substrates with
the catalytic copper centers in 1 and 2 was also investigated by
comparing enthalpy, which indicated that such a combination
process was more favored in 2 (Figure S24).
Encouraged by the excellent performance in F−C reaction of
2, we then attempted to apply this solid cooperative catalyst to
Henry reaction. The reaction was performed between
aldehydes and nitromethane by using 1.0 mol % catalyst. It
was satisfactory that benzaldehyde and its derivatives bearing
electron-poor groups reacted with nitromethane efficiently to
give the β-nitro alcohols in 86−97% yield (entry 1, 3, 5−7 of
Table 3). From the control experiments from entries 2 and 4 in
Table 3, it was found that the mononuclear complex 1 (the
same loading of CuL units as 2) only gives yields of 48 and
53%. Such a difference in catalytic activity is getting more
obviously upon the C/S ratio decreasing from 1:100 to
1:10000 (as shown in Table 4). These results displayed that
catalyst 2 had a higher activity in catalyzing Henry reaction.
Considering that carbonyl moiety of the aldehydes and nitro
group of nitromethanes can also be activated by a Lewis acid
E
DOI: 10.1021/acs.inorgchem.8b02434
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
cooperative catalysis could extend to other porous materials,
like covalent organic frameworks, with chemical tunability,
high porosity, and ordered structural integrities.21
Table 3. Results of Henry Reaction between Aldehydes and
Nitromethane Catalyzed by 1 and 2
a
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b02434.
■
b
c
entry
cat
Ar
time (h)
yield (%)
S
1
2
3
4
5
6
7
2
1
2
1
2
2
2
Ph
Ph
12
12
12
12
12
12
12
86
48
96
53
94
97
4-NO2-Ph
4-NO2-Ph
3-NO2-Ph
2-NO2-Ph
4-CN-Ph
Experimental details and spectral data (PDF)
Accession Codes
CCDC 1863032−1863033 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
95
a
b
For reaction details, please see Supporting Information. 1.0 mol %
loading of 2 based on the empirical formula. For monomeric complex
c
1, the loading of CuL units is same with 2. Determined by 1HNMR.
Table 4. Catalytic Performance of 1 and 2 in Henry
Reaction at Different Catalyst/Substrate Ratios
a
AUTHOR INFORMATION
Corresponding Authors
*(C.Z.) E-mail: ZhuCF@hfut.edu.cn.
*(H.Z.) E-mail: zhou@chem.tamu.edu.
■
b
c
c
ORCID
C/S
catalyst
yield (%)
catalyst
yield (%)
Xiang Wu: 0000-0001-5428-9063
Chengfeng Zhu: 0000-0002-9676-6750
Hong-cai Zhou: 0000-0002-9029-3788
1:100
1:1000
1:10000
1
1
1
53
20
3
2
2
2
96
68
33
a
b
Author Contributions
For reaction conditions, see support information. The molar ratio
of catalyst to 4-NO2-benzaldehyde. The same loading of CuL units for
1 and 2 at different C/S ratios. Determined by HNMR.
#C.L. and H.T. contributed equally.
c
1
Notes
The authors declare no competing financial interest.
catalyst, the enhanced catalytic performance in Henry reaction
could be attributed to a dual activation process, which means
both the aldehydes and the nitromethane were synergistically
activated by the bimetallic (CuL)2 catalyst fixed on the wall of
the channel of MOF 2. However, for the homogeneous system,
especially at a low concentration of CuL catalyst, a dual
catalytic activation mode is less likely happen due to the
probability for the collision of activated nucleophile and
electrophile is very low. Again, this finding highlights the
superiority of cooperative bimetallic catalyst confined in MOF
structure.
ACKNOWLEDGMENTS
■
This work was supported by the National Science Foundation
of China (Nos. 21401037, 21672049, 51771001, and
21471001) and by the Robert A. Welch Foundation through
an endowed chair to H.C.Z. (A-0030). We thank the staff from
BL17B beamline of National Facility for Protein Science in
Shanghai (NFPS) at Shanghai Synchrotron Radiation Facility,
for assistance during data collection.
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In summary, a porous 2-fold interpenetrated heterometallic
MOF was assembled step by step from a carboxylate-
functionalized tridentate Schiff base ligand. The MOF
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