Metal-Organic Framework Anchored with a Lewis Pair as a New Paradigm for Catalysis

Article abstract of DOI:10.1016/j.chempr.2018.08.018

Lewis pair (LP) chemistry has shown broad applications in the catalysis field. However, one significant challenge has been recognized as the instability for most homogeneous LP catalysts upon recycling, thus inevitably leading to dramatic loss in catalytic activity. Additionally, current heterogeneous LP catalysts suffer from low surface area, which largely limits their catalytic efficiency, thereby restricting their potential applications. In this work, we report the successful introduction of LPs, classical and frustrated, into a metal-organic framework (MOF) that features high surface and ordered pore structure via a stepwise anchoring strategy. Not only can the LP be stabilized by the strong coordination interaction between the LP and MOF, but the resultant MOF-LP also demonstrates excellent catalysis performance with interesting size and steric selectivity. Given the broad applicability of LPs, our work therefore paves a way for advancing MOF-LP as a new paradigm for catalysis. Lewis pairs (LPs), classical and frustrated, are excellent prospects in catalysis, organic syntheses, biology, and material sciences. However, the instability of most LP catalysts leads to a dramatic loss in activities, thereby largely restricting their industrial applications. As robust porous materials, metal-organic frameworks (MOFs) offer a platform to stabilize homogeneous catalysts. Here, we show a strategy that grafts the LP catalyst on the MOF to minimize loss of LPs during catalysis and recycling. Our work reveals the enormous potential of MOFs as an appealing paradigm for the construction of efficient heterogeneous catalysts with interesting steric and size selectivity worthy of exploration. In addition, the strategies for anchoring a LP into a MOF as contributed herein can be readily applied for the task-specific design of functional catalysis materials for various applications. Lewis pairs (LPs), classical and frustrated, have been successfully introduced into and stabilized in a metal-organic framework (MOF). Benefiting from the robust framework and tunable porous structure of MOFs, the resultant MOF-LP demonstrates not only great recyclability but also excellent performance in the catalytic reduction of imines and hydrogenation of alkenes. The combination of LP and MOF therefore lays a foundation for developing a MOF-LP as a new paradigm for catalysis, particularly heterogeneous catalysis.

Full text of DOI:10.1016/j.chempr.2018.08.018

Article
Metal-Organic Framework Anchored with a
Lewis Pair as a New Paradigm for Catalysis
Zheng Niu, Wilarachchige D.C.
Bhagya Gunatilleke, Qi Sun, ...,
Yuchuan Cheng, Briana Aguila,
Shengqian Ma
sqma@usf.edu
HIGHLIGHTS
A stepwise strategy was
developed to anchor the Lewis
pair (LP) in a MOF
MOF-LP shows excellent catalytic
performance for reduction of
imines and alkenes
MOF-LP-catalyzed reduction
reactions show unique size and
steric selectivity
MOF-LP demonstrates excellent
recycling performance
Lewis pairs (LPs), classical and frustrated, have been successfully introduced into
and stabilized in a metal-organic framework (MOF). Benefiting from the robust
framework and tunable porous structure of MOFs, the resultant MOF-LP
demonstrates not only great recyclability but also excellent performance in the
catalytic reduction of imines and hydrogenation of alkenes. The combination of LP
and MOF therefore lays a foundation for developing a MOF-LP as a new paradigm
for catalysis, particularly heterogeneous catalysis.
Niu et al., Chem 4, 1–13
November 8, 2018 ª 2018 Elsevier Inc.
https://doi.org/10.1016/j.chempr.2018.08.018

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Article
Metal-Organic Framework
Anchored with a Lewis Pair
as a New Paradigm for Catalysis
Zheng Niu,1 Wilarachchige D.C. Bhagya Gunatilleke,1 Qi Sun,1 Pui Ching Lan,1 Jason Perman,1
Jian-Gong Ma,2 Yuchuan Cheng,3 Briana Aguila,1 and Shengqian Ma1,4,
*
SUMMARY
The Bigger Picture
Lewis pairs (LPs), classical and
frustrated, are excellent prospects
in catalysis, organic syntheses,
biology, and material sciences.
However, the instability of most LP
catalysts leads to a dramatic loss
in activities, thereby largely
restricting their industrial
Lewis pair (LP) chemistry has shown broad applications in the catalysis field.
However, one significant challenge has been recognized as the instability for
most homogeneous LP catalysts upon recycling, thus inevitably leading to dra-
matic loss in catalytic activity. Additionally, current heterogeneous LP catalysts
suffer from low surface area, which largely limits their catalytic efficiency,
thereby restricting their potential applications. In this work, we report the suc-
cessful introduction of LPs, classical and frustrated, into a metal-organic frame-
work (MOF) that features high surface and ordered pore structure via a stepwise
anchoring strategy. Not only can the LP be stabilized by the strong coordination
interaction between the LP and MOF, but the resultant MOF-LP also demon-
strates excellent catalysis performance with interesting size and steric selec-
tivity. Given the broad applicability of LPs, our work therefore paves a way
for advancing MOF-LP as a new paradigm for catalysis.
applications. As robust porous
materials, metal-organic
frameworks (MOFs) offer a
platform to stabilize
homogeneous catalysts. Here, we
show a strategy that grafts the LP
catalyst on the MOF to minimize
loss of LPs during catalysis and
recycling. Our work reveals the
enormous potential of MOFs as an
appealing paradigm for the
construction of efficient
INTRODUCTION
Increasing attention has recently been drawn to Lewis pair (LP) chemistry because of
its broad applicability, from theoretical chemistry to catalysis and synthetic chemis-
try.1–6 One important part of LPs is that they exhibit wide applications in catalytic
polymerization,7,8 C–H bond activation,9 and catalytic reduction fields.10,11 How-
ever, the instability of most LP catalysts upon recycling inevitably leads to the dra-
matic loss in catalytic activity. Meanwhile, the low surface area of current heteroge-
neous LP catalysts limits their catalytic efficiency, thereby restricting their industrial
applications. Loading the catalysts into porous materials12,13 would be a short-cut
method to obtain better recycling performance and catalytic efficiency than current
catalysts but has not yet been achieved, presumably because of the lack of
anchoring sites to hold and stabilize the LPs in most porous material supports.
heterogeneous catalysts with
interesting steric and size
selectivity worthy of exploration.
In addition, the strategies for
anchoring a LP into a MOF as
contributed herein can be readily
applied for the task-specific
design of functional catalysis
materials for various applications.
As one attractive type of porous crystalline material, metal-organic frameworks
(MOFs) have shown enormous potential in catalysis areas due to their well-defined
yet tunable pores and functionalized walls.14–18 The suitable pore size, high surface
area, and spacious pore space of MOFs can provide the substrate molecules enough
accessibility with the catalytic centers inside MOFs.19 Via directly utilizing20–23
or post-synthetically functionalizing24–26 the linkers or secondary building units
(SBUs), MOFs have been extensively investigated as single-site heterogeneous cat-
alysts.27–30 Meanwhile, taking advantage of their spacious pore space and tunable
pore sizes, MOFs have also been widely used to support various catalytic species,
such as organometallic complexes,31–33 nanoparticles,34–39 and enzymes,40–42 for
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Scheme 1. Introduction of the LP into the MOF
(A) Illustration of the stepwise anchoring strategy for grafting the LP on the open metal site of
the MOF.
(B) Construction pathway of MIL-101(Cr)-LP.
heterogeneous catalysis. The merits of MOFs would make them a promising type of
scaffold to stabilize LPs, which has recently been suggested by computational
studies43 but has not yet been realized experimentally. This could be likely due to
the fact that most LP catalysts are very water sensitive,44–46 and thereby the active
species would decompose during the MOF synthesis process when a ligand func-
tionalized with LP moieties is used.
RESULTS
Introduction of the LP into the MOF
To tackle the above issues, we contribute a strategy that allows the successful graft-
ing of the LP into the MOF. As illustrated in Scheme 1A, the custom-designed base
moiety of the LP is first anchored to the open metal sites within the MOF through co-
ordination interaction, which is followed by the introduction of the corresponding
acid moiety of the LP. Given the strong coordination interaction, it is anticipated
that the LP would be stabilized within the MOF yet accessible to substrates.
1Department of Chemistry, Institution University
of South Florida, 4202 E. Fowler Avenue, Tampa,
FL 33620, USA
2Department of Chemistry, Institution Key
Laboratory of Advanced Energy Materials
Chemistry and Collaborative Innovation Center
of Chemical Science and Engineering, Nankai
University, Tianjin 300071, P.R. China
3Ningbo Institute of Materials Technology and
Engineering, Chinese Academy of Sciences,
Ningbo, Zhejiang 315201, P.R. China
As a proof of concept, we selected dehydrated MIL-101(Cr), Cr3(OH)O(BDC)3
(BDC = 1,4-benzenedicarboxylate), to anchor LP considering its large pores or ca-
ges, abundant open metal sites upon activation, and high stability47–50; the LP,
composed of B(C6F5)3 and 1,4-diazabicyclo[2.2.2]octane (DABCO) that features
one potential coordination site, is used as a model for grafting into the MOF. This
4Lead Contact
*Correspondence: sqma@usf.edu
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Figure 1. Structure Characterization of MIL-101(Cr)-LP
(A) PXRD of MIL-101(Cr) and MIL-101(Cr)-LP.
(B) N2 sorption isotherms of MIL-101 and MIL-101(Cr)-LP at 77 K.
LP reacts not only with neutral borane pinacolborane (HBPin) to afford a borenium
salt capable of reducing imine compounds10 but also with H2 gas to catalyze the hy-
drogenation of alkylidene malonates as a frustrated LP (FLP).51 Scheme 1B shows the
stepwise grafting of DABCO and B(C6F5)3 into MIL-101 (Cr) and the resultant
MIL-101 (Cr) anchored with B(C6F5)3/DABCO pair is named MIL-101(Cr)-LP. With
the LP anchored and stabilized within the nanospace of the MOF, MIL-101(Cr)-LP
can efficiently catalyze both the imine reduction with interesting size and steric selec-
tivity and the hydrogenation of alkylidene malonates directly by using H2. Our work
represents a unique example of an active and reusable LP-based heterogeneous
catalyst with both size and steric selectivity, thereby opening a paradigm for
catalysis.
MIL-101(Cr) was synthesized and activated according to the literature.52 MIL-
101(Cr)-LP was prepared through a stepwise method: the activated MIL-101(Cr)
was soaked into a Lewis base toluene solution, then washed, filtered, and dried un-
der vacuum; subsequently, the solution of B(C6F5)3 (Lewis acid) was added to the
sample, stirred for several hours, and then washed with fresh toluene several times
until no LP traces could be detected in the filtrate by liquid nuclear magnetic reso-
nance (NMR) measurement. This process excludes the influence of the adsorbed
free LP molecules in the pores of the framework. Since the theoretical maximum
loading amount of anchored LP on MIL-101(Cr) is 1.47 mmol LP per 1 g MIL-101(Cr)
(Figure S1), MIL-101(Cr)-LP (1 mmol LP per 1 g MIL-101(Cr) or 0.34 LP per unsatu-
rated Cr site) was used in the following characterization.
Structure Characterization of MIL-101(Cr)-LP
The phase purity of MIL-101(Cr)-LP was verified through powder X-ray diffraction
(PXRD) measurements, and the porosity of the materials was studied by N2 gas sorp-
tion at 77 K. As shown in Figure 1A, the PXRD patterns of MIL-101(Cr)-LP are consis-
tent with the calculated ones and those of the pristine MIL-101(Cr), indicating the
preservation of the framework structure during the loading process. The N2 sorption
studies (Figure 1B) reveal that, in comparison with MIL-101, there is an obvious
decrease in the Brunauer-Emmett-Teller surface area (from 2,724 to 1,013 m2/g)
and reduction in pore sizes (Figures S6 and S7) for MIL-101(Cr)-LP because the LP
molecules are grafted onto the pores of MIL-101(Cr) (Figure 1B).
Characterization of Anchored LP in MIL-101(Cr)-LP
The combination of LP and MIL-101 was confirmed by Fourier transform infrared
spectroscopy (FTIR) analysis. The spectrum of MIL-101(Cr)-LP exhibits characteristic
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Figure 2. Characterization of Anchored LP in MIL-101(Cr)-LP
(A) Infrared spectra of MIL-101(Cr), LP, and MIL-101(Cr)-LP.
(B) Cr(III)2p XPS spectra of MIL-101.
(C) Cr(III)2p XPS spectra of MIL-101(Cr)-LP.
(D) Survey XPS spectra of MIL-101 and MIL-101(Cr)-LP.
peaks from the LP. As shown in Figure 2A, the presence of new bands at high wave-
numbers (2,800–3,200 cmÀ1) is due to the aliphatic C–H stretching vibrations of the
LP. Some obvious shifts are observed between MIL-101(Cr)-LP and the original LP
complex (from 2,899 and 2,950 to 2,970 and 3,021 cmÀ1), indicating a coordination
interaction between the N atom of the LP and the metal site of MIL-101(Cr).53,54 The
characteristic LP band at 1,455 cmÀ1 is slightly shifted to 1,463 cmÀ1 after the LP is
loaded within the MOF (Figures S10 and S11), further suggesting possible coordina-
tion interaction between the LP and the MIL-101 framework.
X-ray photoelectron spectroscopy (XPS) spectra of MIL-101(Cr) and MIL-101(Cr)-LP
were used to further verify the coordination interaction between the LP and the Cr
open metal sites in MIL-101(Cr). As shown in Figures 2B and 2C, the Cr(2p) spectra
of MIL-101(Cr)-LP are obviously different from the Cr(2p) spectra of MIL-101(Cr). The
Cr (2p1/2) and Cr (2p3/2) peaks of MIL-101(Cr)-LP are shifted by about 0.58–2 eV to-
ward higher binding energies, compared with those of MIL-101(Cr). Such shifts indi-
cate an increase in the electron density of Cr(III), which can be attributed to the inter-
action between Cr and DABCO. The survey spectra of MIL-101(Cr)-LP indicate the
existence of the F and N elements of the LP molecule within the MOF (Figure 2D).
Scanning electron microscopy (SEM) and transmission electron microscopy (TEM)
were used to investigate the morphology of MIL-101(Cr)-LP. As shown in Figures
3A and 3B, the SEM and TEM images exhibited regular octahedral crystals of MIL-
101(Cr)-LP with an average diameter of ꢀ100 nm. For investigating the distribution
of LP in MIL-101(Cr)-LP, high-angle annular dark-field scanning TEM (HAADF-STEM),
and energy dispersive X-ray spectroscopy (EDS) elemental mapping analyses were
4
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Figure 3. Morphology Characterization and Element Distribution of MIL-101(Cr)-LP
SEM image (A), TEM image (B), and HAADF-STEM image of MIL-101(Cr)-LP with the corresponding
elemental mapping images (C).
used. As presented in Figure 3D, the Cr, F, and N elements are evenly distributed
inside the octahedral crystal of MIL-101(Cr)-LP, suggesting the integration of LP in-
side the pores of the MOF. Furthermore, SEM and EDS elemental mapping analyses
(Figure S12) also revealed an even distribution of B, F, and N in large scale. These
results indicated that the LP was homogeneously distributed in the MIL-101(Cr)
pores without the presence of accumulation in the particular regions.
Affection of LP Loading Amount to Catalysis Performance
Compared with the expensive and potentially toxic metal-based catalyst, the metal-
free LPs are shown to have enhanced catalytic performance in the synthesis of B-con-
taining organic compounds.55,56 To examine the catalytic performances of LP
loaded MIL-101(Cr), we prepared MIL-101(Cr)-LP-1(0.5 mmol LP per 1 g MIL-
101(Cr) or 0.17 LP per unsaturated Cr site) and MIL-101(Cr)-LP-2 (0.75 mmol LP
per 1 g MIL-101(Cr) or 0.26 LP per unsaturated Cr site) to investigate the optimal up-
take amount for catalyzing the imine reduction reactions. The loading amount of LP
in MIL-101(Cr)-LP was quantified with 1H NMR by decomposing MIL-101(Cr)-LP in
5 wt % NaOH D2O solution (Figures S2–S4) and later confirmed by elemental anal-
ysis (Table S1). The pore size of these catalysts was studied through N2 isotherms at
77 K (Figures S5, S8, and S9). The catalytic performances of the MIL-101(Cr)-LP cat-
alysts were evaluated by exposing N-tert-butyl-1-phenylmethanimine (1a) to 1.2
equiv of HBPin and 20 mg catalyst in toluene to result in the reduction to the corre-
sponding pinacolboramide after 2 hr. As shown in Table 1, the lowest LP uptake
amount sample, MIL-101(Cr)-LP-1, gave 83% yield, whereas MIL-101(Cr)-LP ex-
hibited full conversion. Therefore, MIL-101(Cr)-LP was chosen for subsequent
studies. The control experiment using pristine MIL-101(Cr) was conducted, but no
reduction product was detected in the reaction solvent even after 48 hr, meaning
that MIL-101 is inactive for the imine reduction reaction.
Catalysis Studies for Reduction of Different Imine Compounds
Imine compounds with different substituting groups were used to investigate the
difference in catalysis property between the heterogeneous MIL-101(Cr)-LP and ho-
mogeneous LP counterpart. As shown in Table 2, the reaction yields catalyzed by
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Table 1. Investigation of MIL-101(Cr) and MIL-101(Cr)-LP in the Catalytic Reduction of Imine 1a
Entry
Catalyst
Yielda
0%b
1
2
3
4
MIL-101(Cr)
MIL-101(Cr)-LP-1
MIL-101(Cr)-LP-2
MIL-101(Cr)-LP
83%
91%
100%
Reaction conditions: 20 mg catalyst, 55 mg (0.43 mmol) HBPin, 56 mg (0.35 mmol) 1a, 3 mL toluene, room
temperature, 2 hr.
aYields were determined by liquid NMR.
bReaction time: 48 hr.
MIL-101(Cr)-LP from related imine compounds are 38% for N-benzylideneaniline
(2a), 100% for N-benzylidene-1-phenylmethanamine (3a), and 22% for acridine
(4a). For comparison, the yields are 87%, 85%, and 91% catalyzed by 3.5 mol % ho-
mogeneous LP catalyst, respectively. The comparison of the yields of these products
reveals a very interesting phenomenon: the reduction product yields of 1a and 3a are
similar or even higher than the homogeneous LP catalyst, which clearly indicates that
MIL-101(Cr)-LP has comparable performance with the homogeneous LP for catalytic
imine reduction with HBPin at the same conditions; however, the reduction product
yields of 2a and 4a are much less than the homogeneous LP catalyst. The catalysis
results reveal that the steric effect close to the N atoms shows more obvious selec-
tivity in MIL-101(Cr)-LP than LP homogeneous catalyst. This kind of steric selectivity
could presumably be due to the confinement effect imparted by the porous MOF
structure, which restricts the accessibility of buried C=N double bonds to the LP
active centers that are anchored on the pore walls of MOF.
With the increase in the size of the imine substrate, the reduction reaction yield
decreased. The reaction yield catalyzed by homogeneous LP for N-benzhydryl-1-
phenylmethanimine (5a) and N-(diphenylmethylene)-1,1-diphenylmethanamine
(6a) are 60% and 29%, respectively. However, the reaction yield for 5a is 42% and
none of the product could be observed in the heterogeneous MIL-101(Cr)-LP-cata-
lyzed reaction for 6a even after 24 hr. The high size selectivity performance can be
attributed to the window structure in MIL-101(Cr)-LP. Considering the existence
of the coordinated LP on the window, the size of the window (with diameter of
ꢀ1.0 nm) could be smaller than the diameter of the 6a molecule (with diameter of
1.3 nm), thus the large imine molecule could not enter into the pore of MIL-
101(Cr)-LP.
Catalysis Studies for Hydrogenation of Alkylidene Malonate Compounds
FLPs show remarkable reactivity toward the activation of the H2 molecule.57 How-
ever, the FLP-promoted catalytic hydrogenation of electron-poor unsaturated
compounds still faces a significant challenge.58 In view of the interesting catalytic
performance of MIL-101(Cr)-LP in the above imine reduction reactions, MIL-
101(Cr)-LP was used to achieve the catalytic hydrogenation of alkylidene malo-
nates, which is one important kind of electron-poor unsaturated compound,
6
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Table 2. Catalysis Studies for Reduction of C=N Double Bonds
Entry
Substrate
Code
2a
Yield
Yielda
1
38%b
87%b
2
3
4
3a
4a
5a
100%b
22%b
42%c
85%b
91%b
60%c
5
6a
0%c
29%c
Reaction conditions: 20 mg MIL101(Cr)-LP (3.5 mol % LP) for heterogeneous or 3.5 mol % LP for homoge-
neous catalytic reaction, 55 mg (0.43 mmol) HBPin, 0.35 mmol substrate, 3 mL toluene.
aYield for the reaction catalyzed by the homogeneous catalyst; this yield is relative to substrate (see Sup-
plemental Information).
bReaction time: 3 hr.
cReaction time: 24 hr.
directly using H2 gas. Alkylidene malonate compounds with different substituted
groups were used to investigate the catalytic performance of the heterogeneous
MIL-101(Cr)-LP. As presented in Table 3, the reaction yields catalyzed by MIL-
101(Cr)-LP from related alkylidene malonate compounds are 95% for diethyl
2-benzylidenemalonate (7a), 84% for diethyl 2-(2-methylpropylidene)malonate
(8a), 83% for diethyl 2-hexylidenemalonate (9a), and 88% for diethyl 2-(cyclohexyl-
methylene)malonate (10a). For comparison, the yields are 92%, 79%, 81%, and
79% when catalyzed by 10 mol % homogeneous FLP catalyst, respectively. The
comparison of the yields of these products indicates the MIL-101(Cr)-LP can be
used as a porous FLP catalyst and exhibits excellent catalytic performance in the
FLP-promoted hydrogenation.
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Table 3. Catalysis Studies for Hydrogenation of Alkylidene Malonates
Entry
Substrate
Code
7a
Yielda
Yieldb
1
95%
91%
2
3
8a
9a
83%
81%
80%
79%
4
10a
88%
82%
Reaction conditions: 20 mg MIL101(Cr)-LP (10 mol % LP) for heterogeneous or DABCO/B(C6F5)3
(10 mol %) for homogeneous catalytic reaction, 0.12 mmol substrate, 3 mL toluene, 80ꢁC, 24 hr, H2 60 bar.
aYields of isolated product.
bYield for the reaction catalyzed by the homogeneous catalyst; this yield is relative to the substrate (see
Supplemental Information).
Tentative Catalytic Mechanism of MIL-101(Cr)-LP
Toinvestigatetheimpactoftheporousframeworkstructureonthe catalysisperformance
of LP, we examined the kinetics of the reduction of imine 1a with HBPin using heteroge-
neous MIL-101(Cr)-LP and homogeneous LP counterpart. As shown in Figure S13, the re-
actions catalyzed by both heterogeneous and homogeneous catalysts are on the same
order of magnitude; MIL-101(Cr)-LP showed a slower reaction rate as expected due to
the additional diffusion process needed for the substrates and products throughout
the MOF pores. It is noteworthy that the similar kinetics behaviors of MIL-101(Cr)-LP
and the homogeneous LP counterpart implies that MIL-101(Cr)-LP catalyst shares a
similar reaction mechanism with that of a homogeneous reaction system.10 As shown
in Scheme 2, the reaction starts with the generation of DABCO-borenium ion, then the
borenium ion part transfers from DABCO-borenium ion to imine. The resulting B-acti-
vated iminium ion is then reduced by HBPin with assistance from the Lewis base of
MIL-101(Cr)-LP. Reduction of imine-Bpin ion regenerates the borenium ion catalyst,
which then reenters the catalytic cycle. The solid-state 19F NMR of MIL-101(Cr)-LP was
used to investigate [HB(C6F5)3]À in the catalytic reactions. After 10 min of starting the re-
action, the MIL-101(Cr)-LP was separated and dried for the solid-state 19F NMR
8
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Scheme 2. Tentative Mechanism for the Reduction Reaction Catalyzed by MIL-101(Cr)-LP
measurement. As shown in Figure S14, the peaks from À125 to 170 ppm match the re-
ported 19F NMR of [HB(C6F5)3]À.10 The results of solid-state 19F NMR confirmed the in-
termediate [HB(C6F5)3]À in the catalytic reactions, thus supporting the above catalyst
mechanism.
Recycling Performance of MIL-101(Cr)-LP
Given that the recyclability and long-term stability of the catalyst are the essential
performance metrics for cost-effective industrial processes, we investigated these prop-
erties for MIL-101(Cr)-LP. Due to the existence of coordination bonds between LP and
MOF, the LP leaching is dispelled, given that there are no observable signals of LP in
the liquid NMR spectrum of the supernatant after the reaction (Figure S17). Moreover,
MIL-101(Cr)-LP can readily be recycled with the steady catalytic performance for at least
seven cycles, thus highlighting the heterogeneous nature of the catalytic process
(Figure 4). The 1H NMR data of the decomposed MIL-101(Cr)-LP in 5 wt % NaOH D2O
solution after seven cycles of catalytic reaction matches the original MIL-101(Cr)-LP,
thus reinforcing the idea that the LP is competently bound to the Cr sites (Figure S18).
The robustness of the catalyst was further confirmed by the well-retained crystallinity
and pore structure in MIL-101-LP after the catalytic reaction, as shown by PXRD and
N2 adsorption studies, respectively (Figures S15 and S16).
DISCUSSION
In summary, we have demonstrated the successful grafting of a LP into a MOF via a
stepwise anchoring strategy. As a result of the strong coordination interaction be-
tween LP and MOF, the LP is stabilized, thereby exhibiting efficient catalytic activity
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Figure 4. Recycling Performance of MIL-101(Cr)-LP
Reaction conditions: 20 mg MIL-101(Cr)-LP, 55 mg HBPin, 56 mg 1a, 3 mL toluene, room
temperature, 2 hr.
for catalytic reduction of imines with excellent recyclability. Moreover, interesting
size and steric selectivity have also been observed in the catalytic reduction reac-
tions as a result of the confinement imparted by the porous MOF framework struc-
ture. Given the broad applicability of LPs, our work therefore lays a foundation for
developing MOF-LP as a new paradigm for catalysis. Ongoing work in our laboratory
includes grafting different types of LPs into MOFs and exploring the MOF-LP system
for various catalysis applications.
EXPERIMENTAL PROCEDURES
Materials
All chemical reagents were obtained from commercial sources and, unless otherwise
noted, were used as received without further purification. Organic solvents used in
this work were further purified and dried following standard procedures prior to use.
All of the experiments for the LP were performed in the glove box.
Synthesis of MIL-101(Cr)-LP
In the glove box, DABCO (2.5 mg, 0.022 mmol) was dissolved in anhydrous toluene with
degassed MIL-101(Cr) (20 mg) soakedinsideand, after 12hr, the samplewas centrifuged
and washed with anhydrous toluene three times. The tris(pentafluorophenyl)borane
(15.4 mg, 0.03 mmol) anhydrous toluene solution was added to the sample and then
stirred for 12 hr. After that, the sample was centrifuged and washed with anhydrous
toluene three times. The sample was then vacuumed for the following catalysis reaction.
Catalytic Reactions
General Procedure for a MIL-101(Cr)-LP-Catalyzed Imine Reduction Reaction (GP1)
In a N2-filled glove box, MIL-101(Cr)-LP was dispersed into 3 mL of toluene in a 20 mL
vial equipped with a small magnetic stir bar. HBPin was then added to the vial,
and substrates were added to the vial after 10 min. The vial was capped and kept
in the glove box at room temperature for the noted time. The catalyst was separated
by centrifugation. The product was isolated as indicated in the Supplemental
Information.
General Procedure for a LP-Catalyzed Imine Reduction Reaction (GP2)
In a N2-filled glove box, LP was dispersed into 3 mL of toluene in a 20 mL vial equip-
ped with a small magnetic stir bar. HBPin was then added to the vial, and substrates
10
Chem 4, 1–13, November 8, 2018

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were added to the vial after 10 min. The vial was capped and kept in the glove box at
room temperature for the noted time. The product was isolated as indicated in the
Supplemental Information.
General Procedure for a MIL-101(Cr)-LP-Catalyzed Alkylidene Malonate
Hydrogenation Reaction (GP3)
In a N2-filled glove box, MIL-101(Cr)-LP was dispersed into 2 mL of toluene in a 20 mL
vial equipped with a small magnetic stir bar, and then the substrate toluene solution
(1 mL) was added to the vial after stirring for 10 min. The resulting mixture was then
transferred to the autoclave equipped with the magnetic stir bar. At last, the auto-
clave was pressurized with H2 (60 bar) and heated to 80ꢁC for 24 hr. The catalyst
was separated by centrifugation. The product was isolated as indicated in the Sup-
plemental Information.
General Procedure for a LP-Catalyzed Alkylidene Malonate Hydrogenation
Reaction (GP4)
In a N2-filled glove box, LP was dispersed into 2 mL of toluene in a 20 mL vial equip-
ped with a small magnetic stir bar, and then the substrates toluene solution (1 mL)
was added to the vial after stirring for 10 min. The resulting mixture was then trans-
ferred to the autoclave equipped with the magnetic stir bar. The autoclave was then
pressurized with H2 (60 bar) and heated to 80ꢁC for 24 hr. The product was isolated
as indicated in the Supplemental Information.
Characterization
Elemental analysis was performed on a PerkinElmer 240 CHN elemental analyzer.
Infrared spectra were recorded on a PerkinElmer UATR TWO FTIR spectrophotom-
eter. PXRD measurements were recorded on a Bruker D8 Advance X-ray diffractom-
eter with Cu Ka radiation. The simulated powder patterns were calculated with
Mercury 2.0. The XPS data were collected on a PHI5000VersaProbe device, the mi-
cro scan image and EDS elemental mapping analyses were tested on a ZEISS
MERLIN Compact scanning and JEOL JSM-7500F electron microscope. TEM and
HAADF-STEM were performed on a Tecnai G2 F20 microscope (FEI). The NMR tests
were performed on a Varian Unity Inova 400 spectrometer. Gas adsorption measure-
ments were tested by a Micromeritics ASAP 2020 surface area and porosity analyzer.
SUPPLEMENTAL INFORMATION
Supplemental Information includes Supplemental Experimental Procedures, 18 fig-
ures, and 1 table and can be found with this article online at https://doi.org/10.1016/
j.chempr.2018.08.018.
ACKNOWLEDGMENTS
The authors acknowledge the National Science Foundation (DMR-1352065) and the
University of South Florida for financial support of this work. Partial support from the
National Key R&D Program of China (2016YFB0600902), the National Natural Sci-
ence Foundation of China (21573267), and the Youth Innovation Promotion Associ-
ation CAS (2013196) is also acknowledged.
AUTHOR CONTRIBUTIONS
S.M. and Z.N. conceived the research. Z.N. and W.D.C.B.G. synthesized the mate-
rials and substrates. Z.N., J.-G.M., Q.S., and Y.C. performed the structural character-
izations. Z.N. performed the catalyst reactions and kinetics experiments. The paper
Chem 4, 1–13, November 8, 2018
11

Please cite this article in press as: Niu et al., Metal-Organic Framework Anchored with a Lewis Pair as a New Paradigm for Catalysis, Chem (2018),
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was co-written by S.M., Z.N., P.C.L., B.A., and J.P. All authors discussed the results
and commented on the manuscript.
DECLARATION OF INTERESTS
The authors declare no competing interests.
Received: March 26, 2018
Revised: April 24, 2018
Accepted: August 14, 2018
Published: September 27, 2018
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