Promoting Frustrated Lewis Pairs for Heterogeneous Chemoselective Hydrogenation via the Tailored Pore Environment within Metal–Organic Frameworks

Article abstract of DOI:10.1002/anie.201903763

Frustrated Lewis pairs (FLPs) have recently been advanced as efficient metal-free catalysts for catalytic hydrogenation, but their performance in chemoselective hydrogenation, particularly in heterogeneous systems, has not yet been achieved. Herein, we demonstrate that, via tailoring the pore environment within metal–organic frameworks (MOFs), FLPs not only can be stabilized but also can develop interesting performance in the chemoselective hydrogenation of α,β-unsaturated organic compounds, which cannot be achieved with FLPs in a homogeneous system. Using hydrogen gas under moderate pressure, the FLP anchored within a MOF that features open metal sites and hydroxy groups on the pore walls can serve as a highly efficient heterogeneous catalyst to selectively reduce the imine bond in α,β-unsaturated imine substrates to afford unsaturated amine compounds.

Full text of DOI:10.1002/anie.201903763

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Promoting Frustrated Lewis Pair for Heterogeneous
Chemoselective Hydrogenation via Tailored Pore Environment
within Metal-Organic Framework
Authors: Zheng Niu, Weijie Zhang, Pui Ching Lan, Briana Aguila, and
Shengqian Ma
This manuscript has been accepted after peer review and appears as an
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content of this Accepted Article.
To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201903763
Angew. Chem. 10.1002/ange.201903763
Link to VoR: http://dx.doi.org/10.1002/anie.201903763
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10.1002/anie.201903763
Angewandte Chemie International Edition
COMMUNICATION
Promoting
Frustrated
Lewis
Pair
for
Heterogeneous
Chemoselective Hydrogenation via Tailored Pore Environment
within Metal-Organic Framework
Zheng Niu, Weijie Zhang, Pui Ching Lan, Briana Aguila, and Shengqian Ma*
Abstract: Frustrated Lewis pairs (FLPs) have recently been
advanced as efficient metal-free catalysts for catalytic hydrogenation,
but their performance in chemoselective hydrogenation, particularly in
heterogeneous systems, has not yet been achieved. In this work, we
demonstrate that, via delicately tailoring the pore environment within
metal-organic framework (MOF), FLP not only can be stabilized but
also can be rendered with interesting performance in chemoselective
hydrogenation of α, β-unsaturated organic compounds, which
otherwise cannot be achieved for FLP in a homogeneous system.
Specifically, directly using hydrogen gas under moderate pressure,
the FLP anchored within the MOF that features open metal sites and
hydroxyl groups on the pore walls can serve as a highly efficient
heterogeneous catalyst to selectively reduce the imine bond in α, β-
unsaturated imine substrates to afford unsaturated amine compounds.
The density functional theory (DFT) calculation studies indicate that
the –OH group and open metal site in the MOF can preferentially
interact with the C=N group (rather than the C=C group) of α, β-
unsaturated imines to activate it, thus promoting the selectivity for the
formation of unsaturated amines. Our work not only lays a foundation
catalysts in homogeneous systems9 but with the tendency to
preferentially reduce the carbon–carbon double bond, which is
thermodynamically favored. It remains a significant challenge for
FLP catalysts to selectively hydrogenate α, β -unsaturated
organic compounds to afford the relevant olefin product let alone
in heterogeneous systems.10
to develop MOF-FLP as
a new platform for heterogeneous
chemoselective catalysis, but also paves a new avenue for the design
of precious metal-free chemoselective catalysts.
Catalytic hydrogenation of organic compounds containing
unsaturated bonds has drawn comprehensive attention since the
early 20th century,1 given the atom economy and cleanliness of
the transformation as well as its ubiquitousness in industrial
processes.2 Heterogeneous hydrogenation catalysis has been
dominated by precious metals (Pd/Pt/Ru/Rh/Ir). These catalysts
have shown high efficiency and selectivity compared to
stoichiometric reductants.3 However, the scarcity and high cost of
precious metal catalysts cast a shadow on their broader
applications in industry. Moreover, chemoselective hydrogenation,
e.g. selective reduction of α, β-unsaturated organic compounds,
Scheme 1. Illustrative scheme of the pore environment tailoring within MOF to
promote FLP for heterogeneous chemoselective hydrogenation of α, β-
unsaturated organic compounds.
Recently, we demonstrated the successful introduction of
Lewis pair (LP) into metal-organic framework (MOF), which
exhibited enhanced stability and excellent recyclability, as well as
interesting catalysis performance for imine reduction reactions
and hydrogenation of olefins.11 Indeed, the diverse yet tunable
features of MOFs12 make them an efficient platform to engineer
heterogeneous catalysts for various organic transformations that
traditional porous materials simply cannot provide.13 On the basis
of our recent success, to promote FLP for heterogeneous
chemoselective hydrogenation of α, β-unsaturated organic
compounds, we postulate that the MOF pore environment can be
delicately tailored with “ports” to anchor FLP, meanwhile
“activation” groups preferably interact with the targeted double
bond in the substrate, thereby producing selective reduction
during the hydrogenation process (Scheme 1).
We selected the MOF, MIL-101 (Cr) as the platform
considering that it can be prepared with the secondary building
unit (SBU) in the form of Cr3(3-O) (COO)6(OH)(H2O)2; we
envision that upon dehydration, the SBU can expose two Cr(III)
sites with one as the “port“ to anchor FLP and the other, together
with the hydroxyl group residing at the third Cr(III) site, to
preferably interact with the targeted double bond in the substrate
has been
a challenging topic since the early stage of
hydrogenation development,4 which would need the delicate
design and manipulation of the catalyst system.
The emergence of frustrated Lewis pairs (FLPs),5 as pioneered
by Stephan in 2006, suggests different pathways for the activation
6
of H2 and other small molecules7. Tremendous progress has
been witnessed for the development of FLPs as metal-free
catalysts for the hydrogenation of many types of unsaturated
compounds, including olefins, alkynes, imines, esters, ketones,
and aziridines.8 Chemoselective hydrogenation of α,
unsaturated organic compounds has been reported for some FLP
β -
[a]
Dr. Z. Niu, Dr. W. Zhang, P. C. Lan, B. Aguila, Prof. S. Ma
Department Department of Chemistry
University of South Florida
4202 E. Fowler Avenue, Tampa, Florida 33620 United States
E-mail: sqma@usf.edu
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10.1002/anie.201903763
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COMMUNICATION
to facilitate its “activation” (Scheme 2). Bearing this in mind, in this
work we successfully anchored the FLP of B(C6F5)2(Mes)/DABCO
into MIL-101 (Cr), and directly using H2 gas at room temperature
the resultant MIL-101 (Cr)-FLP can selectively hydrogenate the
imine bond in α, β-unsaturated imine compounds to afford
unsaturated amine compounds, which cannot be achieved for
FLP in a homogeneous system. Our work presents the first
example of an FLP-based catalyst capable of selectively
hydrogenating α, β-unsaturated organic compounds, thereby
paving a new avenue for the design of precious metal-free
chemoselective heterogeneous catalysts.
at -22.4 ppm, which is comparable to that of the homogeneous
[HDABCO]+[HB(C6F5)2(Mes)]- (d=-22.5 ppm) and related
[HB(C6F5)2(Mes)][LB] system (d=-22.1 ppm), thus confirming the
formation of the presumed ammonium hydridoborate of FLP.9a
Furthermore, the peaks from the solid state 19F NMR spectrum
(Figure S1) of MIL-101(Cr)-FLP-H2 are consistent with the
formation of a tetracoordinate anionic borate, further confirming
the existence of FLP ammonium hydridoborate within the MOF.8e
The porosity and phase purity of MIL-101(Cr)-LB and MIL-
101(Cr)-FLP-H2 were investigated by N2 gas sorption at 77 K and
powder X-ray diffraction (PXRD) measurements, respectively. As
shown in Figure S2, the PXRD patterns of MIL-101(Cr)-LB and
MIL-101(Cr)-FLP-H2 are in good agreement with the calculated
ones and those of the pristine MIL-101(Cr), indicating the
retention of the framework’s structural integrity during the
stepwise loading process. The N2 sorption studies (Figure S3)
indicated that in comparison with pristine MIL-101(Cr), there is a
steady decrease in the BET surface area (from 2724 to 2196 and
1112 m2/g) and reduction in pore sizes (Figure S4, S5 and S6) for
MIL-101(Cr)-LB and MIL-101(Cr)-FLP-H2 due to the LB and FLP
molecules grafted onto the pore wall of MIL-101(Cr).
The Fourier transform infrared (FT-IR) spectroscopy analysis
was employed to verify the association of FLP and MIL-101(Cr)
framework. The spectrum of MIL-101(Cr)-FLP-H2 shows the
characteristic peaks from LB and LA (Figure S7). As shown in
Figure 1b, the presence of a new band at high wavenumbers
(3000–3030 cm-1) is due to the methyl C–H stretching vibrations
of LA, and the new band around 1466 is owing to the aliphatic C–
H stretching vibrations of LB. The above notable new bands of
MIL-101(Cr)-FLP-H2 further suggest the existence of the FLP-H2
inside the MOF.15
Scheme 2. Illustration of “activation” process based on Cr3(3-O)
(COO)6(OH)(H2O)2 cluster in MIL-101(Cr)-FLP-H2.
MIL-101(Cr)
possessing
the
SBU
of
Cr3(3-O)
(COO)6(OH)(H2O)2 was prepared according to the procedure
reported in the literature14 and was dehydrated before the
introduction of FLP. The Lewis base of 1, 4-
diazabicyclo[2.2.2]octane (DABCO) that features two binding
nitrogens, with one to anchor at the open Cr(III) and the other to
interact with the Lewis acid, was first grafted into MOF to form
MIL-101 (Cr)-LB (Scheme S1). In order to stabilize the anchored
FLP, Lewis acid B(C6F5)2(Mes) was added into MIL-101 (Cr)-LB,
and then reacted with H2 to afford HB(C6F5)2(Mes)-/HDABCO+ pair
anchored MIL-101 (Cr), which is denoted as MIL-101(Cr)-FLP-H2.
(Scheme S2) MIL-101(Cr)-FLP-H2 (1.0 mmol FLP per 1 g MIL-
101(Cr) or 0.34 FLP per unsaturated Cr site) was used in the
following characterizations.
The coordination interaction between FLP and the open Cr(III)
sites in MIL-101(Cr) was investigated by X-ray photoelectron
spectroscopy (XPS) spectra of MIL-101(Cr) and MIL-101(Cr)-
FLP-H2. As presented in Figure S8 and S9, the Cr(2p) spectrum
of MIL-101(Cr)-FLP-H2 is notably different from the Cr(2p)
spectrum of MIL-101(Cr). The Cr (2p1/2) and Cr (2p3/2) peaks of
MIL-101(Cr)-FLP-H2 are shifted by ca. 0.58~2 eV toward higher
binding energies, compared to those of MIL-101(Cr). Such shifts
indicate an change in the electron density of Cr(III), which can be
attributed to the interaction between Cr and DABCO. The survey
spectrum of MIL-101(Cr)-FLP-H2 indicates the existence of the F
and N elements of the FLP molecule within the MOF (Figure S10).
The morphology of MIL-101(Cr)-FLP-H2 was investigated by
scanning electron microscopy (SEM) and transmission electron
microscopy (TEM). As shown in Figure 2a and 2b, the SEM and
TEM images exhibited regular octahedral crystals of MIL-101(Cr)-
FLP-H2 with an average diameter of ~ 100 nm. To investigate the
distribution of FLP-H2 in MIL-101(Cr)-FLP-H2, high-angle annular
dark-field scanning transmission electron microscopy (HAADF-
STEM) and energy dispersive X-ray spectroscopy (EDS)
elemental mapping analyses were conducted. As presented in
Figure 2c, the Cr, F, and N elements are evenly distributed inside
the octahedral crystal of MIL-101(Cr)-FLP-H2, suggesting the
integration of FLP inside the pores of the MOF. These results
indicated that the LP was homogeneously distributed in the MIL-
101(Cr) pores without the presence of accumulation in particular
regions.
Figure 1. (a) 11B NMR spectrum of MIL-101(Cr)-FLP-H2 and FLP; (b) IR spectra
of MIL-101(Cr), FLP, and MIL-101(Cr)-FLP-H2.
11B NMR measurements were carried out to investigate the
FLP with activated hydrogen that is anchored on the pore wall of
MIL-101(Cr)-FLP-H2. As shown in Figure 1a, the 11B NMR
spectrum of the mixture of B(C6F5)2(Mes) and DABCO shows a
peak at 66.1 ppm, indicative of an extremely weak interaction
between LA and LB. After reacting LA with MIL-101(Cr)-LB in
toluene under 10 bar H2 atmosphere and at room temperature for
24 h, the solid-state 11B NMR measurement gave a distinct peak
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Table 1. The catalysis studies for hydrogenation of α, β-unsaturated imine
compoundsa
entry
1
substrate
product
yield
0%b
7a
7b
7b
Figure 2. SEM image (a), TEM image (b), and HAADF-STEM image of MIL-
101(Cr)-FLP-H2 with the corresponding elemental mapping images (c).
2
3
4
5
87%
100%
91%
7a
To investigate the optimal loading amount for catalyzing the
imine reduction reactions, we prepared a series of MOF-FLP with
different FLP uptake amounts, (MIL-101(Cr)-FLP-H2-1(0.5 mmol
FLP per 1 g MIL-101(Cr)) and MIL-101(Cr)-FLP-H2-2 (0.75 mmol
FLP per 1 g MIL-101(Cr))). The loading amount of FLP in the
above samples was quantified with 1H NMR by decomposing
MOF-FLP in 10 wt% NaOH deuterium oxide solution (Figure S11,
S12 and S13). The catalytic performances of the MIL-101(Cr)-
FLP-H2 catalysts were evaluated by exposing N-tert-butyl-1-
phenylmethanimine (1a) to 20 mg catalyst in toluene, resulting in
the reduction to N-Benzyl-tert-butylamine (1b) at 10 bar H2
atmosphere and room temperature after 48 h. As presented in
Table S2, along with the increase of FLP loading amount, the
catalytic yield for the above reaction increased. The MOF-FLP
with the lowest amount of FLP, MIL-101(Cr)-FLP-H2-1, only gave
36% yield, whereas the highest uptake amount of FLP loaded
MIL-101(Cr)-FLP-H2 exhibited complete conversion. Therefore,
MIL-101(Cr)-FLP-H2 was chosen for the following catalysis
studies. The control experiments using pristine MIL-101(Cr), MIL-
101(Cr)-LB, and MIL-101(Cr)/LA were conducted, however no
reduction product was detected in the reaction solvent even after
72h, suggesting MIL-101(Cr), MIL-101(Cr)-LB, and MIL-
101(Cr)/LA are inactive for the imine reduction reaction.
Imine compounds with different substituting groups were
employed to investigate the catalytic property of heterogeneous
MIL-101(Cr)-FLP-H2. As shown in Table S3, the reaction yields
catalyzed by MIL-101(Cr)-FLP-H2 from related imine compounds
are 68% for N-Benzylideneaniline (2a), 91% for N-Benzylidene-1-
phenylmethanamine (3a), and 58% for acridine (4a). The
catalysis results reveal that the steric effect close to the N atoms
can affect the catalytic performance. The steric effect could be
presumably owing to the confinement effect imparted by the
porous structure of the MOF, which restricts the accessibility of
buried C=N double bonds to the FLP active centers that are
anchored on the pore walls of the MOF. Along with the increase
in size of the imine substrate, the reduction reaction yield
decreased. The reaction yield catalyzed by MIL-101(Cr)-FLP-H2
for 5a is 21% and none of the product could be observed for 6a
even after 72 h. Considering the existence of the coordinated FLP
on the window, the size of the window (with diameters of ~1.0 nm)
is smaller than the diameter of 6a molecule (with diameters of 1.3
nm), thus the large imine molecule could not enter into the pores
of MIL-101(Cr)-FLP-H2. Therefore, the observed size selectivity
performance of MIL-101(Cr)-FLP-H2 can be attributed to the
restriction of the window structure in the framework.
8b
8a
9a
9b
81%
10a
10b
6
7
100%
93%
11a
12a
11b
12b
aReaction conditions: 20 mg (10 mol% FLP) MIL-101(Cr)-FLP-H2, 0.13 mmol
substrate, 3 mL toluene, room temperature, 48 h. bReaction conditions: 10 mol%
FLP, 0.13 mmol 7b, 3 mL toluene, room temperature, 48 h.
Considering the interesting catalytic performance of MIL-
101(Cr)-FLP-H2 in the above imine reduction reactions, we
decided to examine MIL-101(Cr)-FLP-H2 for selective catalytic
hydrogenation of the α, β-unsaturated imine compounds directly
using hydrogen gas, which has barely been explored in both
homogeneous and heterogeneous systems.12 Interestingly, for
the substrate 7a, both imine and alkene double bonds were
reduced in the case of homogeneous FLP, and no 7b can be
observed after the catalytic reaction. However, the
heterogeneous MIL-101(Cr)-FLP-H2 shows a yield of 87% 7b for
7a under the same reaction conditions. This excellent selectivity
could be attributed to the hydrogen bonding between the N atoms
of the imines and the -OH groups from the framework and/or the
interaction between the N atoms of the imines and remaining
open metal sites.16 The α, β-unsaturated imine compounds with
different substituted groups were employed to further investigate
the chemoselective catalysis performance of the heterogeneous
MIL-101(Cr)-FLP-H2. As presented in Table 1, the reaction yields
catalyzed by MIL-101(Cr)-FLP-H2 from the related α, β-
unsaturated imine compounds are 100% for 8a, 91% for 9a, 81%
for 10a, 100% for 11a, and 93% for 12a. The high yields of these
products highlight that MIL-101(Cr)-FLP-H2 can serve as a porous
FLP catalyst with excellent catalytic performance in the
chemoselective hydrogenation reactions, which has not been
achieved previously for any FLP-based catalyst.
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On the basis of the reported homogenous FLP catalyzed
hydrogenation reactions and the solid-state 19F NMR of MIL-
101(Cr)-FLP-H2 results, the tentative catalysis mechanism of MIL-
101(Cr)-FLP-H2 catalyzed imine reduction reactions is proposed
as illustrated in Figure S14. The process is initiated by the
reaction between MIL-101(Cr)-FLP-H2 and the imine substrate.
The [HBMes(C6F5)2]- reduces the imine substrate and then
converts to BMes(C6F5)2, while the MIL-101(Cr)-[LBH]+ converts
to MIL-101(Cr)-LB. Subsequently, the MIL-101(Cr)-LB and
BMes(C6F5)2 react with hydrogen to regenerate the MIL-101(Cr)-
FLP-H2 and complete the catalysis cycle.
robustness of the catalyst was further confirmed by the well-
retained crystallinity and pore structure in MIL-101(Cr)-FLP-H2
after the catalytic reaction, as evidenced by PXRD and N2
adsorption studies, respectively (Figure S18 and S19).
In summary, we demonstrated the successful incorporation of
a frustrated Lewis pair (FLP) into the MOF, MIL-101(Cr) with a
tailored pore environment featuring open metal sites and hydroxyl
groups in the SBUs. Such tailored pore environment stabilizes the
FLP through strong coordination interaction between LB and MOF
thereby rendering it with excellent recyclability as well as
interesting catalytic activity for catalytic reduction of imines with
direct utilization of hydrogen gas under moderate pressure.
Moreover, the hydroxyl groups and open metal sites from the
tailored pore environment within the MOF promote the anchored
FLP with great performance in the chemoselective hydrogenation
of α, β-unsaturated imines through the preferential interaction
between C=N group (rather than the C=C group) and the –OH
group and remaining open Cr(III) site in catalyst, as indicated by
DFT calculations. Our work not only lays a foundation to develop
MOF-FLP as a new generation of efficient chemoselective
heterogeneous catalyst, but also paves a new avenue for the
design of precious metal-free chemoselective catalysts.
Density functional theory (DFT) calculations were employed to
understand the tentative mechanism of the chemoselective
hydrogenation of α, β-unsaturated imine compounds. As shown
in Figure 3, the unsaturated Cr(III) sites and the hydroxyl group in
Cr3O(OH)(COO)6H2O trimers of MIL-101 can serve as
“active“ sites that interact with the C=N group of 7a molecule. The
calculated adsorption energy of 7a over the unsaturated Cr(III)
sites and the hydroxyl group in Cr3O(OH)(COO)6H2O trimers is -
33.77 and -34.83 kJ mol-1, respectively; that is, binding between
the
trimers
and 7a through
NHO
and
NCr
is
thermodynamically favored, with the interaction between 7a and
hydroxyl group in Cr3O(OH)(COO)6H2O trimers being slightly
stronger (Figure S15 and S16). Therefore, different from the
homogenous catalyst, the MOF catalyst can selectively reduce
the C=N bond in α, β-unsaturated imine compounds. These
results support the experimental observation that the –OH groups
and remaining open Cr(III) sites in MIL-101(Cr) preferentially
interact with the C=N group (rather than the C=C group)
of 7a thus to activate it, giving rise to the improved selectivity for
the formation of product 7b.
Acknowledgements
The authors acknowledge NSF (DMR-1352065) and the
University of South Florida for financial support of this work.
Keywords: Frustrated Lewis Pair • MOF • heterogeneous •
chemoselective catalysis • hydrogenation
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10.1002/anie.201903763
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
The FLP anchored MOF that features
activation groups including open metal
sites and hydroxyl groups has been
synthesized, and it can serve as a
highly efficient heterogeneous catalyst
to selectively reduce the imine bond in
α, β-unsaturated imine substrates to
afford unsaturated amine compounds
by directly using hydrogen gas under
moderate pressure.
Dr. Z. Niu, Dr. W. Zhang, P. C. Lan, B.
Aguila, and Prof. S. Ma*
Page No. – Page No.
Promoting Frustrated Lewis Pair for
Heterogeneous Chemoselective
Hydrogenation via Tailored Pore
Environment within Metal-Organic
Framework
This article is protected by copyright. All rights reserved.