Use of steric encumbrance to develop conjugated nanoporous polymers for metal-free catalytic hydrogenation

Article abstract of DOI:10.1039/c6cc06372a

The design and synthesis of metal-free heterogeneous catalysts for efficient hydrogenation remains a great challenge. Here we report a novel approach to create conjugated nanoporous polymers with efficient hydrogenation activities toward unsaturated ketones by leveraging the innate steric encumbrance. The steric bulk of the framework as well as the local sterics of the Lewis basic sites within the polymeric skeleton result in the generation of the putative catalyst. This approach opens up new possibilities for the development of innovative metal-free heterogeneous catalysts.

Full text of DOI:10.1039/c6cc06372a

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Communication
Use of Steric Encumbrance to Develop Conjugated Nanoporous
Polymers for Metal-free Catalytic Hydrogenation
-
-Received 00th January 20xx,
a
a
b
c
c
d
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Chengcheng Tian, Xiang Zhu,* Carter W. Abney, Ziqi Tian, De-en Jiang, Kee Sung Han,
b
d
ab
Shannon M. Mahurin, Nancy M. Washton and Sheng Dai*
The design and synthesis of metal-free heterogeneous catalysts for efficient hydrogenation remains a great challenge.
Here we report a novel approach to create conjugated nanoporous polymers with efficient hydrogenation activities
toward unsaturated ketones by leveraging the innate steric encumbrance. The steric bulk of the framework as well as the
local sterics of the Lewis basic sites within the polymeric skeleton result in the generation of the putative catalyst. This
approach
opens up new possibilities for the development of innovative metal-free heterogeneous catalysts.
The use of advanced nanoporous materials for traditional Lewis adducts due to the inherent framework
heterogeneous catalysis, driven by their facile recyclability and structure, rather than through installation of bulky
excellent chemical tunability, has inspired extensive substituents surrounding the Lewis base, yet can cooperatively
development of novel nanoporous solids with readily activate H
1
accessible and highly designable active sites. Conjugated
2
and facilitate the hydrogenation of ketones.
nanoporous polymers (CNPs), formed by linking multidentate
organic building blocks through covalent bonds, have recently
come into the limelight as new versatile platforms for
application in heterogeneous catalysis, due to the extent to
which they can be synthetically tuned to enable molecular
design of catalytic sites and access to diverse nano-pore
regimes. Despite the large number of CNPs that have been
2
studied in the context of heterogeneous catalysis, the use of
CNPs that enable metal-free hydrogenation of unsaturated
substrates, like ketones, is rarely reported. Catalytic
hydrogenation of unsaturated substrates is one of most
fundamental transformations in chemistry and plays a crucial
3
role in the chemical industry. Conventional hydrogenation
processes mediated by using metal-based catalysts also result
Scheme 1. One-pot synthesis route (a) and proposed
frameworks (b, c) of CNPs.
in increased production costs and significant environmental
4
pollution. Herein, we report a novel approach to generating
metal-free CNP-based catalysts by designing appropriately
encumbered microenvironments capable of forming frustrated
Lewis pairs (FLPs) within the supporting polymeric architecture.
When loaded with an appropriate Lewis acid, these
heterogeneous systems are prevented from forming
We designed and synthesized a model CNP-1 by a one-step
FeCl
3
-catalyzed oxidative reaction of 2,6-carbazole-substituted
Scheme 1). Carbazole
pyridine-based building block (2,6-Cz
,
serves as the substituted moiety for the reason that it is a
highly reactive scaffold for the construction of porous rigid
5
frameworks. The persistence of Lewis-basic pyridine sites in
1
3
the final framework material was verified by the solid state
C
cross-polarization magic-angle spinning (CP/MAS) NMR (Figure
S1). The peaks at 153 and 140 ppm arise from the aromatic
6
carbons in pyridine ring, as has been determined previously.
Nitrogen adsorption-desorption isotherms exhibit a Type I
adsorption profile (Figure S2), and the corresponding
Brunauer-Emmett-Teller (BET) surface area was determined to
2
-1
be 1368 m g . To form the desired FLP microenvironment, we
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6 5 3
introduced a typical bulky Lewis acid, B(C F ) , into the CNP above, no catalytic activity was observed for CNP-2/B. From
scaffold through a facile wet impregnation technique (Details this we conclude formation of the active CNP-1/B catalyst
are provided in the supporting information). Powder x-ray requires inclusion of the pyridyl functionality inside the porous
diffraction (PXRD) patterns (Figure S3 and S4) reveal no framework. We propose this Lewis basic site may act
residual characteristic peaks of B(C
6
F
5
)
3
,
indicating the cooperatively with B(C
6
F
5
)
3
to form
a
favourable FLP
and inducing
material (CNP-1/B). A concomitant decrease in surface area catalytic hydrogenation of AP. To confirm this hypothesis, we
molecular Lewis acid is highly dispersed within the hybrid microenvironment, capable of activating H
2
9
1
1
was also observed for CNP-1/B, exhibiting negligible porosities performed solid state B NMR on the CNP-1/B catalyst and
1
. Different B quadrupolar lineshapes and chemical
1
which suggests extensive incorporation of sterically hindered B(C
6
F
5
)
3
Lewis acidic sites.
shifts were observed for these two samples (see Figure
1
1), similar to what has been shown in other B NMR work on
1
Our initial hydrogenation studies focused on the reduction
1
of acetophenone (AP), as we sought to assess the activity of borane-amine Lewis pairs. These data may support the
0
our CNP-based catalyst as well investigate the effects of the interaction between CNP-1 and B(C F ) in the CNP-1/B system.
6 5 3
intrinsic steric hindrance on hydrogenation. CNP-1/B (0.2
mmol B) was combined with AP (2.5 mmol) and hexane (5 mL)
in a high pressure hydrogenation vessel. Upon sealing, the
reactor was charged to a pressure of 5 bar with H and heated
2
at 100 °C for 24 h. The use of hexane eliminates the potential
for generating unintentional catalytic sites. Indeed, previous
reports have demonstrated that catalytic hydrogenation of AP
7
can be achieved by using B(C F ) in ethereal solvents. After
6 5 3
24 h the conversion of AP to ethylbenzene (EB) on CNP-1/B
reached as high as 36.7 %, almost tripling the 14% for the
7
homogeneous FLP system reported under similar conditions.
b
We also performed the hydrogenation of AP with the
8
homogeneous 2,6-diphenyl pyridine/B(C F ) . After 24 h, the
6
5 3
conversion of AP only reached 9.5% and the styrene and EB
yield were 6.0% and 3.2%, respectively, while control reactions
Figure 2. The complexes structures of B(C F ) -2,6-Cz (a) and
6
5 3
using either CNP-1 or B(C F ) displayed no activity. Although
6 5 3
B(C F ) -3,5-Cz (b). We mark π-π stacking with red dotted line.
6 5 3
inductively coupled plasma optical emission spectroscopy
revealed 0.02 wt% of CNP-1 was residual Fe from the
framework synthesis, the absence of any appreciable catalyst
activity in the aforementioned control experiment reveals the
Fe is functionally innocent in the FLP-system.
Color code: H, white; C, grey; N, blue; B, brown; F, cyan.
To obtain a better understanding regarding the influence of
framework sterics on hydrogenation performance, we further
2
-1
prepared CNP-3 (SBET = 1538 m g ) from 3,5-carbazole-
substituted pyridine (3,5-Cz, Scheme 1) and tested it for AP
hydrogenation. Despite a lower conversion of AP to EB (17.2%)
with CNP-3/B, it was nevertheless superior to the activity of
7
b
the analogous homogeneous system. We then performed
density functional theory (DFT) calculations to model the
interaction between carbazolic-substituted pyridines (2,6-Cz
6 5 3
and 3,5-Cz) and B(C F ) , with results displayed in Figure 2, to
further investigate the effects of sterics on the hydrogenation
activity in CNP catalyst systems (calculation details are
provided in the supporting information). Our initial
expectation had been that the difference in activity was due to
the accessibility of the pyridyl nitrogen in CNP-3, with
carbazole substituents at the 3 and 5 positions oriented to
afford unimpeded access to the nitrogen. In contrast, the CNP-
1
1
Figure 1. Solid-state BNMR of B(C
6 5 3
F ) and CNP-1/B. (*:Spinning
sideband)
1
is much more encumbered, with the carbazole ligands
forming a cleft of 60° with the pyridine located at the vertex.
However, in contrast to our previous hypothesis, the
6 5 3
In an effort to confirm the catalytic sites of CNP-1/B, we calculated structures display association of B(C F ) roughly
prepared an analogous material, CNP-2, where the pyridine orthogonal to the plane of the pyridyl group, and indicate π-π
strut was replaced by a benzylic group (Scheme 1). Despite stacking dominates the non-bonding interaction between
6 5 3 6 5 3
loading with B(C F ) identically to the method described carbazolic moieties and B(C F ) . Inspection of the structures
2
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displayed in Figure 2 reveals a significantly greater torsion and 36.8%, respectively. We further investigated the
angle of the carbazole substituents when bound at the 3,5 hydrogenation of 1-phenylethyl alcohol and styrene with CNP-
positions as compared to the 2,6 positions, while the distances 1/B. After 24 h under the same conditions, the hydrogenation
between positive boron and negative pyridinic nitrogen atoms of styrene completely yielded EB and 1-phenylethyl alcohol
are about 4.5 Å for both structures. Finally, the absolute value gave EB and styrene in 76.3% and 23.7% yield, respectively.
of the binding energy (|BE|) for B(C
higher than that of B(C -3,5-Cz, implying that CNP-1 is
more favourable for binding B(C than CNP-3. Taken inhibitory effect on this homogeneous hydrogenation
cumulatively, these results indicate the catalytic activity of the system. To investigate this possibility, stoichiometric H
CNP material is correlated with the ability of the carbazole- (relative to AP) was added with CNP-1/B at the beginning of
pyridine strut to achieve optimal interactions with the B(C the reaction. To our delight, an 35.7% conversion of AP to EB
while experiencing minimal structural distortion. The B-N obtained after 24 h, clearly demonstrating that CNP-1/B is an
interaction may dominate the formation of FLP efficient water-tolerant heterogeneous system for metal-free
6
F
5
)
3
-2,6-Cz is 1.6 kcal/mol These results are summarized in Table 1 (Entry 2,3).
6
F
5
)
3
We reasoned that the presence of H O might have a potent
2
6 5 3
F )
7
b
2
O
6 5 3
F )
a
microenvironment, and while the bond length is similar in both hydrogenation of AP.
instances, it requires greater structural distortion in the case of
the B(C F ) -3,5-Cz. The non-optimal electronic structure
6 5 3
resulting from these distortions would be expected to
deleteriously affect catalytic activity, and indeed the atomic
charge on nitrogen in B(C F ) -2,6-Cz (-0.435|e|) is much more
Table 1. The catalytic hydrogenation of ketones and other
a
substrates by CNP-1/B.
6
5 3
negative than that in B(C F ) -3,5-Cz (-0.374|e|), suggesting a
6
5 3
1
1
more favourable environment for hydrogenation.
Accordingly, both the intrinsic sterically-frustrated framework
and the local innate steric hindrance on pyridine sites in 2,6-Cz
play a crucial role in achieving efficient AP hydrogenation on
CNP-1/B. As such, both need be considered simultaneously
when designing new CNP-based metal-free catalysts for
hydrogenation.
Figure 3. Effect of reaction time on the AP conversion to EB
catalyzed by CNP-1/B at 100 °C.
Inspired by these promising results, we proceeded to
carefully investigate the effects of reaction time on AP
conversion and ethylbenzene (EB) yield in the presence of
CNP-1/B. After 12 h, the conversion of AP reached 26.6% while
the yield of styrene and EB were 10.2% and 16.4%,
respectively, as evidenced by GC-MS (Figure 3). Both AP
conversion and EB yield increased continuously during the
following 12 h, with minimal styrene observed. After 24 h, the
a
Reaction conditions: 2.5 mmol of substrates, 0.20 g of catalyst
2
(0.2 mmol B), 5 bar H , T = 100 °C, and in 5 mL of hexane.
Conversion and yield estimated by GC-MS.
As recyclability has been considered to be one of most
final conversion of AP was as high as 36.7%, with the advantageous features of heterogeneous catalysts over to
corresponding EB yield of 35.8%. Extending the reaction time homogeneous catalysts, the reuse of CNP-1/B was also
to 48 h, AP conversion and EB increased modestly to 37.7% assessed. When simply collected by filtration, dried, and added
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to a new reaction, the recovered catalyst displayed a
significant drop in activity, achieving 8.6% conversion, while
maintaining the yield of EB at 8.2%. This is due to the partial
2
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the decrease of F within the framework (12.2 vs 2.2 at%), as
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restoration of the catalytic activity was facilely achieved
9
(Table S2). Following this
through the addition of B(C
6
F
5
)
3
approach, the recycled catalyst can consistently give high
conversion of AP (29.4%) and EB yield (28.1%), even after as
many as four recycles.
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heterogeneous catalyst, we investigated the hydrogenation of
a range of other ketone substrates, such as three alkyl ketones
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(
Table 1, Entry 4-6) and three benzyl ketone (Entry 7-9). As
displayed in Table 1, these reactions generally afford
conversion to the desired hydrogenation products, clearly
demonstrating the accessibility of bulky substrates within the
pore structure of the CNP-1/B system and the diversity of this
novel heterogeneous FLP catalyst.
3
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Conclusions
In conclusion, we have developed a novel heterogeneous
system for efficient, metal-free, and water-tolerant
hydrogenation using the steric encumbrance within
conjugated nanoporous polymers. Analysis of structurally
related variants demonstrate the intrinsic steric hindrance
from the framework is what effects formation of a favourable
hydrogenation microenvironment, while computational results
indicate the local sterics surrounding the Lewis base also play a
significant role in the charge distribution and thus the resulting
catalytic activity. As the struts for these assemblies can be
rationally designed and synthetically controlled through
conventional organic techniques, we anticipate that this
innovative new class of materials will enable new areas of
catalysis research, and the successful pursuit of more
challenging hydrogenation reactions.
6
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Acknowledgements
The research was supported financially by the Division of
Chemical Sciences, Geosciences, and Biosciences, Office of
Basic Energy Sciences, US Department of Energy. The NMR
experiments were performed in the Environmental Molecular
Sciences Laboratory,
a national scientific user facility
sponsored by the U.S. Department of Energy's Office of
Biological and Environmental Research and located at Pacific
Northwest National Laboratory (PNNL).
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