Pd-Metalated Conjugated Nanoporous Polycarbazoles for Additive-Free Cyanation of Aryl Halides: Boosting Catalytic Efficiency through Spatial Modulation

Article abstract of DOI:10.1002/cssc.201700329

Transition-metal-catalyzed cyanation of aryl halides is a common route to benzonitriles, which are integral to many industrial procedures. However, traditional homogeneous catalysts for such processes are expensive and suffer poor recyclability, so a heterogeneous analogue is highly desired. A novel spatial modulation approach has been developed to fabricate a heterogeneous Pd-metalated nanoporous polymer, which catalyzes the cyanation of aryl halides without need for ligands. The catalyst displays high activity in the synthesis of benzonitriles, including high product yields, excellent stability and recycling, and broad functional-group tolerance.

Full text of DOI:10.1002/cssc.201700329

Accepted Article
Title: Pd-Metalated Conjugated Nanoporous Polycarbazoles for
Additive-Free Cyanation of Aryl Halides: Boosting Catalytic
Efficiency via Spatial Modulation
Authors: Shunmin Ding, Chengcheng Tian, Xiang Zhu, Carter W.
Abney, Ziqi Tian, Bo Chen, Meijun Li, De-en Jiang, Ning
Zhang, and Sheng Dai
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To be cited as: ChemSusChem 10.1002/cssc.201700329
Link to VoR: http://dx.doi.org/10.1002/cssc.201700329
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10.1002/cssc.201700329
ChemSusChem
COMMUNICATION
Pd-Metalated Conjugated Nanoporous Polycarbazoles for
Additive-Free Cyanation of Aryl Halides: Boosting Catalytic
Efficiency via Spatial Modulation
Shunmin Ding,[a,b,‡] Chengcheng Tian,[b,‡] Xiang Zhu,*[b] Carter W. Abney,[c] ZiqiTian,[d] Bo Chen,[a] Meijun
Li,[b] De-en Jiang,[d] Ning Zhang*[a] and Sheng Dai*[b,c]
Abstract: A novel spatial-modulation approach was developed to
fabricate a new heterogeneous Pd-metalated nanoporous polymer for
efficient and ligand-free cyanation of aryl halides. The resulting
catalyst displays high activity in the synthesis of benzonitriles,
including high product yield, excellent stability and recycle, and broad
scope functional group tolerance.
diverse nano-pore regimes. Nevertheless, the use of CNPs that
enable efficient cyanation of aryl halides towards the synthesis of
benzonitriles, is rarely reported.
Herein, we report a novel spatial-modulation (SM) approach
to generating Pd-metalated CNPs for this important chemical
transformation. The key of our success lies in the use of rationally-
designed polymeric backbones to create a favorable Lewis basic
microenvironment within the nanoporous framework that
encourages strong binding and stabilization of Pd (II), facilitating
formation of the catalytically active species which can work with
to initiate the cyanation. Under a heterogeneous, ligand-free
condition, full conversion of aryl halides and excellent recyclability
were successfully achieved for the resultant Pd-metalated CNP
catalyst.
Benzonitriles, an integral part of dyes, pesticides, natural products
and pharmaceuticals, are prevalent in organic chemistry owing to
the versatile nature of the nitrile moiety, which can serve as an
excellent synthetic handle to install a wide variety of functional
groups such as amines, amides and ketones.[1] Transition-metal-
catalyzed cyanation of aryl halides represents a common and
powerful strategy for the construction of nitriles.[2] Of particular
note are examples of Pd-catalyzed cyanation using K4[Fe(CN)6]
as nontoxic and inexpensive cyanide sources.[3] Nevertheless,
most of these conventional homogenous systems involve the use
of expensive additional ligands and suffer from poor recyclability,
limiting their industrial application. Therefore, developing an
efficient heterogeneous analogue is highly desired.
Conjugated nanoporous polymers (CNPs), formed by linking
multidentate organic scaffolds through covalent bonds, have
attracted tremendous attention in the context of heterogeneous
catalysis,[4] due to the extent to which they can be synthetically
tuned to enable molecular design of catalytic sites and access to
Scheme 1. Synthesis of (a) Pd-CNP-1 and (b) Pd-CNP-2
.
[a]
[b]
[c]
[d]
S. Ding, B. Chen, Prof. N. Zhang
Institute of Applied Chemistry, College of Chemistry
Nanchang University
Nanchang, Jiangxi 330031, P. R. China
E-mail: nzhang.ncu@163.com
S. Ding, Dr. C. Tian, Dr. X. Zhu, Dr. M. Li, Prof. S. Dai
Department of Chemistry
We designed and synthesized a model CNP-1, using a FeCl3-
catalyzed carbazole oxidation reaction (Scheme 1a).[5] We
reasoned that the use of a pyridine-functionalized moiety as the
building block would enable construction of
a
basic
University of Tennessee
microenvironment for the cyanation,[3e] and at the same time
afford electron rich active sites to strongly bind Pd (II) species in
place of conventional expensive ligands.[3e] Through a facile
impregnation approach, 1.3 wt.% Pd was loaded inside the Pd-
metalated network (Pd-CNP-1) according to the Inductively
coupled plasma (ICP) result (Table S1). The relevant Brunauer-
Emmett-Teller (BET) surface area was determined to be 1070 m2
g-1 (Table S1), with a Type I Nitrogen adsorption profile (Figure
S1). The value is 247 m2 g-1 lower than that of the non-metalated
CNP-1. The marked hysteresis loop may be attributed to pore
network effects.[6] The existence of the mesopores can be
suggested from the pore size distribution curves (Figure S2).
Knoxville, TN 37916, United States
E-mail: xiang@utk.edu/zhuxiang.ecust@gmail.com
C. Abney, Prof. S. Dai
Chemical Sciences Division
Oak Ridge National Laboratory
Oak Ridge, TN 37831, United States
E-mail: dais@ornl.gov
Dr. Z. Tian, Prof. D. Jiang
Department of Chemistry
University of California, Riverside
CA 92521, United States
‡ These authors contributed equally.
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/cssc.201700329
ChemSusChem
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We initially focused the cyanation of 1,4-dibromobenzene
(DBB) using K4[Fe(CN)6] as the cyanide source to assess the
activity of our Pd-metalated CNP-based catalyst. Unfortunately,
negligible conversion of DBB to 1,4-dicyanobenzene (DCB) on
Pd-CNP-1 was observed under additive-free condition (4 %, entry
3, Table 1). We reasoned that Pd-loading at the sterically-
hindered pyridinic binding sites may account for this poor catalytic
performance. In this regard, we sought to improve the local
environment of the catalytic Pd (II) species via a spatial
modulation of the framework backbone, and therefore
significantly enhancing the cyanation activity. A new building
block, 3,5-di(9H-carbazol-9-yl)pyridine (3,5-Cz, Scheme 1b), was
rationally synthesized in an effort to confirm our hypothesis. The
resultant CNP-2 exhibits a high BET surface area of 1538 m2 g-1
(Figure S1). Due to the greater accessibility of the pyridine
nitrogen, much higher Pd-loading (8.7 wt %, Table S1) can be
introduced into Pd-CNP-2, commensurate with a significant
reduction in BET surface area (865 m2 g-1). Furthermore, the
existence of acetate-based moieties can be confirmed by the
thermal gravimetric analysis (TGA, Figure S3). Nevertheless, this
porosity remains sufficient for the facile transport of reactants to
access the active sites inside the Pd-metalated network.
The significant differences in Pd-loading prompted us to
pursue a better understanding of the spatial effect on the
interaction between the Pd (II) source and rigid CNP frameworks.
As such, we performed density functional theory (DFT)
calculations to model the interaction between carbazolic-
substituted pyridines (2,6-Cz and 3,5-Cz) and Pd(OAc)2, with
results displayed in Figure 1 (calculation details are provided in
the supporting information).[5e] Importantly, the absolute value of
the binding energy (|BE|) for 3,5-Cz-Pd(OAc)2 complex is 23.2
kcal mol-1, four times larger than that of 2,6-Cz-Pd(OAc)2 (Figure
1), implying that CNP-2 is more favourable for binding Pd(OAc)2
than CNP-1. Accordingly, the spatial modulation of pyridinic sites
play a crucial role in such high incorporation of Pd-containing sites.
Table 1. Performances of different catalysts in cyanation of 1,4-
dibromobenzene (DBB) with K4[Fe(CN)6]. [a]
Entry
Catalyst
t (h)
Yield (%) [f]
TON [g]
1
CNP-1
18
18
18
18
18
18
4
0
-
2
CNP-2
0
-
3[b]
4[c]
5[d]
6[e]
7[C]
8[C]
Pd-CNP-1
Pd-CNP-2
Pd(OAc)2
Pd-CNP-1
Pd-CNP-2
Pd-CNP-2
4
65
241
42
81
241
93
99
19
34
99
38
2
[a] Reaction conditions: 1,4-dibromobenzene (1 mmol), K4[Fe(CN)6] (0.34
mmol), dimethylformamide (DMF, 5 mL), 140 oC under nitrogen; [b] Pd-
CNP-1 (5 mg, Pd containing/0.00061 mmol); [c] Pd-CNP-2 (5 mg, Pd
containing /0.0041 mmol); [d] Pd(OAc)2 (1.0 mg, Pd containing /0.0045
mmol); [e] Pd-CNP-1 (34 mg, Pd containing /0.0042 mmol); [f] Yield was
determined by 1HNMR; [g] TON = mol of product / mol of catalyst.
Taken cumulatively, the combination of surface Pd sites,
significant surface area, high loading, and greater accessibility to
the putative catalytic site suggest that Pd-CNP-2 could function
as a highly active Pd catalyst for cyanation reactions.
To confirm this hypothesis, we subsequently carried out the
cyanation of DBB to DCB using Pd-CNP-2 as the heterogeneous
catalyst. As expected, nearly full conversion (99 % yield) was
achieved after heating at 140 oC for 18 h, with no additional base
or stabilizing ligand added to the system (entry 6, Table 1). In
contrast, the use of homogeneous Pd(OAc)2 afforded a much
lower yield (19 %) with a turnover number (TON) value of 42
(entry 5, Table 1), which are close to the previously-reported
values.[3e] Furthermore, the excellent catalytic activity can also be
maintained after heating for 4 h (entry 7, Table 1). These results
clearly indicate that the catalytic cyanation activity of Pd-CNPs
can be dramatically enhanced by spatial modulation of the
intrinsic Lewis basic frameworks.
Figure 1. The complexes structures of 3,5-Cz-Pd(OAc)2 (a) and 2,6-Cz-
Pd(OAc)2 (b). Color code: H, white; C, grey; N, blue; Pd, green; O, red.
The incorporated Pd-containing species were then studied by
the powder x-ray diffraction (PXRD). No obvious peak can be
observed from the PXRD pattern (Figure S4), indicating Pd is
incorporated as a molecular complex. We further performed X-ray
photoelectron spectroscopy (XPS) to obtain
a
deeper
understanding of these sites (Figure 2). The Pd3d XPS spectra
of Pd-CNP-2 shows two deconvoluted peaks for Pd (Pd 3d5/2
peak at 338.3 eV and Pd 3d3/2 peak at 343.6 eV), confirming that
the Pd(II) species are located on the surface of Pd-CNP-2.[7]
Figure 2. Pd 3d XPS spectra of Pd-CNP-2.
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10.1002/cssc.201700329
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Recyclability is one of the most advantageous features of
heterogeneous catalysts, particularly when precious metals are
used.[3e] Accordingly, the reuse and stability of Pd-CNP-2 was
assessed for the model transformation of DBB to DCB. When
simply collected by filtration, dried, and added to a new reaction,
the recovered catalyst displayed negligible drop in activity after
three recycles (Figure 3a), ICP analysis of the regenerated
material reveals no significant change in Pd-content (Table S1),
indicating excellent stability for the heterogeneous Pd-CNP-2.
The morphology of Pd-CNP-2 after the reaction maintains very
well (Figure S5). It should be noted that the recyclability of Pd-
CNP-2 is better than that of Pd-CNP-1 (Figure S6), which may
be attributed to the stronger Pd-binding affinity of Pd-CNP-2.
Furthermore, the reaction can be completely halted by removal of
the Pd-CNP-2, confirming catalysis is performed by the material
itself, and not by some unidentified solution-phase component
(Figure 3b). Accordingly, these results show that the as-prepared
nanoporous Pd-CNP-2 is a robust and efficient heterogeneous
catalyst for the cyanation reaction.
Table 2. The cyanation of various aryl halides with K4[Fe(CN)6]
catalyzed by Pd-CNP-2 [a]
.
Entry
1
Ar-X
Ar-CN
Yield (%)
96
TON
234
2
3
4
5
6
99
97
99
99
86
241
236
241
241
209
7
99
241
[a] reaction conditions: Ar-X (1 mmol), Pd-CNP-2 (5.0 mg, Pd containing
/0.0041 mmol), DMF (5 mL) and K4[Fe(CN)6] (0.17 mmol, entry 1-3; 0.34
Acknowledgements
Figure 3. Pd-CNP-2 activity as a function of recycle (a) and leaching
experiment (b) of Pd-CNP-2 in the cyanation of DBB with K4Fe(CN)6.
The research was supported financially by the Division of
Chemical Sciences, Geosciences, and Biosciences, Office of
Basic Energy Sciences, US Department of Energy. S. Ding, B.
Chen and N. Zhang thank the National Natural Science
Foundation of China (No. 21062013) and the Natural Science
Foundation of Jiangxi Province (No. 2011ZBAB203013). This
manuscript has been authored by UT-Battelle, LLC under
Contract No. DE-AC05-00OR22725 with the U.S. De-partment of
Energy. The United States Government retains and the publisher,
by accepting the article for publication, acknowledges that the
United States Government retains a non-exclusive, paid-up,
irrevocable, worldwide license to publish or reproduce the
published form of this manuscript, or allow others to do so, for
United States Government pur-poses. The Department of Energy
will provide public access to these results of federally sponsored
research in accord-ance with the DOE Public Access Plan
(http://energy.gov/downloads/doe-public-access-plan).
Seeking to probe the generality of our Pd-CNP-2
heterogeneous catalyst, we extended this new protocol to a range
of other aryl halides. As shown in Table 2, these reactions
generally afford full conversion to the desired benzonitriles,
clearly demonstrating the diversity of this novel heterogeneous
catalyst. It is worth noticing that this new approach shows great
potential to convert other aromatic halides like less reactive
chlorobenzene and tribromobenzene, which are generally difficult
to be transformed under similar reaction conditions.[3e]
In conclusion, we have developed a novel heterogeneous
system through spatially modulating the backbones of conjugated
nanoporous polymers to achieve efficient and additive-free
cyanation of aryl halides. The resulting catalyst displays high
porosity, electron rich Pd(II)-containing active sites, and exhibits
promising catalytic performance, including a high yield for all
products, excellent recycle properties and stability, and a broad
functional group tolerance. These exciting results demonstrate
the great potential for application of metalated porous polymers in
the context of heterogeneous catalysis.
Keywords: Pd-metalated • Conjugated nanoporous polymers •
additive-free • cyanation of aryl halides • spatial-modulation
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10.1002/cssc.201700329
ChemSusChem
COMMUNICATION
COMMUNICATION
A
novel
modulation approach was
developed to fabricate
new heterogeneous Pd-
metalated nanoporous
spatial-
Shunmin Ding, Chengcheng Tian, Xiang Zhu,*
Carter W. Abney, ZiqiTian, Bo Chen, Meijun Li, De-
en Jiang, Ning Zhang* and Sheng Dai*
a
polymer for efficient and
ligand-free cyanation of aryl
Page No. – Page No.
Pd-Metalated
Conjugated
Nanoporous
halides.
catalyst
The
displays
resulting
high
Polycarbazoles for Additive-Free Cyanation of
Aryl Halides: Boosting Catalytic Efficiency via
Spatial Modulation
activity in the synthesis of
benzonitriles, including high
product yield, excellent
stability and recycle, and
broad scope functional
group tolerance.
This article is protected by copyright. All rights reserved.