Two-Dimensional Metal-Organic Layers for Electrochemical Acceptorless Dehydrogenation of N-Heterocycles

Article abstract of DOI:10.1002/asia.201900391

The catalytic acceptorless dehydrogenation (CAD) is an attractive synthetic route to unsaturated compounds because of its high atomic efficiency. Here we report electrochemical acceptorless dehydrogenation of N-heterocycles to obtain quinoline or indole derivatives using metal-organic layer (MOL) catalyst. MOL is the two-dimensional version of metal-organic frameworks (MOF), and it can be constructed on conductive multi-walled carbon nanotubes via facile solvothermal synthesis to overcome the conductivity constraint for MOFs in electrocatalysis. TEMPO-OPO₃ ^2? was incorporated into the system through a ligand exchange with capping formate on the MOL surface to serve as the active catalytic centers. The hybrid catalyst is efficient in the organic conversion and can be readily recycled and reused.

Full text of DOI:10.1002/asia.201900391

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: Two-Dimensional Metal–Organic Layers for Electrochemical
Acceptorless Dehydrogenation of N-Heterocycles
Authors: Ling Yang, Fa-Xue Ma, Fan Xu, Dong Li, Liangmei Su, Hai-
Chao Xu, and Cheng Wang
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To be cited as: Chem. Asian J. 10.1002/asia.201900391
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COMMUNICATION
Two-Dimensional Metal–Organic Layers for Electrochemical
Acceptorless Dehydrogenation of N-Heterocycles
Ling Yang[a], Fa-Xue Ma[a], Fan Xu[a], Dong Li[a], Liangmei Su[a], Hai-Chao Xu*[a], Cheng Wang*[a]
Abstract: The catalytic acceptorless dehydrogenation (CAD) is an
attractive synthetic route to unsaturated compounds because of its
high atomic efficiency. Here we report electrochemical acceptorless
dehydrogenation of N-heterocycles to obtain quinoline or indole
derivatives using metal–organic layer (MOL) catalyst. MOL is the two-
dimensional version of metal−organic frameworks (MOF), and it can
be constructed on conductive multi-walled carbon nanotubes via facile
solvothermal synthesis to overcome the conductivity constraint for
diffusivity of organic substrates in the 3D framework also presents
an obstacle for its use in electrosynthesis.[17]
In this regard, we and others have recently constructed a
series of metal−organic layers (MOLs)[18], which are two-
dimensional (2D) analogs of MOFs. There are several distinct
advantages in using MOL as catalysts for electroorganic
synthesis: First, the thinness of MOLs down to a monolayer can
overcome the limitation of diffusivity of organic substrates and
electric conductivity; Second, the regular MOL lattice affords high
densities of uniform catalytic sites on the electrodes; Third, the
softness of the MOL thin film makes it possible to assemble with
conductive materials such as multiwalled carbon nanotubes
(CNTs) for the construction of an electrode; Fourth, the chemical
tunability of its surface via post-synthetic modification provides a
convenient way to fine tune the catalytic centers on MOLs to
create a desirable microenvironment for substrate conversion.
Fifth, the stable heterogeneous MOLs catalysts in organic
aqueous environments is immobilized on the electrode easy to
recycled and reused.
2-
MOFs in electrocatalysis. TEMPO-OPO3 was incorporated into the
system via ligand exchange with capping formate on the MOL surface
to serve as the active catalytic centers. The hybrid catalyst is efficient
in the organic conversion and can be readily recycled and reused.
The catalytic acceptorless dehydrogenation (CAD) of N-
heterocycles is an atomically efficient way to synthesize quinoline
and indole rings[1], which are widely found in pharmaceuticals and
bioactive molecules[2]. Moreover, these N-heterocycles are also
proposed as H2 carriers for liquid organic H2 storage[3].
Conventional dehydrogenations require oxidants or sacrificial
hydrogen acceptors, which usually produce byproducts such as
toxic metal salts[4]. Recent development of acceptorless
dehydrogenation produces H2 to avoid the use of an oxidant. A
number of catalysts based on noble metals such as iridium[5],
palladium[6], platinum[6b, 7] and ruthenium[8], and nom-noble metals
such as iron[9], cobalt[10] and nickel[11], and organic borane
compounds[12] are active for this transformation. However, most
of the above catalysts are homogeneous complexes that requires
catalyst removal after the reaction, increasing the overall cost of
synthesis. Therefore, it is still of interest to perform
dehydrogenation in the absence of these homogeneous catalysts,
especially metal complexes, to avoid the tedious separation step
or the contamination of the product. Electrosynthesis has risen as
a new scalable transformation method that can use electricity as
the reductant or oxidant[13]
. Electrochemical acceptorless
2−
Scheme 1. CNT/MOL-TEMPO-OPO3
for electrochemical acceptorless
dehydrogenation (ECAD) oxidizes the N-heterocycles to quinoline
or indole derivatives at the anode and produces H2 at the cathode
to complete the overall conversion. This process may be
accelerated by functionalizing the anode with a catalyst for N-
heterocycle oxidation.
dehydrogenation of N-Heterocycles
Based on the above considerations, we herein report the
synthesis of MOL/CNT hybrid electrode for efficient
a
Metal-organic frameworks (MOFs)[14] are constructed from
organic ligands and metal nodes or secondary-building units
(SBUs). The well-defined structures of MOFs enable the
electrochemical CAD. Hf−NTB−MOL used in this study is
composed of three-connected 4,4′,4″-nitrilotribenzoate (NTB)
bridging ligands and six-connected Hf6(μ3-O)4(μ3-OH)4(HCO2)6
SBU, following our previous publication.[19] The Hf−NTB−MOL is
tethered to multiwalled CNTs with surface functionalized with
carboxylic acid to form a CNT/MOL assembly, which can
significantly improve the electric conductivity of the MOL. The
CNT/MOL was then modified with TEMPO-OPO3H2 through a
linkage between the phosphate moiety and the Hf6 SBU of the
MOL (abbreviate as CNT/MOL-TEMPO-OPO32−) to act as an
efficient ECAD electrocatalyst (scheme 1).
integration of functional molecular entities for electrocatalysis[15]
.
However, most of the MOFs are insulators with limited electron
migration rate on the framework.[15a, 16] In addition, the limited
[a]
L. Yang, F.-X. Ma, F. Xu, D. Li, L. Su, Prof H.-C. Xu, Prof C. Wang
College of Chemistry and Chemical Engineering, iCHEM, State Key
Laboratory of Physical Chemistry of Solid Surface
Xiamen University, Xiamen 361005 (China)
E-mail: haichao.xu@xmu.edu.cn wangchengxmu@xmu.edu.cn
Hf−NTB−MOL with the chemical formula of Hf6(μ3-O)4(μ3-
OH)4(μ1-OH)2(μ1-OH2)2(HCO2)4(NTB)2 was prepared via
a
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
solvothermal reaction with H3NTB and HfOCl2·8H2O in a solvent
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mixture of N,N-dimethylformamide (DMF), formic acid, and water
at 120 °C for 2 days. The CNT/MOL was obtained by heating a
DMF solution of H3NTB and HfOCl2·8H2O in the presence of
0.2 nm (Figure 1c). The AFM image of the MOL functionalized
with the TEMPO-OPO32- gave a thickness of 2.8 ± 0.2 nm (Figure
1d), which is thicker than the unfunctionalized MOL, possibly due
to the size of the TEMPO molecule on the MOL surface. TEM
image showed Hf−NTB−MOL wrapping around CNTs as a coat
(Figure 1e, Figure S1), and after the postsynthetic modification
with TEMPO-OPO3H2, the morphology of the CNT/MOL hybrid
remained the same (Figure 1f). Conductivity measurement of the
CNT/MOL-TEMPO-OPO32− assembly gave a value of 3.35 S·m−1
(Figure 2a), which is significantly increased as compared to the
1.6x10-7 S·m−1 of the Hf−NTB−MOL alone.
carboxyl-functionalized CNTs following a similar procedure as
2−
synthesizing Hf−NTB−MOL. CNT/MOL-TEMPO-OPO3
was
then prepared by post-synthetic modification of CNT/MOL with 4-
phosphonooxy-2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO-
OPO3H2) in water at 60 °C for 1 day.
Figure 2. a) Conductivity of Hf−NTB−MOL and CNT/MOL-TEMPO-OPO32−. b)
2−
CV curve of CNT/MOL-TEMPO-OPO3
.
We then tested electrochemical CAD activity of the
2−
CNT/MOL-TEMPO-OPO3
for
N-Heterocycles.
Cyclic
voltammetry (CV) scan of CNT/MOL-TEMPO-OPO32− showed the
redox peaks of TEMPO+/TEMPO with an oxidative wave at 0.39
V (Ep/2 vs Cp2Fe+/0) (Figure 2b), providing support for the
successful attachment of the TEMPO-OPO3H2 to the
Hf−NTB−MOL. The electrochemically generated TEMPO+ can
oxidize 1,2,3,4-tetrahydroquinaldine (1a), as confirmed by the
catalytic current in the presence of 1a (Figure 2b). As expected,
2−
the CNT/MOL-TEMPO-OPO3
acceptorless
electrochemical method under relatively mild conditions.
can effectively catalyze the
of N-heterocycles with
dehydrogenation
b
]
Table 1. Optimization of Reaction Conditions. [a,
Figure 1. a) PXRD patterns of simulated (black line), Hf-NTB-MOL (red line),
CNT/MOL (pink line) and CNT/MOL-TEMPO-OPO32. (blue line). b) TEM image
of Hf-NTB-MOL. c) Tapping-mode AFM topography of the MOL and the height
profile along the white line. d) Tapping-mode AFM topography of the MOL
Entry
Deviation from the standard conditions
Yieldc.(%)
79
2-
functionalized with the TEMPO-OPO3 and the height profile along the white
1
2
3
4
5
6
7
none
no catalyst
CNT/MOL
no voltage
no H2O
NR
NR
NR
31
32
58
line. e) Hf-NTB-MOL on CNTs. f) TEM image of the CNT/MOL-TEMPO-
OPO32−catalyst.
catalyst suspension
under air
The powder X-ray diffraction (PXRD) patterns of
2−
Hf−NTB−MOL, CNT/MOL and CNT/MOL-TEMPO-OPO3
aReaction conditions: 1a (0.25 mmol), nBu4NBF4 (0.05 M), MeCN (9.0 mL), H2O
indicated that the samples contain the same MOL structure with
two major peaks at 2θ = 5.82° and at 2θ = 10.10°, which
corresponds to (hk) indices of (10) and (11) respectively. These
features are consistent with a 2D structural model (Figure 1a) of
3,6-connected 2D network of kagome dual (kgd) topology. The
broad peak at 2θ = 26° is attributed to CNTs. Introduction of the
CNTs and the TEMPO-OPO3H2 thus preserves the crystalline
structure of the Hf−NTB−MOL. Transmission electron microscopy
(TEM) image showed Hf−NTB−MOL as ultra-thin wrinkled films
(Figure 1b). Atomic force microscopy (AFM) measurements
verified the monolayer structures with an edge thickness of 1.2 ±
2-
(1 mL), nitrogen atmosphere, RT, 4 h, 7 mA, 45 mg CNT/MOL-TEMPO-OPO3
on RVC as anode, Pt cathode. bThe structure of the product was confirmed by
1H NMR analysis. cIsolated yield. NR = no reaction.
We optimized the reaction conditions and found that the
optimal conditions for the ECAD involved running the electrolysis
using a constant current of 7 mA in a mixed solvent of CH3CN/H2O
(9/1) containing 0.5 M nBu4NBF4 under nitrogen atmosphere at
room temperature. The reaction employed CNT/MOL-TEMPO-
OPO32− catalysts loaded onto a macroporous reticulated vitreous
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carbon (RVC, 100 PPI) as the working electrode and Pt plate as
the counter electrode. Under these conditions using 1a as the
substrate, the desired quinaldine product was isolated in the
highest yield of 79% (Table 1, entry 1). The presence of H2 was
confirmed by the gas chromatography analysis (Figure S2).
Control experiments in the absence of CNT/MOL-TEMPO-
OPO32− (Table 1, entry 2) or with CNT/MOL (Table 1, entry 3) did
not show the conversion of 1a, confirming the catalytic role of the
assembly and specifically the TEMPO moiety. Electricity is
indispensable as the reaction was completely abolished when
there was no electric current (Table 1, entry 4). Similarly, the
reaction conducted with no catalyst on the electrode but the
CNT/MOL-TEMPO-OPO32− suspended in the electrolyte solution
produced only 32% product, probably because only a few
3) afforded the corresponding products in 82% and 85% yield,
respectively. However, the presence of electron-withdrawing
group such as the nitro group leads to a lower yield of 35% (Table
2, entry 4). The ECAD method would be attractive to synthesize
the indole derivatives[20]. Indole 2e was obtained in an isolated
yield of 79% (Table 2, entry 5). In the case of the 2-methylindoline
(Table 2, entry 6) and 5-bromoindoline (Table 2, entry 7), higher
yields of 2-methylindole (81%) and 5-bromoindole (70%) were
observed, respectively. CNT/MOL-TEMPO-OPO32−-modified
electrode was reused three times with 1a as the substrate (Figure
S3) without loss of activity. The PXRD pattern and the TEM image
of the recovered catalysts showed that the morphology of MOL
was retained (Figure S4).
In conclusion, by attaching the ultra-thin MOLs to conductive
CNTs via facile one-pot solvothermal synthesis, followed by
postsynthetic modification CNT/MOL assembly with TEMPO-
OPO3H2, we have obtained an efficient electrocatalyst for CAD of
N-heterocycles under mild reaction condition. This straightforward
synthetic strategy provides a feasible way to use non-conductive
MOFs for electrochemical applications. Moreover, compared with
the frequently used covalently tethered functional electrode, a
higher density of catalytical centers can be obtained on this MOL-
functionalized CNT.
2−
CNT/MOL-TEMPO-OPO3 can contact with the electrode even
under vigorous stirring (Table 1, entry 6). Other control
experiments showed that the reaction became less efficient
without water in the solution (Table 1, entry 5), or under air (Table
1, entry 7).
Table 2. Scope of ECAD reaction for six-membered and five-
membered N-heterocycles [a]
Acknowledgements
We thank Prof. Wenbin Lin from University of Chicago for
insightful discussion. We acknowledge funding support from the
National Natural Science Foundation and Ministry of Science and
Technology of the P. R. China (NSFC21671162,
2016YFA0200702, NSFC21721001).
Entry
1
Substrates
Productsb
Yieldc (%)
79
2
82
Keywords:
catalytic
acceptorless
dehydrogenation
•
•
metal−organic
layers
•
metal-organic frameworks
3
4
5
6
85
35
79
81
electrosynthesis • N-heterocycles
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
COMMUNICATION
Ling Yang[a], Fa-Xue Ma[a], Fan Xu[a], Dong
Li[a], Liangmei Su[a], Hai-Chao Xu*[a], Cheng
Wang*[a]
A
metal–organic
layer
functionalized with TEMPO
was used to construct active
catalytic electrodes with
Page No. – Page No.
carbon nanotubes
electrochemical
acceptorless
for
Two-Dimensional Metal–Organic Layers
for
Electrochemical
Acceptorless
Dehydrogenation of N-Heterocycles
dehydrogenation of N-
heterocycles
quinoline
to
or
obtain
indole
derivatives.
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