Ligand-tuned cobalt-containing coordination polymers and applications in water

Article abstract of DOI:10.1039/d0gc02341h

Ligands play a key role in modern catalysis research and occasionally determine whether a reaction will take place under specific conditions, such as in water. In this experiment, ligands containing an indole-based diacid moiety were employed to prepare the corresponding cobalt coordination polymer material (Co-CIA) and porous oval polymer material (Co-NCIA). Interestingly, it was observed that Co-CIA could promote the alkylation of ketones with alcohols and alcohols with alcohols, while Co-NCIA was effective for the synthesis of 1-benzyl-2-aryl-1H-benzo[d]imidazoles from various phenylenediamine and benzyl alcohols through borrowing hydrogen and dehydrogenation strategies. Other mechanism explorations, such as deuterium labeling experiments and a kinetics study, were conducted to better understand Co-CIA and Co-NCIA systems and the related transformations. Our studies provided an efficient method for the development of highly active cobalt coordination polymer materials with excellent recovery performance for dehydrogenation and borrowing hydrogen reactions under water and base-free conditions.

Full text of DOI:10.1039/d0gc02341h

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DOI: 10.1039/D0GC02341H
Journal Name
ARTICLE
Ligand-Tuned Cobalt-Containing Coordination Polymers and
Applications in Water†
Received 00th January 20xx,
Accepted 00th January 20xx
Rong Tao,a,b Yike Yang, Haiyan Zhu, Xinyu Hu, and Dawei Wang*
c
b
b
a,b
DOI: 10.1039/x0xx00000x
Ligand plays a key role in modern catalysis research area and occasionally determines whether a
reaction will take place under specific condition, such as in water. In this experiment, ligands
containing indole-based diacid moiety were employed to prepare the corresponding cobalt
coordination polymer material (Co-CIA) and porous oval polymer material (Co-NCIA) tuned.
Interestingly, it was observed that Co-CIA could promote the alkylation of ketones with alcohols,
alcohols with alcohols, while Co-NCIA was effective for the synthesis of 1-benzyl-2-aryl-1H-
benzo[d]imidazoles from various phenylenediamine and benzyl alcohols through borrowing
hydrogen and dehydrogenation strategy. Other mechanism explorations, such as the deuterium
labeling experiments and the kinetic study, were conducted to better understand Co-CIA and Co-
NCIA systems, and the related transformations. Our studies provided an efficient method for the
development of high active cobalt coordination polymer materials with excellent recovery
performance for the dehydrogenation and borrowing hydrogen reactions under water and base-free
conditions.
www.rsc.org/
Keywords: cobalt; borrowing hydrogen; ligand; dehydrogenation; water
6
7
8
9
metals, for example Fe, Co, Ni, Mn, have been used in the
catalysts for hydrogenation, dehydrogenation,,
dehydrogenation cyclization and borrowing hydrogen
Introduction
1
0
During the past several years, borrowing hydrogen and reactions. As one of the typical inexpensive and earth-
dehydrogenation reactions have gained tremendous interests abundant metals, Cobalt has attracted considerable attentions
and became one of the most promising areas in organic in the past. For example, Kirchner group developed a novel
1
,2
chemistry and material science research.
noble metals, such as Rh, Ru, Ir, Au and Pd have been used in catalyst to achieve alkylations of amines in the presence of
Traditionally, cobalt complex and successfully applied it as an efficient
1
1a
the catalysts for C-C, C-N, and C-O bonds formations alcohols. In 2015, Kempe et al demonstrated another Co-
through borrowing hydrogen and dehydrogenation catalyzed alkylation reaction of amines with alcohols in
3
,4
5
pathways. In order to achieve the goal of sustainable and which the novel Co complex was stabilized by a PN P ligand
1
2a
economical chemistry, it is urgent to develop novel catalysts under mild conditions. Later, they reported Co-catalyzed C-
employing earth-abundant and inexpensive metals instead of alkylation of unactivated amides and esters with alcohols
1
2b
noble metals for these reactions, especially for large-scale which was catalyzed by PN
5
P-stabilized Co complexes.
5
productions. Recently, earth-abundant and inexpensive Zheng and Zhang described Co-catalyzed synthesis of
aromatic, aliphatic, and cyclic sec-amines through alkylation
of amines with amines by using a cobalt pincer complex
[(PNHPCy)Co(CH SiMe )][BAr ]. In 2018, Madhu and
2 3 4
a.Chemical Engineering College, Xinjiang Agricultural University, Urumqi
F
13a
8
30052, China. E-mail: wangdw@jiangnan.edu.cn
Balaraman developed a phosphine-free Co-NNN pincer
complex which showed high activity in catalyzing direct N-
alkylation of amines with alcohols through borrowing
hydrogen pathway and This novel catalyst was also employed
in the synthesis of imines through dehydrogenative coupling
b.
The Key Laboratory of Synthetic and Biological Colloids, Ministry of
Education, School of Chemical and Material Engineering, Jiangnan
University, Wuxi 214122, China.
Department of Criminal Science and Technology, Henan Police College,
Zhengzhou, China.
c.
1
4a
†
Footnotes relating to the title and/or authors should appear here.
of benzylamines with secondary alcohols.
Zhang group
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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reported direct and selective C–H alkylation of 2- Teflon stopper and heated for 72 hours at 110 °C.
View Article Online
DOI: 10.1039/D0GC02341H
2
alkylquinolines catalyzed by a reusable doped TiO -supported Subsequently, the resulting mixture was cooled to room
1
5a
nanocobalt oxides.
Sundararaju et al reported the a- temperature and the obtained solid was washed three
alkylation of ketones with alcohols employing a phosphine- times with anhydrous ethanol, centrifuged three times,
1
6
free and well-defined cobalt catalyst. Very recently, Kundu and dried to gain the material Co-CIA (1a) in 85% yield.
group reported the synthesis of a variety of methyl-substituted Using the same hydrothermal synthesis method, under
o
N-heteroarenes through alkylation of various methyl- the condition of a high temperature 140 C for 72 hours,
substituted N-heteroarene derivatives in the presence of a then material Co-NCIA (1b) was gained in 77% yield
1
7
phosphine-free and stable Co(NNN) catalyst. Quintard and from diacids (L
Rodriguez also indicated that well-defined Co is a valuable
2
)
(For details, please see SI).
O
alternative to expensive noble metals for the development of
OH
sustainable and economic catalyst and its application in
CoCl2 6H2O
Co-CIA (1a)
borrowing hydrogen reactions could lead to a great number of
Self-assembly
N
O
1
8
innovative transformations. Xu and coworkers developed
transition-metal free alkylation reactions of ketones with
alcohols to selectively afford ketone or alcohol products
under KOH or NaOH conditions, which is an excellent base
L1 OH
CoCl2 6H2O
O
N
Co-NCIA (1b)
4
k
metal catalyzed systems for borrowing hydrogen reactions.
O
1,4-bisimidazole
Self-assembly
OH
Based on the aforementioned discoveries, it is undoubtable
that the development of recyclable cobalt catalytic system for
borrowing hydrogen reactions is in urgent needs to achieve
OH
L2
Scheme 1. The synthesis of Co-CIA and Co-NCIA catalysts.
1
9
sustainable and economic chemistry goals.
Recently, we have developed several triazole-skeleted
Characterization of composite Co-CIA and Co-NCIA.
2
0
ligands
and successfully applied them in borrowing
2
1
hydrogen and dehydrogenation transformations, However,
The materials Co-CIA and Co-NCIA were both carefully
noble metals were commonly used in most of the catalysts characterized through scanning electron microscope (SEM),
and the resulting metal composites usually demonstrated poor energy dispersive X-ray (EDX), X-ray power diffraction
reactivity, stability and reusability. Furthermore, it is also (XRD), and X-ray photoelectron spectrometry (XPS).
challenging to apply these catalysts in water or solvent-free
conditions at high temperature. Herein, indole-skeleted-
diacids based cobalt coordinationed polymer material (Co-
CIA) and porous oval polymer material (Co-NCIA) were
developed. Interestingly, it was observed that Co-CIA could
realize the alkylation of ketones with alcohols, alcohols with
alcohols through borrowing hydrogen strategy, while Co-
NCIA was effective for the synthesis of 1-benzyl-2-aryl-1H-
benzo[d]imidazoles from various phenylenediamine and
benzyl alcohols through dehydrogenative cyclization reaction
in water.
Results and discussion
Synthesis of Co-CIA and Co-NCIA catalysts.
1 2
Two indole- based diacids (L ) and (L ) were prepared
Fig. 1. (a) (b): SEM images of Co-CIA; (c) (d): SEM images
of Co-NCIA.
according to the similar experimental procedures used in our
triazole acids synthesis (For details, see supporting
2
2
information). Co-CIA and Co-NCIA catalysts were
obtained by using different substituted diacids and auxiliary
ligand 1,4-bisimidazole through hydrothermal method
In Figure 1 (a) and (b) shows the SEM images of the
material Co-CIA, which indicate a regular square structure on
the surface of the coordination polymer. Fig. (a) shows the
overall morphology of the prepared material, while Fig. (b)
shows more direct and clear material map formed by the self-
assembly of the ligand and metal. Figure (c) and (d) shows
the full and partial enlarged the scanning electron
microscopes images of Co-NCIA, respectively. The Co-NICA
processes a porous and elliptical structure as shown in Fig. (c).
(
Scheme 1). For the synthesis of material 1a (Co-CIA): a
mixture of the dissolved diacid (1-(carboxymethyl)-1H-
indole-3-carboxylic acid, L , 0.1 mmol) and CoCl ·6H
0.05 mmol) in 8 mL DMF-ethanol-water (5: 2: 1, v/v/v)
1
2
2
O
(
was stirred for 30 minutes at room temperature. Then, the
mixture was placed in a 15 mL autoclave lined with a
2
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Furthermore, this porous and elliptical structure is composed at 2θ = 32°, which further proves the successfuVliepwrAerpticaleraOtniloinne
of sheet-like agglomerate as shown in Fig (d) From the SEM of the material (Co-NCIA). The differeDnOtI: l1i0g.1a0n3d9s/Da0pGpCl0ie23d41iHn
images, it can be seen that the morphology of these two these two materials showed different polymer states and
materials differs greatly from one to the other due to the different positions of characteristic peaks, resulting in distinct
change of ligands which enables controllable material catalytic activities.
synthesis.
Fig. 2. (a) EDX pattern of Co-CIA; (b) EDX pattern of Co-NCIA.
In Figure 2, images of (a) and (b) shows the EDX pattern
of Co-CIA and Co-NCIA, respectively. The spatial
distribution of different elements was studied through
elemental analysis. From Fig. (a), it was observed that C, O
and Co elements of Co-CIA are evenly distributed in the
material from Fig. (a). The characteristic peaks of Co were
found at 1.0 keV and 1.5 keV in EDS spectrum, which further
proved the existence of cobalt in the prepared material. The
distribution of C, N, O and Co elements in the material Co-
NCIA could be determined from figure (b). The characteristic
peaks of Co were found at 0.7 keV and 1.7 keV in EDS
spectrum, which proved the existence of Co-NCIA. EDS
images revealed that the element assignments of these two
materials are different, making them two kinds of materials.
Fig. 4. (a) XPS spectrum of Co-CIA
spectrum, (c) high-resolution C 1s spectrum, (d) high-resolution O
s.
.
, (b) high-resolution Co 2p
1
Fig. 3. (a) XRD spectrum of Co-CIA; (b) XRD spectrum of Co-
NCIA.
Fig. 5. (a) XPS spectrum of Co-NCIA, (b) high-resolution Co 2p
spectrum, (c) high-resolution C 1s spectrum, (d) high-resolution O
1
s.
X-ray diffraction (XRD) was performed to determine the
position at which the metal degree, thereby, appeared to prove
the crystallinity of the materials (Fig. 3). The XRD pattern (a)
shows two diffraction peaks of cobalt at 2θ = 44.2° and 51.5°,
The chemical state and element composition on the surface
of Co-CIA and Co-NCIA coordination polymer materials
were also tested by XPS (Figs. 4-5). The full spectrum of Co-
CIA and Co-NCIA showed that the materials are composed of
C, N, O and Co, respectively. Fig. 4b and Fig. 5b revealed the
presence of Co 2P3/2 and Co 2P1/2 peaks. The binding energies
of Co-CIA were 781.06, 785.68, 796.74 and 802.25 eV, while
binding energies of Co-NCIA were 781.32, 785.64, 797.20
respectively. The diffraction peak of C in the ligand (L
also shown in Figure 3(a). Obviously, the diffraction peak at
θ = 10° of the XRD pattern (a) is the characteristic peak of
the ligand (L ). The results were shown in the XRD pattern
a) indicated the existence form of metal Co in Co-CIA.
1
) is
2
1
(
Figure (b) shows a more distinct characteristic peak of Cobalt
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and 802.13 eV, respectively. The binding energies of Co-CIA
at 785.68 and 802.25 eV were attributed to metal Co, while
the binding energies of 781.06 and 796.74 eV were attributed
View Article Online
DOI: 10.1039/D0GC02341H
1
1
5
6
-
AgNTf2 /KF
TBAB
-
<5
Co-CIA
-
-
-
38
2
+
to Co , Similar results were observed for Co-NCIA. Four
major peaks were seen from C 1s spectrum (Fig 4c), which
are assigned to C=C (284.7 eV), C-N (285.8 eV), C=O (287.8
eV), and O-C=O (288.7eV). The O 1s spectrum (Fig. 4d)
showed two peaks, which are attributed to C=O (530.7eV), C-
OH (532.1eV). The surface element composition of Co-CIA
and Co-NCIA proved the successful synthesis of cobalt-
containing coordination polymers synthesis.
17
TBAB
<5
86d
1
8
Co-NCIA AgNTf2 /KF
>20:1
a
Conditions: 2a (1.0 mmol), 3a (1.2 mmol), cat.(10 mg), additive
o
b
c
(
2
1.0 mmol), H O (3 mL), 16 h, 90 C. Isolated yields. TBAF (0.2
d
e
mmol). TBAB (0.2 mmol). TBAI (0.2 mmol).
In the optimization of conditions, AgNTf
2
is an auxiliary
agent, which converts its coordination complex (Co-CIA)
from neutral to cationic form, and the reaction is much
smooth. Through screening, it was found that the combined
Catalytic activity
additive of AgNTf
2
and KF can achieve a higher yield.
With Co-CIA and Co-NCIA in hand, the classic borrowing
After optimizing the reaction conditions, the substrate
hydrogen reaction of ketone with benzyl alcohol was selected scope of borrowing hydrogen reaction between ketones and
as a model reaction to examine catalytic activities of these benzyl alcohols were investigated and the results were shown
two materials. The reaction of acetophenone and benzyl in Table 2. From this study, it was found showed that the
alcohol was attempted with Co-CIA as a catalyst in water. As substrate with an electron-withdrawing group is less reactive
can be seen from Table 1, it was found that the reaction could than that with electron-donating group. For example,
take place in water without extra base, however, an additive substrate
was needed to promote the reaction. After screening a series methoxyphenyl)ethan-1-one gave the desired products in 84%
of conditions, it was found that the combination of AgNTf
and 87% yields, respectively. However, the substrates with
1-(p-tolyl)ethan-1-one
and
1-(4-
2
and KF as additives produced a better result. Further halogen group resulted in slightly lower yields, such as 4l, 4q,
conditions screening revealed that phase transfer catalyst 4r. Generally, the catalyst Co-CIA demonstrated good
TBAB greatly promoted this reaction and the reaction yield selectivity for cross-coupling reaction of ketones with
reached 93%.. Meanwhile, it was found that Co-NCIA could alcohols though moderate yields were commonly seen.
also promote this reaction with a slightly lower yield.
Therefore, the optimal reaction condition was established to
Table 2. Substrate expansion of borrowing hydrogen
have AgNTf
2
and KF as additives, Co-CIA/TBAB as catalyst
a,b
reaction.
o
and stirring in water at 90 C for 16h, which realizes the goal
of sustainable development in green chemistry.
O
O
OH
Co-CIA, AgNTf2
KF, TBAB, 90 oC, H2O
R1
R2
R1
R2
Table 1. Optimization of reaction conditions.a,b
2
3
16 h
4
4
a, R1 = H, 93%
4b, R1 = 4-Me, 84%
c, R1 = 4-OMe, 87%
4d, R1 = 3-Me, 76%
e, R1 = 3-OMe, 85%
O
O
4f, R1 = 2-Me, 71%
4g, R = 2-Br, 78%
R2 4h, R2 = 4-OMe, 88%
4i, R2 = 4-Me, 84%
O
2
4
O
O
R1
OH
[
Co], additives
+
4
Ph
Ph
+
Ph
Ph
H2O
O
O
O
2a
3a
4a
5a
O
O
O
Cl
entry
cat
-
additives
ratio(4a/5a) yield(4a)[%]
4j, 90%
4
k, 86%
4l, 74%
O
O
O
1
2
3
5
6
7
8
9
-
-
-
<5
23
<5
11
<5
27
31
48
54
73
90c
Co-CIA
Co-CIA
Co-CIA
Co-CIA
Co-CIA
Co-CIA
Co-CIA
Co-CIA
Co-CIA
Co-CIA
Co-CIA
Co-CIA
-
O
O
O
4m, 76%
4n, 83%
4o, 81%
LiCl
KF
-
-
O
O
O
O
O
Cl
Br
I
2
-
Br
O
Br
4q, 82%
4r, 78%
4p, 79%
AgOTf
AgBF
AgSbF
AgNTf
-
O
O
O
O
4
-
O
Cl
O
4s, 83%
4t, 80%
4u, 79%
6
13:1
>20:1
>20:1
>20:1
>20:1
>20:1
a
Reagents and conditions: 2 (1.0 mmol), 3 (1.2 mmol), Co-CIA (10
2
mg), AgNTf (1.0 mmol), KF (1.0 mmol), TBAB (0.2 mmol), H O
3 mL), 16 h. Yields of isolated product.
1
1
1
1
1
0
1
2
3
4
2
2
AgNTf2 /KF
AgNTf2 /KF
AgNTf2 /KF
AgNTf2 /KF
b
(
93d
Next, alkylation between alcohols was studied to further
evaluate the activities of Co-CIA and Co-NCIA catalysts..
The results showed that most of the alcohol substrates reacted
81e
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smoothly and the corresponding substituted ketones were
obtained in moderate to good yields with Co-CIA as catalyst
View Article Online
DOI: 10.1039/D0GC02341H
2
R
NH2
NH2
(Table 3). When substrate with para-methoxy group was
OH
Co-NCIA, AgNTf2
R2
R1
+
N
N
KF, TBAB, 90 oC, R1
employed in this reaction, the highest yield of the desired
product was 90%. It should be noted that only moderate yield
was achieved when Co-NCIA was used as catalyst in this
reaction.
R2
H2O, 36 h
7
2
8
Cl
Cl
Cl
N
N
Table 3. Substrate expansion.a,b
N
N
N
N
OH
O
Cl
8
a, 80%, <5%c
8b, 84%
8
c, 77%
OH
Co-CIA, AgNTf2
KF, TBAB, 90 oC, H2O
R1
+
R2
R1
R2
Cl
Me
MeO
6
3
24 h
4
O
O
O
N
N
N
N
N
N
OMe
Me
Cl
MeO
4a, 86%
4b, 76 %
4c, 76 %
8d, 83%
8
e, 79%
8
f, 85%
O
O
O
F
Me
F
Br
O
N
N
N
N
4
d, 86%
4g, 86%
4h, 83%
N
N
Me
F
O
O
O
F
8g, 81%
8h, 75%
8i, 73%
O
O
4
i, 74 %
O
4j, 90%
4
k, 86%
Br
F
O
O
O
N
N
N
N
N
Br
F
O
O
Br
N
4n, 83%
4o, 81%
4r, 78%
8j, 86%
8k, 86%
8l, 83%
a
Reagents and conditions: 6 (1.0 mmol), 3 (1.2 mmol), Co-CIA (10
2
mg), AgNTf (1.0 mmol), KF (1.0 mmol), TBAB (0.2 mmol), H O
a
Reagents and conditions: 7 (1.0 mmol), 2 (3.0 mmol), Co-NCIA
2
(10 mg), AgNTf
2
(1.0 mmol), KF (1.0 mmol), TBAB (0.2 mmol),
b
(3 mL), 24 h. Yields of isolated product.
O (3 mL), 36 h. b Yields of isolated product. Conditions: NaOH
c
H
2
o
(1 mmol), toluene, 110 C, 24 h.
1
H-benzo[d]imidazole derivatives were known as valuable
building blocks due to their antidiabetic, antiviral,
Electrocatalytic water decomposition is known as be an
2
3
antimalarial, antibiotic and inhibitory activities. For example,
our group recently found that 2-aryl-1H-benzo[d]imidazoles
and 1-benzyl-2-aryl-1H-benzo[d] imidazoles display certain
fungicidal activities against Rhizoctonia solani, Fusaium
graminearum, Penicillium digitatum, Magnaporthe grisea
efficient and simple hydrogen production method, which is
widely used for hydrogen production.. The efficiency of water
decomposition is often determined by oxygen evolution. This
is mainly because the oxygen evolution reaction occurs
undergo
a 4e transformation process, which usually
2
4
and Penicillium italicum. Research on the synthesis of 1H-
benzo[d]imidazoles has attracted considerable attentions
during the past four years. However, synthesis of these
derivatives under water or solvent-free conditions is still
accompanied by the cleavage of the O-H bond and the
formation of the O-O bond. Therefore, the oxygen evolution
reaction requires a larger overpotential. IrO and RuO are
2 2
generally recognized as the most efficient catalysts for
oxygen evolution reaction, however, these noble metals are
relatively expensive. Therefore, designing and developing an
efficient and inexpensive oxygen evolution catalyst is in
urgent need to further improve the oxygen evolution reaction.
To better understand the feasibility and advantages of Co-CIA
and Co-NCIA catalysts in oxygen evolution reaction, the
electrochemistry properties of these two catalysts were
investigated and the results were concluded in Fig. 5.
2
5,26
challenging. . Based on our previous synthesis of 1-benzyl-
2
4
2
-aryl-1H-benzo[d] imidazole derivatives,
herein, we
employed the Co-CIA and Co-NCIA catalysts in the base-free
reactions of phenylenediamines and substituted benzyl
alcohols in water to achieve 1-benzyl-2-aryl-1H-
benzo[d]imidazole derivatives. To our delight, the Co-NCIA
was found to be an effective catalyst in this reaction leading
to high product yields (Table 4). It should be noted that this
reaction couldn’t happen without this catalyst under strong
base conditions. However, the Co-CIA was inactive in
The electrocatalytic activities of Co-CIA and Co-NCIA for
promoting this transformation, which indicated that the ligand water oxidation reaction were studied in alkaline solution (1.0
impact on the activity of the catalysts is dominant.
M, KOH) using linear sweep voltammetry (LSV) with
standard three-electrode system in which rotating disk
electrode (RDE) acted as a working electrode. The detailed
procedure of electrochemical experiments and calculations
Table 4. Substrate expansion.a,b
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were presented in the supporting information. Figure 5a Fig.5. (a) LSV curves of IrO
showed the linear sweep voltammetry (LSV) polarization in electrolyte solution of 1M KOH. (b) TDhOeI:c1o0r.1r0es3p9o/Dn0dGinCg02T3a4f1eHl
curves of Co-CIA, Co-NCIA, IrO and CoCl2 ` 6H
O catalysts fitting plots of IrO , Co-NCIA and Co-CIA.
2 2 2
View Article Online
, CoCl ·6H O, Co-NCIA and Co-CIA
2
2
2
with a scan rate of 5 mV/s. Furthermore,, the LSV curve for
these catalysts revealed that the overpotential of 320, 309,
As indicated above, the Co-CIA and Co-NCIA could be
utilized for efficient electrocatalytic oxygen evolution
3
03, and 334 mV was required to produce an anodic current
reactions. Compared with IrO
very similar potentials and Tafel slopes compared to the
precious metal IrO . Meanwhile, it was observed that Co-
2
, Co-CIA and Co-NCIA have
2
density of 10 mA/cm , respectively.
2
The reaction kinetics of water oxidation catalyzed by Co-
CIA and Co-NCIA were determined by Tafel plot which
correlates the catalytic current density with overpotential. In
the alkaline solution (1.0 M, KOH), the Tafel plot exhibited
linear behavior within a broad range of potential values. The
slope of the Tafel plot for Co-CIA and Co-NCIA was found
NCIA showed better catalytic oxidation performance than
Co-CIA, which provided another clue to explain the different
catalytic activities of these two catalysts found in the
dehydrogenation and borrowing hydrogen transformations.
Based on the literature reports, the driving force of catalytic
reaction was caused by the morphology, surface functional
groups, active sites, and/or surface structures of a catalytic
-1
-1
to be 74.8 mV dec and 69.1 mV dec respectively (Figure
b), indicating the high efficiency of these two catalysts.
5
Meanwhile, these results were comparable to the values of material.27 The morphology of catalytic the cobalt
some Co-based water oxidation catalysts reported recently. coordination polymers Co-CIA and Co-NCIA with two
Furthermore, the Tafel plot of the costly water oxidation different organic ligands lead to different catalytic active
catalyst IrO
2
was also studied in the alkaline solution (1.0 M, sites. The indole-skeletoned diacid ligand is acidic and cobalt
was found to ions could easy diffuse in the solution area near the surface of
be 69.1 mV dec . Therefore, the Tafel plot studies show that the ligand, and some cobalt ions are adsorbed on the surface.
KOH) for comparison. The Tafel slope for IrO
2
-1
Cobalt acts as a nucleation site and reacts to the ligand, with
the reaction progresses, if the relative amounts of indole
skeleton diacids n(L)/n(M) and c(L) increase, they could be
transformed into a hollow structure (Co-CIA and Co-NCIA
with different shells), such as (Fig. 1). Since there is no
further proof (the single crystal structure), X-ray diffraction
the catalyst Co-CIA and Co-NCIA could serve as an efficient
electrocatalyst for the water oxidation reaction.
(
XRD) and XPS tests were performed on it, which initially
2
+
proved that the catalytic active center is metal Co , which
was consistent with the literatures.
2
7
To further explore Co-CIA and Co-NCIA, several control
experiments and mechanism studies were carried out in water
under base-free conditions. Catalytic experiments revealed
that Co-CIA could catalyze borrowing hydrogen of ketones
with alcohols, alcohols with alcohols, while Co-NCIA was
effective for the synthesis of 1-benzyl-2-aryl-1H-
benzo[d]imidazoles from various phenylenediamine and
benzyl alcohols. It was noted that cobalt slats, such as CoCl
2
,
CoBr , failed to these two reactions in water under base-free
2
conditions. The intermolecular competitive reaction in the
basic steps was involved in kinetics, aiming at explaining the
decision-making steps at a competitive rate. Kinetic isotope
H D
effect values (KIE: K /K = 2.01) was obtained by the first
order reaction plots of ln [3a-d2] and ln [3a] (Scheme 2). The
results indicated that dehydrogenation of benzyl alcohol (3a)
is the rate-determining step during this borrowing hydrogen
transformation.
O H/D
O
D
D
D/H
Co-NCIA, AgNTf2
KF, TBAB
OH
+
H2O, 90 o
C
2a
3a-d2
4a-d2
6
| J. Name., 2013, 00, 1-9
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benzyl alcohol was also examined and similar results were
View Article Online
DOI: 10.1039/D0GC02341H
seen
O
O
Co-CIA, AgNTf2
+
Ph
OH
Ph
KF, TBAB, 90 oC, H2O
Ph
Ph
4a
2
a
3a
0
Recycled times
Yields
1
2
3
4
5
93%
92%
92%
90%
89%
87%
OH
O
Co-CIA, AgNTf2
+
Ph
OH
Ph
Ph
Ph
KF, TBAB, 90 oC,
H O
2
4a
6a
3a
0
Recycled times
Yields
1
2
3
4
5
86%
84%
84%
83%
82%
81%
Scheme 2. Kinetic plot of acetophenone and benzyl alcohol.
: original concentration of substrate. A : concentration of
substrate at time t. k: rate constant.
NH2
NH2
N
A
0
t
Co-NCIA, AgNTf2
Ph
Ph
+
Ph
OH
N
KF, TBAB, 90 oC, H O
2
2a
7a
8a
In addition, the kinetic studies were conducted and the
possible reaction plots were achieved to better understand
these cobalt catalyzed reactions (Scheme 3). The results
showed that the formation of benzaldehyde intermediate was
the key of this reaction, which also proved the borrowing
hydrogen process during this coupling reaction.
Recycled times
Yields
0
1
2
3
4
5
80%
78%
77%
76%
74%
73%
Scheme 4. Recycled experiments.
O
O
Co-CIA, AgNTf2
KF, TBAB
OH
During this catalytic system, it was noted that the additives
were employed, such as, silver reagent, potassium fluoride
and TBAB. Therefore, it was necessary to investigate the role
of these additives. It was reported many transformations
involving silver salts are actually belonged to silver-assisted
+
H2O, 90 o
C
2a
3a
4a
2
8
metal catalysis or bimetallic catalysis. Silver nanoparticle
or silver halide) play a key role in some cases. Here, we also
conducted several reactions to explore the effects of additives
Table 5). It was observed that silver salt couldn’t catalyze
(
(
this reaction. After removing silver halide through celite,
almost the same yield was obtained, which indicated the
catalytic activity of cobalt catalyst was enhanced by non-
coordinating anions and it was observed that silver slat does
not play a role during the synthesis of 1-benzyl-2-aryl-1H-
benzo[d]imidazoles, which was consistent with the relevant
2
9
literatures. .
Table 5. The additives experiments.a, b
Scheme 3. Kinetic investigations for the synthesis of 4a.
NH2
N
N
Co/additives
90 oC, H2O
Ph
Ph
+
Ph
OH
Reusability of the catalysts
NH2
2a
7a
8a
Furthermore, the reusability of the Co-CIA was
examined under water condition. The used catalyst was
washed with ethanol and deionized water, centrifuged for
four times, and dried. The catalyst was recycled for five
times. At each time, the recovery experiment for the
alkylation of an alcohol with another alcohol was
surveyed in water. It was observed that the yield of the
desired product decreased slightly as the increasing of recycle
time (Scheme 4). At the same time, the reusability of Co-
NCIA catalyst for the reaction of phenylenediamine with
Conditionsc
Entry
Catalyst
Yield (8a) [%]
1
2
3
4
5
6
7
Co-NCIA
-
-
-
-
-
16
<5
<5
<5
<5
47
62
-
AgBr
AgNTf
TBAB
Co-NCIA/AgNTf
2
2
Co-NCIA/ AgNTf
2
/KF
-
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-9 | 7
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Journal Name
explored to better display these two cobalt catalysts. It
was found that Co-CIA and Co-NCIA could be utilized
for efficient electrocatalytic oxygen evolution reactions
and Co-CIA and Co-NCIA have very similar potentials
2
and Tafel slopes compared to the noble metal IrO .
View Article Online
DOI: 10.1039/D0GC02341H
8
9
Co-NCIA/ AgNTf
Co-NCIA/ AgNTf
2
2
/KF
/KF
No filtration
After filtration
-
80
76
<5
1
0
2
AgNTf /KF/TBAB
a
Conditions: 7a (1.0 mmol), 2a (3.0 mmol), cat.(10 mg), additive
1.0 mmol), H O (3 mL), 16 h, 90 C. Isolated yields. The silver
2
o
b
c
(
sediment was filtrated through celite.
Acknowledgements
We gratefully acknowledge financial support of this
work by the National Natural Science Foundation of
China (21776111, 21861039) and the Fundamental
Research Funds for the Central Universities (JUSRP
Finally, the summary of the synthesis of 1-benzyl-2-aryl--
H-benzo[d]imidazole was summarized in Scheme 5. The
1
results indicated that Co-NCIA catalytic system shows high
atomic efficiency, which enables effective green
transformation under water and base-free conditions in terms
51627B).
2
4-26
of recovery.
In general, this cobalt catalytic system
provided an efficient method for the reactions of ketones with Notes and references
alcohols, alcohols with alcohols and the synthesis of 1-
1
. Recent reviews: (a) L. Alig, M. Fritz and S. Schneider,
benzyl-2-aryl-1H-benzo[d]imidazoles
from
various
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NH2
OH
catalyst
conditions
+
N
N
NH2
7
2
8a
Catalyst
TAP-Cu
Solvent
Toluene
Conditions and yields
Cycles Reference
t-BuOK, 120 oC, 70%
-
-
-
-
-
24a
25a
25e
25f
2
t-BuOK, N , 80 oC, 92%
[
(Cp*)Ir(LSe-S)*Cl]/A Toluene
2
Mn(CO)5Br
Toluene
CH3CN
t-BuOK, N2, 130 oC, 67%
Pd(OAc)2/Xantphos
Ni-Cat./L
80 oC, 48 h, 65%
t-BuOK,140 oC, 24 h, 91%
K CO , 110 oC, 90%
Toluene
Toluene
25g
25h
26a
Zr-IDA
5
-
2
3
alcohol excess t-BuOK, N2, 140 oC, 79%
Mn-NNS
90 oC, 40%
base-free, 90 oC, 80%
Pd-HAP
Toluene
-
26b
Co-NCIA
H2O
5
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cobalt coordination polymers Co-CIA and Co-NCIA
using different substituted diacids and auxiliary ligand
1
,4-bisimidazole.
These
materials
were
fully
characterized by SEM, EDS, XRD, XPS and TG.
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1
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