A ru-complex tethered to a N-rich covalent triazine framework for tandem aerobic oxidation-knoevenagel condensation reactions

Article abstract of DOI:10.3390/molecules26040838

Herein, a highly N-rich covalent triazine framework (CTF) is applied as support for a RuIII complex. The bipyridine sites within the CTF provide excellent anchoring points for the [Ru(acac)2(CH3CN)2]PF6 complex. The obtained robust RuIII@bipy-CTF material was applied for the selective tandem aerobic oxidation-Knoevenagel condensation reaction. The presented system shows a high catalytic performance (>80% conversion of alcohols to α, β-unsaturated nitriles) without the use of expensive noble metals. The bipy-CTF not only acts as the catalyst support but also provides the active sites for both aerobic oxidation and Knoevenagel condensation reactions. This work highlights a new perspective for the development of highly efficient and robust heterogeneous catalysts applying CTFs for cascade catalysis.

Full text of DOI:10.3390/molecules26040838

molecules
Article
A Ru-Complex Tethered to a N-Rich Covalent Triazine
Framework for Tandem Aerobic Oxidation-Knoevenagel
Condensation Reactions
Geert Watson 1,†, Parviz Gohari Derakhshandeh 1,†, Sara Abednatanzi 1,*, Johannes Schmidt 2, Karen Leus 1 and
Pascal Van Der Voort 1,
*
1
2
Department of Chemistry, Center for Ordered Materials, Organometallics and Catalysis (COMOC), Ghent
University, Krijgslaan 281, Building S3 (Campus Sterre), 9000 Gent, Belgium; geert.watson@ugent.be (G.W.);
parviz.gohariderakhshandeh@ugent.be (P.G.D.); Karen.Leus@ugent.be (K.L.)
Institut für Chemie-Funktionsmaterialien, Technische Universität Berlin, Hardenbergstraße 40,
10623 Berlin, Germany; johannes.schmidt@tu-berlin.de
*
†
Correspondence: sara.abednatanzi@ugent.be (S.A.); pascal.vandervoort@ugent.be (P.V.D.V.)
These authors contributed equally to this work.
Abstract: Herein, a highly N-rich covalent triazine framework (CTF) is applied as support for a
RuIII complex. The bipyridine sites within the CTF provide excellent anchoring points for the
[Ru(acac)2(CH3CN)2]PF6 complex. The obtained robust RuIII@bipy-CTF material was applied for the
selective tandem aerobic oxidation-Knoevenagel condensation reaction. The presented system shows
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
a high catalytic performance (>80% conversion of alcohols to α, β-unsaturated nitriles) without
ꢀꢁꢂꢃꢄꢅꢆ
the use of expensive noble metals. The bipy-CTF not only acts as the catalyst support but also
provides the active sites for both aerobic oxidation and Knoevenagel condensation reactions. This
work highlights a new perspective for the development of highly efficient and robust heterogeneous
catalysts applying CTFs for cascade catalysis.
Citation: Watson, G.; Gohari
Derakhshandeh, P.; Abednatanzi, S.;
Schmidt, J.; Leus, K.; Van Der Voort, P.
A Ru-Complex Tethered to a
N-Rich Covalent Triazine
Keywords: covalent triazine frameworks; heterogeneous catalysis; tandem catalysis; aerobic oxidation-
knoevenagel condensation
Framework for Tandem Aerobic
Oxidation-Knoevenagel
Condensation Reactions. Molecules
2021, 26, 838. https://doi.org/
10.3390/molecules26040838
1. Introduction
Academic Editors: Igor Djerdj and
Sergio Navalo
Conventional porous materials including silica, zeolite and activated charcoal have
attracted extensive interest in large-scale industrial applications, most importantly in
heterogeneous catalysis [1–3]. However, the poor chemical versatility of their chemical
Received: 23 December 2020
Accepted: 2 February 2021
Published: 5 February 2021
structure has increased the need for alternative porous materials with tailorable properties.
In recent years, the focus in heterogeneous catalysis lies in the development of novel
and efficient porous supports with tailor-made functionalities rather than prefabricated
materials for targeted liquid phase reactions [
particularly metal–organic frameworks (MOFs) and covalent organic frameworks (COFs)
have led to excellent progress in the field [ ]. These materials possess a large surface area
with regular and accessible pores, and an adjustable skeleton, making them attractive
for several purposes of interest [ ]. In contrast to MOFs fabricated from inorganic nodes
(metal ions/clusters), COFs are purely organic materials and constructed from covalently
linking light atoms (C, O, N, P, B, Si) [ ]. Covalent Triazine Frameworks (CTFs) are a class
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
published maps and institutional affil-
iations.
4,5]. The recently emerging porous materials,
6
7
8,9
Copyright:
© 2021 by the authors.
of COFs discovered in 2008 by Thomas and Antonietti [10]. CTFs are formed through a
trimerization reaction followed by the subsequent oligomerization of aromatic nitriles [11].
The robust aromatic covalent bonds endow CTFs with excellent stability compared to
coordinative-linked porous materials [12]. Additionally, CTFs contain a high amount of
nitrogen functionalities in their networks, allowing them to be outstanding candidates as
supports for various catalytic active centers [13].
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
Molecules 2021, 26, 838. https://doi.org/10.3390/molecules26040838
https://www.mdpi.com/journal/molecules

Molecules 2021, 26, 838
2 of 12
Among a wide range of catalytic processes, tandem catalysis has attracted increasing
research attention [14]. In tandem catalysis, several consecutive catalytic reactions occur
consecutively in one reaction vessel, using only one multifunctional catalyst. Therefore,
there is no need for the separation, purification, and transfer of intermediates produced
in each step. Tandem catalysis significantly reduces the amount of waste and minimizes
the use of harmful solvents [15]. Great efforts have been made to design heterogeneous
catalysts for tandem reactions through the immobilization of metal complexes and nanopar-
ticles on the surface of various porous supports [14,16]. α, β-unsaturated nitriles are key
intermediates for the synthesis of pharmaceuticals and fine chemicals [17]. These inter-
mediates are generally produced through the Knoevenagel condensation of aldehydes
or ketones with nitriles catalyzed by common bases [18]. However, the catalytic process
suffers from limited substrate scope due to the high price or unavailability of some alde-
hydes [18]. In this regard, the development of highly efficient multifunctional catalysts
to prepare α, β-unsaturated nitriles through the tandem aerobic oxidation-Knoevenagel
condensation reaction significantly boosts the synthesis efficiency.
Highly efficient oxidation catalysts for the selective conversion of alcohols to aldehydes
are a key step in designing an appropriate heterogeneous catalyst for the tandem oxidation-
Knoevenagel condensation reaction. Traditional oxidation processes employ stoichiometric
amounts of sometimes toxic and expensive inorganic oxidants, mainly iodosylbenzene,
sodium hypochlorite and chromium trioxide [19–22].
Many papers have appeared on the design of various homogeneous and heteroge-
neous catalysts containing noble metals, such as Au, Pd, Pt and Ir, for the selective aerobic
oxidation of alcohols [23–26]. Ru catalysts are economically attractive in comparison to
other noble-metal catalysts, which are rather expensive. RuIII complexes are well docu-
mented as efficient oxidation catalysts for various substrates, such as alcohols, aldehydes
and sulfides. Nevertheless, the majority of studies using ruthenium, either homo- or het-
erogeneously, utilize non-green oxidants (3-dichloroiodanyl-benzoic acid, periodic acid
and iodosylbenzene) [19,27,28].
Recently, our group reported on the immobilization of a RuIII complex onto a periodic
mesoporous organosilica (PMO) [29]. Although the catalyst was highly active for the
selective oxidation of alcohols using periodic acid, no activity was observed using oxygen
as the green oxidant. To date, only a limited amount of studies have been reported on the
application of RuIII-based catalysts in the aerobic oxidation of alcohols [30
–
32]. Moreover,
many of these catalysts exhibit fundamental drawbacks as high catalyst loadings (5 mol%
[Ru]) or a large excess of oxygen (20 atm) is required as the oxidant [33 34]. Thus, the
,
development of greener and more atom-efficient methods that adopt recyclable catalysts
and molecular oxygen as the sole oxidant is a great alternative to the existing systems.
We introduce here an efficient catalytic system for the tandem aerobic oxidation-
Knoevenagel condensation reaction. A highly N-rich CTF containing bipyridine (bipy)
building blocks (bipy-CTF) is used as the catalyst support. The bipy building units provide
excellent docking sites for immobilization of a RuIII complex, initially examined in the
selective aerobic oxidation of alcohols to aldehydes. The bipy-CTF material not only acts
as an anchoring point but also promotes the sequential reaction of aldehydes and nitriles
due to the presence of N-rich basic functionalities. Our results indicate that the synergistic
effects between the N-rich bipy-CTF and the RuIII complex are beneficial to obtaining a
highly active and selective catalyst for tandem catalysis in the absence of any co-oxidant.
2. Results and Discussion
2.1. Synthesis and Characterization of the Modified Bipy-CTF with the Ru Complex
(RuIII@bipy-CTF)
0
0
0
We targeted a CTF with free 2,2 -bipyridine building blocks (5,5 -dicyano-2,2 -bipyridine),
which forms excellent anchoring points. The bipy-CTF material was synthesized following
the typical reported ionothermal procedure [35]. After the synthesis, the remaining ZnCl2
is removed by extensive washing with water, followed by refluxing at 120 ◦C in 1 M HCl.

Molecules 2021, 26, 838
3 of 12
The obtained bipy-CTF material was post-modified with the (Ru(acac)2(CH3CN)2)PF6
complex through a simple wet impregnation method, as depicted in Figure 1.
Figure 1. Schematic representation of the ideal ordered structure of RuIII@bipy-covalent triazine framework (CTF) material.
In the diffuse reflectance infrared Fourier transform (DRIFT) spectrum of the bipy-
CTF (Figure 2a), the characteristic bands of the triazine fragment appear at 1356 and
1521 cm−1. The absence of the intense nitrile band at around 2330 cm−1 demonstrates the
complete consumption of monomer and formation of triazine linkages. The doublet band
at around 1602–1626 cm−1 is ascribed to the C=N vibrations of the bipy moiety. In the
DRIFT spectrum of the RuIII@bipy-CTF material, the vibration bands of the bipy moiety
are shifted (~10 cm−1) to a lower frequency, which may be due to coordination with the Ru
complex. A similar observation is reported in previous studies [36].
The pristine bipy-CTF material displays a rapid uptake of N2 at low relative pressures
which is indicative of a highly microporous material (Figure 2b). This profile is assigned to
−1
a type I isotherm and exhibits a Brunauer–Emmett–Teller (BET) surface area of 787 m2 g
.
The total pore volume was found to be 0.40 cm3 g−1 at P/P0 = 0.99. After the introduction
of the Ru complex, the BET surface area decreases moderately to 556 m2 g−1, indicating
that most of the pores are still accessible.
The powder X-ray diffraction (PXRD) patterns of the pristine bipy-CTF and RuIII@bipy-
CTF are shown in Figure 2c. As known from most of the CTFs that were prepared ionother-
mally, the bipy-CTF materials were found to be predominantly amorphous. The broad
◦
peaks at 2
θ
~13 and 25 are assigned to the 00l reflection showing a “graphitic” layer stack-
ing. It is important to note that the exact structure of these amorphous materials cannot be
determined since the harsh synthesis conditions result in carbonization and blackening
of the material, making it difficult to fully characterize. To estimate the carbonization
degree and purity of the materials, we applied elemental analysis (Table 1). The CHN data
obtained from the bipy-CTF material reveal a C/N ratio of 2.9 and the theoretical value
for C/N in the bipy-CTF sample is calculated to be 2.6. Therefore, partial carbonization of
around 10% occurs, while 90% of the structural composition is preserved.
Table 1. Elemental analysis of the pristine and modified CTF materials.
Ru b (mmol g
)
−1
Sample
C a (wt.%)
58.92
N a (wt.%)
20.27
C/N
2.9
bipy-CTF
RuIII@bipy-
CTF
-
59.6
15.7
3.8
0.15
a
b
Determined by elemental analysis. Determined by ICP-OES analysis.

Molecules 2021, 26, 838
4 of 12
The thermal stability of both materials was determined by thermogravimetric analysis
(TGA). The TGA profile of the bipy-CTF displayed a high thermal stability up to approxi-
mately 550 ◦C (Figure 2d). A first weight loss of about 8% below 150 ◦C corresponds to
the loss of water and organic solvent molecules. The TGA profile of the modified sample
shows that the RuIII@bipy-CTF material is thermally stable up to 300 ◦C, and gradually
decomposes at higher temperatures.
Figure 2. Structural characterization of bipy-CTF and RuIII@bipy-CTF materials. (
transform (DRIFT) spectra. ( ) Nitrogen adsorption/desorption isotherms. ( ) Powder X-ray diffraction (XRD) patterns.
(d) Thermogravimetric analysis (TGA) curves.
a) Diffuse reflectance infrared Fourier
b
c
Further structural characterization was done by applying X-ray photoelectron spec-
troscopy (XPS). In the N 1S spectrum of the bipy-CTF material (Figure 3a), a peak at
398.39 eV confirms the existence of the pyridinic nitrogen in the framework. Besides, the
peaks at 399.58 and 400.38 eV are attributed to the pyrrolic-and quaternary-N species,
respectively. These nitrogen functionalities are formed during the synthesis at high tem-
peratures, as reported by Osadchii et al. [37]. In the N 1s spectrum of the RuIII@bipy-CTF
material (Figure 3b), a peak shift towards a higher binding energy is observed for the
pyridinic N species which overlaps with the peak of pyrrolic-N sites. Such a N 1s shift
towards higher binding energies can be attributed to the slight transfer of electrons to the
immobilized Ru complexes [38]. The Ru 3p peaks for the RuIII@bipy-CTF are located at
around 463 and 485 eV, which corresponds to Ru in the (+3) oxidation state (Figure 3c).
Moreover, the Ru 3d peak is seen at 285 eV (Figure 3d). Based on the inductively coupled
plasma (ICP) analysis, the loading of Ru in the modified material is 0.15 mmol g−1, and
around 3% of the total bipyridine sites are coordinated to the Ru complex.

Molecules 2021, 26, 838
5 of 12
Figure 3. Structural characterization of bipy-CTF and RuIII@bipy-CTF materials. (
N 1s XPS spectrum of the RuIII@bipy-CTF. ( ) Ru 3p XPS spectrum of the RuIII@bipy-CTF. (
RuIII@bipy-CTF.
a
) N 1S XPS spectrum of the bipy-CTF. (
b)
c
d) Ru 3d XPS spectrum of the
2.2. Catalytic Activity of the RuIII@bipy-CTF Catalyst in the Tandem Aerobic
oxidation-Knoevenagel Condensation Reaction
The catalytic activity of heterogeneous Ru catalysts for oxidation reactions using O2 or
air as the green oxidant remains a challenge. Initially, the catalytic activity of the RuIII@bipy-
CTF catalyst was tested under aerobic conditions for the selective oxidation of benzyl
alcohol to benzaldehyde. The catalytic results are presented in Table 2. The RuIII@bipy-
CTF catalyst displays a moderate activity with a conversion of 37% using toluene as the
reaction medium (Table 2, entry 1). To further enhance the catalytic performance of the
catalyst, different bases were applied (Table 2, entries 2–4). The catalytic conversion of
benzyl alcohol increases in the presence of K2CO3 and Cs2CO3 with conversions of 78
and 99%, respectively. Notably, no product of over-oxidation (benzoic acid) was detected,
proving the high selectivity of the Ru catalyst towards benzaldehyde. In the absence of
the catalyst, no conversion of benzyl alcohol was observed (<1% after 12 h). Moreover,
the conversion of benzyl alcohol toward benzaldehyde decreased to 3% under an Ar
atmosphere, which confirms the essential need for oxygen as the oxidant (entry 7 in
Table 2). The catalytic activity of the RuIII@bipy-CTF catalyst was further compared with
its homogeneous counterpart (Table 2, entry 8). Under the same reaction conditions, the
(Ru(acac)2(CH3CN)2)PF6 complex showed lower catalytic conversion (54% using Cs2CO3).
The improved activity of the RuIII@bipy-CTF catalyst can be attributed to the contributing
role of the bipy-CTF support (see mechanistic studies in the Supplementary Materials,
Table S1 and Figure S1). A control experiment was performed using the pristine bipy-CTF
material. A conversion of 39% was obtained using the pristine CTF as the catalyst under

Molecules 2021, 26, 838
6 of 12
identical reaction conditions (Table 2, entry 9). It has been proven that nitrogen-rich carbon
materials are effective catalysts for aerobic oxidation reactions [39]. We recently showed
the unique properties of CTFs to proceed with aerobic oxidation reactions assisted by
nitrogen functionalities [40,41]. More specifically, CTFs with quaternary N species can
activate molecular O2 to generate oxygen radicals (superoxide) which further promote the
oxidation reaction.
Table 2. Catalytic performance of different catalysts in the oxidation of benzyl alcohol.
Entry
Catalyst
Base
Conversion (%)
TON a
RuIII@bipy-CTF
RuIII@bipy-CTF
RuIII@bipy-CTF
RuIII@bipy-CTF
RuIII@bipy-CTF b
No catalyst
1
2
3
4
5
6
7
8
9
No base
Na2CO3
K2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
37
41
78
99
64
<1
3
37
41
78
99
267
-
3
54
39
RuIII@bipy-CTF c
[Ru(acac)2(CH3CN)2]PF6
bipy-CTF d
54
39
Reaction conditions: 1 mol% catalyst (based on Ru, obtained from ICP-OES analysis), 0.33 mmol benzyl alcohol,
a
0.4 mmol base, 500
µ
L toluene, O2, 100 ◦C, 12 h. mmol of product formed per mmol of Ru in the catalyst.
b
c
d
0.24 mol% catalyst was used. Under Ar atmosphere. 17 mg catalyst was used. All catalysts displayed >99%
selectivity toward benzaldehyde.
Our further studies focused on the catalytic performance of the RuIII@bipy-CTF
catalyst in the tandem aerobic oxidation-Knoevenagel condensation reaction. For this
purpose, the optimized reaction condition for the oxidation of benzyl alcohol was selected
(1 mol% RuIII@bipy-CTF, Cs2CO3, O2, 100 ◦C, 12 h). Different substituted benzyl alcohols
and malononitrile were examined and the obtained results are listed in Table 3. No product
was formed in the absence of the catalyst. As shown in Table 3, the RuIII@bipy-CTF
catalyst was found to be highly active in the Knoevenagel condensation reaction under
mild reaction conditions. A high conversion was obtained for all substrates at a low
◦
temperature (70 C) and only after 1 h. Moreover, complete selectivity was observed
towards the corresponding product.
Table 3. Catalytic performance of RuIII@bipy-CTF catalyst in tandem aerobic oxidation-Knoevenagel condensation reaction.
Substrate
Product
Conversion of 1a–f (%)
Yield of 2a–f (%)
99
99
99
99

Molecules 2021, 26, 838
7 of 12
Table 3. Cont.
99
99
97
99
80
97
99
80
Reaction conditions: 1 mol% catalyst (based on Ru, obtained from ICP-OES analysis), 0.33 mmol alcohol, 0.4 mmol Cs2CO3, 500 µL toluene,
0.33 mmol malononitrile, O2, 100 ◦C, 12 h (1st step) and 70 ◦C, 1 h (2nd step). All substrates displayed >99% selectivity toward the
corresponding product.
The recyclability of the RuIII@bipy-CTF catalyst was investigated for the tandem
aerobic oxidation-Knoevenagel condensation reaction. The recyclability studies showed
that the RuIII@bipy-CTF catalyst maintains almost its full catalytic performance after four
consecutive cycles with no obvious loss of activity or selectivity (Figure 4). Moreover, the
recycled catalyst showed no detectable Ru leaching (analyzed by ICP-OES).
Figure 4. Recyclability of the RuIII@bipy-CTF catalyst (1 mol% catalyst, 0.33 mmol benzyl a◦lcohol,
◦
0.4 mmol Cs2CO3, 500
(2nd step).
µ
L toluene, 0.33 mmol malononitrile, O2, 100 C, 12 h (1st step) and 70 C, 1 h

Molecules 2021, 26, 838
8 of 12
A comparison of the catalytic performance of the presented system is made with
various catalysts for the tandem aerobic oxidation-Knoevenagel condensation reaction. It is
challenging to make a fair comparison since, in almost all the studies, no turnover number
(TON) or turnover frequency (TOF) values were reported. Therefore, a comparison can
only be made in terms of conversion to provide an overall overview. As can be seen from
Table 4, a high yield of benzylmalononitrile is obtained over the RuIII@bipy-CTF catalyst
using O2 as the green oxidant and in the absence of any co-oxidant. Moreover, the present
system has the advantage of a high catalytic performance without the use of expensive
noble metals.
Table 4. Comparison of the RuIII@bipy-CTF catalyst with other heterogeneous catalysts for selective tandem oxidation-
Knoevenagel condensation reaction.
◦
Entry
Catalyst
Oxidant/Temp. ( C)
Time (h) a
Conv./Yield (%)
Ref
1
2
3
4
5
6
7
Au@Cu(II)-MOF
Au@MIL-53(NH2)
UoB-2 (Ni-MOF)
Cu3TATAT-3 MOF
Pd/COF-TaPa-Py
5CoOx/tri-g-C3N4
RuIII@bipy-CTF
a
Air/110
O2/100
TBHP/65
O2, TEMPO/75
O2/80
15 + 7
13
1.5
12
4 + 1.5
6
99/99
99/99
94
95/95
98/98
96.4/96.4
99/99
[42]
[43]
[44]
[45]
[46]
O2/80
O2/100
[47]
12 + 1
This work
“m + n” refers to a step-by-step reaction without separation of benzaldehyde as the intermediate.
3. Materials and Methods
3.1. Materials and Instrumentation
X-ray powder diffraction (XRPD) patterns were collected on a Thermo Scientific ARL
X’Tra diffractometer, operated at 40 kV, 40 mA, using Cu
−
K
α
radiation (λ = 1.5406 Å).
◦
Nitrogen sorption studies were performed at
−
196 C using a Belsorp-mini II gas analyzer.
◦
Before the adsorption experiments, the samples were degassed under vacuum at 120 C to
remove adsorbed water. An ultra-fast GC equipped with a flame ionization detector (FID)
and a 5% diphenyl/95% polydimethylsiloxane column, with 10 m length and 0.10 mm
internal diameter, was used to follow the conversion of the products during the catalytic
tests. Helium was used as the carrier gas and the flow rate was programmed as 0.8 mL/min.
The reaction products were identified with a TRACE GC
×
GC (Thermo, Interscience,
Waltham, MA, USA), coupled to a TEMPUS TOF-MS detector (Thermo, Interscience,
Waltham, MA, USA). Thermogravimetric analysis (TGA) was carried out to determine
the stability of the CTF materials using a NET-ZSCH STA 409 PC/PGTG instrument. The
samples were heated from 30 to 1000 ◦C in air at a constant rate of 10 ◦C/min. The X-ray
photoelectron spectroscopy (XPS) measurements were performed on a K-alpha Thermo
Fisher Scientific spectrometer with a monochromatic Al Kα X-ray source. Metal content
was determined by an ICP-OES Optima 8000 (inductively coupled plasma optical emission
spectroscopy) atomic emission spectrometer. The nitrogen content of the materials was
determined with a Thermo Flash 200 elemental analyzer using V2O5 as the catalyst.
All chemicals were purchased from Sigma-Aldrich, abcr or TCI Europe and used
without further purification. 5,50-dicyano-2,20-bipyridine was synthesized following the
procedure described in the literature [48].
3.2. Synthesis of Bipy-CTFs and RuIII@bipy-CTF Materials
The preparation of the bipy-CTF material was achieved following the standard proce-
dure described in the literature applying the typical ionothermal conditions [32]. Typically,
0
0
a glass ampule was filled with 5,5 -dicyano-2,2 -bipyridine (100 mg, 0.48 mmol) and ZnCl2
(332 mg, 2.40 mmol) in a glovebox. The ampule was flame-sealed under vacuum and
placed in an oven at 400 ◦C for 48 h with a heating rate of 60 ◦C/h. After cooling to room
temperature, the ampule was opened and the black-colored solid was stirred in 120 mL
H2O overnight at 60 ◦C, filtered and washed with H2O and acetone. The solid was then

Molecules 2021, 26, 838
9 of 12
stirred at 120 ◦C in 1 M HCl (150 mL) overnight, filtered, and subsequently washed with
1 M HCl (3
×
75 mL), H2O (15
×
75 mL), THF (3
×
75 mL), and acetone (3
×
75 mL). Finally,
the powder was dried under vacuum overnight at 90 ◦C (Found for bipy-CTF: C, 58.92; H,
3.51; N, 20.27%).
The (Ru(acac)2(CH3CN)2)PF6 complex was prepared according to the literature
method [49]. For this, (Ru(acac)3) (400 mg, 1 mmol) was dissolved in 30 mL of CH3CN and
to this solution, 10 mL of a 1% H2SO4/CH3CN solution was slowly added while stirring.
The solution was stirred at room temperature until the wine-red solution turned deep blue
(approx. 12 h). Next, the solution was concentrated to 3 mL by evaporating the solvent.
NH4PF6 (0.5 g, 3 mmol) in 5 mL of cold water was added to the deep-blue solution. The
resulting deep-blue precipitate was collected by filtration, washed with cold water and
n-hexane and dried under vacuum.
The post-modification of the bipy-CTF was performed as follows: (Ru(acac)2(CH3CN)2)PF6
(13.6 mg, 0.026 mmol) was added to 4 mL dry toluene. Afterward, 120 mg bipy-CTF mate-
rial was added and stirred for 48 h at 80 ◦C. The prepared material was stirred in CH3CN
for 24 h to remove the unreacted residue of the Ru complex. Then, the modified material
was filtered, and washed thoroughly with CH3CN and acetone, followed by drying under
vacuum overnight (Found for RuIII@bipy-CTF, C, 59.6; H, 3.2; N, 15.7%).
3.3. Catalytic Reactions
The procedure used to perform the tandem reaction is as follows: The oxidation
of benzyl alcohol was carried out in a 20 mL Schlenk tube. During a typical catalytic
test, the catalyst (1 mol % Ru), Cs2CO3 (0.4 mmol, placed in a porous membrane) as
base, benzyl alcohol (0.33 mmol), dodecane as internal standard (0.33 mmol) and toluene
(500
µ
L) were added to the Schlenk tube. The tube was purged with pure oxygen, sealed
◦
and heated to 100 C for 12 h. Samples were withdrawn after 12 h. Upon cooling to
room temperature and dilution with solvent, the samples were analyzed using a gas
chromatograph. Hereafter, the porous membrane containing Cs2CO3 was removed and
nitrile substrates (0.33 mmol) were added to the previous reaction mixture. The tube was
sealed without purging oxygen. The reaction was heated from room temperature to 70 ◦C
for an additional 1 h. Upon cooling to room temperature and dilution with toluene, the
samples were analyzed using a gas chromatograph. After each catalytic run, the catalyst
was recovered by filtration and washed with toluene, water, and acetone. The catalyst was
then used directly in the subsequent runs. Conversion, selectivity and yield are calculated
through Equations (S1)–(S3), respectively (see section catalytic reactions in SI, Figure S2).
4. Conclusions
In conclusion, an efficient heterogeneous catalyst is developed by applying a highly
N-rich covalent triazine framework. The presence of bipyridine docking sites within the
CTF provides excellent anchoring centers for the (Ru(acac)2(CH3CN)2)PF6 complex. The
potential application of the obtained RuIII@bipy-CTF catalyst was studied in the selective
tandem aerobic oxidation-Knoevenagel condensation reaction. The catalyst showed a
very high conversion of various benzyl alcohol derivatives (80–99%) with full selectivity
towards the corresponding α, β-unsaturated nitriles using O2 as the sole oxidant. The
N-rich functionalities not only act as basic sites for Knoevenagel condensation reaction but
also promote the aerobic oxidation of alcohols through oxygen activation. The obtained
results revealed the high catalytic performance of the RuIII@bipy-CTF catalyst, exceeding
its homogeneous counterpart. To the best of our knowledge, this is one of the rare reports
on the application of Ru-based catalysts for tandem oxidation catalysis using O2 as the
sole oxidant.
Supplementary Materials: The following are available online, Table S1: Various control experiments
to obtain insights into the reaction mechanism for the oxidation of benzyl alcohol, Figure S1: Proposed
mechanism for bipy-CTF-catalyzed aerobic oxidation of benzyl alcohol, Figure S2: The GC-MS spectra

Molecules 2021, 26, 838
10 of 12
of the
α
,
β-unsaturated nitriles. Equation (S1): Conversion calculation of catalytic reaction. Equation
(S2): yield calculation of catalytic reaction. Equation (S3): selectivity calculation of catalytic reaction.
Author Contributions: Conceptualization, S.A. and P.V.D.V.; formal analysis, J.S.; investigation,
S.A. and P.V.D.V.; methodology, G.W. and P.G.D.; supervision, S.A. and P.V.D.V.; validation, K.L.;
writing—original draft, P.G.D. and S.A.; writing—review and editing, P.V.D.V. All authors have read
and agreed to the published version of the manuscript.
Funding: This work was financially supported by UGent concerted action grant BOFGOA2017000303,
Ghent University BOF doctoral grant 01D04318 and the Research Foundation Flanders (FWO-
Vlaanderen) grant no. G000117N.
Data Availability Statement: The data and compounds presented in this study are available on
request from the corresponding authors.
Acknowledgments: We are grateful to J. Goeman for experimental help with the GC–mass spectrom-
etry analyses.
Conflicts of Interest: The authors declare that they have no competing interests.
References
1.
2.
3.
4.
5.
6.
Liang, J.; Liang, Z.B.; Zou, R.Q.; Zhao, Y.L. Heterogeneous catalysis in zeolites, mesoporous silica, and metal-organic frameworks.
Adv. Mater. 2017, 29, 1701139. [CrossRef] [PubMed]
Lehman, S.E.; Larsen, S.C. Zeolite and mesoporous silica nanomaterials: Greener syntheses, environmental applications and
biological toxicity. Environ. Sci. Nano. 2014, 1, 200–213. [CrossRef]
Gopinath, K.P.; Vo, D.V.N.; Prakash, D.G.; Joseph, A.A.; Viswanathan, S.; Arun, J. Environmental applications of carbon-based
materials: A review. Environ. Chem. Lett. 2020. [CrossRef]
Huang, N.; Wang, K.C.; Drake, H.; Cai, P.Y.; Pang, J.D.; Li, J.L.; Che, S.; Huang, L.; Wang, Q.; Zhou, H.C. Tailor-made pyrazolide-
based metal-organic frameworks for selective catalysis. J. Am. Chem Soc. 2018, 140, 6383–6390. [CrossRef]
Fried, D.I.; Brieler, F.J.; Froba, M. Designing inorganic porous materials for enzyme adsorption and applications in biocatalysis.
ChemCatChem 2013, 5, 862–884. [CrossRef]
Rogge, S.M.J.; Bavykina, A.; Hajek, J.; Garcia, H.; Olivos-Suarez, A.I.; Sepulveda-Escribano, A.; Vimont, A.; Clet, G.; Bazin, P.;
Kapteijn, F.; et al. Metal-organic and covalent organic frameworks as single-site catalysts. Chem Soc. Rev. 2017, 46, 3134–3184.
[CrossRef]
7.
8.
Thomas, A. Functional materials: From hard to soft porous frameworks. Angew. Chem. Int. Ed. 2010, 49, 8328–8344. [CrossRef]
Geng, K.Y.; He, T.; Liu, R.Y.; Dalapati, S.; Tan, K.T.; Li, Z.P.; Tao, S.S.; Gong, Y.F.; Jiang, Q.H.; Jiang, D.L. Covalent organic
frameworks: Design, synthesis, and functions. Chem. Rev. 2020, 120, 8814–8933. [CrossRef]
9.
Diercks, C.S.; Yaghi, O.M. The atom, the molecule, and the covalent organic framework. Science 2017, 355. [CrossRef] [PubMed]
10. Kuhn, P.; Antonietti, M.; Thomas, A. Porous, covalent triazine-based frameworks prepared by ionothermal synthesis. Angew.
Chem. Int. Ed. 2008, 47, 3450–3453. [CrossRef] [PubMed]
11. Artz, J. Covalent triazine-based frameworkstailor-made catalysts and catalyst supports for molecular and nanoparticulate species.
ChemCatChem 2018, 10, 1753–1771. [CrossRef]
12. Wang, H.; Jiang, D.N.; Huang, D.L.; Zeng, G.M.; Xu, P.; Lai, C.; Chen, M.; Cheng, M.; Zhang, C.; Wang, Z.W. Covalent triazine
frameworks for carbon dioxide capture. J. Mater. Chem. A 2019, 7, 22848–22870. [CrossRef]
13. Chan-Thaw, C.E.; Villa, A.; Katekomol, P.; Su, D.S.; Thomas, A.; Prati, L. Covalent triazine framework as catalytic support for
liquid phase reaction. Nano Lett. 2010, 10, 537–541. [CrossRef] [PubMed]
14. Jin, R.H.; Zheng, D.S.; Liu, R.; Liu, G.H. Silica-supported molecular catalysts for tandem reactions. ChemCatChem 2018, 10,
1739–1752. [CrossRef]
15. Cho, H.J.; Xu, B.J. Enabling selective tandem reactions via catalyst architecture engineering. Trends Chem. 2020, 2, 929–941.
[CrossRef]
16. Huang, Y.B.; Liang, J.; Wang, X.S.; Cao, R. Multifunctional metal-organic framework catalysts: Synergistic catalysis and tandem
reactions. Chem Soc. Rev. 2017, 46, 126–157. [CrossRef] [PubMed]
17. Huang, L.S.; Lai, Y.H.; Yang, C.; Xu, D.Z. Iron-catalyzed one-pot oxidation/Knoevenagel condensation reaction using air as an
oxidant. Appl. Organomet. Chem. 2019, 33, e4910. [CrossRef]
18. Chen, C.; Yang, H.M.; Chen, J.Z.; Zhang, R.; Guo, L.; Gan, H.M.; Song, B.N.; Zhu, W.W.; Hua, L.; Hou, Z.S. One-pot tandem
catalytic synthesis of alpha, beta-unsaturated nitriles from alcohol with nitriles in aqueous phase. Catal. Commun. 2014, 47, 49–53.
[CrossRef]
19. Yang, Z.W.; Kang, Q.X.; Quan, F.; Lei, Z.Q. Oxidation of alcohols using iodosylbenzene as oxidant catalyzed by ruthenium
complexes under mild reaction conditions. J. Mol. Catal. A Chem. 2007, 261, 190–195. [CrossRef]
20. Zhang, Y.C.; Zhou, Q.; Ma, W.C.; Zhao, J.Q. Enantioselective oxidation of racemic secondary alcohols catalyzed by chiral
Mn(III)-salen complex with sodium hypochlorite as oxidant. Catal. Commun. 2014, 45, 114–117. [CrossRef]

Molecules 2021, 26, 838
11 of 12
21. Lou, J.D.; Xu, Z.N. Selective oxidation of primary alcohols with chromium trioxide under solvent free conditions. Tetrahedron Lett
2002, 43, 6095–6097. [CrossRef]
22. Parmeggiani, C.; Matassini, C.; Cardona, F. A step forward towards sustainable aerobic alcohol oxidation: New and revised
catalysts based on transition metals on solid supports. Green Chem. 2017, 19, 2030–2050. [CrossRef]
23. Sheldon, R.A.; Arends, I.W.C.E.; Ten Brink, G.J.; Dijksman, A. Green, catalytic oxidations of alcohols. Acc. Chem. Res. 2002, 35,
774–781. [CrossRef] [PubMed]
24. Piera, J.; Backvall, J.E. Catalytic oxidation of organic substrates by molecular oxygen and hydrogen peroxide by multistep electron
transfer—A biomimetic approach. Angew. Chem. Int. Ed. 2008, 47, 3506–3523. [CrossRef]
25. Lu, T.L.; Du, Z.T.; Liu, J.X.; Ma, H.; Xu, J. Aerobic oxidation of primary aliphatic alcohols over bismuth oxide supported platinum
catalysts in water. Green Chem. 2013, 15, 2215–2221. [CrossRef]
26. Abednatanzi, S.; Derakhshandeh, P.G.; Abbasi, A.; Van der Voort, P.; Leus, K. Direct synthesis of an Iridium(III) bipyridine
metal-organic framework as a heterogeneous catalyst for aerobic alcohol oxidation. ChemCatChem 2016, 8, 3672–3679. [CrossRef]
27. Zeng, X.M.; Chen, J.M.; Yoshimura, A.; Middleton, K.; Zhdankin, V.V. SiO2–supported RuCl3/3–(dichloroiodo)benzoic acid:
Green catalytic system for the oxidation of alcohols and sulfides in water. RSC Adv. 2011, 1, 973–977. [CrossRef]
28. Ganesamoorthy, S.; Tamizh, M.M.; Shanmugasundaram, K.; Karvembu, R. Immobilization of Ru(III) complex on silica: A
heterogenized catalyst for selective oxidation of alcohols in water at room temperature. Tetrahedron Lett. 2013, 54, 7035–7039.
[CrossRef]
29. Clerick, S.; De Canck, E.; Hendrickx, K.; Van Speybroeck, V.; Van der Voort, P. Heterogeneous Ru(III) oxidation catalysts via ’click’
bidentate ligands on a periodic mesoporous organosilica support. Green Chem. 2016, 18, 6035–6045. [CrossRef]
30. Ganesh Babu, S.; Krishnamoorthi, R.; Thiruneelakandan, R.; Karvembu, R. V2O5 Anchored RuO2: An efficient nanocatalyst for
aerial oxidation of alcohols. Catal. Lett. 2014, 144, 1245–1252. [CrossRef]
31. Wang, S.S.; Zhang, J.; Zhou, C.L.; Vo–Thanh, G.; Liu, Y. An ionic compound containing Ru(III)–complex cation and phospho-
tungstate anion as the efficient and recyclable catalyst for clean aerobic oxidation of alcohols. Catal. Commun. 2012, 28, 152–154.
[CrossRef]
32. Zahmakiran, M.; Özkar, S. Zeolite confined nanostructured dinuclear ruthenium clusters: Preparation, characterization and
catalytic properties in the aerobic oxidation of alcohols under mild conditions. J. Mater. Chem. 2009, 19, 7112–7118. [CrossRef]
33. Guo, H.J.; Liu, W.D.; Yin, G.C. Aerobic oxidation of alcohols to aldehydes and ketones using ruthenium(III)/Et3N catalyst. Appl
Organomet. Chem. 2011, 25, 836–842. [CrossRef]
34. Sodhi, R.K.; Paul, S.; Clark, J.H. A comparative study of different metal acetylacetonates covalently anchored onto amine
functionalized silica: A study of the oxidation of aldehydes and alcohols to corresponding acids in water. Green Chem. 2012, 14,
1649–1656. [CrossRef]
35. Hug, S.; Tauchert, M.E.; Li, S.; Pachmayr, U.E.; Lotsch, B.V. A functional triazine framework based on N–heterocyclic building
blocks. J. Mater. Chem. 2012, 22, 13956–13964. [CrossRef]
36. Bagherzadeh, M.; Karimi, H.; Amini, M. Immobilization of dioxomolybdenum(VI) Schiff base complex on graphene oxide
nanosheets and its catalytic activity for oxidation of sulfides. J. Coord. Chem. 2017, 70, 2986–2998. [CrossRef]
37. Osadchii, D.Y.; Olivos–Suarez, A.I.; Bavykina, A.V.; Gascon, J. Revisiting nitrogen species in covalent triazine frameworks.
Langmuir 2017, 33, 14278–14285. [CrossRef] [PubMed]
38. Chen, L.Y.; Huang, B.B.; Qiu, X.; Wang, X.; Luque, R.; Li, Y.W. Seed–mediated growth of MOF–encapsulated Pd@Ag core–shell
nanoparticles: Toward advanced room temperature nanocatalysts. Chem. Sci. 2016, 7, 228–233. [CrossRef] [PubMed]
39. Watanabe, H.; Asano, S.; Fujita, S.; Yoshida, H.; Arai, M. Nitrogen–doped, metal–free activated carbon catalysts for aerobic
oxidation of alcohols. ACS Catal. 2015, 5, 2886–2894. [CrossRef]
40. Abednatanzi, S.; Derakhshandeh, P.G.; Leus, K.; Vrielinck, H.; Callens, F.; Schmidt, J.; Savateev, A.; Van der Voort, P. Metal–free
activation of molecular oxygen by covalent triazine frameworks for selective aerobic oxidation. Sci. Adv. 2020, 6, eaaz2310.
[CrossRef]
41. Abednatanzi, S.; Derakhshandeh, P.G.; Tack, P.; Muniz–Miranda, F.; Liu, Y.Y.; Everaert, J.; Meledina, M.; Vanden Bussche, F.;
Vincze, L.; Stevens, C.V.; et al. Elucidating the promotional effect of a covalent triazine framework in aerobic oxidation. Appl.
Catal. B Environ. 2020, 269, 118769. [CrossRef]
42. Wang, J.S.; Jin, F.Z.; Ma, H.C.; Li, X.B.; Liu, M.Y.; Kan, J.L.; Chen, G.J.; Dong, Y.B. Au@Cu(II)–MOF: Highly efficient bifunctional
heterogeneous catalyst for successive oxidation–condensation reactions. Inorg. Chem. 2016, 55, 6685–6691. [CrossRef]
43. Qi, Y.; Luan, Y.; Peng, X.; Yang, M.; Hou, J.Y.; Wang, G. Design and synthesis of an Au@MIL–53(NH2) catalyst for a one–pot
aerobic oxidation/knoevenagel condensation reaction. Eur. J. Inorg. Chem. 2015, 2015, 5099–5105. [CrossRef]
44. Aryanejad, S.; Bagherzade, G.; Farrokhi, A. Efficient and recyclable novel Ni–based metal–organic framework nanostructure
as catalyst for the cascade reaction of alcohol oxidation-Knoevenagel condensation. Appl. Organomet. Chem. 2018, 32, e3995.
[CrossRef]
45. Miao, Z.C.; Luan, Y.; Qi, C.; Ramella, D. The synthesis of a bifunctional copper metal organic framework and its application in the
aerobic oxidation/Knoevenagel condensation sequential reaction. Dalton Trans. 2016, 45, 13917–13924. [CrossRef] [PubMed]
46. Sun, Q.; Aguila, B.; Ma, S.Q. A bifunctional covalent organic framework as an efficient platform for cascade catalysis. Mater.
Chem. Front. 2017, 1, 1310–1316. [CrossRef]

Molecules 2021, 26, 838
12 of 12
47. Wu, J.Q.; Hua, W.M.; Yue, Y.H.; Gao, Z. A highly efficient bifunctional catalyst CoOx/tri–g–C(3)N(4)for one–pot aerobic
oxidation-Knoevenagel condensation reaction. Catalysts 2020, 10, 712. [CrossRef]
48. Liao, L.Y.; Kong, X.R.; Duan, X.F. Reductive couplings of 2–halopyridines without external ligand: Phosphine–free nickel–
catalyzed synthesis of symmetrical and unsymmetrical 2,2’–bipyridines. J. Org. Chem. 2014, 79, 777–782. [CrossRef]
49. Koiwa, T.; Masuda, Y.; Shono, J.; Kawamoto, Y.; Hoshino, Y.; Hashimoto, T.; Natarajan, K.; Shimizu, K. Synthesis, characterization,
and detailed electrochemistry of binuclear ruthenium(III) complexes bridged by bisacetylacetonate. Crystal and molecular
structures of [{Ru(acac)(2)}(2)(tae)] (acac=2,4–pentanedionate ion, tae=1,1,2,2–tetraacetylethanate dianion). Inorg. Chem. 2004, 43,
6215–6223.