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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 1and

Pascal Van Der Voort 1,*





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

Framework for Tandem Aerobic

Oxidation-Knoevenagel

Condensation Reactions. Molecules

2021,26, 838. https://doi.org/

10.3390/molecules26040838

Academic Editors: Igor Djerdj and

Sergio Navalo

Received: 23 December 2020

Accepted: 2 February 2021

Published: 5 February 2021

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4.0/).

1Department of Chemistry, Center for Ordered Materials, Organometallics and Catalysis (COMOC), Ghent

University, Krijgslaan 281, Building S3 (Campus Sterre), 9000 Gent, Belgium; [email protected] (G.W.);

[email protected] (P.G.D.); [email protected] (K.L.)

2Institut für Chemie-Funktionsmaterialien, Technische Universität Berlin, Hardenbergstraße 40,

10623 Berlin, Germany; [email protected]

*Correspondence: [email protected] (S.A.); [email protected] (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

III

complex. The bipyridine sites within the CTF provide excellent anchoring points for the

[Ru(acac)

(CH

CN)

]PF

complex. The obtained robust Ru

III

@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

-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.

Keywords:

covalent triazine frameworks; heterogeneous catalysis; tandem catalysis; aerobic oxidation-

knoevenagel condensation

1. Introduction

Conventional porous materials including silica, zeolite and activated charcoal have

attracted extensive interest in large-scale industrial applications, most importantly in

heterogeneous catalysis [

–

]. However, the poor chemical versatility of their chemical

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 [

]. The recently emerging porous materials,

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

of COFs discovered in 2008 by Thomas and Antonietti [

]. CTFs are formed through a

trimerization reaction followed by the subsequent oligomerization of aromatic nitriles [

The robust aromatic covalent bonds endow CTFs with excellent stability compared to

coordinative-linked porous materials [

]. 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].

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 [

]. 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 [

]. 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 [

-unsaturated nitriles are key

intermediates for the synthesis of pharmaceuticals and fine chemicals [

]. These inter-

mediates are generally produced through the Knoevenagel condensation of aldehydes

or ketones with nitriles catalyzed by common bases [

]. However, the catalytic process

suffers from limited substrate scope due to the high price or unavailability of some alde-

hydes [

]. In this regard, the development of highly efficient multifunctional catalysts

to prepare

-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 [

–

]. Ru catalysts are economically attractive in comparison to

other noble-metal catalysts, which are rather expensive. Ru

III

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 Ru

III

complex onto a periodic

mesoporous organosilica (PMO) [

]. 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 Ru

III

-based catalysts in the aerobic oxidation of alcohols [

–

]. 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 [

]. 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 Ru

III

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 Ru

III

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)

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 [

]. After the synthesis, the remaining ZnCl

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)

(CH

CN)

)PF

complex through a simple wet impregnation method, as depicted in Figure 1.

Molecules 2021, 26, x FOR PEER REVIEW 3 of 12

2. Results and Discussion

2.1. Synthesis and Characterization of the Modified Bipy-CTF with the Ru Complex (RuIII@bipy-

CTF)

We targeted a CTF with free 2,2’-bipyridine building blocks (5,5′-dicyano-2,2′-bipyr-

idine), which forms excellent anchoring points. The bipy-CTF material was synthesized

following the typical reported ionothermal procedure [35]. After the synthesis, the re-

maining ZnCl2 is removed by extensive washing with water, followed by refluxing at 120

°C in 1 M HCl. 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) mate-

rial.

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 com-

plete 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 a type I isotherm and exhibits a Brunauer–Emmett–Teller (BET) surface area of 787

m2g−1. The total pore volume was found to be 0.40 cm3 g−1 at P/P0 = 0.99. After the intro-

duction of the Ru complex, the BET surface area decreases moderately to 556 m2g−1, indi-

cating 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 pre-

pared ionothermally, the bipy-CTF materials were found to be predominantly amor-

phous. The broad peaks at 2θ ~ 13 and 25° are assigned to the 00l reflection showing a

‘‘graphitic’’ layer stacking. It is important to note that the exact structure of these amor-

phous materials cannot be determined since the harsh synthesis conditions result in car-

bonization and blackening of the material, making it difficult to fully characterize. To es-

timate 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,

Figure 1.

Schematic representation of the ideal ordered structure of Ru

III

@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 Ru

III

@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 N

at low relative pressures

which is indicative of a highly microporous material (Figure 2b). This profile is assigned to

a type I isotherm and exhibits a Brunauer–Emmett–Teller (BET) surface area of 787 m

−1

The total pore volume was found to be 0.40 cm

−1

at P/P

= 0.99. After the introduction

of the Ru complex, the BET surface area decreases moderately to 556 m

−1

, indicating

that most of the pores are still accessible.

The powder X-ray diffraction (PXRD) patterns of the pristine bipy-CTF and Ru

III

@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

~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.

Sample C a(wt.%) N a(wt.%) C/N Ru b(mmol g−1)

bipy-CTF 58.92 20.27 2.9 -

RuIII@bipy-

CTF 59.6 15.7 3.8 0.15

aDetermined by elemental analysis. bDetermined 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 Ru

III

@bipy-CTF material is thermally stable up to 300

◦

C, and gradually

decomposes at higher temperatures.

Molecules 2021, 26, x FOR PEER REVIEW 4 of 12

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.

Sample C

a (Wt.%) N a (Wt.%) C/N Ru b (mmol g−1)

bipy-CTF 58.92 20.27 2.9 -

RuIII@bipy-CTF 59.6 15.7 3.8 0.15

a Determined by elemental analysis. b Determined by ICP-OES analysis.

The thermal stability of both materials was determined by thermogravimetric analy-

sis (TGA). The TGA profile of the bipy-CTF displayed a high thermal stability up to ap-

proximately 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 sam-

ple 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. (a) Diffuse reflectance infrared Fourier

transform (DRIFT) spectra. (b) Nitrogen adsorption/desorption isotherms. (c) Powder X-ray diffraction (XRD) patterns.

(d) Thermogravimetric analysis (TGA) curves.

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, respec-

tively. These nitrogen functionalities are formed during the synthesis at high tempera-

Figure 2.

Structural characterization of bipy-CTF and Ru

III

@bipy-CTF materials. (

) Diffuse reflectance infrared Fourier

transform (DRIFT) spectra. (

) Nitrogen adsorption/desorption isotherms. (

) Powder X-ray diffraction (XRD) patterns.

(d) Thermogravimetric analysis (TGA) curves.

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. [

]. In the N 1s spectrum of the Ru

III

@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 [

]. The Ru 3p peaks for the Ru

III

@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

Molecules 2021, 26, x FOR PEER REVIEW 5 of 12

tures, as reported by Osadchii et al. [37]. In the N 1s spectrum of the RuIII@bipy-CTF ma-

terial (Figure 3b), a peak shift towards a higher binding energy is observed for the pyri-

dinic 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 immobi-

lized 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.

Figure 3. Structural characterization of bipy-CTF and RuIII@bipy-CTF materials. (a) N 1S XPS spectrum of the bipy-CTF.

(b) N 1s XPS spectrum of the RuIII@bipy-CTF. (c) Ru 3p XPS spectrum of the RuIII@bipy-CTF. (d) Ru 3d XPS spectrum of

the RuIII@bipy-CTF.

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 tol-

uene as the reaction medium (Table 2, entry 1). To further enhance the catalytic perfor-

mance 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 conver-

sions 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

Figure 3.

Structural characterization of bipy-CTF and Ru

III

@bipy-CTF materials. (

) N 1S XPS spectrum of the bipy-CTF. (

)

N 1s XPS spectrum of the Ru

III

@bipy-CTF. (

) Ru 3p XPS spectrum of the Ru

III

@bipy-CTF. (

) Ru 3d XPS spectrum of the

RuIII@bipy-CTF.

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 O

air as the green oxidant remains a challenge. Initially, the catalytic activity of the Ru

III

@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 Ru

III

@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 K

and Cs

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 Ru

III

@bipy-CTF catalyst was further compared with

its homogeneous counterpart (Table 2, entry 8). Under the same reaction conditions, the

(Ru(acac)

(CH

CN)

)PF

complex showed lower catalytic conversion (54% using Cs

The improved activity of the Ru

III

@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 [

]. We recently showed

the unique properties of CTFs to proceed with aerobic oxidation reactions assisted by

nitrogen functionalities [

]. More specifically, CTFs with quaternary N species can

activate molecular O

to generate oxygen radicals (superoxide) which further promote the

oxidation reaction.

Table 2. Catalytic performance of different catalysts in the oxidation of benzyl alcohol.

Molecules 2021, 26, x FOR PEER REVIEW 6 of 12

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

identical reaction conditions (Table 2, entry 9). It has been proven that nitrogen-rich car-

bon materials are effective catalysts for aerobic oxidation reactions [39]. We recently

showed the unique properties of CTFs to proceed with aerobic oxidation reactions as-

sisted by nitrogen functionalities [40,41]. More specifically, CTFs with quaternary N spe-

cies can activate molecular O2 to generate oxygen radicals (superoxide) which further pro-

mote the oxidation reaction.

Table 2. Catalytic performance of different catalysts in the oxidation of benzyl alcohol.

Entry Catalyst Base Conversion (%) TON

1 RuIII@bipy-CTF No base 37 37

2 RuIII@bipy-CTF Na2CO3 41 41

3 RuIII@bipy-CTF K2CO3 78 78

4 RuIII@bipy-CTF Cs2CO3 99 99

5 RuIII@bipy-CTF b Cs2CO3 64 267

6 No catalyst Cs2CO3 <1 -

7 RuIII@bipy-CTF c Cs2CO3 3 3

8 [Ru(acac)2(CH3CN)2]PF6 Cs2CO3 54 54

9 bipy-CTF

d Cs2CO3 39 39

Reaction conditions: 1 mol% catalyst (based on Ru, obtained from ICP-OES analysis), 0.33 mmol benzyl alcohol, 0.4 mmol

base, 500 μL toluene, O2, 100 °C, 12 h. a mmol of product formed per mmol of Ru in the catalyst. b 0.24 mol% catalyst was

used. c Under Ar atmosphere. d 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 cata-

lyst in the tandem aerobic oxidation–Knoevenagel condensation reaction. For this pur-

pose, 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 reac-

tion 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 cor-

responding product.

Entry Catalyst Base Conversion (%) TON a

1RuIII@bipy-CTF No base 37 37

2RuIII@bipy-CTF Na2CO341 41

3RuIII@bipy-CTF K2CO378 78

4RuIII@bipy-CTF Cs2CO399 99

5RuIII@bipy-CTF bCs2CO364 267

6 No catalyst Cs2CO3<1 -

7RuIII@bipy-CTF cCs2CO33 3

8 [Ru(acac)2(CH3CN)2]PF6Cs2CO354 54

9bipy-CTF dCs2CO339 39

Reaction conditions: 1 mol% catalyst (based on Ru, obtained from ICP-OES analysis), 0.33 mmol benzyl alcohol,

0.4 mmol base, 500

L toluene, O

, 100

◦

C, 12 h.

mmol of product formed per mmol of Ru in the catalyst.

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 Ru

III

@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% Ru

III

@bipy-CTF, Cs

, O

, 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 Ru

III

@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 Ru

III

@bipy-CTF catalyst in tandem aerobic oxidation-Knoevenagel condensation reaction.

Molecules 2021, 26, x FOR PEER REVIEW 7 of 12

Table 3. Catalytic performance of RuIII@bipy-CTF catalyst in tandem aerobic oxidation–Knoevenagel condensation reac-

tion.

Substrate Product Conversion of 1a–f (%) Yield of 2a–f (%)

99 99

97 97

99 99

80 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 aer-

obic oxidation–Knoevenagel condensation reaction. The recyclability studies showed that

the RuIII@bipy-CTF catalyst maintains almost its full catalytic performance after four con-

secutive cycles with no obvious loss of activity or selectivity (Figure 4). Moreover, the re-

cycled catalyst showed no detectable Ru leaching (analyzed by ICP-OES).

Substrate Product Conversion of 1a–f (%) Yield of 2a–f (%)

Molecules 2021, 26, x FOR PEER REVIEW 7 of 12

Table 3. Catalytic performance of RuIII@bipy-CTF catalyst in tandem aerobic oxidation–Knoevenagel condensation reac-

tion.

Substrate Product Conversion of 1a–f (%) Yield of 2a–f (%)

99 99

97 97

99 99

80 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 aer-

obic oxidation–Knoevenagel condensation reaction. The recyclability studies showed that

the RuIII@bipy-CTF catalyst maintains almost its full catalytic performance after four con-

secutive cycles with no obvious loss of activity or selectivity (Figure 4). Moreover, the re-

cycled catalyst showed no detectable Ru leaching (analyzed by ICP-OES).

Molecules 2021, 26, x FOR PEER REVIEW 7 of 12

Table 3. Catalytic performance of RuIII@bipy-CTF catalyst in tandem aerobic oxidation–Knoevenagel condensation reac-

tion.

Substrate Product Conversion of 1a–f (%) Yield of 2a–f (%)

99 99

97 97

99 99

80 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 aer-

obic oxidation–Knoevenagel condensation reaction. The recyclability studies showed that

the RuIII@bipy-CTF catalyst maintains almost its full catalytic performance after four con-

secutive cycles with no obvious loss of activity or selectivity (Figure 4). Moreover, the re-

cycled catalyst showed no detectable Ru leaching (analyzed by ICP-OES).

99 99

Molecules 2021, 26, x FOR PEER REVIEW 7 of 12

Table 3. Catalytic performance of RuIII@bipy-CTF catalyst in tandem aerobic oxidation–Knoevenagel condensation reac-

tion.

Substrate Product Conversion of 1a–f (%) Yield of 2a–f (%)

99 99

97 97

99 99

80 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 aer-

obic oxidation–Knoevenagel condensation reaction. The recyclability studies showed that

the RuIII@bipy-CTF catalyst maintains almost its full catalytic performance after four con-

secutive cycles with no obvious loss of activity or selectivity (Figure 4). Moreover, the re-

cycled catalyst showed no detectable Ru leaching (analyzed by ICP-OES).

Molecules 2021, 26, x FOR PEER REVIEW 7 of 12

Table 3. Catalytic performance of RuIII@bipy-CTF catalyst in tandem aerobic oxidation–Knoevenagel condensation reac-

tion.

Substrate Product Conversion of 1a–f (%) Yield of 2a–f (%)

99 99

97 97

99 99

80 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 aer-

obic oxidation–Knoevenagel condensation reaction. The recyclability studies showed that

the RuIII@bipy-CTF catalyst maintains almost its full catalytic performance after four con-

secutive cycles with no obvious loss of activity or selectivity (Figure 4). Moreover, the re-

cycled catalyst showed no detectable Ru leaching (analyzed by ICP-OES).

99 99

Molecules 2021,26, 838 7 of 12

Table 3. Cont.

Molecules 2021, 26, x FOR PEER REVIEW 7 of 12

Table 3. Catalytic performance of RuIII@bipy-CTF catalyst in tandem aerobic oxidation–Knoevenagel condensation reac-

tion.

Substrate Product Conversion of 1a–f (%) Yield of 2a–f (%)

99 99

97 97

99 99

80 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 aer-

obic oxidation–Knoevenagel condensation reaction. The recyclability studies showed that

the RuIII@bipy-CTF catalyst maintains almost its full catalytic performance after four con-

secutive cycles with no obvious loss of activity or selectivity (Figure 4). Moreover, the re-

cycled catalyst showed no detectable Ru leaching (analyzed by ICP-OES).

Molecules 2021, 26, x FOR PEER REVIEW 7 of 12

Table 3. Catalytic performance of RuIII@bipy-CTF catalyst in tandem aerobic oxidation–Knoevenagel condensation reac-

tion.

Substrate Product Conversion of 1a–f (%) Yield of 2a–f (%)

99 99

97 97

99 99

80 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 aer-

obic oxidation–Knoevenagel condensation reaction. The recyclability studies showed that

the RuIII@bipy-CTF catalyst maintains almost its full catalytic performance after four con-

secutive cycles with no obvious loss of activity or selectivity (Figure 4). Moreover, the re-

cycled catalyst showed no detectable Ru leaching (analyzed by ICP-OES).

Molecules 2021, 26, x FOR PEER REVIEW 7 of 12

Table 3. Catalytic performance of RuIII@bipy-CTF catalyst in tandem aerobic oxidation–Knoevenagel condensation reac-

tion.

Substrate Product Conversion of 1a–f (%) Yield of 2a–f (%)

99 99

97 97

99 99

80 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 aer-

obic oxidation–Knoevenagel condensation reaction. The recyclability studies showed that

the RuIII@bipy-CTF catalyst maintains almost its full catalytic performance after four con-

secutive cycles with no obvious loss of activity or selectivity (Figure 4). Moreover, the re-

cycled catalyst showed no detectable Ru leaching (analyzed by ICP-OES).

99 99

Molecules 2021, 26, x FOR PEER REVIEW 7 of 12

Table 3. Catalytic performance of RuIII@bipy-CTF catalyst in tandem aerobic oxidation–Knoevenagel condensation reac-

tion.

Substrate Product Conversion of 1a–f (%) Yield of 2a–f (%)

99 99

97 97

99 99

80 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 aer-

obic oxidation–Knoevenagel condensation reaction. The recyclability studies showed that

the RuIII@bipy-CTF catalyst maintains almost its full catalytic performance after four con-

secutive cycles with no obvious loss of activity or selectivity (Figure 4). Moreover, the re-

cycled catalyst showed no detectable Ru leaching (analyzed by ICP-OES).

Molecules 2021, 26, x FOR PEER REVIEW 7 of 12

Table 3. Catalytic performance of RuIII@bipy-CTF catalyst in tandem aerobic oxidation–Knoevenagel condensation reac-

tion.

Substrate Product Conversion of 1a–f (%) Yield of 2a–f (%)

99 99

97 97

99 99

80 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 aer-

obic oxidation–Knoevenagel condensation reaction. The recyclability studies showed that

the RuIII@bipy-CTF catalyst maintains almost its full catalytic performance after four con-

secutive cycles with no obvious loss of activity or selectivity (Figure 4). Moreover, the re-

cycled catalyst showed no detectable Ru leaching (analyzed by ICP-OES).

97 97

Molecules 2021, 26, x FOR PEER REVIEW 7 of 12

Table 3. Catalytic performance of RuIII@bipy-CTF catalyst in tandem aerobic oxidation–Knoevenagel condensation reac-

tion.

Substrate Product Conversion of 1a–f (%) Yield of 2a–f (%)

99 99

97 97

99 99

80 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 aer-

obic oxidation–Knoevenagel condensation reaction. The recyclability studies showed that

the RuIII@bipy-CTF catalyst maintains almost its full catalytic performance after four con-

secutive cycles with no obvious loss of activity or selectivity (Figure 4). Moreover, the re-

cycled catalyst showed no detectable Ru leaching (analyzed by ICP-OES).

Molecules 2021, 26, x FOR PEER REVIEW 7 of 12

Table 3. Catalytic performance of RuIII@bipy-CTF catalyst in tandem aerobic oxidation–Knoevenagel condensation reac-

tion.

Substrate Product Conversion of 1a–f (%) Yield of 2a–f (%)

99 99

97 97

99 99

80 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 aer-

obic oxidation–Knoevenagel condensation reaction. The recyclability studies showed that

the RuIII@bipy-CTF catalyst maintains almost its full catalytic performance after four con-

secutive cycles with no obvious loss of activity or selectivity (Figure 4). Moreover, the re-

cycled catalyst showed no detectable Ru leaching (analyzed by ICP-OES).

99 99

Molecules 2021, 26, x FOR PEER REVIEW 7 of 12

Table 3. Catalytic performance of RuIII@bipy-CTF catalyst in tandem aerobic oxidation–Knoevenagel condensation reac-

tion.

Substrate Product Conversion of 1a–f (%) Yield of 2a–f (%)

99 99

97 97

99 99

80 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 aer-

obic oxidation–Knoevenagel condensation reaction. The recyclability studies showed that

the RuIII@bipy-CTF catalyst maintains almost its full catalytic performance after four con-

secutive cycles with no obvious loss of activity or selectivity (Figure 4). Moreover, the re-

cycled catalyst showed no detectable Ru leaching (analyzed by ICP-OES).

Molecules 2021, 26, x FOR PEER REVIEW 7 of 12

Table 3. Catalytic performance of RuIII@bipy-CTF catalyst in tandem aerobic oxidation–Knoevenagel condensation reac-

tion.

Substrate Product Conversion of 1a–f (%) Yield of 2a–f (%)

99 99

97 97

99 99

80 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 aer-

obic oxidation–Knoevenagel condensation reaction. The recyclability studies showed that

the RuIII@bipy-CTF catalyst maintains almost its full catalytic performance after four con-

secutive cycles with no obvious loss of activity or selectivity (Figure 4). Moreover, the re-

cycled catalyst showed no detectable Ru leaching (analyzed by ICP-OES).

80 80

Reaction conditions: 1 mol% catalyst (based on Ru, obtained from ICP-OES analysis), 0.33 mmol alcohol, 0.4 mmol Cs

, 500

L toluene,

0.33 mmol malononitrile, O

, 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 Ru

III

@bipy-CTF catalyst was investigated for the tandem

aerobic oxidation-Knoevenagel condensation reaction. The recyclability studies showed

that the Ru

III

@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).

Molecules 2021, 26, x FOR PEER REVIEW 8 of 12

Figure 4. Recyclability of the RuIII@bipy-CTF catalyst (1 mol% catalyst, 0.33 mmol benzyl 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).

A comparison of the catalytic performance of the presented system is made with var-

ious 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 Au@Cu(II)-MOF Air/110 15 + 7 99/99 [42]

2 Au@MIL-53(NH2) O2/100 13 99/99 [43]

3 UoB-2 (Ni-MOF) TBHP/65 1.5 94 [44]

4 Cu3TATAT-3 MOF O2, TEMPO/75 12 95/95 [45]

5 Pd/COF-TaPa-Py O2/80 4 + 1.5 98/98 [46]

6 5CoOx/tri-g-C3N4 O2/80 6 96.4/96.4 [47]

7 RuIII@bipy-CTF O2/100 12 + 1 99/99 This work

a “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−Kα 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 cat-

alytic tests. Helium was used as the carrier gas and the flow rate was programmed as 0.8

Figure 4.

Recyclability of the Ru

III

@bipy-CTF catalyst (1 mol% catalyst, 0.33 mmol benzyl alcohol,

0.4 mmol Cs

, 500

L toluene, 0.33 mmol malononitrile, O

, 100

◦

C, 12 h (1st step) and 70

◦

C, 1 h

(2nd step).

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 Ru

III

@bipy-CTF catalyst

using O

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 Ru

III

@bipy-CTF catalyst with other heterogeneous catalysts for selective tandem oxidation-

Knoevenagel condensation reaction.

Entry Catalyst Oxidant/Temp. (◦C) Time (h) aConv./Yield (%) Ref

1 Au@Cu(II)-MOF Air/110 15 + 7 99/99 [42]

2 Au@MIL-53(NH2) O2/100 13 99/99 [43]

3 UoB-2 (Ni-MOF) TBHP/65 1.5 94 [44]

4 Cu3TATAT-3 MOF O2, TEMPO/75 12 95/95 [45]

5 Pd/COF-TaPa-Py O2/80 4 + 1.5 98/98 [46]

6 5CoOx/tri-g-C3N4O2/80 6 96.4/96.4 [47]

7RuIII@bipy-CTF O2/100 12 + 1 99/99 This work

a“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

−

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 K

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 V2O5as the catalyst.

All chemicals were purchased from Sigma-Aldrich, abcr or TCI Europe and used

without further purification. 5,5

-dicyano-2,2

-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 [

]. Typically,

a glass ampule was filled with 5,5

-dicyano-2,2

-bipyridine (100 mg, 0.48 mmol) and ZnCl

(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

O overnight at 60

◦

C, filtered and washed with H

O 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), H

O (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)

(CH

CN)

)PF

complex was prepared according to the literature

method [

]. For this, (Ru(acac)

) (400 mg, 1 mmol) was dissolved in 30 mL of CH

CN and

to this solution, 10 mL of a 1% H

/CH

CN 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.

(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.

Thepost-modificationofthebipy-CTFwasperformedasfollows: (Ru(acac)

(CH

CN)

)PF

(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 CH

for 24 h to remove the unreacted residue of the Ru complex. Then, the modified material

was filtered, and washed thoroughly with CH

CN 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), Cs

(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

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 Cs

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

◦

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)

(CH

CN)

)PF

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

-unsaturated nitriles using O

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 Ru

III

@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 O

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

-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.

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