A Ru-Complex Tethered to a N-Rich Covalent Triazine Framework for Tandem Aerobic Oxidation-Knoevenagel Condensation Reactions

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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 1 and Pascal Van Der Voort 1,*

1 Department 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.) 2 Institut 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 RuIII complex. The bipyridine sites within the CTF provide excellent anchoring points for the III [Ru(acac)2(CH3CN)2]PF6 complex. The obtained robust Ru @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 Citation: Watson, G.; Gohari Derakhshandeh, P.; Abednatanzi, S.; provides the active sites for both aerobic oxidation and Knoevenagel condensation reactions. This Schmidt, J.; Leus, K.; Van Der Voort, P. work highlights a new perspective for the development of highly efﬁcient and robust heterogeneous A Ru-Complex Tethered to a catalysts applying CTFs for cascade catalysis. N-Rich Covalent Triazine Framework for Tandem Aerobic Keywords: covalent triazine frameworks; heterogeneous catalysis; tandem catalysis; aerobic oxidation- Oxidation-Knoevenagel knoevenagel condensation Condensation Reactions. Molecules 2021, 26, 838. https://doi.org/ 10.3390/molecules26040838 1. Introduction Academic Editors: Igor Djerdj and Conventional porous materials including silica, zeolite and activated charcoal have Sergio Navalo attracted extensive interest in large-scale industrial applications, most importantly in Received: 23 December 2020 Accepted: 2 February 2021 heterogeneous catalysis [1–3]. However, the poor chemical versatility of their chemical 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

Publisher’s Note: MDPI stays neutral and efficient porous supports with tailor-made functionalities rather than prefabricated with regard to jurisdictional claims in materials for targeted liquid phase reactions [4,5]. The recently emerging porous materials, published maps and institutional affil- particularly metal–organic frameworks (MOFs) and covalent organic frameworks (COFs) iations. have led to excellent progress in the field [6]. These materials possess a large surface area with regular and accessible pores, and an adjustable skeleton, making them attractive for several purposes of interest [7]. In contrast to MOFs fabricated from inorganic nodes (metal ions/clusters), COFs are purely organic materials and constructed from covalently

Copyright: © 2021 by the authors. linking light atoms (C, O, N, P, B, Si) [8,9]. Covalent Triazine Frameworks (CTFs) are a class Licensee MDPI, Basel, Switzerland. of COFs discovered in 2008 by Thomas and Antonietti [10]. CTFs are formed through a This article is an open access article trimerization reaction followed by the subsequent oligomerization of aromatic nitriles [11]. distributed under the terms and The robust aromatic covalent bonds endow CTFs with excellent stability compared to conditions of the Creative Commons coordinative-linked porous materials [12]. Additionally, CTFs contain a high amount of Attribution (CC BY) license (https:// nitrogen functionalities in their networks, allowing them to be outstanding candidates as creativecommons.org/licenses/by/ supports for various catalytic active centers [13]. 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 nanoparticles 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 intermediates 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 aldehydes [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 heterogeneous 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) We targeted a CTF with free 2,20-bipyridine building blocks (5,50-dicyano-2,20-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, 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-

Molecules 2021, 26, 838 idine), which forms excellent anchoring points. The bipy-CTF material was synthesized3 of 12 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. The obtained bipy-CTF material was post-modified with the (Ru(acac)The obtained2(CH3CN) bipy-CTF2)PF6 complex material through was post-modiﬁed a simple wet withimpregnation the (Ru(acac) method,2(CH as3CN) depicted2)PF6 incomplex Figure through1. a simple wet impregnation method, as depicted in Figure1.

FigureFigure 1. 1. SchematicSchematic representation ofof thethe idealideal ordered ordered structure structure of of Ru RuIII@bipy-covalentIII@bipy-covalent triazine triazine framework framework (CTF) (CTF) material. material. In the diffuse reflectance infrared Fourier transform (DRIFT) spectrum of the bipy- CTFIn (Figure the diffuse2a), the reflectance characteristic infrared bands Fourie of ther transform triazine (DRIFT) fragment spectrum appear atof 1356the bipy- and CTF1521 (Figure cm−1. The2a), absencethe characteristic of the intense bands nitrile of the band triazine at around fragment 2330 appear cm−1 demonstratesat 1356 and 1521 the cmcomplete−1. The absence consumption of the of intense monomer nitrile and band formation at around of triazine 2330 cm linkages.−1 demonstrates The doublet the com- band pleteat around consumption 1602–1626 of monomer cm−1 is ascribed and formation to the C=N of triazine vibrations linkages. of the The bipy doublet moiety. band In the at aroundDRIFT 1602–1626 spectrum ofcm the−1 is Ru ascribedIII@bipy-CTF to the C=N material, vibrations the vibration of the bipy bands moiety. of the In bipy the DRIFT moiety spectrumare shifted of (~10 the cm Ru−III1)@bipy-CTF to a lower frequency,material, the which vibration may be duebands to coordinationof the bipy withmoiety the are Ru shiftedcomplex. (~10 A cm similar−1) to observation a lower frequency, is reported which in previous may be studiesdue to coordination [36]. with the Ru complex.The pristineA similar bipy-CTF observation material is reported displays in a previous rapid uptake studies of N[36].2 at low relative pressures whichThe is pristine indicative bipy-CTF of a highly material microporous displays material a rapid uptake (Figure 2ofb). N This2 at low profile relative is assigned pressures to − whicha type is I isothermindicative and of exhibitsa highly amicroporous Brunauer–Emmett–Teller material (Figure (BET) 2b). surface This profile area of is 787 assigned m2 g 1 . 3 −1 toThe a totaltype poreI isotherm volume and was exhibits found toa beBrunauer–Emmett–Teller 0.40 cm g at P/P0 = 0.99.(BET) After surface the introductionarea of 787 2 −1 mof2g the−1. The Ru complex,total pore the volume BET surface was found area to decreases be 0.40 cm moderately3 g−1 at P/P to0 = 556 0.99. m Afterg , indicatingthe intro- ductionthat most of ofthe the Ru pores complex, are still the accessible. BET surface area decreases moderately to 556 m2g−1, indi- III catingThe that powder most of X-ray the pores diffraction are still (PXRD) accessible. patterns of the pristine bipy-CTF and Ru @bipy- CTFThe are shownpowder in FigureX-ray2 c.diffraction As known (PXRD) from most pa oftterns the CTFs of the that pristine were prepared bipy-CTF ionother- and Rumally,III@bipy-CTF the bipy-CTF are shown materials in Figure were 2c. found As known to be predominantly from most of the amorphous. CTFs that Thewere broad pre- ◦ paredpeaks ationothermally, 2θ~13 and 25 theare bipy-CTF assigned tomaterial the 00ls reflection were found showing to be a “graphitic”predominantly layer amor- stack- phous.ing. It isThe important broad peaks to note at that2θ ~ the 13 exact and 25° structure are assigned of these to amorphous the 00l reflection materials showing cannot bea ‘‘graphitic’’determined layer since stacking. the harsh Itsynthesis is important conditions to note resultthat the in exact carbonization structure andof these blackening amor- phousof the materials material, cannot making be it determined difficult to fullysince characterize.the harsh synthesis To estimate conditions the carbonizationresult in car- bonizationdegree and and purity blackening of the materials, of the material, we applied making elemental it difficult analysis to fully (Table characterize.1). The CHN To data es- timateobtained the fromcarbonization the bipy-CTF degree material and purity reveal of the a C/N materials, ratio of we 2.9 applied and the elemental theoretical analysis value for C/N in the bipy-CTF sample is calculated to be 2.6. Therefore, partial carbonization of (Table 1). The CHN data obtained from the bipy-CTF material reveal a C/N ratio of 2.9 around 10% occurs, while 90% of the structural composition is preserved. and the theoretical value for C/N in the bipy-CTF sample is calculated to be 2.6. Therefore,

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- 59.6 15.7 3.8 0.15 CTF a Determined by elemental analysis. b Determined by ICP-OES analysis. 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)

Molecules 2021bipy-CTF, 26, 838 58.92 20.27 2.9 - 4 of 12 RuIII@bipy-CTF 59.6 15.7 3.8 0.15 a Determined by elemental analysis. b Determined by ICP-OES analysis.

The thermalthermal stabilitystability ofof bothboth materials materials was was determined determined by by thermogravimetric thermogravimetric analysis analy- (TGA).sis (TGA). The The TGA TGA profile profile of the of bipy-CTFthe bipy-CTF displayed displayed a high a high thermal therma stabilityl stability up to up approxi- to ap- ◦ ◦ matelyproximately 550 C550 (Figure °C (Figure2d). A2d). first A weightfirst weight loss ofloss about of about 8% below8% below 150 150C corresponds°C corresponds to theto the loss loss of waterof water and and organic organic solvent solvent molecules. molecules. The The TGA TGA profile profile of theof the modified modified sample sam- III ◦ showsple shows that that the the Ru Ru@bipy-CTFIII@bipy-CTF material material is is thermally thermally stable stable up up to to 300 300 °C,C, andand graduallygradually decomposes at higher temperatures.temperatures.

FigureFigure 2.2. StructuralStructural characterizationcharacterization ofof bipy-CTFbipy-CTF andand RuRuIIIIII@bipy-CTF@bipy-CTF materials. (a) DiffuseDiffuse reﬂectancereflectance infraredinfrared FourierFourier transformtransform (DRIFT)(DRIFT) spectra.spectra. ((bb)) NitrogenNitrogen adsorption/desorptionadsorption/desorption isotherms. isotherms. ( (cc)) Powder Powder X-ray diffraction (XRD) patterns. ((dd)) ThermogravimetricThermogravimetric analysisanalysis (TGA)(TGA) curves.curves.

Further structural characterization was donedone by applyingapplying X-rayx-ray photoelectron spectroscopy (XPS). (XPS). In In the the N N 1S 1S spectrum spectrum of the of thebipy-CTF bipy-CTF material material (Figure (Figure 3a), a3 a),peak a peakat 398.39 at 398.39eV confirms eV conﬁrms the existence the existence of the ofpyridinic the pyridinic nitrogen nitrogen in the inframework. the framework. Besides, Besides, the peaks the peaksat 399.58 at 399.58and 400.38 and 400.38eV are eVattributed are attributed to the pyrrolic-and to the pyrrolic-and quaternary-N quaternary-N species, species,respec- respectively.tively. These Thesenitrogen nitrogen functionalities functionalities are fo arermed formed during during the synthesis the synthesis at high at hightempera- tem- peratures, as reported by Osadchii et al. [37]. In the N 1s spectrum of the RuIII@bipy-CTF material (Figure3b), 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 (Figure3c). Moreover, the Ru 3d peak is seen at 285 eV (Figure3d). 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, 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 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, Molecules 2021, 26, 838 the Ru 3d peak is seen at 285 eV (Figure 3d). Based on the inductively coupled plasma5 of 12 (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.

FigureFigure 3.3.Structural Structural characterization characterization of of bipy-CTF bipy-CTF and and Ru RuIII@bipy-CTFIII@bipy-CTF materials. materials. (a )(a N) N 1S 1S XPS XPS spectrum spectrum of theof the bipy-CTF. bipy-CTF. (b) III III N(b 1s) N XPS 1s XPS spectrum spectrum of the ofRu theIII Ru@bipy-CTF.@bipy-CTF. (c) Ru (c) 3pRu XPS 3p XPS spectrum spectrum of the of Ru theIII Ru@bipy-CTF.@bipy-CTF. (d) Ru(d) 3dRu XPS 3d XPS spectrum spectrum of the of III RutheIII Ru@bipy-CTF.@bipy-CTF.

2.2. Catalytic Activity of the RuIII@bipy-CTF Catalyst in the Tandem Aerobic Oxidation– 2.2. Catalytic Activity of the RuIII@bipy-CTF Catalyst in the Tandem Aerobic oxidation-KnoevenagelKnoevenagel Condensation Condensation Reaction Reaction The catalytic activity of heterogeneous Ru catalysts for oxidation reactions using O2 The catalytic activity of heterogeneous Ru catalysts for oxidation reactions using O2 or airor asair the as greenthe green oxidant oxidant remains remains a challenge. a challenge. Initially, theInitially, catalytic the activity catalytic of theactivity RuIII @bipy-of the CTFRuIII@bipy-CTF catalyst was catalyst tested was under tested aerobic under conditions aerobic conditions for the selective for the selective oxidation oxidation of benzyl of alcoholbenzyl toalcohol benzaldehyde. to benzaldehyde. The catalytic The resultscatalytic are results presented are inpresented Table2. Thein Table Ru III @bipy-2. The CTFRuIII@bipy-CTF catalyst displays catalyst a moderatedisplays a activity moderate with activity a conversion with a conversion of 37% using of toluene37% using as thetol- reactionuene as mediumthe reaction (Table medium2, entry (Table 1). To 2, further entry 1). enhance To further the catalyticenhance performancethe catalytic ofperfor- the catalyst,mance of different the catalyst, bases different were applied bases (Tablewere 2applied, entries (Table 2–4). The2, entries catalytic 2–4). conversion The catalytic of benzylconversion alcohol of benzyl increases alcohol in the increases presence in ofthe K presence2CO3 and of CsK22COCO33 andwith Cs conversions2CO3 with conver- of 78 andsions 99%, of 78 respectively. and 99%, respectively. Notably, no productNotably, of no over-oxidation product of over-oxidation (benzoic acid) (benzoic was detected, acid) provingwas detected, the high proving selectivity the high of theselectivity Ru catalyst of the towards Ru catalyst benzaldehyde. towards benzaldehyde. In the absence In the 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 Table2 ). The catalytic activity of the RuIII@bipy-CTF catalyst was further compared with its homogeneous counterpart (Table2, 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, 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 Molecules 2021, 26, 838 role of the bipy-CTF support (see mechanistic studies in the Supplementary6 of 12 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 reactionidentical conditions reaction (Table conditions2, entry 9). (Table It has 2, been entry proven 9). It thathas nitrogen-richbeen proven carbonthat nitrogen-rich car- materials are effectivebon materials catalysts are for effective aerobic oxidationcatalysts for reactions aerobic [39 oxidation]. We recently reactions showed [39]. We recently the unique propertiesshowed ofthe CTFs unique to proceedproperties with of aerobicCTFs to oxidation proceed with reactions aerobic assisted oxidation by reactions as- nitrogen functionalitiessisted by nitrogen [40,41]. Morefunctionalities speciﬁcally, [40,41]. CTFs More with specifically, quaternary CTFs N species with canquaternary N spe- activate molecularcies Ocan2 to activate generate molecular oxygen radicalsO2 to generate (superoxide) oxygen which radicals further (superoxide) promote which the further pro- oxidation reaction.mote the oxidation reaction.

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

Entry EntryCatalyst Catalyst Base Base ConversionConversion (%) (%) TON a TON a 1 1 RuIII@bipy-CTFRuIII@bipy-CTF NoNo base base 37 37 37 37 III III 2 2 Ru @bipy-CTFRu @bipy-CTF NaNa2CO2CO3 3 4141 41 41 III 3 3 RuIII@bipy-CTFRu @bipy-CTF KK2CO2CO3 3 7878 78 78 4 RuIII@bipy-CTF Cs CO 99 99 III 2 2 3 3 4 Ru @bipy-CTFIII b Cs CO 99 99 5 Ru @bipy-CTF Cs2CO3 64 267 5 RuIII@bipy-CTF b Cs2CO3 64 267 6 No catalyst Cs2CO3 <1 - III c 2 3 6 7 NoRu catalyst@bipy-CTF CsCs2COCO3 3<1 3 - III c 7 8Ru [Ru(acac)@bipy-CTF2(CH3CN) 2]PF6 CsCs2CO2CO3 3 543 54 3 d 8 9 [Ru(acac)2(CHbipy-CTF3CN)2]PF6 CsCs2CO2CO3 3 3954 39 54 9 Reaction conditions:bipy-CTF 1 mol% catalyst d (based on Ru, obtainedCs2 fromCO3 ICP-OES analysis), 0.3339 mmol benzyl alcohol, 39 0.4 mmol base, 500 µL toluene, O , 100 ◦C, 12 h. a mmol of product formed per mmol of Ru in the catalyst. Reaction conditions: 1 mol% catalyst (based2 on Ru, obtained from ICP-OES analysis), 0.33 mmol benzyl alcohol, 0.4 mmol b 0.24 mol% catalyst was used. c Under Ar atmosphere. d 17 mg catalyst was used. All catalysts displayed >99% a b base, 500 μselectivityL toluene, toward O2, 100 benzaldehyde. °C, 12 h. mmol of product formed per mmol of Ru in the catalyst. 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 catalyst in the tandemOur aerobicfurther studies oxidation-Knoevenagel focused on the catalytic condensation performance reaction. of the For Ru thisIII@bipy-CTF cata- purpose, the optimizedlyst in the reaction tandem condition aerobic foroxidation–Knoevenagel the oxidation of benzyl condensation alcohol was selectedreaction. For this pur- III ◦ (1 mol% Ru @bipy-CTF,pose, the optimized Cs2CO3,O reaction2, 100 C,condition 12 h). Different for the oxidation substituted of benzylbenzyl alcoholsalcohol was selected (1 and malononitrilemol% were Ru examinedIII@bipy-CTF, and Cs the2CO obtained3, O2, 100 results °C, 12 are h). listed Different in Table substituted3. No product benzyl alcohols and was formed inmalononitrile the absence ofwere the examined catalyst. and As shownthe obtained in Table results3, the are Ru listedIII@bipy-CTF in Table 3. No product MoleculesMolecules 2021 2021, 26, 26, x, xFOR FOR PEER PEER REVIEW REVIEWcatalyst was foundwas formed to be highly in the activeabsence in of the the Knoevenagel catalyst. As shown condensation in Table reaction 3, the Ru7 under7ofIII of @bipy-CTF12 12 catalyst MoleculesMolecules Molecules 2021 2021 2021, ,26 26, ,26 ,x x ,FOR xFOR FOR PEER PEER PEER REVIEW REVIEW REVIEW 77 of 7of of12 12 12 mild reaction conditions.was found to A be high highly conversion active in the was Knoevenagel obtained for condensation all substrates reaction at a low under mild reac- ◦ temperature (70tionC) conditions. and only A after high 1 h.conversion Moreover, was complete obtained selectivity for all substrates was observed at a low temperature towards the corresponding product. TableTable 3. 3. Catalytic Catalytic performance performance of of Ru RuIIIIII@bipy-CTF@bipy-CTF(70 °C)catalyst catalyst and in onlyin tandem tandem after aerobic aerobic1 h. Moreover, oxidation–Knoevenagel oxidation–Knoevenagel complete selectivity condensation condensation was observedreac- reac- towards the cor- TableTable 3. 3. Catalytic Catalytic performance performance of of Ru RuIIIIII@bipy-CTFIII@bipy-CTF catalyst catalyst in in tandem tandem aerobic aerobic oxidation–Knoevenagel oxidation–Knoevenagel condensation condensation reac- reac- tion.tion.Table 3. Catalytic performance of Ru @bipy-CTFresponding catalyst product. in tandem aerobic oxidation–Knoevenagel condensation reaction.tion.Tabletion. 3. Catalytic performance of RuIII@bipy-CTF catalyst in tandem aerobic oxidation-Knoevenagel condensation reaction.

SubstrateSubstrate Product Product Conversion Conversion of of 1a–f 1a–f (%) (%) Yield Yield of of 2a–f 2a–f (%) (%) SubstrateSubstrateSubstrateSubstrate Product Product Product Product Conversion Conversion Conversion Conversion of of 1a–fof of 1a–f 1a–f 1a–f (%) (%) (%) (%) Yield Yield Yield ofof of 2a–fof 2a–f 2a–f 2a–f (%) (%) (%) (%)

9999 9999 99999999 99999999

999999 999999 999999 999999

9999 9999 999999 999999

9797 9797 979797 979797

9999 9999 999999 999999

8080 8080 808080 808080

ReactionReaction conditions: conditions: 1 1mol% mol% catalyst catalyst (based (based on on Ru, Ru, obtain obtaineded from from ICP-OES ICP-OES analysis), analysis), 0.33 0.33 mmol mmol alcohol, alcohol, 0.4 0.4 mmol mmol Cs Cs2CO2CO3,3 , ReactionReactionReaction conditions: conditions: conditions: 1 1 mol% 1mol% mol% catalyst catalyst catalyst (based (based (based on on on Ru, Ru, Ru, obtain obtain obtainededed from from from ICP-OES ICP-OES ICP-OES analysis), analysis), analysis), 0.33 0.33 0.33 mmol mmol mmol alcohol, alcohol, alcohol, 0.4 0.4 0.4 mmol mmol mmol Cs Cs Cs2CO2CO2CO3,3 ,3 , 500500 μ μLL toluene, toluene, 0.33 0.33 mmol mmol malononitrile, malononitrile, O O2,2 100, 100 °C, °C, 12 12 h h (1st (1st step) step) and and 70 70 °C, °C, 1 1h h (2nd (2nd step). step). All All substrates substrates displayed displayed >99% >99% 500500 μ μLL toluene, toluene, 0.33 0.33 mmol mmol malononitrile, malononitrile, O O2,2 ,1002 100 °C, °C, 12 12 h h (1st (1st step) step) and and 70 70 °C, °C, 1 1 h h (2nd (2nd step). step). All All substrates substrates displayed displayed >99% >99% selectivityselectivity500 μL toluene, toward toward 0.33 the the correspondingmmol corresponding malononitrile, product. product. O , 100 °C, 12 h (1st step) and 70 °C, 1 h (2nd step). All substrates displayed >99% selectivityselectivityselectivity toward toward toward the the the corresponding corresponding corresponding product. product. product. TheThe recyclability recyclability of of the the Ru RuIIIIII@bipy-CTF@bipy-CTF catalyst catalyst was was investigated investigated for for the the tandem tandem aer- aer- TheTheThe recyclability recyclability recyclability of of of the the the Ru Ru RuIIIIII@bipy-CTFIII@bipy-CTF@bipy-CTF catalyst catalyst catalyst was was was investigated investigated investigated for for for the the the tandem tandem tandem aer- aer- aer- obicobic oxidation–Knoevenagel oxidation–Knoevenagel condensation condensation reac reaction.tion. The The recyclability recyclability studies studies showed showed that that obicobicobic oxidation–Knoevenagel oxidation–Knoevenagel oxidation–Knoevenagel condensation condensation condensation reac reac reaction.tion.tion. The The The recyclability recyclability recyclability studies studies studies showed showed showed that that that thethe Ru RuIIIIII@bipy-CTF@bipy-CTF catalyst catalyst maintains maintains almost almost its its full full catalytic catalytic performance performance after after four four con- con- thethethe Ru Ru RuIIIIII@bipy-CTFIII@bipy-CTF@bipy-CTF catalyst catalyst catalyst maintains maintains maintains almost almost almost its its its full full full catalytic catalytic catalytic performance performance performance after after after four four four con- con- con- secutivesecutive cycles cycles with with no no obvious obvious loss loss of of activity activity or or selectivity selectivity (Figure (Figure 4). 4). Moreover, Moreover, the the re- re- secutivesecutivesecutive cycles cycles cycles with with with no no no obvious obvious obvious loss loss loss of of of activity activity activity or or or selectivity selectivity selectivity (Figure (Figure (Figure 4). 4). 4). Moreover, Moreover, Moreover, the the the re- re- re- cycledcycled catalyst catalyst showed showed no no detectable detectable Ru Ru leaching leaching (analyzed (analyzed by by ICP-OES). ICP-OES). cycledcycledcycled catalyst catalyst catalyst showed showed showed no no no detectable detectable detectable Ru Ru Ru leaching leaching leaching (analyzed (analyzed (analyzed by by by ICP-OES). ICP-OES). ICP-OES).

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

TableTable 3. 3. Catalytic Catalytic performance performance of of Ru RuIIIIII@bipy-CTF@bipy-CTF catalyst catalyst in in tandem tandem aerobic aerobic oxidation–Knoevenagel oxidation–Knoevenagel condensation condensation reac- reac- TableTable 3. 3. Catalytic Catalytic performance performance of of Ru RuIIIIII@bipy-CTF@bipy-CTF catalyst catalyst in in tandem tandem aerobic aerobic oxidation–Knoevenagel oxidation–Knoevenagel condensation condensation reac- reac- Tabletion.Tabletion. 3. 3. Catalytic Catalytic performance performance of of Ru RuIIIIII@bipy-CTF@bipy-CTF catalyst catalyst in in tandem tandem aerobic aerobic oxidation–Knoevenagel oxidation–Knoevenagel condensation condensation reac- reac- Tabletion.tion.Table 3. 3. Catalytic Catalytic performance performance of of Ru RuIIIIII@bipy-CTF@bipy-CTF catalyst catalyst in in tandem tandem aerobic aerobic oxidation–Knoevenagel oxidation–Knoevenagel condensation condensation reac- reac- tion.tion. tion.tion.

SubstrateSubstrate Product Product Conversion Conversion of of 1a–f 1a–f (%) (%) Yield Yield of of 2a–f 2a–f (%) (%) SubstrateSubstrate Product Product Conversion Conversion of of 1a–f 1a–f (%) (%) Yield Yield of of 2a–f 2a–f (%) (%) Molecules 2021SubstrateSubstrate, 26, x FOR PEER REVIEW Product Product Conversion Conversion of of 1a–f 1a–f (%) (%) Yield Yield of of 2a–f 2a–f (%) (%) 7 of 12 Molecules 2021SubstrateSubstrate, 26, 838 Product Product Conversion Conversion of of 1a–f 1a–f (%) (%) Yield Yield of of 2a–f 2a–f (%) (%) 7 of 12 9999 9999 9999 9999 9999 9999 Table 3. Catalytic performance of RuIII@bipy-CTF catalyst in tandem aerobic 99oxidation–Knoevenagel99 condensation9999 reac-

tion. Table 3. Cont.

9999 9999 9999 9999 9999 9999 9999 9999

Substrate Product Conversion9999 of 1a–f (%) Yield99 of99 2a–f (%) 9999 9999 9999 9999 999999 999999

99 99

9797 9797 9797 9797 979797 979797 9797 9797

99 99

9999 9999 999999 999999 9999 9999 9999 9999

99 99 808080 808080 8080 8080 8080 8080 8080 8080

ReactionReactionReaction conditions: conditions:conditions: 1 mol%1mol% mol% catalyst catalyst catalyst (based (based (based on Ru,on on Ru, obtained Ru, obtain obtain fromeded from ICP-OES from ICP-OES ICP-OES analysis), analysis), analysis), 0.33 mmol 0.33 0.33 alcohol, mmol mmol 0.4 alcohol, alcohol, mmol Cs 0.4 20.4CO mmol mmol3, 500 Csµ CsL2CO toluene,2CO3, 3, ◦ ◦ ReactionReaction0.33 mmol conditions: conditions: malononitrile, 1 1mol% mol% O ,catalyst 100catalystC, (based 12 (based h (1st on on step) Ru, Ru, andobtain obtain 70 edC,ed from 1 from h (2nd ICP-OES ICP-OES step). analysis), All analysis), substrates97 0.33 0.33 displayedmmol mmol alcohol, alcohol, >99% selectivity0.4 0.4 mmol mmol Cs toward 97Cs2CO 2CO3 the,3 , 500500 μ μLL toluene, toluene, 0.33 0.33 mmol mmol malononitrile, 2malononitrile, O O2, 2100, 100 °C, °C, 12 12 h h (1st (1st step) step) and and 70 70 °C, °C, 1 1h h (2nd (2nd step). step). All All substrates substrates displayed displayed >99% >99% ReactionReactioncorresponding conditions: conditions: product. 1 1mol% mol% catalyst catalyst (based (based on on Ru, Ru, obtain obtaineded from from ICP-OES ICP-OES analysis), analysis), 0.33 0.33 mmol mmol alcohol, alcohol, 0.4 0.4 mmol mmol Cs Cs2CO2CO3, 3, Reaction500selectivity500Reaction μ μLL toluene, toluene, conditions: conditions:toward 0.33 0.33 the mmol1 mmol 1mol%corresponding mol% malononitrile, malononitrile, catalyst catalyst (based product. (based O O2 on,2 100,on 100 Ru, Ru, °C, °C, obtain obtain12 12 h h (1sted (1sted from step)from step) ICP-OES andICP-OES and 70 70 °C, °C,analysis), analysis), 1 1h h (2nd (2nd 0.33 step).0.33 step). mmol mmol All All substratesalcohol, substrates alcohol, 0.4 0.4 displayed displayedmmol mmol Cs Cs 2>99%CO >99%2CO3, 3, 500500selectivity μ μLL toluene, toluene, toward 0.33 0.33 themmol mmol corresponding malononitrile, malononitrile, product. O O2, 2100, 100 °C, °C, 12 12 h h(1st (1st step) step) and and 70 70 °C, °C, 1 1h h(2nd (2nd step). step). All All substrates substrates displayed displayed >99% >99% 500selectivityselectivity500 μ μLL toluene, toluene, toward toward 0.33 0.33 the themmol mmol corresponding corresponding malononitrile, malononitrile, product. product. O O2, 2100, 100 °C, °C, 12 12 h h(1st (1st step) step) and and 70 70 °C, °C, 1 1h h(2nd (2nd step). step). All All substrates substrates displayed displayed >99% >99% selectivityselectivity toward toward the the corresponding corresponding product. product. III selectivityselectivity toward toward the the corresponding correspondingTheTheThe product.recyclability product.recyclability recyclability of of the of the theRu RuIII RuIII@bipy-CTF@bipy-CTF@bipy-CTF catalyst catalyst catalyst was was investigated was investigated investigated for for the the for tandem tandem the tandem aer- aer- TheThe recyclability recyclability of of the the Ru RuIIIIII@bipy-CTF@bipy-CTF catalyst catalyst was was investigated investigated for for the the tandem tandem aer- aer- obicobicaerobic oxidation–KnoevenagelThe oxidation–KnoevenagelThe recyclability oxidation-Knoevenagelrecyclability of of the the Ru condensationRu condensationIIIIII@bipy-CTF@bipy-CTF condensation reac reaccatalyst catalysttion.tion. reaction.99 Thewas Thewas recyclabilityinvestigated recyclability Theinvestigated recyclability studies for studies for the the studies showedtandem 99showedtandem showed aer-that aer-that obicobic oxidation–KnoevenagelThe oxidation–KnoevenagelThe recyclability recyclabilityIII of of the the Ru condensationRu condensationIIIIII@bipy-CTF@bipy-CTF reac reaccatalyst catalysttion.tion. Thewas Thewas recyclability investigatedrecyclability investigated studies forstudies for the the showedtandem showedtandem thataer- thataer- obictheobicthethat Ru oxidation–KnoevenagelRu oxidation–KnoevenagelIII theIII@bipy-CTF@bipy-CTF Ru @bipy-CTF catalyst catalyst catalyst maintains condensationmaintains condensation maintains almost almost reac reac almostits itstion. tion.full full The catalytic itsThe catalytic fullrecyclability recyclability catalytic performance performance performancestudies studies after after showed showed four four after con-that con-that four thethe Ru RuIIIIII@bipy-CTF@bipy-CTF catalyst catalyst maintains maintains almost almost its its full full catalytic catalytic performance performance after after four four con- con- Molecules 2021, 26, x FOR PEER REVIEWobicobicconsecutive oxidation–Knoevenagel oxidation–KnoevenagelIIIIII cycles with no condensation obvious condensation loss ofreac reac activitytion.tion. The orThe selectivity recyclability recyclability (Figure studies studies4). Moreover,showed showed that that8 theof 12 thesecutivethesecutive Ru Ru@bipy-CTF cycles@bipy-CTF cycles with with catalyst nocatalyst no obvious obvious maintains maintains loss loss of ofalmost activityalmost activity its its or fullor fullselectivity selectivity catalytic catalytic (Figureperformance (Figureperformance 4). 4). Moreover, Moreover, after after four four the the con- con-re- re- thesecutivesecutivethe Ru RuIIIIII@bipy-CTF cycles@bipy-CTF cycles with with catalyst nocatalyst no obvious obvious maintains maintains loss loss of ofalmost activityalmost activity its itsor orfull fullselectivity selectivity catalytic catalytic (Figure performance(Figure performance 4). 4). Moreover, Moreover, after after four four the the con- con-re- re- secutivecycledsecutivecycledrecycled catalyst catalystcycles cycles catalyst showedwith showedwith showed no no obviousno obviousno detectable nodetectable detectable loss loss of Ruof Ruactivity activity leachingRu leaching leaching or or selectivity(analyzed (analyzedselectivity (analyzed by(Figure by(Figure ICP-OES). byICP-OES). ICP-OES). 4). 4). Moreover, Moreover, the the re- re- secutivecycledcycledsecutive catalyst catalyst cycles cycles showedwith showedwith no no noobvious noobvious detectable detectable loss loss of Ruof Ru activity activityleaching leaching or or (analyzedselectivity (analyzedselectivity by(Figure by(Figure ICP-OES). ICP-OES). 4). 4). Moreover, Moreover, the the re- re- cycledcycled catalyst catalyst showed showed no no detectable detectable Ru Ru leaching leaching (analyzed (analyzed by by ICP-OES). ICP-OES). cycledcycled catalyst catalyst showed showed no no detectable detectable Ru Ru leaching leaching 80(analyzed (analyzed by by ICP-OES). ICP-OES). 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 re- cycled catalyst showed no detectable Ru leaching (analyzed by ICP-OES).

FigureFigure 4.4. RecyclabilityRecyclability of the Ru III@bipy-CTF@bipy-CTF catalyst catalyst (1 (1 mol% mol% catalyst, catalyst, 0.33 0.33 mmol mmol benzyl benzyl alcohol, alcohol, 0.4 mmol Cs2CO3, 500 μL toluene, 0.33 mmol malononitrile, O2, 100◦ °C, 12 h (1st step) and 70◦ °C, 1 0.4 mmol Cs2CO3, 500 µL toluene, 0.33 mmol malononitrile, O2, 100 C, 12 h (1st step) and 70 C, 1 h (2ndh (2nd step). step).

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

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 Table4, a high yield of benzylmalononitrile is obtained over the Ru III@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] III 7 Ru @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 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 procedure 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 µL) 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.

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