Recyclable Covalent Triazine Framework-based Ru Catalyst for

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A Recyclable Covalent Triazine Framework Based Ru Catalyst for Transfer Hydrogenation of Carbonyl Compounds in Water Sudakar Padmanaban, Gunniya Hariyanandam Gunasekar, Mearae Lee, and Sungho Yoon ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 9, 2019

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ACS Sustainable Chemistry & Engineering

A Recyclable Covalent Triazine Framework Based Ru Catalyst for Transfer Hydrogenation of Carbonyl Compounds in Water Sudakar Padmanaban,† Gunniya Hariyanandam Gunasekar,§ Mearae Lee,† and Sungho Yoon*† †Department of Applied Chemistry, Kookmin University, 861-1, Jeongneung-dong, Seongbukgu, Seoul 136-702, Republic of Korea. E-mail: [email protected]. §Clean Energy Research Centre, Korea Institute of Science and Technology, P. P. Box 131, Cheongryang, Seoul 136-791, Republic of Korea. Keywords: Green Chemistry, Heterogeneous catalysis, Covalent triazine framework, Transfer hydrogenation, Ru catalysis

ABSTRACT: A heterogeneous Ru catalyst, [(bpy-CTF)RuCl3], which is synthesized by the coordinative immobilization of RuCl3.xH2O on a bipyridyl-functionalized covalent triazine framework (bpy-CTF), is an efficient catalyst for the transfer hydrogenation (TH) of various carbonyl compounds into alcohols in water using HCOONa as a benign reducing agent. The asprepared heterogeneous catalyst exhibits a very high turnover number (~990) and an initial

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turnover frequency (44.8 h−1) as well as excellent recyclability, which makes it a greener catalyst for industrial applications in TH.

Introduction Transition metal complex catalyzed transfer hydrogenation (TH) of carbonyl compounds is given much importance while conducting research related to the synthesis of fine chemicals and pharmaceutical agents.1-9 The TH of hydrogen acceptors that use hydrogen donors apart from molecular hydrogen is an attractive alternative to the traditional hydrogenation techniques that are dependent on the usage of hazardous metal-hydride reagents and high-pressure hydrogen gas.4, 10 Several homogeneous transition-metal complexes have been developed to conduct the TH of carbonyl compounds.4 Among them, Ir-, Rh-, and Ru-based half-sandwich complexes are the most extensively studied entities.4,7,11-14 However, from an industrial viewpoint, TH of carbonyl compounds using heterogeneous catalysts in aqueous media is attractive because of the following factors: 1) Using water as a solvent eliminates the environmental problems that are associated with the usage of organic solvents and allows safe, clean, economical, and green conditions with reaction-specific pH selectivity.15,16 2) The usage of heterogeneous catalysts, with their simple operating procedures, avoids the problems that are associated with product separation and catalyst recycling.17-19 Consequently, numerous heterogeneous catalysts have been developed for TH by immobilizing the active transition-metal complexes onto various support materials, including carbon materials, such as carbon nanotubes, graphene, and activated carbon;20-23 inorganic supports, such as silica,2426

titanium dioxide,27-29 aluminum,30,31 and zirconium;32,33 polymeric supports;34-38 and other

magnetic materials.39,40 However, metal leaching during reactions, the usage of organic solvents


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in some cases, and reduced activities upon recycling have prevented the extensive commercialization of these catalysts.17 Recently, the covalent triazine frameworks (CTFs) have attracted a colossal attention because of their robustness in both acidic and basic media under a wide range of temperatures and pressures, their high surface areas, and their enhanced porosities with tunable pore sizes.41-44 Especially, the functionalized CTFs with a large number of coordination sites in their pore walls were considered to be excellent alternatives to perform the coordinative incorporation of metal ions.45-48 These supporting materials provide strongly bound metal complexes, ensuring minimal or zero metal leaching during catalysis and enhanced reusability. Indeed, CTF-heterogenizedmetal complexes have been recently applied to several catalytic transformations.49-62 Based on this context, we have recently reported CTFs bearing bipyridyl units (bpy-CTF) that generate heterogenized Rh and Ir half-sandwich complexes to conduct the TH of carbonyl compounds.63 These Rh and Ir catalysts (chart S1), namely [bpy-CTF-(Cp*RhCl)]Cl (1) and [bpyCTF-(Cp*IrCl)]Cl (2), depict maximal activities in case of the TH of various carbonyl compounds using an HCOOH/HCOONa buffer in water at a pH of 3.5. However, despite its high activity, catalyst 1 exhibited reduced activity during the recycling experiments. On the other hand, catalyst 2 exhibited excellent recyclability but lower reaction rates. Furthermore, the pH of the system should be compulsorily maintained at approximately 3.5 because any change in the pH of the solution would significantly affect the activities of both 1 and 2. Therefore, in commercial grounds, the CTF -based metal catalytic systems with relatively stable and inexpensive metal precursors that are active in water medium are generally observed to be more viable. In this regard, Ru complexes containing arene and non-arene ligands have been extensively used in recent decades because they are relatively inexpensive as compared to both Rh and Ir metal precursors.4 Among


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them, the Ru-complexes consisting non-arene ligands are considered as easily accessible candidates for larger scale TH reactions. Therefore, designing a new type of CTF heterogenized Ru catalyst without arene-ligands for effective, industrially viable, and greener aqueous phase TH reactions is highly desirable from both environmental and synthetic viewpoints. Interestingly, we have recently reported that the incorporation of simple RuCl3.xH2O into a bpy-CTF would generate a heterogeneous catalyst [(bpy-CTF)RuCl3] (3).64 In this context, we hypothesized that 3 could act as an active catalyst for the aqueous-phase TH of carbonyl compounds (Figure 1a). Here, we investigate a simple CTF-based heterogeneous Ru complex to perform the efficient TH of carbonyl compounds using HCOONa in water medium (Scheme 1).

O R

OH

[(bpy-CTF)RuCl3] R'

HCOONa, H2O, 80 °C

R

R'

R, R' = Aryl, Alkyl or H

Scheme 1. TH of carbonyl compounds using 3 Results and Discussion Synthesis and Characterization of 3 The porous bpy-CTF was prepared as per a reported procedure (Scheme S1).63 The active catalyst 3 was prepared by the metalation of bpy-CTF with RuCl3.xH2O in refluxing methanol for 13 h (Scheme S1). The resulting black solid was filtered and washed well using an excess of methanol followed by acetone and was then dried under vacuum at 25 °C for 24 h. The scanning electron microscopy (SEM) analysis of bpy-CTF and 3 depicts an irregular block-shaped morphology (Figure S1 and Figure 1b and Figure S2). The energy dispersive spectroscopy (SEMEDS) mapping of 3 depicts that the Ru and Cl atoms are uniformly distributed throughout the bpy-


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Figure 1. (a) Structural representation of catalyst 3; (b) SEM image of 3; EDS mapping of (c) Ru and (d) Cl in 3; (e) N2 sorption measurements of bpy-CTF and 3. Deconvoluted X-ray photoelectron spectra of 3 (f) C-1s and Ru-3d and (g) N-1s core level. CTF support in a ratio of 1:3 (Figure 1c–d)). The actual amount of Ru that was incorporated into 3 was 1.97 wt%, as revealed using the inductively coupled plasma optical emission spectrometry (ICP-OES) (Table S1). The attenuated total reflection infrared (ATR-IR) spectrum of 3 is comparable with that of bpy-CTF (Figure S3). Similarly, Figure S5 shows that the powder X-ray diffraction pattern of 3 is identical to that of the support. The absence of reflexes for RuCl3 in the PXRD pattern of 3 indicates that the Ru metal is well dispersed in the support.65 The surface properties of bpy-CTF and 3 were investigated by the N2 adsorption–desorption isotherm and Barrett–Joyner–Halenda (BJH) method. Figure 1e shows the N2 sorption isotherm of bpy-CTF and 3 in which both bpy-CTF and 3 exhibit type 1 isotherm with both micro and meso-pores. The Brunauer–Emmett–Teller (BET) surface area analyses of bpy-CTF and 3 depict that incorporation of RuCl3.xH2O results in a decrease in the surface area of 3 from 684 to 477 m2g-1 (Table S1). Similarly, the RuCl3.xH2O incorporation resulted in a decrease in the pore volume of 3 from 0.4 to 0.17 cm3 g−1 indicating the partial occupancy of the pores by the incorporated Ru precursor


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(Table S1). Regardless, 3 is still porous enough for the facile diffusion of substrates and reducing agents into catalytic sites to ensure effective catalysis. To identify the coordination environment of Ru in 3, X-ray photoelectron spectroscopy (XPS) analysis was performed.

Figure 2. (a) Catalytic activity of 3 for the TH of acetophenone at 80 °C in HCOOH/HCOONa buffer at different pH conditions for a period of 2.5 h. (b) Hot filtration test for the TH of acetophenone using catalyst 3; Black line indicates original reaction with catalyst and red line indicates reaction in filtrate after removing catalyst. (c) Recyclability of catalyst 3 for the TH of acetophenone. Reaction conditions: S/C = 200, 80 °C, 5 h. Figure 1f shows the Ru-3d core level spectrum that is partially overlapped with the C-1s core level spectrum. The Ru3d5/2 line is observed at a binding energy (BE) of 281.5 eV in the deconvoluted C-1s and Ru-3d core level spectrum demonstrating that the oxidation state of Ru in 3 is +3.66 The Ru-3d3/2 peak has been overlapped with the peak of the C–N species at 285.5 eV. The N-1s spectrum shows two peaks at 398.2 eV and 399.2 eV (Figure 1g); the peak at 398.2 eV corresponds to the pyridine N species, and the peak at 399.2 eV corresponds to the metal-bound N species.67 The increased BE of metal-bound N species is indicative of electron donation from bipyridyl sites to metal center in 3.


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Catalytic activity of 3 in the TH of acetophenone After successfully preparing, the catalytic ability of 3 for TH was assessed using acetophenone as a substrate. In our previous study, 1 and 2 were observed to exhibit their highest activities at a pH of 3.5.58 Consequently, the catalytic activity of 3 for the TH of acetophenone with a substrate to catalyst ratio (S/C) of 200 was investigated using an HCOOH/HCOONa buffer at a pH of 3.5 and at 40 °C. However, 3 exhibits no catalytic activity under these conditions. A further increase in the temperature to 80 °C would cause a conversion of only 2.2% after 2 h; even after 12 h, the conversion was observed to remain below 10% (Figure 2a). Subsequently, to identify the optimal conditions the pH dependency of the TH activity of 3 has been evaluated in a pH range between 3.5 and 7.7 over 2.5 h using 200 eq. of acetophenone in aqueous HCOOH/HCOONa buffer at 80 °C. The resulting pH-dependent activity profile for 3 is depicted in Figure 2a. It was observed that the conversion was very slow at lower pH values, however, at above pH 5.0, the reaction rate increased drastically, which may be ascribed to the availability of more formate anions instead of formic acid with the increasing pH of the reaction medium. In particular, the conversion of substrates reached a maximum at pH 7.0. At this pH, the Ru metal center is likely to be bound with the abundant water molecules that are labile enough to facilitate the formation of vacant sites for coordination of substrates and formate to Ru center. Additionally, with further increment in pH of the medium, the less labile hydroxide ions may competitively bind with the Ru metal and hinder the coordination of formate and substrate to the metal center which lead to lower reactivity.68 Notably, 3 depicts the highest activity at an approximate pH of 7, which indicates that it may be active in a simple aqueous medium. This is highly significant because achieving high activity for heterogeneous transition-metal-complex catalysts in a neutral water medium would avoid metal leaching that is frequently observed when using acidic reaction media.


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Table 1. TH of various carbonyl compounds using catalyst 3a Time Entry

Substrate

Conversion TONc

Product (h)

(%)b

1

4

99.0

198

2d

11

99.0

990

3

10

99.0

198

4

10

99.0

198

5

24

97.0

198

6

16

82.0

164

7

4

99.0

198

8

5

86.3

173

9

5

99.0

198

10

6

50.1

100

11

5

99.0

198

a)Reaction

conditions: S/C = 200, 80 °C, 2.0 mL of aq. HCOONa. b)Determined by 1H-NMR spectroscopy. c)no. of moles of product per no. of moles of Ru; Based on 1H-NMR spectroscopic conversion. d) S/C = 1000, 80 °C, 5.0 mL of Aq. HCOONa.


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Interestingly, this factor ensures greater catalyst recyclability, and facilitates its use on an industrial scale in an eco-friendly manner because there is no release of acid into the environment. Therefore, we verified the ability of 3 to reduce carbonyl compounds in water using HCOONa instead of an HCOOH/HCOONa buffer. Initially, 200 eq. of acetophenone in 2.0 mL aqueous HCOONa solution was treated at 80 °C with catalyst 3 that contained 0.4 µM of Ru, which resulted in 11.2% conversion in the initial 30 minutes of reaction time. Notably, upon increasing the reaction time to 4 h, the conversion was observed to increase to approximately 99.0% with a turnover number (TON) of 198(table 1, entry 1). The initial turnover frequency (TOF) was found to be 44.8 h−1 at ~11% conversion Once again, increasing both the temperature and amount of reducing agent is feasible in case of 3; however, this was not observed in case of catalysts 1, 2, and other related complexes that are active in acidic media because, in those cases, a simultaneous increment in the acidity and reaction temperature can lead to more rapid metal leaching and catalyst deactivation. Additionally, upon increasing the S/C to 1000, catalyst 3 shows an increased TON of ~990 (table 1, entry 2). Consequently, the heterogeneous nature of the catalytic process with 3 was confirmed by performing a hot filtration test (Figure 2b). During the reduction of acetophenone, after an approximate conversion of 40%, a portion of the reaction mixture was collected by filtration, and the reaction was continued in the filtrate. As expected, no further conversion was observed in the filtrate, even after 20 h, while complete conversion was obtained in the original reaction mixture within a period of 4 h (Figure 2b). This hot filtration test proved that the heterogenized catalyst 3 acts as a true heterogeneous catalyst in aqueous medium. Substrate scope of catalyst 3 The substrate scope of 3 was verified against various carbonyl compounds, and the results are summarized in Table 1. Catalyst 3 is highly active in case of the TH of various ketones and


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aldehydes. Aromatic ketones with both electron-withdrawing and electron-donating groups were cleanly converted into the corresponding alcohols with excellent conversion rates (entry 1–6). Notably, the sterically hindered α-hydroxy-ketone benzoin was converted into hydrobenzoin in a good yield over a period of 16 h (table 1, entry 6). The aliphatic ketone cyclohexanone was efficiently reduced to cyclohexyl alcohol within a period of 4 h with a TON of 198 (table 1, entry 7). Acetone was also reduced to isopropanol with a TON of 173 in 5 h (table 1, entry 8). Furthermore, aromatic aldehydes were observed to be amenable substrates for 3 in case of TH (table 1, entry 9-11). Catalyst 3 completely converted benzaldehyde into benzyl alcohol within a period of 5 h (table 1, entry 9). The hydroxyl-substituted benzaldehyde was observed to exhibit only 50% conversion over 6 h with a TON of 100 (table 1, entry 10). Interestingly, furfuraldehyde was cleanly converted to furfuryl alcohol, which was an important chemical to synthesize furan resins, in 5 h with a TON of 198 (table 1, entry 11). Reusability of catalyst 3 To be effective on an industrial scale, the recyclability of the catalyst must be excellent. Consequently, the reusability of 3 was tested with 200 eq. of acetophenone under optimized conditions. After the initial run, the catalyst was simply filtered, washed well with water and acetone, and then dried under vacuum at 60 °C for 6 h. The dried catalyst was then directly used in the subsequent run. Figure 2c depicts that 3 is highly active during the recycling experiments and that its catalytic activity is well maintained, decreasing to approximately 92% during the fifth cycle. The ICP-OES analysis of the filtrate during the fifth cycle had shown a very small amount of metal leaching (

Recyclable Covalent Triazine Framework-based Ru Catalyst for

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