Directly Knitted Ruthenium Pincer Complexes with Enhanced Activity as Recyclable Single-Site Catalysts for Hydrogenation of CO ₂ to Methanol

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLES16 Aug 2022Directly Knitted Ruthenium Pincer Complexes with Enhanced Activity as Recyclable Single-Site Catalysts for Hydrogenation of CO2 to Methanol Daheng Wen, Jiangbo Chen, Qingshu Zheng, Siqi Yang and Tao Tu Daheng Wen Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Department of Chemistry, Fudan University, Shanghai 200438 , Jiangbo Chen Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Department of Chemistry, Fudan University, Shanghai 200438 , Qingshu Zheng Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Department of Chemistry, Fudan University, Shanghai 200438 , Siqi Yang Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Department of Chemistry, Fudan University, Shanghai 200438 and Tao Tu *Corresponding author: E-mail Address: [email protected] Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Department of Chemistry, Fudan University, Shanghai 200438 State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032 Green Catalysis Center and College of Chemistry, Zhengzhou University, Zhengzhou 450001 https://doi.org/10.31635/ccschem.022.202202233 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Considering the importance of the valorization of CO2, a number of phosphine-containing ruthenium pincer complexes have been successfully heterogenized using a "direct knitting" strategy without any premodification. The resulting porous organometallic polymers (POMPs) with high specific-surface areas, hierarchical pores, and uniformly dispersed Ru single-sites exhibited outstanding catalytic activity toward the N-formylation of diverse amines with CO2. Besides excellent turnover number (TON, 5 × 105) and turnover frequency (TOF, 5592 h−1), the obtained formamides were readily hydrogenated to methanol with the same catalyst. Consequently, an amine-assisted direct hydrogenation system of CO2 to methanol was established by POMPs with higher activity and TON (1.46 × 104) than their molecular precursors, shedding light on the direct valorization of CO2 and carbon neutral recycling. Download figure Download PowerPoint Introduction Since first reported in the 1970s,1,2 pincer complexes have attracted broad attention in catalysis,3,4 pharmaceutical,5 and material sciences.6,7 Transition-metal pincer complexes containing phosphine ligands have demonstrated excellent catalytic activity and selectivity in diverse transformations, including hydrogenation, dehydrogenation, reductive amination, cross-coupling,8–11 and even the valorization of CO2.12,13 Therefore, considerable efforts have been devoted to the heterogenization of pincer complexes to strengthen their practical application in industry.14–16 However, a conventional strategy to anchor pincer catalysts on solid supports15,16 is plagued by various disadvantages including random anchoring, tedious ligand modification processes, and reduced catalytic activity.17 Porous materials are widely recognized as excellent scaffolds for catalyst immobilization due to their intrinsic porosity, high surface areas, and easy modification.18–20 Compared with the corresponding homogeneous catalytic precursors, comparable catalytic efficiency has been achieved by metal–organic frameworks, covalent organic frameworks, covalent triazine frameworks, hypercrosslinked polymers,21–28 and so on. However, these materials are less studied for the immobilization of phosphine-containing metal pincer complexes.14 Huang and co-workers29 reported the first example of hypercrosslinked porous organometallic polymers (POMPs) based on phosphine-containing pincer complexes, which exhibited excellent activity and reusability toward hydrogenation of CO2 to formic acid, although a pre-synthesized dimeric Ph-PN3P-Ru is required instead of a mono pincer Ru complex. Despite these advances, there is still no general approach for the immobilization of pincer complexes with excellent feasibility in challenging reactions,30 especially the valorization of CO2 to methanol. Because of our interest in developing single-site organometallic porous polymers and exploring their catalytic applications,31–36 we examined the possibility of directly constructing novel POMPs based on phosphine-containing pincer complexes and investigated their catalytic activity in many challenging valorization transformations of CO2. In this study, we successfully fabricated a series of POMPs with high specific surface areas, hierarchical pores, and single-site Ru centers from several viable phosphine-containing ruthenium pincer complexes ( 1–5, Figure 1) after direct knitting with different aromatic comonomers ( a–d, Figure 1). The obtained POMPs exhibited outstanding catalytic activity toward the N-formylation of amines with CO2. Broad substrate scope, high turnover number (TON, 5 × 105), and turnover frequency (TOF, 5592 h−1) were obtained. More importantly, the subsequent hydrogenation of formamides to methanol could be accomplished by the same catalyst. Therefore, a system of amine-assisted direct hydrogenation of CO2 to methanol with enhanced catalytic activity could be established. An excellent TON of 1.46 × 104 was achieved, and the solid POMPs could be reused for more than 10 runs for the direct hydrogenation of CO2 to methanol, highlighting their feasibility in CO2 utilization and carbon neutral recycling. Figure 1 | Fabrication of hierarchical POMPs via direct knitting of pincer complexes 1–5 in the presence of FeCl3 and FDA in DCE with different comonomers a–d. Download figure Download PowerPoint Experimental Methods General procedure for the synthesis of POMPs Pincer complex and comonomers were dissolved in 1,2-dichloroethane (DCE). After stirring for several minutes, formaldehyde dimethyl acetal (FDA) and anhydrous FeCl3 were added. The resulting mixture was heated to 80 °C and stirred for 24 h under a nitrogen atmosphere. After cooling to room temperature, the precipitate was successively washed by methanol, distilled water, acetone, and dichloromethane. Further purification of the polymer was carried out by Soxhlet extraction from methanol for 48 h. The product was dried under vacuum for 24 h at 60 °C to give the resulting polymer. General procedure for N-formylation of diverse amines with CO2 To a stainless-steel autoclave (125 mL) equipped with a magnetic stir bar was added amines (10 mmol), catalyst (45 ppm), and tetrahydrofuran (THF; 2 mL). The autoclave was sealed and purged three times with CO2 gas, then pressurized to 35 atm with CO2, and finally charged with 35 atm of H2 gas. The vessel was heated to 140 °C (oil bath temperature) and stirred therein for the specified time, and then cooled to room temperature in a water bath. The residual gases were released carefully in a hood to give a clear solution. Mesitylene (120 mg, 1 mmol) was added to the reaction mixture as an internal standard for 1H NMR analysis to determine the yield. General procedure for hydrogenation of formamide In a glove box, a 125 mL Parr autoclave was charged with catalyst, KOt-Bu, THF, and N-formylmorpholine 6b. The reaction vessel was sealed and then purged three times with H2, and the pressure in the autoclave was adjusted to 50 atm. The vessel was heated at 150 °C (oil bath temperature) for the specified time with stirring, and then cooled in an ice-water bath for 1.5 h. The residual H2 was released carefully in a hood and the mixture was analyzed by 1H NMR with mesitylene (120 mg, 1 mmol) as the internal standard. Typical procedure for the tandem direct conversion of CO2 to methanol In a glove box, a 125 mL Parr autoclave with a magnetic stirring bar was charged with POMP 1c (136 mg), pentaethylenehexamine (PEHA) (5 mmol), and triglyme (10 mL). The autoclave was sealed and purged three times with CO2 gas, then pressurized to 20 atm with CO2, and finally charged with 60 atm of H2 gas. The vessel was heated to 155 °C (oil bath temperature) for 48 h, and then cooled in an ice-water bath for 1.5 h. The residual gases were released carefully in a hood, the internal standard 1,3,5-trimethoxybenzene (84 mg, 0.5 mmol) and water (10 mL) were added, the mixture was analyzed by 1H NMR in D2O. The reaction mixture was filtered and then washed with EtOH and dichloromethane, and the remaining solid was dried for 6 h at 60 °C in a vacuum oven. The recovered solid was reused as the catalyst for the next run of the reaction without any additional activation steps. Results and Discussion Fabrication of POMPs based on ruthenium pincer complexes As an abundant, inexpensive, and renewable carbon resource, catalytic valorization of CO2 to useful bulky chemicals is regarded as an ideal solution for its fixation and utilization, especially with recyclable catalysts.37–45 Regarding their excellent catalytic activity toward the hydrogenation of CO2 in the literature,12,13 several privileged phosphine-containing Ru-pincer complexes 1–5 were selected to form hypercrosslinked polymers with aromatic comonomers a–d via direct knitting. FDA was selected as an external cross-linker, and FeCl3 was employed as an efficient catalyst toward the Friedel–Craft reaction to generate a series of POMPs 1a–5a in good yields (Table 1). Table 1 | Fabrication of POMPs by Direct Knitting of Ruthenium Pincer Complexesa Entry POMPs Comonomer (equiv) Ru-Con. (wt %)b SBET (m2/g)c 1 POMP 1a a (6) 0.18 849 2 POMP 1b b (6) 0.13 325 3 POMP 1c c (6) 0.15 1155 4 POMP 1c′ c (3) 0.33 1048 5 POMP 1d d (6) 0.20 105 6 POMP 2a a (6) 0.44 675 7 POMP 3a a (6) 0.19 543 8 POMP 4a a (6) 0.42 944 9 POMP 5a a (6) 1.35 360 aReaction was carried out with 0.3 mmol pincer complex, 3–6 equiv comonomers, 20 equiv FeCl3, and 20 equiv FDA in 3 mL dry DCE under a N2 atmosphere in a sealed tube at 80 °C for 24 h. bThe content of ruthenium ions (Ru-Con.) was determined by ICP-AES and given in wt %. cBET specific-surface areas were calculated from the N2 adsorption isotherms at 77 K. Initially, the pincer complex 1 (MACHO-Ru), a privileged bifunctional catalyst showing good activity in diverse hydrogenation reactions,46–48 was chosen to react with different comonomers a–d in the presence of FDA and FeCl3 in 3 mL of dry DCE under N2 atmosphere at 80 °C for 24 h (Table 1, see the Supporting Information for details). Although increasing the size of the comonomers' aromatic ring showed an adverse impact on the porosity (325 vs 849 m2 g−1 for POMPs 1b vs 1a, respectively, entries 1 and 2, Table 1), the tetrahedral aromatic tetra-phenylmethane ( c) provided the highest Brunauer–Emmett–Teller (BET) specific-surface area (1155 m2 g−1 for POMP 1c, entry 3, Table 1). A lower BET value (105 m2 g−1, entry 5, Table 1) was observed with POMP 1d when the bulkier tetrahedral reagent 1,3,5,7-tetraphenyladamantane ( d) was involved. By increasing the quantity of comonomers, the BET values of the resulting POMPs could be increased simultaneously, but the Ru content was reduced ( Supporting Information Table S1). Delightedly, by decreasing the amount of c to 3 equiv, a similar BET value with higher Ru content (1155 m2 g−1 with 0.15% Ru vs 1048 m2 g−1 with 0.33%) was observed in POMP 1c′. The Ru content in POMPs 1a and 3a was relatively lower than those in POMPs 4a and 5a because the presence of benzyl, N-phenyl, and benzimidazole groups in complexes 4 and 5 are more easily knitted than PPh2 and pyridine moieties in complexes 1 and 3 via Friedel–Crafts reactions. The volume of solvent also showed impact on the metal content: low Ru content was observed with more DCE addition ( Supporting Information Table S1). Consequently, the condition of 6 equiv benzene, 20 equiv FeCl3, and 20 equiv FDA in 3 mL DCE was selected to fabricate POMPs 2a–5a with 0.3 mmol Ru pincer complexes 2–5. The highest ruthenium content was observed with POMP 2a (0.44%, entry 6, Table 1); and a high ruthenium content and high BET value were found with POMP 4a (0.42% with 944 m2 g−1, entry 8, Table 1). Notably, no Fe was detected by inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis in all the obtained POMPs after Soxhlet extraction with methanol for 48 h. Characterization of POMPs based on ruthenium pincer complexes With these fabricated POMPs in hand, their morphology was subsequently investigated. Similar amorphous morphologies with porous structure were found by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) for all the samples (Figure 2a,b, Supporting Information Figures S1–S12), which were further confirmed by powder X-ray diffraction analysis ( Supporting Information Figures S23–S30). Due to the low Ru content (<0.2%) in POMP 1c, which hampered its detailed analysis, the POMP 1c′ with high BET value and Ru content (0.33%) was used for further characterization. Energy dispersive X-ray (EDX) spectroscopy indicated that Ru, N, and P were uniformly dispersed in the POMP 1c′, and no Ru nanoparticles were observed (Figure 2c and Supporting Information Figures S15–S21). Subsequent high-resolution high-angle annular dark-field scanning TEM (HAADF-STEM) indicated POMP 1c′ may function as a single-site catalyst with individually well dispersed Ru centers on the sub-nanometer scale (Figure 2d). Thermogravimetric analysis revealed that the POMP 1c′ did not undergo predominant decomposition until 380 °C ( Supporting Information Figure S43), which was much higher than that of the pincer complex 1 (250 °C, Supporting Information Figure S44). Similar outcomes were observed with POMP 1c (details see Supporting Information Figure S14). These results indicated that the Ru complexes became more stable once embedded in the POMPs, highlighting the advantage of the direct knitting strategy. Figure 2 | (a) SEM image, (b) TEM image, (c) EDX mapping image, and (d) high-resolution HAADF-STEM image (the well dispersed white dots represented Ru atoms) of POMP 1c′. (e) Nitrogen adsorption and desorption isotherms measured at 77 K with the corresponding pore width distribution determined by quenched solid density functional theory as insets. (f) The solid-state 13C NMR spectra, (g) FT-IR spectra, and (h) XPS spectra of POMP 1c′ and complex 1. Download figure Download PowerPoint The porosity of POMP 1c′ was then analyzed by nitrogen sorption analysis (Figure 2e). Steep nitrogen gas uptake at a relative low-pressure region (P/Po < 0.1) was observed, indicating the presence of typical microporous structures. The capillary condensation hysteresis at relative high pressure (P/P0 = 0.6–1.0) suggested the presence of mesopores. Further computational analysis of the pore size distribution using quenched solid density functional theory (QSDFT) confirmed the existence of both micro- and mesopores. Similar porosities were observed with other POMPs (details see Supporting Information Table S1, Figures S31–S40). Combined with the observed macropores observed with SEM and TEM, we envisaged that these POMPs had hierarchical porous structures as shown in Figure 1. In view of the possible impact of the gas adsorption capacity of POMPs toward its catalytic performance, the CO2 adsorption of POMPs was measured ( Supporting Information Figure S41). The POMPs 1c′ exhibited a CO2 uptake of 23 cm3 g−1, which was consistent with our previous POMPs constructed from bis-N-heterocyclic carbene iridium complexes and benzene (20–32 cm3 g−1).31 The good capacity of POMP 1c′ toward CO2 adsorption laid the foundation for further transformations with CO2. To investigate the coordination environment around the Ru centers after direct knitting, solid-state 13C and 31P NMR, Fourier-transform infrared (FT-IR), as well as X-ray photoelectron spectroscopy (XPS, Figure 2 and Supporting Information Figure S45) were performed. The solid-state 13C NMR spectra of POMP 1c′ showed peaks for CAr (120–150 ppm, aromatic rings) and CMe (20–60 ppm, pincer skeleton and bridging methylene), which matched well the corresponding signals of complex 1 (Figure 2f). The quaternary carbon signal originating from the aromatic comonomer at 73 ppm was also observed (Figure 2f). Similar but slightly broadened P signals were also found in the solid-state 31P NMR spectrum of POMP 1c′ as compared to that of complex 1 ( Supporting Information Figure S46). These results confirmed that the complex 1 and comonomers were successfully knitted. There were two predominate peaks at 1916 and 1972 cm−1 in the FT-IR spectrum of complex 1, which are typical signals for the carbonyl group with two different configurations. Pleasingly, no obvious shift was found in the FT-IR spectrum of POMP 1c′, but it was less intense due to the low content of the complex 1 fragment knitted in the POMPs matrix (Figure 2g). Furthermore, the peaks at 1120 cm−1, corresponding to P–Ar stretching vibrations, was also found in the FT-IR spectrum of POMP 1c′, indicating the air-sensitive phosphine moiety was intact under the hypercrosslinking conditions. Considering the interference from carbon substrates arising from the overlap of the C 1s and Ru 3d core-levels, the Ru 3p state was selected for analysis instead of the typical 3d spectra (Figure 2h). The Ru 3p region of complex 1 revealed the presence of two broad peaks for Ru 3p3/2 and Ru 3p1/2 centered at 462.4 and 484.6 eV, respectively. In the case of POMP 1c′, the signals were slightly shifted to 462.8 and 484.2 eV, respectively. These outcomes indicate the coordination environment was well preserved after knitting, and Ru sites were present in the +2 oxidation state in both cases. Catalytic performance of POMPs toward N-formylation of amines with CO2 With these POMPs in hand, their catalytic performance toward hydrogenation of CO2 was then investigated. Because the direct reduction of CO2 by H2 to methanol is extremely challenging due to the inertness of CO2,49,50 and based on our previous work on the highly efficient N-formylation of amines,31 we first exploited the stepwise conversion of CO2 to methanol via formamide. Initially, the N-formylation of piperidine with CO2 and H2 was employed as a model reaction (Figure 3). Pleasingly, the POMPs 1a–1d and 5a all provided the N-formylation product 7 in excellent yields (>80%, Figure 3) under 35 bar CO2 and 35 bar H2 pressure at 140 °C for 24 h at a low catalyst loading of 45 ppm (3.5–35 mg). Remarkably, the best yield (97%) was achieved by POMP 1c′. Up to 5592 h−1 TOF could be achieved by elevating the reaction temperature to 180 °C for 3 h. Pleasingly, even at 2 ppm catalyst loading (1.2 mg), full conversion with excellent TONs (5 × 105) could be achieved simply by extending the reaction time to 168 h. Other POMPs 2a, 3a, and 4a gave inferior results due to the similar low catalytic activity of the corresponding pincer precursors (Figure 3, Supporting Information Table S2). In comparison, an additional POMP 6, which was prepared by a post-modification approach from porous organic polymers and metal precursors (details see the Supporting Information Scheme S1), led to very poor yield of formamide 7 under the otherwise identical reaction conditions. This might be attributed to the random anchoring or absorption of Ru species within the solid matrix of POMP- 6,15,16 further confirming the advantage of the direct knitting strategy. Figure 3 | Screening of solid catalysts in the N-formylation of piperidine. Reaction conditions: 10 mmol piperidine and 45 ppm catalyst were added the and the reaction mixture was heated at 140 °C under 35 bar CO2 and 35 bar H2 pressure for 24 h. was determined by 1H Download figure Download PowerPoint the substrate many and amines was examined in the reaction with CO2 and H2 ( Supporting Information Table Figures which further confirmed the catalytic activity and feasibility of our The corresponding formamides were obtained in good to excellent yields with high amines including and were all well to corresponding formamides in high yields Furthermore, the amines were also with the the in good to excellent yields Remarkably, an group on was also well ( were also which may the for CO2 and Due to their in all selected including water, methanol, and and so these solid catalysts were readily recovered from the reaction mixture by and with additional methanol. To our the solid POMP catalysts could be reused for more than 20 runs in the reaction under the reaction ( Supporting Information Figure The and reusability of the POMPs were further by ICP-AES analysis of the ( Supporting Information Table and the TEM, and XPS of the recovered catalyst ( Supporting Information Figures and 0.18 Ru was observed for the first run ( Supporting Information Table which might be attributed to a amount of Ru pincer complex or in the In the subsequent the of Ru was which further indicated the of the POMP even in formamide. The yields of product 7 after the run might be by the catalyst at low catalyst loading the and Further TEM of the recovered POMP 1c showed identical morphologies with the prepared POMP 1c ( Supporting Information Figure The and dispersive spectroscopy mapping revealed no ruthenium nanoparticles were within the recovered POMP 1c after the run ( Supporting Information Figure The recovered POMP 1c′ was also via FT-IR and and identical were observed with the prepared POMP 1c′ ( Supporting Information Figures and confirming the and reusability of the Catalytic performance of POMPs in hydrogenation of formamides to methanol In view of the importance of methanol and bifunctional of pincer complex 1, we to investigate the feasibility of solid POMPs toward hydrogenation of formamides to methanol. As shown in Figure after of the reaction hydrogenation of formamide 7 to methanol and corresponding piperidine was carried out in the presence of POMP under 50 bar of H2 at 150 °C for 24 h. The hydrogenation efficiency of the POMPs was by both the catalyst precursors and The POMPs 1c and 1c′ provided yields of methanol and than other solid The gas product mixture was analyzed by gas and no other including formaldehyde was detected ( Supporting Information Figure The low yield of methanol may be by its on a which is also consistent with the were to to yield of methanol with solid 1c by simply increasing the amount of substrate and the reaction time to 48 h (Figure 4 and Supporting Information Table Figure 4 | for the hydrogenation of formamide 7 to aReaction conditions: 1 mmol 2 mL of anhydrous THF, 0.5 mmol KOt-Bu, and POMPs were added the autoclave and the reaction mixture was heated at 150 °C under 50 bar H2 pressure for 24 h was determined by 1H NMR with mesitylene as an internal 10 mmol 10 mL of anhydrous THF, and 2 mmol for 48 h. Download figure Download PowerPoint Direct catalytic conversion of CO2 to methanol by POMPs on the transformations, including N-formylation of CO2 to and subsequent hydrogenation of formamide to methanol, a direct conversion system of CO2 to methanol was then investigated. After many amines and reaction ( Supporting Information Table the was finally selected instead of piperidine and for the amine-assisted direct of may not and CO2 but also function as a to the of the the of CO2 to methanol. be out that high temperature is for the CO2 formamide was observed at lower temperature °C, entry 10 in Supporting Information Table After catalyst with all POMPs, solid catalyst 1c exhibited the highest TON for under 20 bar CO2 and 60 bar H2 pressure at 155 °C for 48 h even at the catalyst loading as low as ppm (136 solid 1c to 5 mmol see the Supporting which is two of higher than the corresponding number obtained from pincer 1 Figure A slightly lower TON was found with POMP 1c′ Figure but it much higher than that obtained from complex 1. attributed the enhanced catalytic activity to the hierarchical porous which are to the adsorption and desorption of CO2 and The catalytic environment by the hierarchical porous may be to the Figure 5 | Direct catalytic conversion of CO2 to (a) Catalytic performance of POMPs ppm complex 1, 1c′ and (b) and reusability of POMP Reaction conditions: 5 mmol 10 mL of and solid 1c were added the autoclave and the reaction mixture was under 20 bar CO2 and 60 bar H2 pressure at 155 °C for 48 h. was determined by 1H Download figure Download PowerPoint A reaction for the tandem conversion of CO2 to methanol was ( Supporting Information Figure The formamide in the reaction mixture within the first 5 h, and then hydrogenated to methanol. the amount of formamide was the of methanol acid, a possible was also found in the reaction The gas were analyzed and a amount of and methanol were detected by gas and 1H NMR, ( Supporting Information Figures The catalytic activity of the recovered POMP from different was investigated within h ( Supporting Information Table Similar were the excellent reusability of solid the solid POMP 1c could be reused for 10 runs without in activity and The after was analyzed by and ruthenium was observed as well ( Supporting Information Table confirming the of the POMPs catalyst. Remarkably, to 1.46 × 104 TONs was achieved with 6 solid 1c simply by extending the reaction time to 168 h, which is consistent with the TONs 10 runs TON = Figure and much higher than catalytic for methanol from CO2 via an amine-assisted (details see Supporting Information Figure highlighting our catalytic have successfully heterogenized a series of phosphine-containing Ru pincer complexes solid POMPs via a direct knitting revealed that the materials high specific-surface areas, hierarchical pores, and single-site Ru centers with a preserved coordination environment as molecular The resulting POMPs function as solid single-site catalysts toward N-formylation of hydrogenation of and direct conversion of CO2 to methanol. Besides excellent catalytic activity and TOF values to

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