English

• 0.   Introduction

  To solve the ever-increasing energy and environmental issues, hydrogen energy serves as an important supplement due to its numerous advantages^[1-3]. It is of great importance for hydrogen energy utilization to develop safe methods for hydrogen storage. Recently, formic acid has emerged as a promising liquid organic hydrogen carrier due to its low price, low toxicity, environmental friendliness, biodegradability, etc^[4-7]. Catalytic hydrogenation of CO₂ to formic acid is a green chemical reaction with high atom economy. However, CO₂ hydrogenation to formic acid is strongly limited by thermodynamic constraints (ΔG_{298 K}^⊖=33 kJ·mol^-1), and bases are widely used to neutralize formic acid and drive the reaction equilibrium toward the formate product. To realize efficient CO₂ hydrogenation, it is essential to develop superior catalysts^[8]. Many homogeneous catalysts have exhibited the potential for CO₂ hydrogenation reactions^[8-10].

  To date, one of the best-performing catalysts for CO₂ hydrogenation to formate is the iridium-pyridine-based PNP-pincer catalyst developed by Nozaki and colleagues^[11], which achieved a remarkable turnover number (TON) of up to 3 500 000. Another notable catalyst is the ruthenium-pyridine-based PNP-pincer catalyst reported by Pidko and colleagues^[12], exhibiting a high turnover frequency (TOF) of up to 1 100 000 h^-1. However, even though PNP-pincer ligand-based homogeneous catalysts have demonstrated high efficiency for CO₂ hydrogenation, these complexes are prone to oxidation due to the alkyl phosphines, which hinders their practical utilization. To solve this problem, pincer ligands without an alkylphosphine structure were introduced, including hydroxyl, amino, and heterocyclic rings containing N or O atoms, where C, N, or O atoms serve as the coordination sites^[13-15]. However, these catalytic systems exhibit low catalytic efficiency. Therefore, we are committed to synthesizing non-phosphine pincer ligands and complexes that can efficiently and stably catalyze the hydrogenation of CO₂ to formate under a wider range of conditions. It is generally believed that the catalytic hydrogenation activity is related to the vacant coordination sites provided by the active species during the reaction process. Typically, the easier it is for a complex to provide vacant sites, the higher its reactivity tends to be^[16]. However, strongly rigid pincer ligands can form a stable complex structure during the reaction, making it difficult to create vacant coordination sites. On the other hand, one advantage of strongly rigid pincer ligands is their ability to stably chelate with the metal center throughout the reaction process, which effectively reduces the likelihood of metal center clustering, thereby enhancing catalyst stability. As a result, the complexes bearing rigid ligands often exhibit comparatively lower catalytic activity, while the complexes with non-rigid ligands face difficulties in balancing high activity with stability. To date, although various complexes integrating rigid and non-rigid ligands have been explored in catalyzing the hydrogenation of carbon dioxide^[17-22], the complexes that have both good stability and high activity are rare^[23]. Hence, it remains a tough challenge to synthesize complexes with high catalytic activity and excellent stability.

  Herein, we synthesized Ru complexes bearing rigid pincer-type tridentate NNN ligands^[24-25] and weakly coordinated triphenylphosphine (PPh₃) (Scheme 1). PPh₃ in the complexes is easily dissociated and forms the vacant coordination sites. The vacant coordination sites can, in return, enhance their catalytic activity. Furthermore, the presence of NNN ligands plays a crucial role in maintaining the stability of the Ru center, contributing to the overall robustness of the catalytic system. As a result, the synthesized Ru(Ⅱ)-NNN complex [Ru(L-NNN)Cl(PPh₃)₂]Cl (1, L-NNN=2,6-bis(5-methyl-1H-pyrazol-3-yl)pyridine) was not only quite stable in both water and air, but also showed high activity for CO₂ hydrogenation towards formate, achieving a TON of up to 150 000.

  Scheme 1

  

  Scheme 1.  Constructing Ru-NNN complexes with both high catalytic activity and stability by using rigid ligands and weak ligands simultaneously

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  1.   Experimental

  1.1   Materials and methods

  NMR spectra were recorded on a Bruker AV Ⅱ-400 spectrometer at 400 MHz (¹H NMR) and 162 MHz (³¹P NMR). Chemical shifts were reported downfield from internal Me₄Si and external 85% H₃PO₄, respectively. All the solvents used for reactions were distilled under argon after drying over an appropriate drying agent. All other commercially available reagents were purchased from Aaladin, Adamas, Aldrich, and Alfa Aesar Chemical Company.

  1.2   Preparation and characterization of NNN-type ligands

  The ligand L-NNN was synthesized using pyridine-2, 6-dicarboxylic acid as the precursor^[25]. In a round-bottom flask with a 250 mL capacity equipped with a reflux condenser apparatus, pyridine 2, 6-dicarboxylate (15 g) and anhydrous ethanol (120 mL) were combined and treated with the addition of acetyl chloride (25 mL), followed by a reaction at room temperature for 24 h. Upon completion of the reaction, the solid was removed by filtration, and the filtrate was neutralized with sodium carbonate solution, then extracted with ethyl acetate to yield a white solid (diethyl pyridine 2, 6-dicarboxylate). The diethyl pyridine 2, 6-dicarboxylate and ethanol sodium (8.8 g) were added to a toluene (60 mL) solution under an ice bath, followed by the gradual addition of a toluene solution of acetone (8.7 mL) to produce a yellowish turbid mixture. The reaction was then allowed to proceed at room temperature for 12 h. The mixture was filtered after the reaction to collect a light yellow solid, and the pH of the solid was neutralized to 1 with dilute hydrochloric acid before being extracted with ethyl acetate to obtain yellow solid 1, 1′-(pyridine-2, 6-diyl) bis(butane-1, 3-dione). Subsequently, in a 100 mL round-bottom flask, 1, 1′-(pyridine-2, 6-diyl)bis(butane-1, 3-dione) (2 mmol), glacial acetic acid (4 mL), and ethanol (30 mL) were stirred at room temperature for 30 min, followed by the addition of an ethanolic solution containing 4 mmol of hydrazine hydrate via a pressure-equalizing dropping funnel. The temperature was then raised to 65 ℃ to continue for 24 h. Upon completion, the solution was concentrated to around 10 mL, neutralized to pH=8 with sodium carbonate solution, and extracted with ethyl acetate to yield a faint yellow solid L-NNN. ¹H NMR (400 MHz, DMSO-d₆): δ 12.79 (s, 2H), 7.83 (t, J=7.7 Hz, 1H), 7.72 (d, J=7.8 Hz, 2H), 6.75 (s, 2H), 2.28 (s, 6H).

  Ligand 2,6-bis(1, 5-dimethylpyrazol-3-yl)pyridine (L-NNN-Me) was prepared analogously to ligand L-NNN, except for using methyl hydrazine as the hydrazine reagent^[24].

  1.3   Synthesis of Complex 1

  Complex 1 was synthesized according to the reported method^[25]. A mixture of ligand L-NNN (1 mmol) and RuCl₃·3H₂O (1 mmol) was refluxed in EtOH (60 mL) for 5 h to give a reddish-brown precipitate. After the mixture had cooled to room temperature, it was filtered, washed with diethyl ether, and dried under vacuum. EtOH (30 mL) and PPh₃ (2 mmol) were added to the filtered precipitate, and the mixture was refluxed for 6 h. After completion of the reaction, the solvent was removed under vacuum to get a reddish-brown solid. The resulting solid was dissolved in dichloromethane (DCM), followed by filtration to remove insoluble impurities, yielding a red solution. Upon addition of ether to the solution, orange solid Complex 1 was crystallized from the solution, with a yield of 50%. ¹H NMR (400 MHz, DMSO-d₆): δ 13.50 (s, 2H), 7.43-7.38 (m, 1H), 7.29 (t, J=7.2 Hz, 6H), 7.25-7.13 (m, 24H), 7.00 (d, J=7.8 Hz, 2H), 6.44 (s, 2H), 2.14 (s, 6H). ³¹P NMR (162 MHz, DMSO-d₆): δ 33.6, 24.4, -6.9. Despite repeated attempts, the low solubility of the complex in all suitable NMR solvents prevented us from obtaining a high-quality ¹³C NMR spectrum.

  1.4   Synthesis of complex [Ru₂(L-NNN)₂Cl₂(PPh₃)₂]Cl₂ (2)

  A mixture of ligand L-NNN (1 mmol) and RuCl₃·3H₂O (1 mmol) was refluxed in EtOH (60 mL) for 5 h to get a reddish-brown precipitate. After the mixture was cooled to room temperature, it was filtered, washed with diethyl ether, and dried under vacuum. EtOH (30 mL) and PPh₃ (1 mmol) were added to the filtered precipitate, and the mixture was refluxed for 6 h. After completion of the reaction, the solvent was removed under vacuum to obtain a reddish-brown solid. The resulting solid was dissolved in DCM, followed by filtration to remove insoluble impurities, yielding a red solution. Upon addition of ether to the solution, orange solid Complex 2 was crystallized from the solution, with a yield of 57%. ¹H NMR (400 MHz, CD₂Cl₂): δ 15.09 (s, 4H), 7.33-7.17 (m, 32H), 6.89 (d, J=7.8 Hz, 4H), 6.03 (s, 4H), 2.45 (s, 12H). ³¹P NMR (162 MHz, CD₂Cl₂): δ 52.8. Due to the same reasons as those for Complex 1, high-quality ¹³C NMR spectra could not be obtained.

  1.5   Synthesis of complex [Ru(L-NNN-Me)Cl(PPh₃)₂]Cl (Ru-NNN-Me)

  A mixture of ligand L-NNN-Me (1 mmol) and RuCl₃·3H₂O (1 mmol) was refluxed in EtOH (60 mL) for 5 h to get a reddish-brown precipitate. After cooling the precipitate to room temperature, it was filtered, washed with diethyl ether, and dried under vacuum. EtOH (30 mL) and PPh₃ (2 mmol) were added to the filtered precipitate, and the mixture was refluxed for 6 h. After completion of the reaction, the solvent was removed under vacuum to obtain a reddish-brown solid. The resulting solid was dissolved in DCM, followed by filtration to remove insoluble impurities, yielding a red solution. Upon addition of ether to the solution, an orange solid complex, Ru-NNN-Me, was crystallized from the solution, with a yield of 61%. ¹H NMR (400 MHz, CD₂Cl₂): δ 7.33 (t, J=7.8 Hz, 1H), 7.29-7.08 (m, 14H), 7.04-6.97 (m, 15H), 6.94-6.85 (m, 13H), 6.72 (s, 2H), 3.12 (s, 6H), 2.13 (s, 6H). ³¹P NMR (162 MHz, CD₂Cl₂): δ 43.8, 17.9, -5.6. For the same reason as for Complex 1, we were unable to obtain a high-quality ¹³C NMR spectrum.

  1.6   Catalytic hydrogenation of CO₂ with H₂

  Catalytic CO₂ hydrogenation was carried out in a Hastelloy Autoclave Reactor system equipped with a 25 mL cylinder. The catalyst was dissolved in a degassed aqueous solution (5 mL) of CsOH, along with the addition of 1 mL THF. The reactor was pressurized with 5 MPa of CO₂/H₂ (1:1, p/p) and heated at 90-170 ℃ for the appropriate time. DMF was added as an internal standard, while D₂O was added as the solvent. Then, the formate was quantified by ¹H NMR spectroscopy (Fig.S1, Supporting information).

  1.7   General procedure for the in-situ NMR measurement

  To uncover the underlying mechanism, stoichiometric reactions were conducted with 30 μmol Ru-NNN Complex 1 and 4 mmol CsOH under 5 MPa of CO₂/H₂ (1:1, p/p) in the mixed solvent of THF-d₈ and D₂O (1 and 10 mL, respectively) at 140 ℃, in which the intermediates were monitored by NMR in situ.

  2.   Results and discussion

  2.1   Structure description of Complex 1

  The Ru(Ⅱ)-NNN Complex 1 was synthesized by refluxing L-NNN in ethanol with an equal amount of RuCl₃·3H₂O and two equivalents of PPh₃, following the method reported in the literature^[25]. Ru(Ⅲ) can be reduced to Ru(Ⅱ) by ethanol under heating reflux conditions and form stable Ru(Ⅱ)-NNN complexes with ligands. Afterward, Complex 1 was crystallized by adding diethyl ether to the solution of dichloromethane^[25]. The structure of Complex 1 has already been umambiguously confirmed by single-crystal X-ray diffraction in our previous work^[25]. The crystal structure of Complex 1 is shown in Fig.1. The Ru(Ⅱ) center adopts a distorted octahedral coordination geometry with a meridionally coordinated L-NNN ligand and a chloride occupying the equatorial sites, while two trans-located PPh₃ occupy the axial positions. The average Ru—P bond length is 0.238 4 nm, and the Ru—N bond length is 0.198 1 nm (pyridine). The complex exhibits a distinct mirror-symmetric structure with N—Ru—N as the σ_v symmetry plane, and the orientation of the phenyl groups in PPh₃ also exhibits a symmetric structure. In the ³¹P NMR spectrum of Complex 1, a weakly characteristic peak of triphenylphosphine located at δ of -6.9 was observed, while the other weak single peak at δ of 33.6 might originate from the species (Ⅱ) after PPh₃ dissociation (Fig.2a). Noteworthily, this phenomenon becomes more pronounced with increasing temperature (Fig.2b). After adding PPh₃, the ³¹P NMR spectrum of Complex 1 (Fig.2c) only showed the characteristic peak of PPh₃ (δ=-6.9) and the main peak of the Complex 1 (δ=23.8), indicating that the dissociation process was reversible. The rigidity of the aromatic heterocyclic structure in the NNN pincer ligands prevents bending or twisting, meaning their dissociation from the ruthenium ion requires simultaneous rupture of two or three coordination bonds. This kinetically high-energy barrier ensures strong chelation with the Ru center in a meridional fashion, greatly augmenting the complex′s stability. The influence of the trans-effect of phosphine and the cis-labilizing effect of the chloride facilitates the dissociation of PPh₃ to provide a vacant site^[25]. Probably, the PPh₃, which is weakly coordinated compared to the rigid ligand L-NNN, can easily dissociate to provide vacant sites, contributing to the catalytic activity of Complex 1.

  Figure 1

  

  Figure 1.  Crystal structure of Complex 1 and active intermediate (Ⅱ) (Crystal structure of Complex 1 was obtained from the reference [25].)

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  Figure 2

  

  Figure 2.  (a) ³¹P NMR spectrum of Complex 1; (b) ³¹P NMR spectrum of Complex 1 at 50 ℃; (c) ³¹P NMR spectrum of the mixture of Complex 1 and PPh₃ (162 MHz, CDCl₃)

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  2.2   Hydrogenation of CO₂ to formate catalyzed by Complex 1

  The catalytic performances of Ru Complex 1 were investigated in the mixed solvent of H₂O/THF (5 mL/1 mL) for CO₂ hydrogenation. Initially, we varied the type of base and screened multiple organic and inorganic bases, including 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), triethylamine, pentaethylene-hexamine, CsOH, NaOH, and KOH, based on the different types of bases with high reactivity reported in previous work^{[11-12,17-22]}. We found that the complex exhibited the highest catalytic activity in the presence of CsOH, and there was a positive correlation between catalytic activity and the concentration of the base (Table 1 and S3). Cs⁺ ions exhibit stronger basicity in aqueous solutions compared to other alkali metal ions, which may more effectively facilitate the heterolytic cleavage of H₂, thereby enhancing the reaction activity. Subsequently, we tested the catalytic reactivity at different reaction temperatures, increasing the temperature from 90 to 170 ℃ (Table 1, entries 4-12). The activity of Complex 1 varied significantly with temperature and peaked at 140 ℃, with a maximum TOF of 574 h^-1 at 140 ℃ (Table 1, entry 10). The rigid L-NNN ligand endows the Complex 1 with high thermal stability. Afterward, we used 0.001 to 10 μmol of Complex 1 to react for 16 h at 140 ℃, and the TOF increased significantly with the decrease of complex concentration, reaching a maximum TOF of 9 347 h^-1 (Table 1, entries 10, 16-19). Meanwhile, stability is another crucial parameter for ruthenium complexes, apart from their activity. We tested the catalytic activity of Complex 1 for CO₂ hydrogenation at 140 ℃ at different time intervals ranging from 0.5 to 16 h. The TOF of Complex 1 did not change significantly (less than 5%) after 0.5, 1, 2, 4, 8, and 16 h (Table 1, entries 10, 13-15; Table S3, entries 3-4), indicating its excellent stability. Using 0.001 μmol of the catalyst, the TON of formate reached approximately 150 000 after 16 h, which was a high record among all Ru complexes (Table S2)^[17-22]. Considering its excellent stability and high activity, Ru-NNN Complex 1 can be considered one of the best CO₂ hydrogenation catalysts among all Ru complexes.

  Table 1

  

  Table 1.  Catalytic activity of complex 1 for hydrogenation of CO₂ to formate^a

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  Entry	nCat. / μmol	Base	T / ℃	t / h	TONb	TOFc / h-1
  1	1	NEt3	150	16	1 465	92
  2	1	DBU	150	16	2 218	139
  3	1	KOH	150	16	3 412	213
  4	1	CsOH	150	16	6 213	388
  5	1	CsOH	90	16	178	11
  6	1	CsOH	100	16	512	32
  7	1	CsOH	110	16	1 513	95
  8	1	CsOH	120	16	2 746	172
  9	1	CsOH	130	16	2 280	143
  10	1	CsOH	140	16	9 176	574
  11	1	CsOH	160	16	7 322	458
  12	1	CsOH	170	16	4 894	306
  13	1	CsOH	140	2	1 096	548
  14	1	CsOH	140	4	2 245	561
  15	1	CsOH	140	8	4 635	579
  16	0.001	CsOH	140	16	149 558	9 347
  17	0.01	CsOH	140	16	68 196	4 262
  18	0.1	CsOH	140	16	32 116	2 007
  19	10	CsOH	140	16	584	37
  20d	1	CsOH	140	16	10 304	644
  a General conditions: complex 1, base (10 mmol), pH2=pCO2=2.5 MPa, VTHF=1 mL, VH2O=5 mL; b TON=nformate/nRu; c TOF=TON/t; d 1 mmol LiOAc as additive.

  Furthermore, we increased the CO₂ and H₂ pressures, and the results showed that an increase in gas pressure could further enhance the reaction activity. When the CO₂ and H₂ pressure reached 3 MPa, the TOF increased to 704 h^-1 (Table S3, entry 5) compared to the original conditions (Table 1, entry 10; Table S3, entry 12). These results suggest that the activity of Complex 1 has the potential for further improvement for CO₂ hydrogenation.

  After optimizing the reaction conditions, we aimed to further enhance the CO₂ hydrogenation performance of Complex 1 with the help of additives. Various salt additives, including K₃PO₄, LiOAc, LiOTf, and KNO₃, were introduced into the reaction system to improve its performance^[26]. The results indicated that LiOAc exhibited the most significant promotional effect (Table 1, entry 20; Table S3, entries 6-8), achieving a maximum TOF of 644 h^-1, representing a 12% improvement compared to the original value (Table 1, entry 10). The promotional effect of LiOAc in the reaction process could be attributed to two aspects: Li⁺ may function as a Lewis acid, while OAc^- may act as an internal base. Bernskoetter′s work found that Li ions^[27], as Lewis acids, can activate the M-OCOH intermediate, thereby enhancing the reaction activity. Hopmann′s work^[28], through density functional theory (DFT) calculations, demonstrated that Li ions activate the process of CO₂ insertion into the M—H bond, improving the reaction. Jessop′s research found that OAc^- in RuCl(OAc)(PMe₃)₄ can serve as an internal base to promote the generation of the active species [RuH(PMe₃)₄]^+[29]. In contrast, NO₃^- cannot have the same effect as OAc^- due to its conjugate acid′s low pK_a and the anion′s difficulty in donating electrons to the Ru center. Therefore, it can be concluded that the promotional effect of LiOAc on the reaction is a result of the combined action of both Lewis acid and internal base.

  2.3   Reaction mechanism

  To investigate the reaction mechanism of the hydrogenation of CO₂, we conducted activity tests for the catalytic reaction using stoichiometric Complex 1 in a mixed solvent of THF-d₈ and D₂O. During the catalytic reaction, the in-situ formation of several new species was revealed by the ¹H NMR spectrum, exhibiting signals in the high-field region. This observation indicates the formation of metal-H species during the reaction (Fig.3a). The triplet peaks at around -9 {-9.17 (t, J=26.1 Hz, 1H), -9.65 (t, J=26.1 Hz, 1H), -9.90 (t, J=26.1 Hz, 1H)} and the broad peak at -17 indicate the formation of multiple distinct new species with significantly different coordination environments during the reaction. Based on literature reports^[30], it is reasonable to infer that the triplet peaks at around -9 correspond to the Ru complex with symmetric two PPh₃ groups formed by H substitution of Cl (intermediate Ⅴ_A, δ=-9.17), as well as several structurally similar complexes formed by deprotonation of N—H on the pyrazole ring under alkaline conditions (Ⅴ_B, δ=-9.65; Ⅴ_C, δ=-9.90). The broad peak at -17 can be attributed to the Ru complex coordinated with hydrogen molecules (intermediate Ⅲ). Relevant evidence was also detected in the in situ HPLC-HRMS (Fig.3b). Peaks at m/z 640.097 8 and 604.117 8, corresponding to intermediate Ⅲ ([C₃₁H₃₀ClN₅PRu]⁺, Exact mass: 640.097 08) and intermediate Ⅳ ([C₃₁H₂₉ N₅PRu]⁺, Exact mass: 604.120 41) were identified in the HPLC-HRMS data. We speculate that the active species exists as a vacant coordination intermediate (Ⅱ) during the reaction, rather than as a Ru-Cl₂ complex formed by the exchange of PPh₃ and free chloride ions. This is because vacant coordination intermediates (Ⅱ) may possess higher reactivity than the Ru-Cl₂ complex. To verify this speculation, we attempted to obtain Ru-Cl₂ Complex 2. Interestingly, we observed that Complex 1 forms a reddish-brown solid when left to stand in solution for more than a week. So, we repeatedly recrystallized Complex 1 with DCM and diethyl ether. By taking advantage of the difference in solubility between PPh₃ and the complex, we obtained Complex 2. Complex 2 was characterized using ¹H NMR and ³¹P NMR spectra (Fig.S7 and S8) as well as single-crystal X-ray diffraction (Fig.S2). The single crystal structure reveals that Complex 2 is a dimer of two intermediate Ⅱ. The chemical shift of the ³¹P spectrum of Complex 2 (Fig.S8) also differs from that of the intermediate products generated in situ from Complex 1 (δ=33.6). When Complex 2 was added to the reaction along with two equivalents of PPh₃, the catalytic activity for CO₂ hydrogenation was significantly lower than that of Complex 1, with a TOF of only 63% of Complex 1 (Table S3, entry 9). On the other hand, adding excess PPh₃ to the catalytic system of Complex 1 to inhibit its in-situ dissociation so that a decrease in reactivity was observed, with a TOF of only 75% of the original conditions (entry 10 in Table S3 vs entry 10 in Table 1). Therefore, it can be inferred that the high activity of Complex 1 is due to the formation of a vacant coordination site from the dissociation of PPh₃ during the reaction. This dissociation results in the formation of intermediate Ⅱ, which serves as the active species. To eliminate the effect of the N—H functionality on the pyrazole ring during the reaction, we methylated the N—H on the pyrazole ring. Using a similar method to the synthesis of Complex 1, we synthesized Ru-NNN-Me. The catalytic activity of Ru-NNN-Me in the hydrogenation of CO₂ to formate is comparable to that of Complex 1 and did not show significant deactivation (Table S3, entry 11). Therefore, we can conclude that the N—H functionality on the pyrazole does not have a positive effect on the catalytic reaction.

  Figure 3

  

  Figure 3.  (a) ¹H NMR spectrum and (b) HPLC-HRMS spectrum of the reaction mixture

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  Data of the crystal structures show that the Ru—N bond lengths in complexes 1 and 2 are similar (Table S4), indicating that PPh₃ has little influence on the charge of the central atoms in the complexes. It can be inferred that the abnormal changes in the Ru—P bond length in the Ru-NNN complexes 1 and 2 in this study are due to their special structures. This is supported by the crystal data of NNN complexes previously reported by the Yu′s group^[31]. When the symmetry of the complex is broken, the bond length of the Ru—P bond is shortened^[32] (Table S4). The slightly longer Ru—P bond length in Complex 1 (0.238 4 nm) compared to Complex 2 (0.228 3 nm) by about 4.4% may be attributed to the higher symmetry of Complex 1. The longer bond length in Complex 1 leads to the facile breaking of the Ru—P bond, contributing to the formation of the active intermediate Ⅱ with vacant coordination sites.

  Based on the aforementioned characterization, we hypothesized potential intermediates and calculated their Gibbs free energies, along with those of the transition states, using Gaussian 09 and Shermo. The optimizations were performed at the PBE0/def2-SVP level. The Gibbs free energies of the intermediates were calculated at T=298.15 K and p=101.325 kPa using the Shermo program (Fig.4). DFT calculations reveal that the hydrogenation of carbon dioxide is thermodynamically feasible, with no excessively high energy barriers to prohibit the reaction. The free energy difference between the formation of the Ru-H₂ (Ⅲ) intermediate from the Ru-PPh₃ intermediate (Ⅱ) and the dimerization to form Complex 2 is minimal, favoring the latter. This suggests a slightly higher likelihood of forming Complex 2, which hinders the reaction′s progression. This aligns with the observed decrease in reactivity as Complex 2 forms.

  Figure 4

  

  Figure 4.  Gibbs free energy profiles for CO₂ hydrogenation catalyzed by the Ru-NNN complex

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  Based on the above results, we inferred the possible catalytic mechanism (Fig.5). First, Complex 1 partially dissociates one molecule of PPh₃ and generates one molecule of active intermediate Ⅱ in solution. Intermediate Ⅱ may form the low-activity Complex 2 during the reaction. But due to the low concentration of intermediate Ⅱ, it is more likely to react with H₂ to form the hydrogen molecular complex Ⅲ. Under alkaline conditions, CsOH assists intermediate Ⅲ in eliminating one molecule of HCl, yielding intermediate Ⅳ, H₂O, and CsCl. Ⅳ is complexed with the free PPh₃ in solution to form Ⅴ. The C=O bond in CO₂ is activated by the Ru—H bond in intermediate Ⅴ through electrostatic interactions. Carbon dioxide is then inserted into the Ru—H bond of Ⅴ to form the Ru-OCOH intermediate Ⅵ. The HCOO^- of Ⅵ is exchanged with Cl^- in the solution to obtain the product formate and complete the catalyst regeneration.

  Figure 5

  

  Figure 5.  Possible catalytic mechanism for CO₂ hydrogenation catalyzed by the Ru-NNN complex

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  3.   Conclusions

  In summary, we designed and synthesized a Ru-NNN Complex 1, which was applied to the hydrogenation of CO₂ to formate, achieving relatively high catalytic activity (TON=150 000) compared to similar Ru complexes. By combining the crystal structure of Complex 2 with in-situ NMR and MS spectra, we inferred a potential reaction mechanism involving intermediate Ⅱ with empty coordination sites as the active intermediate. To further validate the plausibility of these intermediates, we conducted catalytic tests under various control conditions. By performing catalytic activity tests with an excess of PPh₃ in a mixture with Complex 1, we confirmed that the high activity of Complex 1 was due to the vacant coordination sites generated by its in-situ dissociation. At the same time, the easy dissociation of PPh₃ may be related to the symmetric structure of Complex 1, which requires further confirmation. This study showed that combining rigid and weak ligands in the catalysts simultaneously is a good strategy to solve the paradox of high activity and stability.

  Supporting information is available at http://www.wjhxxb.cn

  
  
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  Scheme 1  Constructing Ru-NNN complexes with both high catalytic activity and stability by using rigid ligands and weak ligands simultaneously

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  Figure 1  Crystal structure of Complex 1 and active intermediate (Ⅱ) (Crystal structure of Complex 1 was obtained from the reference [25].)

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  Figure 2  (a) ³¹P NMR spectrum of Complex 1; (b) ³¹P NMR spectrum of Complex 1 at 50 ℃; (c) ³¹P NMR spectrum of the mixture of Complex 1 and PPh₃ (162 MHz, CDCl₃)

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  Figure 3  (a) ¹H NMR spectrum and (b) HPLC-HRMS spectrum of the reaction mixture

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  Figure 4  Gibbs free energy profiles for CO₂ hydrogenation catalyzed by the Ru-NNN complex

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  Figure 5  Possible catalytic mechanism for CO₂ hydrogenation catalyzed by the Ru-NNN complex

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  Table 1.  Catalytic activity of complex 1 for hydrogenation of CO₂ to formate^a

  Entry	nCat. / μmol	Base	T / ℃	t / h	TONb	TOFc / h-1
  1	1	NEt3	150	16	1 465	92
  2	1	DBU	150	16	2 218	139
  3	1	KOH	150	16	3 412	213
  4	1	CsOH	150	16	6 213	388
  5	1	CsOH	90	16	178	11
  6	1	CsOH	100	16	512	32
  7	1	CsOH	110	16	1 513	95
  8	1	CsOH	120	16	2 746	172
  9	1	CsOH	130	16	2 280	143
  10	1	CsOH	140	16	9 176	574
  11	1	CsOH	160	16	7 322	458
  12	1	CsOH	170	16	4 894	306
  13	1	CsOH	140	2	1 096	548
  14	1	CsOH	140	4	2 245	561
  15	1	CsOH	140	8	4 635	579
  16	0.001	CsOH	140	16	149 558	9 347
  17	0.01	CsOH	140	16	68 196	4 262
  18	0.1	CsOH	140	16	32 116	2 007
  19	10	CsOH	140	16	584	37
  20d	1	CsOH	140	16	10 304	644
  a General conditions: complex 1, base (10 mmol), pH2=pCO2=2.5 MPa, VTHF=1 mL, VH2O=5 mL; b TON=nformate/nRu; c TOF=TON/t; d 1 mmol LiOAc as additive.

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