English

• 0.   Introduction

  The increasing global energy demand and the overconsumption of fossil fuels, which have led to rising carbon dioxide (CO₂) concentration in the atmosphere, have drawn significant concern from the global community. CO₂ is not only a greenhouse gas, but also an important C1 feedstock for producing chemicals and fuels due to its abundant, renewable, low-cost, and non-toxic nature^[1-2]. Thus, the conversion of CO₂ into valuable fuel-related C1 chemicals, such as formic acid, formate, formaldehyde, methanol, and methane, provides a pathway to a sustainable society^[1-7]. Among various CO₂-derived products, formate is a particularly promising target, serving as a chemical feedstock, a commodity chemical for food and agriculture, a liquid fuel, and a potential hydrogen (H₂) storage medium^[8-10]. By utilizing CO₂ as an energy vector for H₂ storage, the hydrogenation of CO₂ into formate using transition-metal complexes is one of the most attractive approaches to simultaneously address both CO₂ chemical fixation and hydrogen storage problems^[11-12].

  Since the pioneering work by Inoue et al. in 1976^[13], the homogeneous catalytic hydrogenation of CO₂ to formic acid/formate, catalyzed by transition- metal complexes based on Ru^[14-16], Rh^[17-18], Ir^[19-22], Fe^[23], Co^[24], and Ni^[25], has been extensively investigated. To date, the state-of-the-art in homogeneous hydrogenation of CO₂ remains dominated by noble metal catalysts, particularly phosphorus (P)-containing ruthenium and iridium complexes, which are the most active^{[16,19-20,26-27]}. Significantly, Pidko′s group reported an exceptional turnover frequency (TOF) of 1.89×10⁶ h^-1 by Ru-PNP complex^[16]. Nozaki and coworkers developed an unprecedentedly high activity, achieving a turnover number (TON) of 3.5×10⁶ by an iridium complex with tridentate pyridine-based pincer phosphine ligands (Ir-PNP)^[19]. However, most Ir-based complexes do not give a satisfying TON due to the rapid deactivation of the catalyst^[28-32]. In order to meet the requirements for industrial applications, there remains a strong demand for the development of a catalytic system that is both highly active and stable in the hydrogenation of CO₂. N-heterocyclic carbenes (NHCs) are one of the most promising ligands due to their strong σ‑ donating property, which enables the formation of strong coordination bonds with metal centers, therefore stabilizing the active metal species and enhancing the stability of metal complexes during CO₂ hydrogenation^[27,33-38]. Recently, our group reported that unsymmetric Ru-CNP and Ru-CNN complexes^[34,37], incorporating functionalized NHC ligands (CNP and CNN), exhibited high activity and excellent stability in CO₂ hydrogenation to formate, with high TONs of 1.7×10⁵ and 6.5×10⁵, respectively. To further explore stable and efficient catalysts and improve the catalytic performance of CO₂ hydrogenation based on NHC metal complexes, we developed a new class of iridium(Ⅰ) complexes bearing nitrogen-phosphine functionalized NHC ligands (CNP) and 1, 5-cyclooctadiene molecule (cod), which exhibit high activity and long catalytic lifetime for the hydrogenation of CO₂ (Fig. 1).

  Figure 1

  

  Figure 1.  Design of Ir-CNP complexes with high activity and excellent stability for efficient CO₂ hydrogenation into formate

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  1.   Experimental

  1.1   Materials and methods

  All operations were carried out under a nitrogen atmosphere or in a glovebox. All solvents were purified with standard methods. Reagents and [Ir(cod)Cl]₂ were purchased from commercial suppliers and used as received. 3-(2-{[2-(diphenylphosphaneyl)benzylidene]amino}ethyl)-1-methyl-1H-imidazol-3-ium chloride (L1-Cl)^[39], 3-(2-{[2-(diphenylphosphaneyl)benzylidene]amino}ethyl)-1-methyl-1H-imidazol-3-ium hexafluorophosphate (L1-PF₆)^[39], and 3-(2-{[2-(diphenylphosphaneyl)benzyl]amino}ethyl)-1-methyl-1H-imidazol-3-ium chloride (L2) were synthesized according to the reported methods^[34]. ¹H NMR, ¹³C NMR, and ³¹P NMR spectra were measured on the Bruker AVANCE Ⅲ HD-400 MHz in DMSO-d₆ or CD₂Cl₂. HRMS spectra were recorded on a SHIMADZU LCMS-IT-TOF mass spectrometer.

  1.2   Synthesis of [Ir(cod)(κ³-CN^imP)]Cl (1-Cl)

  The addition of [Ir(cod)Cl]₂ to the silver-NHC complexes generated from L1-Cl, L1-PF₆, and L2 with Ag₂O, without isolation, afforded the corresponding NHC‑based iridium(Ⅰ) complexes, respectively (Scheme 1).

  Figure 1

  

  Figure 1.  Synthetic routes of Ir(Ⅰ)-CNP complexes

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  Ligand L1-Cl (0.30 mmol, 129.9 mg), silver oxide (0.15 mmol, 34.8 mg), and dichloromethane (20 mL) were added to a two-necked flask in a glovebox. The mixture was stirred in the dark at room temperature for 10 h, then filtered through celite to remove a small amount of unreacted Ag₂O. Subsequently, [Ir(cod)Cl]₂ (0.15 mmol, 101 mg) and toluene (30 mL) were added to the filtrate and stirred at 60 ℃ for 9 h. At the end of the reaction, the mixture was filtered, and the solvent of the filtrate was concentrated to about 10 mL. Ether (10 mL) was then added to the solution to precipitate the desired product. The precipitate was filtered, washed with ether (5 mL×2), and dried under vacuum to obtain complex 1-Cl as a bright yellow solid (141 mg, 64% yield). ¹H NMR (600 MHz, DMSO-d₆): δ 8.82 (s, 1H), 7.96 (dd, J=7.7, 3.9 Hz, 1H), 7.92-7.88 (m, 2H), 7.74-7.71 (m, 3H), 7.67 (t, J=7.5 Hz, 1H), 7.60 (t, J=7.6 Hz, 1H), 7.26-7.01 (m, 6H), 6.51 (t, J=5.56 Hz, 2H), 4.71 (t, J=10.9 Hz, 1H), 4.39 (dd, J=12.1, 2.9 Hz, 1H), 4.31 (dt, J=13.8, 3.6 Hz, 1H), 3.87 (s, 3H), 2.93-2.81 (m, 4H), 2.33-1.93 (m, 7H), 1.75 (m, 2H). ³¹P NMR (162 MHz, DMSO-d₆): δ 2.39 (s). HRMS (ESI-TOF) m/z: Calcd. for C₃₃H₃₆N₃Pir ([M-Cl]⁺) 698.227 6, Found 698.223 4.

  1.3   Synthesis of [Ir(cod)(κ³-CN^imP)]PF₆ (1-PF₆)

  Ligand L1-PF₆ (0.30 mmol, 162.9 mg), silver oxide (0.15 mmol, 34.8 mg), and dichloromethane (20 mL) were added to a two-necked flask in a glovebox. The mixture was stirred in the dark at room temperature for 10 h, then filtered through celite. Subsequently, [Ir(cod)Cl]₂ (0.15 mmol, 101 mg) and toluene (30 mL) were added to the filtrate and stirred at 60 ℃ for 9 h. The reaction mixture was then filtered, and the solvent of the filtrate was reduced to about 10 mL under vacuum, and ether (10 mL) was added to it to precipitate the desired product. The precipitate was filtered, washed with ether (5 mL×2), and dried under vacuum to yield complex 1-PF₆ as a brick-red solid (167 mg, 66% yield). ¹H NMR (400 MHz, CD₂Cl₂): δ 8.69 (s, 1H), 8.03-7.93 (m, 2H), 7.89 (ddd, J=7.6, 4.0, 1.3 Hz, 1H), 7.75-7.58 (m, 4H), 7.57-7.50 (m, 1H), 7.23-7.14(m, 2H), 7.09 (td, J=7.8, 2.2 Hz, 2H), 6.92 (d, J=2.1 Hz, 1H), 6.69 (d, J=2.0 Hz, 1H), 6.60-6.52 (m, 2H), 4.96-4.80 (m, 1H), 4.55-4.40 (m, 1H), 4.25 (dt, J=13.8, 3.4 Hz, 1H), 3.95 (s, 3H), 3.10-2.91 (m, 4H), 2.36-1.70 (m, 8H), 1.41-1.23 (m, 1H). ³¹P NMR (162 MHz, CD₂Cl₂): δ 2.64 (s), -144.41 (hept, J=711.0 Hz). ¹³C{¹H} NMR (100.6 MHz, CD₂Cl₂): δ 166.44, 159.24 (d, J_C—P=11.4 Hz, carbon atom of the NHC), 137.35 (d, J=9.6 Hz), 136.72 (d, J=15.5 Hz), 134.95 (d, J=42.7 Hz), 134.23 (d, J=6.9 Hz), 132.21 (d, J=6.1 Hz), 131.87, 131.85, 131.21 (d, J=1.6 Hz), 130.02 (d, J=10.7 Hz), 129.27 (d, J=2.1 Hz), 128.98 (d, J=10.3 Hz), 128.41 (d, J=9.0 Hz), 127.85 (d, J=32.3 Hz), 126.92 (d, J=35.7 Hz), 123.3, 122.2, 64.40 (d, J=3.3 Hz), 51.04, 39.27, 0.76. HRMS (ESI‑TOF) m/z: Calcd. for C₃₃H₃₆N₃PIr ([M-PF₆]⁺) 698.227 6, Found 698.223 2.

  1.4   Synthesis of [Ir(cod)(κ³-CN^HP)]Cl (2)

  Ligand L2 (0.30 mmol, 130.6 mg), silver oxide (0.15 mmol, 34.8 mg), and dichloromethane (20 mL) were added to a two-necked flask in a glovebox. The mixture was stirred in the dark at room temperature for 10 h, and then was filtered with celite. [Ir(cod)Cl]₂ (0.15 mmol, 101 mg) and toluene (30 mL) were added to the filtrate and stirred at 60 ℃ for 9 h. Then, the reaction mixture was filtered, and the filtrate was concentrated to about 10 mL under vacuum. The addition of 10 mL of ether to the filtrate generated the desired product. The precipitate was filtered, washed with ether (5 mL×2), and dried under vacuum to give complex 2 as a yellow-brown solid (110.3 mg, 50% yield). ¹H NMR (400 MHz, DMSO-d₆): δ 8.11-7.93 (m, 2H), 7.76-7.58 (m, 3H), 7.46-7.34 (m, 5H), 7.29-7.12 (m, 3H), 6.92-6.79 (m, 1H), 6.65 (t, J=8.76 Hz, 2H), 6.27 (brs, 1H), 4.15-4.10 (m, 1H), 4.05-3.93 (m, 3H), 3.72 (s, 3H), 3.40-3.35 (m, 1H), 2.98 (dd, J=13.2, 10.2 Hz, 1H), 2.68-2.60 (m, 3H), 2.21-2.05 (m, 6H), 1.98-1.83 (m, 1H), 1.57 (d, J=8.1 Hz, 2H). ³¹P NMR (162 MHz, DMSO-d₆): δ 2.00 (s). HRMS (ESI-TOF) m/z: Calcd. for C₃₃H₃₈N₃PIr ([M-Cl]⁺) 700.243 3, Found 700.239 4.

  1.5   General procedure for catalytic hydrogenation of CO₂ with H₂

  A fresh stock solution of the Ir complexes was prepared before the experiment. For example, the catalyst (40 μmol) was dissolved in degassed EtOH (100.0 mL) and diluted to 0.10 μmol·mL^-1 with EtOH. Catalytic CO₂ hydrogenation was carried out in a Hastelloy Autoclave Reactor system equipped with a 50 mL cylinder. Under a nitrogen atmosphere, 1 mL of a stock solution of the complex and 10 mL of KOH aqueous solution (or other base aqueous solution) were added to the 50 mL autoclave. The autoclave was then sealed, pressurized with CO₂, and stirred at room temperature until the pressure of CO₂ no longer dropped, after which the CO₂ pressure was adjusted to the required level. Then, H₂ was filled to the desired pressure. The reaction mixture was vigorously stirred and heated to the desired temperature (50-170 ℃) for the appropriate time (5-150 h). After the reaction, the mixture was cooled to room temperature, and slowly released the pressure. 50-1 000 μL of DMF (N,N-dimethylformamide) was added as an internal standard. Then, the formate was quantified by ¹H NMR spectroscopy with D₂O (formate was the only product; no CO or other gas-phase products were detected by gas chromatography from the residual gas after the reaction). The reactions were run three times, and the average amount of formate (mmol) was used.

  1.6   X-ray crystallography

  A suitable quality single crystal with a dimension of 0.35 mm×0.3 mm×0.25 mm was obtained by slow expanding evaporation of n-hexane into the saturated dichloromethane solution of complex 1-PF₆ at 2 ℃. Single crystal X-ray diffraction data were collected on a Xcalibur Eos diffractometer (Mo Kα, λ=0.071 073 nm) at 293(2) K. The structure was solved by Olex2^[40] with the ShelXT^[41] structure solution program using Intrinsic Phasing and refined with the ShelXL^[42] refinement package using Least Squares minimisation. All non- hydrogen atoms were refined anisotropically. All the hydrogen atoms were positioned geometrically and refined using a riding model. The details of single‑ crystal diffraction data and structure refinement parameters for 1‑PF₆ are provided in Table 1. Selected bond lengths (nm) and angles (°) are displayed in Table 2.

  Table 1

  

  Table 1.  Crystal data and structure refinement for complex 1-PF₆

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  Parameter	1-PF6		Parameter	1-PF6
  Empirical formula	C33H36F6IrN3P2		Dc / (g·cm-3)	1.743
  Formula weight	842.79		μ / mm-1	4.320
  Crystal system	Triclinic		F(000)	832.0
  Space group	P1		2θ range for data collection / (°)	5.854-58.536
  a / nm	0.960 81(7)		Index ranges	-10 ≤ h ≤ 13, -16 ≤ k ≤ 16, -18 ≤ l ≤ 18
  b / nm	1.226 27(7)		Reflection collected	14 343
  c / nm	1.398 78(8)		Independent reflection	7 366 (Rint=0.045 0, Rσ=0.077 6)
  α / (°)	89.612(5)		Data, restraints, number of parameters	7 366, 3, 421
  β / (°)	86.094(5)		Goodness-of-fit on F 2	1.095
  γ / (°)	77.638(5)		Final R indexes [I≥2σ(I)]	R1=0.058 2, wR2=0.137 0
  Volume / nm3	1.606 06(18)		Final R indexes (all data)	R1=0.078 0, wR2=0.152 0
  Z	2

  Table 2

  

  Table 2.  Selected bond lengths (nm) and angles (°) for complex 1-PF₆

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  Ir1—P1	0.232 4(2)	Ir1—N3	0.220 2(6)	Ir1—C1	0.203 9(8)
  Ir1—C26	0.213 8(8)	Ir1—C27	0.210 7(8)	Ir1—C30	0.222 9(7)
  Ir1—C31	0.219 9(8)
  N3—Ir1—P1	80.55(17)	N3—Ir1—C30	117.4(3)	C1—Ir1—P1	90.8(2)
  C1—Ir1—N3	86.5(3)	C1—Ir1—C26	90.8(3)	C1—Ir1—C27	86.5(4)
  C1—Ir1—C30	155.5(4)	C1—Ir1—C31	165.1(3)	C26—Ir1—P1	177.0(2)

  2.   Results and discussion

  2.1   Synthesis and characterization of iridium(Ⅰ) complexes

  The transmetallation reaction via a silver-carbene intermediate (Ag-NHC) was employed to prepare iridium (Ⅰ) complexes: [Ir(cod)(κ³-CN^imP)]Cl (1-Cl), [Ir(cod)(κ³-CN^imP)]PF₆ (1-PF₆), and [Ir(cod)(κ³-CN^HP)]Cl (2) (Scheme 1). The silver-NHC complexes generated from L1-Cl, L1-PF₆, and L2 with Ag₂O reacted with [Ir(cod)Cl]₂ in a mixed solvent of dichloromethane and toluene at 60 ℃ to form the corresponding NHC-based iridium(Ⅰ) complexes 1-Cl, 1-PF₆, and 2, respectively. ¹H NMR, ³¹P NMR, ¹³C NMR, HRMS, and single crystal X-ray diffraction analyses were performed to verify the successful synthesis of iridium(Ⅰ) complexes 1-Cl, 1-PF₆, and 2. In the ¹H NMR spectrum of 1-Cl (Fig. S2, Supporting information), the proton of the imine moiety (—CH=N—) on the ligand backbone produced a broad singlet at δ 8.82. Meanwhile, only one singlet at δ 2.39 was observed in the ³¹P NMR spectrum of complex 1-Cl (Fig. S3), while its HRMS spectrum (Fig. S4) displayed the exact molecular ion peak of [M-Cl]⁺ (C₃₃H₃₆N₃PIr) at m/z 698.223 4. The ¹H NMR spectrum of complex 1-PF₆ (Fig. S5) in CD₂Cl₂ showed a characteristic signal at δ 8.69, which was assigned to the proton of the imine moiety (—CH=N—) on the ligand backbone. The ³¹P NMR spectrum (Fig. S6) in CD₂Cl₂ exhibited a singlet at δ 2.64, while the counterion hexafluorophosphate (PF₆^-) gave rise to a septet at δ= -144.41 (J=711.0 Hz). In the ¹³C NMR spectrum of 1-PF₆ (Fig. S7), a doublet signal at δ 159.24 with a small coupling constant of phosphorus to carbene carbon (J_C—P=11.4 Hz), corresponding to the Ir-C{NHC} resonance, was observed. The intense signal at m/z 698.223 2 corresponds well to the cationic fragment [M-PF₆]⁺ of complex 1-PF₆ (Fig. S8). For complex 2, the ¹H NMR spectrum (Fig. S9) in DMSO-d₆ exhibited a broad resonance at δ 6.27 for the proton of the NH moiety. The ³¹P NMR spectrum of complex 2 in DMSO-d₆ exhibited a singlet at δ 2.00 (Fig. S10), with an exact molecular ion peak at m/z 700.239 4 ([M-Cl]⁺, C₃₃H₃₈N₃PIr) in the HRMS spectrum (Fig. S11), which was consistent with that of the desired complex 2.

  2.2   Crystal structure of 1-PF₆

  The molecular structure of complex 1-PF₆ was determined by single-crystal X-ray diffraction. Complex 1-PF₆ belong to triclinic crystal system with space group P1 (Table 1), and its asymmetric unit consists of one Ir(Ⅰ) ion, one L1 ligand and one cod ligand (Fig. 2). As shown in Fig. 2, the coordination geometry around the Ir(Ⅰ) center adopts a slightly distorted trigonal bipyramidal configuration with two olefinic bonds of a cod molecule and three coordination atoms of the CNP ligand. The Ir1—C1(NHC) bond shows a shorter bond length of 0.203 9(8) nm (Table 2), which is similar to those iridium-carbene bond distances of NHC iridium complexes^[27,33]. This strong Ir1—C1(NHC) bond indicates that the carbene carbon plays a better anchoring role to iridium due to the strong σ-donating character of the NHC moiety, which helps stabilize the active metal species during CO₂ hydrogenation.

  Figure 2

  

  Figure 2.  Crystal structure of complex 1-PF₆ with an ellipsoid probability of 50%

  The counterion is omitted for clarity.

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  2.3   Catalytic hydrogenation of CO₂ to formate

  Initially, the catalytic hydrogenation of CO₂ by iridium complexes 1-Cl, 1-PF₆, and 2 was investigated under 6.0 MPa of H₂/CO₂ (3∶1, V/V) in K₂CO₃ aqueous solution with 0.1 μmol of the Ir complexes (1-Cl, 1-PF₆, and 2) at 150 ℃ for 5 h (Table 3). Changing the counterion from Cl^- in 1-Cl to PF₆^- in 1-PF₆ did not significantly influence the observed catalytic activity (Table 3, entries 1 and 2). Complex 1-Cl showed the same catalytic activity in terms of TON and TOF as complex 2, reaching a TON of 6 340 (Table 3, entries 1 and 3). This result suggested that the N—H structure in Ir-CNP complexes 2 does not influence the performance for the CO₂ hydrogenation reaction. Therefore, complex 1-Cl was chosen for the following catalytic experiments.

  Table 3

  

  Table 3.  Hydrogenation of CO₂ catalyzed by the Ir complexes*

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  Entry	Ir cat.	Base	TON	TOF/h-1
  1	1-Cl	K2CO3	6 340	1 268
  2	1-PF6	K2CO3	5 820	1 164
  3	2	K2CO3	6 340	1 268
  * Reaction conditions: 0.1 μmol Ir complexes in 1 mL EtOH, 6.0 MPa H2/CO2 (3∶1, V/V), K2CO3 (12 mmol), H2O (10 mL), 150 ℃, 5 h; TON=nHCOO-/nIr; TOF=TON/treaction; Reported values are the average of three trials.

  Then, we studied the influence of the total pressure of H₂/CO₂ (1∶1, V/V) on the reaction in a 1.2 mol·L^-1 KOH aqueous solution at 150 ℃ for 5 h. As shown in Table 4, a higher total pressure is beneficial for the CO₂ hydrogenation reaction. Increasing the total pressure of H₂/CO₂ (1∶1, V/V) from 2.0 MPa to 6.0 MPa greatly increased the TON, with a high TON of 4150 achieved at 6.0 MPa (Table 4, entry 5). Moreover, the partial pressures of H₂ and CO₂ also influenced the hydrogenation of CO₂. As shown in Table 5, when the CO₂ pressure was kept at 1.5 MPa, the TON of formate increased significantly with the increasing pressure of H₂ (Table 5, entries 1-4). Conversely, when the H₂ pressure was maintained at 1.5 MPa, the TON showed a slow increase as the CO₂ partial pressure rose (Table 5, entries 1, 5-7). These results indicated a strong dependency of the catalyst activity on the total pressure of the H₂/CO₂ mixture and the partial pressures of H₂ and CO₂. As a result, a high TON of 8 950 was achieved at the optimum ratio of 1∶3 (V/V) of CO₂/H₂ mixture (Table 5, entry 4).

  Table 4

  

  Table 4.  Effect of total pressure on catalytic CO₂ hydrogenation*

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  Entry	ptotal / MPa	TON	TOF / h-1
  1	2.0	1 520	304
  2	3.0	2 130	426
  3	4.0	3 180	636
  4	5.0	3 760	752
  5	6.0	4 150	830
  * Reaction conditions: 0.1 μmol complex 1-Cl in 1 mL EtOH, pCO2∶pH2=1∶1, 1.2 mol·L-1 aqueous solution of KOH (10 mL), 150 ℃, 5 h.

  Table 5

  

  Table 5.  Effect of H₂ and CO₂ partial pressure on catalytic CO₂ hydrogenation*

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  Entry	$ {p}_{\mathsf{C}{\mathsf{O}}_{2}} $ / MPa	$ {p}_{{\mathsf{H}}_{2}} $ / MPa	TON	TOF / h-1
  1	1.5	1.5	2 130	426
  2	1.5	2.5	3 630	726
  3	1.5	3.5	7 240	1 448
  4	1.5	4.5	8 950	1 790
  5	2.5	1.5	2 720	544
  6	3.5	1.5	3 100	620
  7	4.5	1.5	3 490	698
  *Reaction conditions: 0.1 μmol complex 1-Cl in 1 mL EtOH, 1.2 mol·L-1 aqueous solution of KOH (10 mL), 150 ℃, 5 h.

  The choice of base can potentially impact the catalytic activity of CO₂ hydrogenation. So, the catalytic activity of complex 1-Cl for the hydrogenation of CO₂ was tested using several different bases (Table 6). Organic bases, including triethylamine (TEOA), triethanolamine (NEt₃), and 1, 8-diazabicyclo[5.4.0]undec-7-ene (DBU), as well as inorganic bases such as Na₂CO₃, K₂CO₃, Cs₂CO₃, NaOH, and KOH, were all effective in promoting the CO₂ hydrogenation. Remarkably, Cs₂CO₃ exhibited the best promoting effect, achieving a TON of 11 400 and a TOF of 2 280 h^-1.

  Table 6

  

  Table 6.  Catalytic hydrogenation of CO₂ with different bases*

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  Entry	Base	TON	TOF / h-1
  1	KOH	8 950	1 790
  2	NaOH	3 502	700
  3	CsOH	3 360	672
  4	Na2CO3	4 540	908
  5	Cs2CO3	11 400	2 280
  6	Na3PO4	2 840	568
  7	TEOA	3 890	778
  8	NEt3	3 370	674
  9	DBU	7 055	1 411
  *Reaction conditions: 0.1 μmol 1-Cl complexes in 1 mL EtOH, 6.0 MPa H2/CO2 (3∶1, V/V), 12 mmol base in 10 mL H2O (1.2 mol·L-1), 150 ℃, 5 h.

  The effect of temperature on CO₂ hydrogenation was studied at temperatures ranging from 50 to 170 ℃ under 6.0 MPa H₂/CO₂ (3∶1, V/V) in Cs₂CO₃ aqueous solution (Table 7). The catalytic performance of complex 1-Cl was largely dependent on temperature, with higher reaction temperatures being favorable for the reaction. The TON of formate significantly improved from 564 to 30 700 as the reaction temperature increased from 70 to 170 ℃ (Table 7, entries 2-7). The introduction of a strong σ bonded carbene moiety and the tris-chelating nature of the CNP ligand endows the Ir(Ⅰ)-CNP complex with high thermal stability and catalytic activity. The effect of Cs₂CO₃ concentration on the TON was also investigated at 170 ℃ for the initial 5 h. As the Cs₂CO₃ concentration increased from 1.2 to 3.6 mol·L^-1, there was a sharp increase in TON of formate from 23 900 to 66 700 (Table 7, entries 7-10). Control experiments showed that the TON of formate was 9 570 when the reaction was performed in the absence of CO₂ gas (Table 7, entry 11), which manifested a ca. 84% decrease compared to the results at H₂/CO₂ mixture (Table 7, entry 9). These findings suggested that Cs₂CO₃ can also act as the carbon source to afford formate with low activity, while CO₂ serves as the primary source of formate.

  Table 7

  

  Table 7.  Influence of temperature and Cs₂CO₃ concentration on catalytic CO₂ hydrogenation^a

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  Entry	$ {c}_{\mathsf{C}{\mathsf{s}}_{2}\mathsf{C}{\mathsf{O}}_{3}} $ / (mol·L-1)	T / ℃	TON	TOF / h-1
  1	1.2	50	trace	trace
  2	1.2	70	564	113
  3	1.2	90	1 820	364
  4	1.2	110	2 970	594
  5	1.2	130	5 450	1 090
  6	1.2	150	11 400	2 280
  7	1.2	170	30 700	6 140
  8	0.6	170	23 900	4 780
  9	2.4	170	60 200	12 040
  10	3.6	170	66 700	13 340
  11b	2.4	170	9 570	1 910
  a Reaction conditions for entries 1‑10: 0.1 μmol complex 1-Cl in 1 mL EtOH, 6.0 MPa H2/CO2 (3∶1, V/V), 5 h, 10 mL Cs2CO3 aqueous solution; b The H2/CO2 mixture was changed into 4.5 MPa H2, and other reaction conditions were the same as those in entries 1-10.

  In further experiments, we investigated the influence of the catalyst loading for the CO₂ hydrogenation reaction in 3.6 mol·L^-1 Cs₂CO₃ aqueous solution at 170 ℃. An increase in activity for complex 1-Cl was observed by reducing the catalyst loading from 0.2 to 0.025 μmol (Table 8, entries 1-4). A TON of 71 600 and an average TOF of 14 320 h^-1 were achieved with 0.025 μmol of complex 1-Cl after 5 h. The catalytic efficiency of complex 1-Cl was comparable to that of complex 2 at a catalyst loading of 0.025 μmol (Table 8, entries 4 and 5), which was consistent with the results from entries 1-3 in Table 3. For the following experiments, we chose a catalyst loading of 0.025 μmol. The Ir(Ⅰ)-CNP complex 1-Cl exhibited high activity and excellent stability for the hydrogenation of CO₂ in a high-concentration base aqueous solution at a high temperature. For instance, the formate production was significantly enhanced by further prolonging the catalytic reaction time in 3.6 mol·L^-1 Cs₂CO₃ aqueous solution at 170 ℃ with 0.025 μmol of complex 1-Cl (Table 8, entries 4, 6-9), with the values of TON increasing to 9.25×10⁵ after 100 h (Table 8, entry 9). For comparison, the TON of formate was 7.47×10⁵ when the reaction was conducted in KOH aqueous solution after 100 h, indicating that Cs₂CO₃ aqueous solution is more suitable for CO₂ hydrogenation, consistent with our previous results (Table 6). The complex 1-Cl showed excellent stability (Table 8, entry 9) with a long catalytic lifetime of over 100 h at 170 ℃. Furthermore, a maximum TON value of 1.16×10⁶ was achieved after prolonging the reaction time to 150 h in 3.6 mol·L^-1 Cs₂CO₃ aqueous solution (Table 8, entry 12), which represents a relatively high value in the reported hydrogenation of CO₂ catalyzed by iridium and ruthenium complexes (Fig. 3)^{[14,19,27,29,34,36,43-48]}. Besides the activity, the stability of a catalyst is crucial in the reaction. For the sake of comparing the stability of some representative Ru and Ir complexes with our system, we use TOF_average/TOF_start to reflect the stability of a catalyst system. The TOF_average/TOF_start of many highly active Ru and Ir complexes decreased rapidly with extending the reaction time until the catalysts were completely deactivated within a few hours (Fig. 4)^{[14,29,34,43-47]}. In our system, when the reaction time was extended to 150 h, the TOF_average/TOF_start showed a decrease of 46%. In other words, the complex 1-Cl could keep about 54% of its initial activity and had moderate activity with a TOF of 7730 h^-1 after 150 h, achieving a high TON of 1.16×10⁶. The high catalytic activity and excellent stability of the Ir-CNP complex are attributed to the design strategy of the CNP ligand. The anchoring role of the NHC moiety and the chelating effect of the polydentate ligand help stabilize the active metal species and then enhance the stability of metal complexes during CO₂ hydrogenation. In addition, the strong σ electron‑ donating ability of the NHC and phosphine moiety can enhance the electronic density of the Ir metal center, which is beneficial to CO₂ hydrogenation.

  Table 8

  

  Table 8.  Catalytic Hydrogenation of CO₂ over the Ir complexes under various conditions^a

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  Entry	Complex	ncomplex / μmol	t / h	$ {n}_{\mathsf{HCO}{\mathsf{O}}^{-}} $ / mmol	TON	TOF / h-1
  1	1-Cl	0.2	5	11.34	56 700	11 340
  2	1-Cl	0.1	5	6.67	66 700	13 340
  3	1-Cl	0.05	5	3.53	70 500	14 100
  4	1-Cl	0.025	5	1.79	71 600	14 320
  5	2	0.025	5	1.82	72 600	14 520
  6	1-Cl	0.025	10	3.47	139 000	13 900
  7	1-Cl	0.025	20	6.82	273 000	13 650
  8	1-Cl	0.025	50	15.93	637 000	12 740
  9	1-Cl	0.025	100	23.12	925 000	9 250
  10b	1-Cl	0.025	100	18.68	747 000	7 470
  11	1-Cl	0.025	130	26.76	1 070 000	8 230
  12	1-Cl	0.025	150	28.92	1 160 000	7 730
  a Reaction conditions for entries 1-9, 11-12: complex in 1 mL EtOH, 6.0 MPa H2/CO2 (3∶1, V/V), 36 mmol Cs2CO3 in 10 mL H2O (3.6 mol·L-1), T=170 ℃; b The base was changed into 72 mmol KOH, and other reaction conditions were the same as those in entries 1-9, 11-12.

  Figure 3

  

  Figure 3.  TON of formate vs time for several typical Ir and Ru complexes in the catalytic CO₂ hydrogenation (Table S3)^{[14,19,27,29,34,36,43-48]}

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  Figure 4

  

  Figure 4.  Stability of several typical Ir and Ru complexes in the catalytic CO₂ hydrogenation (Table S3)^{[14,29,34,43-47]}

  TOF_average represents the average TOF over the total reaction time, while TOF_start represents the initial TOF during the initial part of the reaction.

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  To further elucidate the catalytic mechanism, we employed in situ techniques to detect reaction intermediates. Quasi in situ HPLC-HRMS analysis (Fig. S12) during CO₂ hydrogenation identified a key active iridium species fragment, C₂₅H₂₆IrN₃P⁺, corresponding to the iridium dihydride species Ir-CNP-(H)₂. Complementary quasi in situ ¹H NMR spectroscopy revealed two doublets at δ=-6.76 and -6.93 (Fig. S13), exhibiting identical coupling constants (J=18.4 Hz), which supports the formation of a novel iridium intermediate characterized by two chemically equivalent hydride ligands coordinated to the iridium center. Based on the identification and analysis of these potential intermediates, we propose a plausible catalytic mechanism for hydrogen storage mediated by complex [Ir(cod)(κ³-CN^imP)]⁺ (1) (Fig. 5). Considering the H₂ hydrogenation process as a representative example, the COD ligand in complex 1 undergoes stepwise dissociation and subsequently reacts with H₂ in the presence of a base, yielding the iridium dihydride species (Ⅱ). Subsequently, the hydride ion executes a nucleophilic attack on the carbon atom of CO₂, generating the Ir-OCHO intermediate (Ⅲ). The HCOO^- moiety in intermediate Ⅲ then dissociates, releasing free formate, while H₂ coordinates to the iridium center, forming intermediate Ⅳ. Finally, intermediate Ⅳ regenerates the iridium dihydride species Ⅱ under basic conditions, thereby completing the catalytic cycle. This proposed mechanism underscores the critical roles of both Ir-H intermediates and ligand dynamics in facilitating CO₂ hydrogenation and subsequent formate production.

  Figure 5

  

  Figure 5.  Proposed catalytic cycle for CO₂ hydrogenation by complex 1

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  3.   Conclusions

  In summary, three new iridium(Ⅰ) complexes containing N-heterocyclic carbene-nitrogen-phosphine ligands were developed for the efficient hydrogenation of CO₂ to formate. The introduction of a strongly bonded carbene moiety dramatically improved the stability of P-containing complexes and maintained their high activity over a long period. As a result, the Ir(Ⅰ)-CNP complex emerged as one of the most effective CO₂ hydrogenation catalysts, achieving a maximum TON of 1.16×10⁶ for formate production after 150 h at 170 ℃. We also proposed a plausible reaction mechanism through quasi in situ HRMS and quasi in situ NMR spectroscopy analyses. This work provides a strategy for developing new and efficient CO₂ hydrogenation systems with exceptional activity and stability.

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

  Figure 1  Design of Ir-CNP complexes with high activity and excellent stability for efficient CO₂ hydrogenation into formate

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  Figure 1  Synthetic routes of Ir(Ⅰ)-CNP complexes

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  Figure 2  Crystal structure of complex 1-PF₆ with an ellipsoid probability of 50%

  The counterion is omitted for clarity.

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  Figure 3  TON of formate vs time for several typical Ir and Ru complexes in the catalytic CO₂ hydrogenation (Table S3)^{[14,19,27,29,34,36,43-48]}

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  Figure 4  Stability of several typical Ir and Ru complexes in the catalytic CO₂ hydrogenation (Table S3)^{[14,29,34,43-47]}

  TOF_average represents the average TOF over the total reaction time, while TOF_start represents the initial TOF during the initial part of the reaction.

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  Figure 5  Proposed catalytic cycle for CO₂ hydrogenation by complex 1

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  Table 1.  Crystal data and structure refinement for complex 1-PF₆

  Parameter	1-PF6		Parameter	1-PF6
  Empirical formula	C33H36F6IrN3P2		Dc / (g·cm-3)	1.743
  Formula weight	842.79		μ / mm-1	4.320
  Crystal system	Triclinic		F(000)	832.0
  Space group	P1		2θ range for data collection / (°)	5.854-58.536
  a / nm	0.960 81(7)		Index ranges	-10 ≤ h ≤ 13, -16 ≤ k ≤ 16, -18 ≤ l ≤ 18
  b / nm	1.226 27(7)		Reflection collected	14 343
  c / nm	1.398 78(8)		Independent reflection	7 366 (Rint=0.045 0, Rσ=0.077 6)
  α / (°)	89.612(5)		Data, restraints, number of parameters	7 366, 3, 421
  β / (°)	86.094(5)		Goodness-of-fit on F 2	1.095
  γ / (°)	77.638(5)		Final R indexes [I≥2σ(I)]	R1=0.058 2, wR2=0.137 0
  Volume / nm3	1.606 06(18)		Final R indexes (all data)	R1=0.078 0, wR2=0.152 0
  Z	2

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  Table 2.  Selected bond lengths (nm) and angles (°) for complex 1-PF₆

  Ir1—P1	0.232 4(2)	Ir1—N3	0.220 2(6)	Ir1—C1	0.203 9(8)
  Ir1—C26	0.213 8(8)	Ir1—C27	0.210 7(8)	Ir1—C30	0.222 9(7)
  Ir1—C31	0.219 9(8)
  N3—Ir1—P1	80.55(17)	N3—Ir1—C30	117.4(3)	C1—Ir1—P1	90.8(2)
  C1—Ir1—N3	86.5(3)	C1—Ir1—C26	90.8(3)	C1—Ir1—C27	86.5(4)
  C1—Ir1—C30	155.5(4)	C1—Ir1—C31	165.1(3)	C26—Ir1—P1	177.0(2)

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  Table 3.  Hydrogenation of CO₂ catalyzed by the Ir complexes*

  Entry	Ir cat.	Base	TON	TOF/h-1
  1	1-Cl	K2CO3	6 340	1 268
  2	1-PF6	K2CO3	5 820	1 164
  3	2	K2CO3	6 340	1 268
  * Reaction conditions: 0.1 μmol Ir complexes in 1 mL EtOH, 6.0 MPa H2/CO2 (3∶1, V/V), K2CO3 (12 mmol), H2O (10 mL), 150 ℃, 5 h; TON=nHCOO-/nIr; TOF=TON/treaction; Reported values are the average of three trials.

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  Table 4.  Effect of total pressure on catalytic CO₂ hydrogenation*

  Entry	ptotal / MPa	TON	TOF / h-1
  1	2.0	1 520	304
  2	3.0	2 130	426
  3	4.0	3 180	636
  4	5.0	3 760	752
  5	6.0	4 150	830
  * Reaction conditions: 0.1 μmol complex 1-Cl in 1 mL EtOH, pCO2∶pH2=1∶1, 1.2 mol·L-1 aqueous solution of KOH (10 mL), 150 ℃, 5 h.

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  Table 5.  Effect of H₂ and CO₂ partial pressure on catalytic CO₂ hydrogenation*

  Entry	$ {p}_{\mathsf{C}{\mathsf{O}}_{2}} $ / MPa	$ {p}_{{\mathsf{H}}_{2}} $ / MPa	TON	TOF / h-1
  1	1.5	1.5	2 130	426
  2	1.5	2.5	3 630	726
  3	1.5	3.5	7 240	1 448
  4	1.5	4.5	8 950	1 790
  5	2.5	1.5	2 720	544
  6	3.5	1.5	3 100	620
  7	4.5	1.5	3 490	698
  *Reaction conditions: 0.1 μmol complex 1-Cl in 1 mL EtOH, 1.2 mol·L-1 aqueous solution of KOH (10 mL), 150 ℃, 5 h.

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  Table 6.  Catalytic hydrogenation of CO₂ with different bases*

  Entry	Base	TON	TOF / h-1
  1	KOH	8 950	1 790
  2	NaOH	3 502	700
  3	CsOH	3 360	672
  4	Na2CO3	4 540	908
  5	Cs2CO3	11 400	2 280
  6	Na3PO4	2 840	568
  7	TEOA	3 890	778
  8	NEt3	3 370	674
  9	DBU	7 055	1 411
  *Reaction conditions: 0.1 μmol 1-Cl complexes in 1 mL EtOH, 6.0 MPa H2/CO2 (3∶1, V/V), 12 mmol base in 10 mL H2O (1.2 mol·L-1), 150 ℃, 5 h.

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  Table 7.  Influence of temperature and Cs₂CO₃ concentration on catalytic CO₂ hydrogenation^a

  Entry	$ {c}_{\mathsf{C}{\mathsf{s}}_{2}\mathsf{C}{\mathsf{O}}_{3}} $ / (mol·L-1)	T / ℃	TON	TOF / h-1
  1	1.2	50	trace	trace
  2	1.2	70	564	113
  3	1.2	90	1 820	364
  4	1.2	110	2 970	594
  5	1.2	130	5 450	1 090
  6	1.2	150	11 400	2 280
  7	1.2	170	30 700	6 140
  8	0.6	170	23 900	4 780
  9	2.4	170	60 200	12 040
  10	3.6	170	66 700	13 340
  11b	2.4	170	9 570	1 910
  a Reaction conditions for entries 1‑10: 0.1 μmol complex 1-Cl in 1 mL EtOH, 6.0 MPa H2/CO2 (3∶1, V/V), 5 h, 10 mL Cs2CO3 aqueous solution; b The H2/CO2 mixture was changed into 4.5 MPa H2, and other reaction conditions were the same as those in entries 1-10.

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  Table 8.  Catalytic Hydrogenation of CO₂ over the Ir complexes under various conditions^a

  Entry	Complex	ncomplex / μmol	t / h	$ {n}_{\mathsf{HCO}{\mathsf{O}}^{-}} $ / mmol	TON	TOF / h-1
  1	1-Cl	0.2	5	11.34	56 700	11 340
  2	1-Cl	0.1	5	6.67	66 700	13 340
  3	1-Cl	0.05	5	3.53	70 500	14 100
  4	1-Cl	0.025	5	1.79	71 600	14 320
  5	2	0.025	5	1.82	72 600	14 520
  6	1-Cl	0.025	10	3.47	139 000	13 900
  7	1-Cl	0.025	20	6.82	273 000	13 650
  8	1-Cl	0.025	50	15.93	637 000	12 740
  9	1-Cl	0.025	100	23.12	925 000	9 250
  10b	1-Cl	0.025	100	18.68	747 000	7 470
  11	1-Cl	0.025	130	26.76	1 070 000	8 230
  12	1-Cl	0.025	150	28.92	1 160 000	7 730
  a Reaction conditions for entries 1-9, 11-12: complex in 1 mL EtOH, 6.0 MPa H2/CO2 (3∶1, V/V), 36 mmol Cs2CO3 in 10 mL H2O (3.6 mol·L-1), T=170 ℃; b The base was changed into 72 mmol KOH, and other reaction conditions were the same as those in entries 1-9, 11-12.

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