CO₂ hydrogenation by phosphorus–nitrogen PN³P-pincer iridium hydride complexes: elucidation of the deactivation pathway

Abstract

PN³P–Ir pincer hydride complexes were synthesized and characterized as catalysts and key intermediates in the direct hydrogenation of CO₂ to formate under mild conditions. The formation of a dearomatized PN³P*–Ir(I)–CO species was identified as a plausible key process accountable for the loss of catalytic activity in the CO₂ hydrogenation.

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Carbon capture and sequestration (CCS) has been the subject of extensive research and commercial efforts.^1–5 The selective hydrogenation of carbon dioxide (CO₂) under mild conditions represents an economical and sustainable method of preparing valuable products and fuels from CO₂.^6,7 In this regard, significant achievements have been made using single-site homogeneous catalysts in CO₂ transformation, especially in the selective conversion of CO₂ and H₂ to formic acid or formate.^8,9 While the hydrogenation of CO₂ to HCOOH is thermodynamically favored in the aqueous phase, the energy barrier is high. As a result, high reaction pressures and temperatures are generally needed. A number of transition-metal complexes, such as rhodium,^10–15 ruthenium^16–27 and iridium,^28–37 have been investigated for the hydrogenation of CO₂. Among these metals, iridium complexes appear very promising.

In recent years, highly efficient and selective hydrogen transfer reactions and dehydrogenation reactions mediated by pincer-ligated complexes have been reported.^38,39 In addition, the catalytic conversion of n-alkanes to alkylaromatics using pincer supported iridium complexes^40–44 and of glycerol to lactic acid^45–47 have been well developed. For the reduction of CO₂, Himeda and Fujita et al. used a dinuclear proton-switchable iridium catalyst bearing an N,N′-type ligand, 4,4′,6,6′-tetrahydroxy-2,2′-bipyrimidine, as a bridging group, to afford an average TOF of 53 800 h⁻¹ in the presence of KHCO₃ at 80 °C under 50 bar.³² When a strong electron-donating PNP ligand was introduced into the CO₂ hydrogenation, high activities of Ir(III) complexes towards CO₂ hydrogenation were demonstrated by the Nozaki group (Fig. 1), and an unprecedentedly high TON of 3 500 000 was achieved.²⁹ Analogous “saturated” systems by Hazari et al. also showed a high TON of 348 000,³⁰ although a much higher temperature and pressure were necessary. Recent mechanistic studies on homogeneous iridium catalytic systems in CO₂ hydrogenation suggested that the CO₂ activation pathways are dependent on the nature of the Ir catalysts and that a hydrido ligand on an Ir(III) center is essential.^48–51

Fig. 1 Examples of PN^XP pincer Ir(III) trihydride catalysts for CO₂ hydrogenation.

We recently demonstrated that PN³(P)-pincer complexes show unique properties in various challenging transformations in organic synthesis and catalytic studies,^52–57 showing diverse catalytic activities and different thermodynamic and kinetic properties.^58,59 Driven by the potential advantages of employing more electron-rich pincer complexes, herein, we present our development on PN³P–Ir(III) trihydride complexes in CO₂ hydrogenation to formate. A probable catalyst-deactivation step in the formation of a dearomatized PN³P*Ir–CO complex is also discussed.

As has been suggested that Ir(III) trihydride complexes serve as effective catalysts in the hydrogenation of CO₂,²⁹ we developed a general route for the synthesis of the Ir(III) complexes based on PN³P ligands (Scheme 1). Cationic iridium(I) complexes 1a and 1b were synthesized by coordinating the PN³PR ligands (R = cyclopentyl (cPe) and t-butyl (^tBu)) with a stoichiometric amount of [Ir(coe)₂Cl]₂, similar to the procedure provided in the literature.⁶⁰ We then introduced H₂ to eliminate the coordinating coe moiety resulting in the formation of cationic iridium(III) complexes 2a and 2b after the oxidative addition of one equivalent of H₂. The NMR data of 2b collected in CDCl₃ at room temperature showed only one N–H (2H, δ 9.27 ppm) signal and one Ir–H (2H, δ −26.87 ppm) signal in the ¹H NMR spectrum and a signal at δ 126.2 ppm in the ³¹P NMR spectrum, suggesting a symmetric conformation enforced by the PN³P pincer ligand. In addition, the solid state molecular structure of 2b was consistent with the NMR details where two hydrides are equivalent (Fig. 2). The structure shows a square pyramidal configuration with a chloride as a dissociated counter anion.

Fig. 2 X-ray structure of complex 2b with 40% probability of thermal ellipsoids. The solvent and hydrogen atoms, except N–H and hydrides, were omitted for clarity. Selected bond lengths (Å): Ir1–N3 2.110(6), Ir1–P1 2.284(2), and Ir1–P2 2.279(2). Selected bond angles (°): (2b) P1–Ir1–P2 163.64(7).

Scheme 1 Synthesis of PN³P–Ir(III) trihydride complexes.

The neutral dearomatized complexes 3a and 3b were obtained by employing one equivalent of KO^tBu to deprotonate one of the NH arms in the PN³P pincer ligand.^61,62 Unlike their –CH₂ analogs,⁶² these dearomatized PN³P* complexes were stable enough to be isolated at room temperature. The ¹H NMR spectrum of 3b in C₆D₆ shows three sets of sp² C–H signals at δ 5.21 (d), 6.85 (d), and 6.92 (t) ppm, in agreement with the dearomatization of the central pyridine ring. As expected, two sets of doublet phosphorus signals were observed in the ³¹P NMR spectrum (²J_P–P = 299 Hz), with an apparent triplet for a hydride (²J_P–H = 12.0 Hz) at δ −24.93 ppm in the ¹H NMR spectrum.^{38,52–56,58,59} The solid-state molecular structure of 3b was obtained with two peaks in the Fourier map, indicative of two hydrides forming a distorted trigonal bipyramidal configuration (Fig. 3).

Fig. 3 X-ray structure of complex 3b with 40% probability of thermal ellipsoids. The solvent and hydrogen atoms, except N–H and hydrides, were omitted for clarity. Selected bond lengths (Å): (3b) Ir1–N3 2.063(6), Ir1–P1 2.2725(8), and Ir1–P2 2.2955(8). Selected bond angles (°): (3b) P1–Ir1–P2 162.79(3).

Finally, the trihydride complexes 4a and 4b were obtained by treating 3a and 3b in the presence of H₂ in THF. The ¹H NMR spectrum of complex 4a in C₆D₆ shows two sets of hydride signals, at δ −18.15 ppm and −11.76 ppm with an integral intensity ratio of 2 : 1, and two sets of signals for three protons in the sp² region (5.60 (d) and 6.81 (t) ppm), indicating the rearomatization of the central pyridine ring. Complex 4a was confirmed by the X-ray diffraction analysis (Fig. 4). The solid-state molecular structure of 4a was consistent with the NMR details. Trihydride 4a has an octahedral geometry with three peaks in the Fourier map assigned to hydrides on the Ir center, consistent with the reported ⁱPr–PNP–Ir trihydride compound.²⁹

Fig. 4 X-ray structures of complex 4a with 40% probability of thermal ellipsoids. The solvent and hydrogen atoms, except N–H and hydrides, were omitted for clarity. Selected bond lengths (Å): (4a) Ir1–N3 2.117(3). Selected bond angles (°): (4a) P1–Ir1–P2 164.07(3).

Several pincer-ligated iridium hydride complexes^29,30 are well known to catalyze CO₂ hydrogenation. Therefore, the catalytic activity of complexes 4a and 4b was examined accordingly in THF/H₂O solution (Table 1). According to the results of entries 1 and 2, the performance of 4b (yield 17.6%) is better than that of 4a (yield 6.3%) under the same conditions. 4b was thus chosen to evaluate the influence of the reaction conditions, such as temperature and reaction time, for the CO₂ hydrogenation reaction. The results of entries 2–4 show that the yield increases (17.6% to 49.6%) in the beginning with increasing temperature (120–130 °C), but then decreases with temperature increasing continually (130–140 °C). These observations suggest that 4b is not stable at high temperature. Prolonging the reaction time to 18 hours did not improve the yield of formate (49.6% to 51.0%, entry 5), indicating that 4b is likely to be deactivated within 12 hours.

Table 1 Hydrogenation of CO₂ catalyzed by Ir(III)-pincer complexes

Entry	Cat.	T (°C)	P (psi)	Time (h)	Yield^a (%)	TON	TOF (h^−1)
a Total pressure at room temperature: 120 psi (H2 : CO2 = 1 : 1). Yields: Calculated by ^1H NMR analysis using sodium 3-(trimethylsilyl)-1-propanesulfonate as an internal standard, based on the added KOH base (1.0 × 10^−3 mol). Catalyst loading: 1.0 × 10^−7 mol. [THF]/[H2O] = 1/4 (ratio by volume).
1	4a	120	120	12	6.3	630	53
2	4b	120	120	12	17.6	1760	147
3	4b	130	120	12	49.6	4960	413
4	4b	140	120	12	30.6	3060	255
5	4b	130	120	18	51.0	5100	283
6	5b	130	120	18	—	—	—

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To gain more insight into the catalyst deactivation, additional experiments were carried out. The treatment of 4b with H₂ and CO₂ (1 : 1, 120 psi) at 140 °C for 24 hours afforded 5b in 78% yield after recrystallization (Scheme 2), and in this process, the water should be generated as a by-product (Scheme 2). Furthermore, the hydrogenation of CO₂ which is catalyzed by complex 5b showed no activity (Table 1, entry 6). The ³¹P NMR spectrum of 5b indicated the presence of two nonequivalent P atoms. In the ¹H NMR spectrum, three sets of sp² C–H signals at 5.11(d), 6.74 (d), and 6.83 (t) ppm were observed, indicating the dearomatization of the central pyridine ring. The molecular structure of 5b shows a slightly distorted square-planar coordination geometry (Fig. 5). The metal–carbon bond distance is 1.837(6) Å, just slightly longer than that in the PNP–Ir–CO complex (Ir–CO: 1.818(2) Å).⁶² The Ir–N bond distance of 5b (2.076(4) Å) and that in the PNP–Ir–CO complex (2.083(2) Å) were practically identical. Consistent with our results, 5b was unable to catalyze the CO₂ hydrogenation under the same conditions shown in Table 1. The formation of the CO ligand implies that the reverse water gas shift (RWGS) reaction also takes place in the presence of the PN³P–Ir complex,^62–66 and this process is likely responsible for the loss of catalytic activity.

Scheme 2 Production complex 5b from complex 4b in the presence of CO₂/H₂.

Fig. 5 X-ray structure of 5b with 40% probability of thermal ellipsoids. The solvent and hydrogen atoms, except N–H, were omitted for clarity. Selected bond lengths (Å): (5b) Ir1–N3 2.076(4), Ir1–C22 1.837(6), Ir1–P1 2.3000(14), and Ir1–P2 2.3008(13). Selected bond angles (°): (5b) P1–Ir1–P2 163.33(5) and C(22)–Ir(1)–N(1) 179.3(2).

In summary, we have developed a general method for synthesizing PN³P pincer Ir(III) trihydride complexes and obtained several key structures involved in the CO₂ hydrogenation process. While the Ir complexes showed favorable activity (a turnover number up to 5100), the formation of a dearomatized PN³P*–Ir(I)–CO species was identified as a plausible key pathway for the catalyst deactivation. Our work reveals that the RWGS should be taken into account for future catalyst design to avoid this unfavorable process to maximize the catalyst activity and lifetime.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by funding from the King Abdullah University of Science and Technology (KAUST).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures, characterization data, and ¹H and ¹³C NMR spectra for the compounds. CCDC 1423984 (4a), 1423985 (2b), 1482628 (5b) and 1914804 (3b). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9dt01319a

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