CO Oxidation by N₂O Homogeneously Catalyzed by Ruthenium Hydride Pincer Complexes Indicating a New Mechanism

Abstract

Both CO and N₂O are important, environmentally harmful industrial gases. The reaction of CO and N₂O to produce CO₂ and N₂ has stimulated much research interest aimed at degradation of these two gases in a single step. Herein, we report an efficient CO oxidation by N₂O catalyzed by a (PNN)Ru–H pincer complex under mild conditions, even with no added base. The reaction is proposed to proceed through a sequence of O-atom transfer (OAT) from N₂O to the Ru–H bond to form a Ru–OH intermediate, followed by intramolecular OH attack on an adjacent CO ligand, forming CO₂ and N₂. Thus, the Ru–H bond of the catalyst plays a central role in facilitating the OAT from N₂O to CO, providing an efficient and novel protocol for CO oxidation.

Because of the widespread use of nitrogenous chemicals in agriculture and industrial processes, N₂O has become the most abundant stratospheric ozone depletion substance and concomitantly one of the most potent greenhouse gases (N₂O is ca. 300 times more potent than CO₂) due to atmospheric concentration increases.¹ To address this environmental issue, efficient destruction/conversion of N₂O, including its degradation or reduction to molecular dinitrogen, is of utmost importance and has drawn much attention.² The reaction of CO and N₂O to produce CO₂ and N₂, which is quite exothermic and strongly driven by formation of dinitrogen, is considered an attractive reductive process due to the destruction of two environmentally harmful industrial gases in one step.³⁻⁶ Although heterogeneously catalyzed reactions have been reported,⁴ reactions homogeneously catalyzed by metal complexes are rare and desirable,⁶ since they may proceed selectively under mild conditions and may provide fundamental mechanistic information.

Experiments aimed at reduction of N₂O by CO involving metal complexes were reported.⁵ In early stoichiometric work by Bottomley,^5a a CO ligand of Cp₂Ti(CO)₂1 was converted to free CO₂ under excess N₂O in 15% yield, and the process was suggested to involve reaction of Cp₂Ti(CO)₂1 with N₂O, affording [(Cp₂Ti)₄(CO₃)₂] 2, which upon further reaction with N₂O produces free CO₂ and [Cp₂TiO] 3. A concerted transient species M1 (Scheme 1) was proposed, in which the O atom interacts with Ti and CO ligand directly. Sita reported stoichiometric photolytic CO oxidation with N₂O in the presence of Cp*M[η²-N(ⁱPr)C(Me)N(ⁱPr)](CO)₂ (M = Mo or W) 4.^5b−5d The reaction was initiated by oxidation of 4 by N₂O forming Cp*M(O)[η²-N(ⁱPr)C(Me)N(ⁱPr)] 5, releasing N₂ and CO. O-atom transfer (OAT) in the presence of CO afforded CO₂. While catalysis was mentioned, no catalytic data were reported. Homogeneously catalyzed reaction of CO with N₂O was reported by Cheng, using metal carbonyl anions.⁶ In this system N₂O was proposed to undergo nucleophilic attack by the anionic metal center of [Rh(CO)₄]⁻, [Fe₂(CO)₈]^2–, and [Ru₄(CO)₁₃]^2– (M3, Scheme 1) followed by intramolecular OAT to form CO₂. Although catalytic, a large excess of base was required, and low turnover numbers (TONs) were obtained (maximum ca. 50). Thus, the development of mild and efficient homogeneously catalyzed N₂O reduction by CO is challenging.

Scheme 1. Homogeneous Reactions of CO and N₂O.

As mentioned above, direct interaction between a CO ligand and N₂O was the proposed pathway for CO oxidation by N₂O to form CO₂ and N₂.^5,6 In 2017, we developed the homogeneously catalyzed hydrogenation of N₂O in high TONs, in which selective mono O-transfer from N₂O into a (PNP)Ru–H bond of the pincer catalyst was the key step in the catalysis.⁷ The efficient O-transfer into Ru–H was studied computationally,⁹ including specifically the (PNP)Ru–H pincer system,^9b,9c and it was concluded to proceed via nucleophilic attack of the hydride ligand on the terminal nitrogen of N₂O, followed by a concerted N₂ liberation.^9,12 This result encouraged us to explore a catalytic CO oxidation by N₂O initiated by O-transfer into a M–H bond.^8,9 We propose that a H–M–CO complex may undergo O-transfer from N₂O into the M–H bond to generate a HO–M–CO species, followed by intramolecular interaction between M–OH and CO ligand to form a M–COOH intermediate (Scheme 1).¹⁰ CO₂ release in the presence of CO regenerates the H–M–CO complex, thus completing the catalytic cycle. To enable such a homogeneously catalyzed CO oxidation, highly selective OAT from N₂O into M–H bond is needed, without decomposing the complex. Herein, we report the development of mild CO oxidation by N₂O catalyzed by a (PNN)Ru–H complex based on such a mechanism. The reaction proceeds smoothly, yielding high TON under mild conditions, even with no added base.

Initially, CO oxidation by N₂O was examined by treating a premixed solution of 0.01 mmol of the (PNP)Ru complex 6 (P = P(ⁱPr)₂,^11a) and 1 equiv of t-BuOK in 4 mL of toluene under 1 atm of CO and 2 atm of N₂O in a 90 mL Fisher-Porter tube (1 atm of gas in 90 mL corresponds to ca. 3.7 mmol at 20 °C). After heating at 100 °C for 22 h, 0.63 mmol of CO₂ (63 TON) was detected by GC in the gas phase. The reaction occurred smoothly, even using catalytic base. In the absence of 6, no CO₂ was formed. Moreover, replacing 6 by RuHCl(CO)(PPh₃)₃ did not lead to CO₂ formation. Other pincer Ru complexes were then screened (Table 1). The complex (PNP)Ru(CO)Cl₂7 (X-ray characterized¹²) did not lead to any CO₂, indicating that the Ru–H bond is crucial for the transformation. With the bulkier complex 8 (P = P(^tBu)₂),^11b only a trace of CO₂ was formed. The PNN complex 9(8c,11c) was less active than the PNNH complex 10,^11d which yielded 62 TON of CO₂. The 2,2-bipyridine-based pincer complexes 12 (P = PPh₂)^11e and 14 (P = P(^tBu)₂)^11f were less efficient and afforded 24 and 22 TON of CO₂, respectively, while use of 13 (P = P(ⁱPr)₂)^11g resulted in 83 TON of CO₂. Interestingly, the catalytic activity of complexes 13 and 14 in THF was higher than in toluene. In THF at 70 °C, 130 and 197 TON were obtained in the gas phase, respectively, and an additional 40 TON (when using 14) was detected in solution. Without base, significantly lower catalytic activity was observed, forming only 30 TON of CO₂.

Table 1. Catalyst Screening for CO Oxidation by N₂O^a–^d.

^aAll reactions were conducted in a 90 mL Fisher-Porter tube using the catalyst (0.01 mmol), t-BuOK (0.01 mmol), 3.7 mmol of CO, and 7.4 mmol of N₂O in 4 mL toluene.

^bThe catalyst and t-BuOK were premixed in the solvent for 20 min and used directly for the reactions.

^cThe TONs are based on the generated CO₂ as measured by GC of the gas phase calibrated by a standard curve.

^dThe amount of CO₂ dissolved in toluene was not determined.^13,14

^eThe reaction was conducted in THF at 70 °C.

^fThe amount of CO₂ dissolved in THF was determined as ca. 0.4 mmol (40 TON).¹³

To gain mechanistic information on this catalytic transformation, individual stoichiometric reactions that may be involved in the catalytic cycle using complex 14 as precatalyst were explored. Reaction of 14 with t-BuOK is known to occur smoothly to afford the dearomatized complex 15.^11g Alarmingly, reaction of 15 with excess N₂O resulted in complete decomposition of the complex. On the other hand, reaction of 15 with excess CO afforded the dearomatized dicarbonyl complex 16, in which CO coordination stabilizes the dearomatized complex (Scheme 2). Importantly, a competitive experiment of 15 under both N₂O and CO (1:1) resulted in the formation of 16 as the major product. Thus, the much faster reaction of 15 with CO prevents the decomposition of 15 caused by over-oxidation by N₂O and enables the catalytic cycle.

Scheme 2. Reactions of Complex 15 and Formation of 16.

Crystals of complex 16 were obtained by recrystallization from pentane at −35 °C. The X-ray structure of 16 (Figure 1) reveals an octahedral geometry, the hydride ligand being located trans to a CO ligand. The dearomatized structure of 16 is clearly indicated by the bond length C(10)–C(11) (1.390(3) Å) being much shorter than a C–C single bond (the corresponding bond length in complex 17 is 1.508 Å), indicating a significant double bond character.

Figure 1.

Crystal structure of complex 16. Atoms are presented as thermal ellipsoids at 50% probability level. Hydrogen atoms, except for Ru–H, are not shown. For selected bond lengths and angles, see SI.

While the corresponding dearomatized PNP pincer complexes of 6 and 8 are stable,^11a,11b the PNN complex 16 is converted slowly, even at room temperature, to the aromatic complex 17 via C–H activation at the pyridine ring (eq 1).^15,16 The Ru–H of 17 appears in ¹H NMR in THF at −5.3 ppm (d, ²J_PH = 17.6 Hz) and ³¹P{¹H} NMR shows a singlet at 88.4 ppm.¹² This transformation is much faster in toluene, providing a possible explanation for the better catalysis in THF than in toluene. Nevertheless, in real-time ³¹P NMR analysis of the catalytic reaction of CO and N₂O using complex 14 in THF, both complexes 16 and 17 were observed. Interestingly, 17 (0.01 mmol) under 1 bar of CO and 2 bar of N₂O also catalyzed the reaction, but with much lower efficiency, affording only 54 TON of CO₂ in the gas phase. Thus, although 16 is the more active catalyst (see below), participation of 17 in the catalysis cannot be excluded.

1

The reaction of complexes 16 or 17 with N₂O (1 equiv or excess) in the absence of CO resulted in decomposition and the Ru–OH complex was not observed. Aiming at generation of (PNN)Ru(OH)X proposed in Scheme 1, the aromatized pincer complex (PNN)RuCl₂(CO) 18 was prepared and crystallographically characterized.¹² Complex 18 was subjected to a reaction with 2.2 equiv of [(18-crown-6)K]OH in THF at room temperature (Scheme 3). Interestingly, while the proposed ruthenium dihydroxo complex M4 and dearomatized ruthenium hydroxo complex M5 were not detected, treatment of the reaction mixture with CO resulted in formation of the dearomatized hydride complex 16, and CO₂ (detected by GC), suggesting that the reaction proceeds through halide substitution by [(18-crown-6)K]OH followed by water elimination via metal–ligand cooperation (MLC)¹⁶ forming in the presence of CO the complex M5, which is unstable and leads to 16 by releasing CO₂ under CO. The relatively fast intramolecular reaction between the OH group and CO results in CO₂ formation, likely via a RuCOOH intermediate M6,¹⁰ and suggests that the catalysis is enabled by O-insertion from N₂O into the Ru–H bond.⁷⁻⁹ These results suggest that the turnover limiting step in the process is the oxygen-atom-transfer step.

Scheme 3. Reaction of Complex 18 with [(18-C-6)K]OH.

On the basis of these observations and reported DFT studies,⁹ a plausible mechanism for this reaction is proposed (Scheme 4). First, the premixed solution of precatalyst 14 and base generates the dearomatized ruthenium complex, which reacts with CO immediately, affording the dicarbonyl complex 16.^11,12 Efficient OAT from N₂O into Ru–H, which is likely initiated by nucleophilic attack of the hydride ligand on N₂O,⁹ results in formation of a hydroxo intermediate M5,⁷⁻⁹ which might undergo intramolecular nucleophilic attack by hydroxide on the adjacent CO to give a RuCOOH intermediate M6,¹⁰ followed by β-H elimination to form CO₂ and regeneration of 16 under CO.¹⁰ In addition, a less efficient mechanism involving complex 17, obtained during catalysis via MLC¹⁶ and C–H activation,¹⁵ might also take place to some extent, by undergoing OAT (M7) and CO₂ formation (M8) (catalytic cycle B). In both processes, the Ru–H bond plays a key role in assisting OAT to CO. Throughout this catalytic cycle the formal metal oxidation state may not change, providing a novel protocol for CO oxidation by N₂O.

Scheme 4. Possible Mechanism.

This plausible mechanism encouraged us to explore the catalytic reaction using complex 16 as catalyst with no added base. Remarkably, full conversion of CO was achieved by using 0.01 mmol of complex 16 under mild, base-free conditions (eq 2). CO₂ was produced in 90% yield (2.72 mmol in the gas phase and 0.6 mmol in solution, for a total of 332 TON) and 86% yield of dinitrogen (3.17 mmol, 317 TON) was determined by GC.

2

In addition, the highest TONs were achieved by using 0.0106 mmol of 16 as catalyst in the presence of excess of CO (50 psi) and N₂O (50 psi). After heating for 22h, 6.16 mmol of dinitrogen (579 TON) together with 5.97 mmol of CO₂ (4.97 mmol in gas phase and ca. 1.0 mmol in solution; 561 TON) were produced (eq 3).

In summary, a new homogeneously catalyzed reaction of CO and N₂O to produce CO₂ and N₂ was developed. High efficiency and high TON were achieved using the ruthenium complex 14 as the precatalyst or the corresponding dearomatized complex 16 as the actual catalyst. The reaction catalyzed by 16 proceeds smoothly under base-free conditions, providing an efficient method for degradation of both CO and N₂O in a single step. The Ru–H bond is necessary for the catalysis. The catalytic cycle is proposed to involve selective O-atom transfer into Ru–H of 16, intramolecular CO insertion into the resulting Ru–OH, and subsequent CO₂ liberation, regenerating the catalyst 16 in the presence of CO. Quite remarkably, while N₂O alone decomposes the catalyst, this is prevented by the presence of the more reactive CO.

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b03927.

• Experimental procedures and spectral data (PDF)

• X-ray data for complexes 7, 16, 17, and 18 (CIF)

The authors declare no competing financial interest.