Nitrous oxide activation by picoline-derived Ni–CNP hydrides

Abstract

Oxygen atom transfer (OAT) from N₂O to the Ni–H bond of proton-responsive picoline-derived CNP nickel complexes has been investigated both experimentally and theoretically. These Ni–CNP complexes efficiently catalyse the reduction of N₂O with pinacolborane (HBpin) under mild conditions.

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Nitrous oxide (N₂O) is an important contributor to the global climate change due to its high greenhouse impact and ozone-depleting properties.^1,2 The increasing concentrations of N₂O in the Earth's atmosphere have been chiefly attributed to anthropogenic sources associated with the use of nitrogen-based fertilizers, combustion of biomass and fossil fuels, and the production of industrial chemicals.³ While thermodynamically very favourable, N₂O degradation to N₂ and O₂ is associated with high kinetic barriers, which explains its average long life (118 years) in the atmosphere.⁴ The known ability of transition metal complexes to react with small molecules, including CO₂, CO or H₂, make them interesting alternatives for N₂O activation and subsequent transformation. However, the reactivity of transition metal complexes towards N₂O is critically hampered by its weak σ-donor and π-acceptor properties as a ligand.⁵ Interestingly, a favourable pathway for N₂O activation by transition metal complexes involves the nucleophilic attack of a metal-bound hydride to the terminal nitrogen of N₂O, thus generating an O-bound oxyldiazene intermediate (M–ONNH). Subsequent nitrogen extrusion from this derivative produces a hydroxy complex (M–OH), this process representing an overall oxygen atom transfer (OAT) to the complex's M–H bond.^6–11 Moreover, the reaction of the hydroxyl derivative with common reductants, such as H₂,^6,7 alcohols,⁸ CO⁹ or silanes,^7,10 might lead to the regeneration of the M–H bond, either in a stoichiometric or catalytic process.

With the exception of a hafnium hydride complex,¹¹ metal systems examined for the OAT from N₂O to M–H bonds are based on electron-rich precious metals (Rh, Ru, Os, Ir).^6–10 In recent years, efficient catalytic systems incorporating Earth-abundant first-row transition metals are in great demand due to their lower economic and environmental costs. As a result of being less hydridic, hydride complexes of first-row late transition metals often lack the high reactivity usually associated with their precious metal counterparts.¹² Nickel hydride pincer complexes have been found to be active catalysts in a plethora of reduction reactions,^13,14 and have been profusely investigated in the activation of CO₂,^15–17 an isoelectronic molecule to N₂O. Moreover, nickel derivatives have been shown to activate N₂O by different modes.¹⁸ Herein, we report on the reactivity of Ni–H pincer complexes based on a picoline-derived CNP (C = N-heterocyclic carbene, P = phosphine) ligand towards N₂O, including the OAT from N₂O to the metal-hydride bond and the catalytic activity of these complexes in the reduction of N₂O with pinacolborane (HBpin).

The reaction of the carbene ligand precursors 1a–b¹⁹ with KHMDS (potassium bis(trimethylsilyl)amide), followed by addition of NiBr₂(dme) (dme = 1,2-dimethoxyethane), afforded the isolation of the bromide complexes 2a–b in good yields (83–87%) (Scheme 1). These derivatives were characterised by NMR spectroscopy and elemental analysis. Aiming to access Ni hydride pincer derivatives, the reaction of the bromide complexes 2a–b with NaBH₄ was targeted. Upon anion exchange using NaBPh₄, the cationic hydride complexes 3a–b were isolated with yields of 68 and 74%, respectively. These derivatives were analytical and spectroscopically characterised. For instance, the hydride region of the ¹H NMR spectrum of 3a exhibits a doublet at −18.0 ppm with a coupling constant of ²J_HP = 72 Hz. The IR spectra of 3a and 3b include a band at 1901 and 1888 cm⁻¹, respectively, attributable to the stretching of the Ni–H bond. Moreover, the proposed structure of complex 3a was further confirmed in the solid state by single crystal X-ray diffraction analysis (Fig. 1a). This complex shows a square planar coordination geometry (Σ(Ni) = 359.9°), with the pincer ligand adopting the expected κ³-(P,N,C) coordination mode (C–Ni–P angle: 170.3°).

Scheme 1 Synthesis of the Ni complexes 2a–b, 3a–b, 4a–b and 5a–b.

Fig. 1 ORTEP drawings at 30% ellipsoid probability of: (a) the cationic fragment of complex 3a, and (b) complex 5b. Most hydrogen atoms have been omitted for the sake of clarity. See ESI† for selected bond lengths and angles.

As result of the expected acidity of the methylene hydrogens of the CNP ligand,²⁰ treatment of complexes 3a–b with a strong base, such as KHMDS, in THF-d₈ produced the instantaneous colour change of the initially pale yellow solutions to dark red. The clean formation of the new species 4a–b, deprotonated at the pincer methylene bridge, was evidenced by ¹H and ¹³C{¹H} NMR spectroscopies. Attempted isolation of complexes 4a–b only led to significant decomposition, likely due to a poor steric stabilization,¹⁴ and consequently they were characterised spectroscopically in solution. In the ¹H NMR spectrum of 4a, deprotonation of the methylene arm of the pincer was evidenced by the presence of a singlet appearing at 3.26 ppm, corresponding to the =CHP methyne bridge, and the up-field shift of the resonances of the dearomatized pincer central N-containing ring that appear between 6.41 and 5.34 ppm. In addition, a doublet resonance appearing at ca. −17.5 ppm (²J_HP = 67 Hz) in the hydride region was detected in the same experiment, which is attributable to the Ni–H hydrogen. Moreover, the stretching mode of the Ni–H bond produces in the IR spectra of 4a–b absorption bands at lower wavelengths than in their protonated counterparts 3a–b (1841 and 1857 cm⁻¹ for 4a and 4b, respectively), in agreement with the expected larger trans influence of the anionic amide central donor group of the pincer in comparison to a neutral pyridine donor.

Next, having access to the Ni hydride complexes 3 and 4, we examined their reactions with N₂O. Pressurisation with N₂O (3 bar) of solutions of complexes 3a–b in THF-d₈ did not produced observable changes in their NMR spectra. Conversely, complexes 4a–b, formed in situ by the reaction of 3a–b with KHMDS, gradually react with N₂O (3 bar) (reaction half-life of 4b: 36 h at r.t. and 5.5 h at 55 °C) (Scheme 1). The ¹H NMR spectrum of the resulting hydroxy derivative 5a includes a doublet signal attributable to the hydroxo moiety appearing at −3.85 ppm with a P–H coupling constant of ³J_HP = 7.4 Hz; meanwhile, significant changes in the deprotonated CNP* pincer ligand were not observed. Similar spectroscopic data were obtained for complex 5b. Analysis by X-ray diffraction of a single crystal of 5b revealed a square-planar coordination geometry (Σ(Ni) = 360.1°, C–Ni–P angle: 166.65(7)°), with quite similar metric parameters to the Ni-pincer framework of complex 3a (Fig. 1b). The Ni atom in 5b resides in a distorted square-planar environment with the hydroxyl oxygen displaced 0.17 Å out of the least-square plane defined by the P1, C1, N3 and Ni atoms. Finally, the Ni–O bond length, 1.8272(18) Å, lies at the lowest extreme of the range observed for related nickel square-planar hydroxo complexes based on anionic pincer ligands (Ni–O distances: 1.83–1.93 Å).²¹ It is remarkable that, to our knowledge, this is the first example of an OAT from N₂O to a base metal hydride complex.

To attain further insight into the observed dissimilar reactivity of complexes 3 and 4 towards N₂O, the mechanism of the N₂O oxygen atom transfer to the Ni–H bonds of 3a and 4a was investigated by performing DFT calculations (B3LYP-D3/def2TZVP) (Fig. 2). In the case of 3a, initial outer-sphere transfer of the hydrido ligand to the terminal nitrogen atom of N₂O leads to the endergonic formation of intermediate A, with an associated barrier TS_(3a→A) of 29.2 kcal mol⁻¹.²² Subsequent N₂ release from A to yield the corresponding hydroxo complex B was found to be largely exergonic, having a relatively high energy barrier of 27.8 kcal mol⁻¹. Interestingly, in the case of 4a, the transition state TS_(4a→A*) associated to the transfer of the Ni–H hydride to N₂O has an energy (ΔG^‡) of 26.0 kcal mol⁻¹, and leads to the formation of A* with an energy return of 4.0 kcal mol⁻¹. The lower barrier for the hydride transfer to N₂O for 4a and the thermodynamically favourable formation of A* resemble reported results regarding the insertion of CO₂ into Ni–H bonds of square-planar pincer nickel complexes.^16,17 These studies have shown lower energy barriers and higher stability of the insertion products as the ligand trans to the hydride becomes a stronger donor. In our case, the different behaviour of the hydrides 3 and 4 towards N₂O can be attributed to the expected larger trans influence of the deprotonated pincer ligand, which produces a weakening of the Ni–H bond and increases the nucleophilic character of the hydride ligand.^16,17 In fact, the calculated Ni–H distances and NBO (natural bond orbital) charges on the hydride for 3a and 4a are in good agreement with a less strong Ni–H bond and a higher nucleophilicity of the Ni–H group in the case of 4a. The Ni–H bond is slightly longer for 4a [d(Ni–H) = 1.48 Å (3a), 1.51 Å (4a)]; whereas the hydride ligand bears a larger negative charge [NBO charges on the Ni: 0.41 (3a) and 0.42 (4a); NBO charges on the hydride: –0.24 (3a), –0.31 (4a)].

Fig. 2 DFT calculated free energy (ΔG in THF, kcal mol⁻¹) profile of the N₂O insertion into the Ni–H bond of complexes 3a (a) and 4a (b).

N₂ extrusion from A* to form the corresponding hydroxo derivative 5a was found to be highly exergonic, having an energy barrier of 29.0 kcal mol⁻¹. However, since complex 4a is generated in situ from 3a in the presence of a slight excess of KHMDS, the participation of the base in the N₂ release step was also investigated. Deprotonation of the Ni–ONNH moiety yields the unusual species C* through a transition state located at 16.1 kcal mol⁻¹. This intermediate can be readily protonated by (Me₃Si)₂NH leading to the hydroxo complex 5a. This last step is exergonic by 28.4 kcal mol⁻¹ from C*, and takes place without energy barrier, as indicated by relaxed potential energy (PES) scans (see ESI†). The overall energy return for the formation of 5a and N₂ from 4a and N₂O is 67.9 kcal mol⁻¹.

Next, the catalytic performance of the Ni(II) hydride derivatives in the reduction of N₂O using HBpin was examined (Table 1).²³ Whilst no reaction was observed in the absence of the Ni–CNP catalysts, HBpin was fully consumed using 2.0 mol% of the cationic complexes 3a–b, leading to the formation of pinBOH and (pinB)₂O, in a 2 : 8 ratio (entries 1 and 2); moreover, generation of N₂ and H₂ was detected, although it could not be quantified (see ESI† for details). By employing in situ formed complexes 4a and 4b, somewhat faster reactions using lower catalyst loadings (0.5 mol%) than for the corresponding complexes 3 were observed (entries 3 and 4). Finally, THF-d₈ solutions of complex 5b, formed in situ, were also found to catalyse the reduction of N₂O with HBpin (entry 5).

Table 1 N₂O reduction with HBpin catalysed by Ni complexes 3 and 4^a

Entry	Cat.	Cat. loading [mol%]	Conv. [%] (time, [h])	pinBOH: (pinB)2O ratio
a Reaction conditions, unless otherwise noted: 2 bar N2O, r.t., THF-d8, [HBpin] = 0.4 M. Complexes 4a–b were formed in situ from 3a–b with KHMDS. Complex 5b was formed in situ from 3b and KHMDS under N2O pressure. Conversion and selectivity were determined by ^1H and ^11B{^1H} NMR spectroscopies using mesitylene as internal standard. N2 and H2 formation were detected by GC-MS analysis of the headspace gas and ^1H NMR spectroscopy, respectively (see ESI).
1	3a	2.0	>99 (6.5)	2 : 8
2	3b	2.0	>99 (2.0)	2 : 8
3	4a	0.5	>99 (3.0)	1 : 9
4	4b	0.5	>99 (2.0)	2 : 8
5	5b	2.0	>99 (2.0)	2 : 8

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The observed fast kinetics for the catalytic reaction are at odds with the relatively slow N₂O insertion in complexes 4 and the lack of reactivity of derivatives 3. In an attempt to obtain further information, a series of control experiments were performed. First, no reaction was observed by NMR spectroscopy between HBpin (ca. 4 equiv.) and the hydride complexes 3 and 4 in the temperature range between −60 °C and 55 °C.²⁴ Moreover, a solution of the in situ formed complex 5b in THF-d₈ was treated with HBpin (0.9 equiv.) at −65 °C. After approximately 0.25 h, 82% conversion of 5b to a mixture of the hydride derivatives 3b and 4b (5 : 1 ratio) was observed, demonstrating that Ni–H bond regeneration from 5 is facile with HBpin. In the ¹¹B{¹H} NMR spectrum of this experiment, a new broad signal was observed at 4.5 ppm. Unfortunately, attempts to isolate and characterise this boron species were unsuccessful. The presence of borane Lewis acids has been shown to impact on both the reaction kinetics and product distribution in the CO₂ hydroboration,²⁵ and has been found to play a role in the CO₂ activation by Ni-PBP hydride complexes. Therefore, we hypothesise that Lewis acid borane species might catalyse or co-catalyse the hydroboration of N₂O in the presence of complexes 3 and 4.²⁶ In fact, complete catalyst inhibition was observed when the reaction catalysed by 3b was performed in the presence of 20 mol% Et₃N, acting as a Lewis acid scavenger.²⁷

To conclude, the OAT from N₂O to hydride Ni complexes based on CNP ligands has been found to be dependent on the trans influence of the pincer. Thus, the reactivity of complexes 3a–b towards N₂O is triggered by a strong base (KHMDS), leading to the formation of the deprotonated Ni–CNP* species 4a–b, which are capable of performing N₂O activation. Moreover, the use of a strong base promotes the N₂ release from the Ni–ONNH intermediate resulting from the N₂O insertion into the Ni–H bond. Finally, both complexes 3 and 4 catalyse the reduction of N₂O with HBpin, in a process that is proposed to be catalysed or co-catalysed by the presence of boron Lewis acids generated under catalytic conditions.

Financial support from MCIN/AEI/10.13039/501100011033/FEDER,UE (PID2019-104159GB-I00, TED2021-129181B-I00, PID2022-136570OB-I00) and CSIC (COOPB20604), and the use of the computational facilities of the Supercomputing Center of Galicia (CESGA) is gratefully acknowledged. J. B. thanks CSIC, Junta de Andalucía and Fondo Social Europeo (FSE) for a Sistema Nacional de Garantía Juvenil contract.

Conflicts of interest

There are no conflicts to declare.

Notes and references

1. J. Hansen and M. Sato, Proc. Natl. Acad. Sci. U. S. A., 2004, 101, 16109; S. A. Montzka, E. J. Dlugokencky and J. H. Butler, Nature, 2011, 476, 43.
2. M. J. Prather, Science, 1998, 279, 1339; A. R. Ravishankara, J. S. Daniel and R. W. Portmann, Science, 2009, 326, 123.
3. E. A. Davidson and D. Kanter, Environ. Res. Lett., 2014, 9, 105012.
4. S. I. Gorelsky, S. Ghosh and E. I. Solomon, J. Am. Chem. Soc., 2006, 128, 278; S. Javoy, R. Mevel and C. E. Paillard, Int. J. Chem. Kinet., 2009, 41, 357.
5. W. B. Tolman, Angew. Chem., Int. Ed., 2010, 49, 1018.
6. A. W. Kaplan and R. G. Bergman, Organometallics, 1997, 16, 1106; A. W. Kaplan and R. G. Bergman, Organometallics, 1998, 17, 5072; J.-H. Lee, M. Pink, J. Tomaszewski, H. Fan and K. G. Caulton, J. Am. Chem. Soc., 2007, 129, 8706; L. E. Doyle, W. E. Piers and J. Borau-Garcia, J. Am. Chem. Soc., 2015, 137, 2187; J. D. Smith, E. Chih, W. E. Piers and D. M. Spasyuk, Polyhedron, 2018, 155, 281; P. Jurt, A. S. Abels, J. J. Gamboa-Carballo, I. Fernández, G. Le Corre, M. Aebli, M. G. Baker, F. Eiler, F. Müller, M. Wörle, R. Verel, S. Gauthier, M. Trincado, T. L. Gianetti and H. Grützmacher, Angew. Chem., Int. Ed., 2021, 60, 25372; I. Ortega-Lepe, P. Sánchez, L. L. Santos, P. Lara, N. Rendón, J. López-Serrano, V. Salazar-Pereda, E. Álvarez, M. Paneque and A. Suárez, Inorg. Chem., 2022, 61, 18590.
7. R. Zeng, M. Feller, Y. Ben-David and D. Milstein, J. Am. Chem. Soc., 2017, 139, 5720.
8. T. L. Gianetti, S. P. Annen, G. Santiso-Quinones, M. Reiher, M. Driess and H. Grützmacher, Angew. Chem., Int. Ed., 2016, 55, 1854; J. Bösken, R. E. Rodríguez-Lugo, S. Nappen, M. Trincado and H. Grützmacher, Chem. – Eur. J., 2023, 29, e202203632.
9. R. Zeng, M. Feller, Y. Diskin-Posner, L. J. W. Shimon, Y. Ben-David and D. Milstein, J. Am. Chem. Soc., 2018, 140, 7061.
10. P. Molinillo, B. Lacroix, F. Vattier, N. Rendón, A. Suárez and P. Lara, Chem. Commun., 2022, 58, 7176.
11. G. A. Vaughan, P. B. Rupert and G. L. Hillhouse, J. Am. Chem. Soc., 1987, 109, 5538.
12. E. S. Wiedner, M. B. Chambers, C. L. Pitman, R. M. Bullock, A. J. M. Miller and A. M. Appel, Chem. Rev., 2016, 116, 8655.
13. V. Arora, H. Narjinari, P. G. Nandia and A. Kumar, Dalton Trans., 2021, 50, 3394.
14. N. A. Eberhardt and H. Guan, Chem. Rev., 2016, 116, 8373.
15. T. J. Schmeier, N. Hazari, C. D. Incarvito and J. A. Raskatov, Chem. Commun., 2011, 47, 1824; J. E. Heimann, W. H. Bernskoetter, J. A. Guthrie, N. Hazari and J. M. Mayer, Organometallics, 2018, 37, 3649; C. Yoo, J. Kim and Y. Lee, Organometallics, 2013, 32, 7195; K. J. Jonasson and O. F. Wendt, Chem. – Eur. J., 2014, 20, 11894; L. J. Murphy, H. Hollenhorst, R. McDonald, M. Ferguson, M. D. Lumsden and L. Turculet, Organometallics, 2017, 36, 3709.
16. H.-W. Suh, T. J. Schmeier, N. Hazari, R. A. Kemp and M. K. Takase, Organometallics, 2012, 31, 8225.
17. P. Ríos, N. Curado, J. López-Serrano and A. Rodríguez, Chem. Commun., 2016, 52, 2114; P. Ríos, A. Rodríguez and J. López-Serrano, ACS Catal., 2016, 6, 5715; J. E. Heimann, W. H. Bernskoetter, N. Hazari and J. M. Mayer, Chem. Sci., 2018, 9, 6629; T. J. Schmeier, A. Nova, N. Hazari and F. Maseras, Chem. – Eur. J., 2012, 18, 6915.
18. T. Yamada, K. Suzuki, K. Hashimoto and T. Ikeno, Chem. Lett., 1999, 1043; P. T. Matsunaga and G. L. Hillhouse, J. Am. Chem. Soc., 1993, 115, 2075; N. D. Harrold, R. Waterman, G. L. Hillhouse and T. R. Cundari, J. Am. Chem. Soc., 2009, 131, 12872; B. Horn, C. Limberg, C. Herwig, M. Feist and S. Mebs, Chem. Commun., 2012, 48, 8243; F. Le Vaillant, A. Mateos Calbet, S. González-Pelayo, E. J. Reijerse, S. Ni, J. Busch and J. Cornella, Nature, 2022, 604, 677; J. Gwak, S. Ahn, M.-H. Baik and Y. Lee, Chem. Sci., 2019, 10, 4767.
19. T. Simler, A. A. Danopoulos and P. Braunstein, Chem. Commun., 2015, 51, 10699.
20. J. I. van der Vlugt, M. Lutz, E. A. Pidko, D. Vogt and A. L. Spek, Dalton Trans., 2009, 1016; D. Oren, Y. Diskin-Posner, L. Avram, M. Feller and D. Milstein, Organometallics, 2018, 37, 2217.
21. T. J. Schmeier, A. Nova, N. Hazari and F. Maseras, Chem. – Eur. J., 2012, 18, 6915; D. Adhikari, S. Mossin, F. Basuli, B. R. Dible, M. Chipara, H. Fan, J. C. Huffman, K. Meyer and D. J. Mindiola, Inorg. Chem., 2008, 47, 10479; D. V. Gutsulyak, W. E. Piers, J. Borau-Garcia and M. Parvez, J. Am. Chem. Soc., 2013, 135, 11776; J. Cámpora, P. Palma, D. del Río and E. Álvarez, Organometallics, 2004, 23, 1652; A. Castonguay, A. L. Beauchamp and D. Zargarian, Inorg. Chem., 2009, 48, 3177; C. Yao, P. Chakraborty, E. Aresu, H. Li, C. Guan, C. Zhou, L.-C. Liang and K.-W. Huang, Dalton Trans., 2018, 47, 16057; K. J. Jonasson, A. H. Mousa and O. F. Wendt, Polyhedron, 2018, 143, 132; N. H. Anderson, J. M. Boncella and A. M. Tondreau, Organometallics, 2018, 37, 4675.
22. H. Yu, G. Jia and Z. Lin, Organometallics, 2008, 27, 3825; J. A. Luque-Urrutia and A. Poater, Inorg. Chem., 2017, 56, 14383; L. Yao, Y. Li, L. Huang, K. Guo, G. Ren, Z. Wu, Q. Lei, W. Fang and H. Xie, Comput. Theor. Chem., 2018, 1128, 48; H. Xie, C. Liu, Y. Yuan, T. Zhou, T. Fan, Q. Lei and W. Fang, Dalton Trans., 2016, 45, 1152; S. Escayola, M. Solà and A. Poater, Inorg. Chem., 2020, 59, 9374; H. Xie, Y. Zhang, C. Xiang, Y. Li, T. Fan, Q. Lei and W. Fang, Dalton Trans., 2018, 47, 15324; G. Liang, M. Zhang and C. E. Webster, Inorganics, 2022, 10, 69; H. Xie, Y. Li, L. Huang, F. Nong, G. Ren, T. Fan, Q. Lei and W. Fang, Dalton Trans., 2016, 45, 16485.
23. Y. Pang, M. Leutzsch, N. Nöthling and J. Cornella, J. Am. Chem. Soc., 2020, 142, 19473; X. Chen, H. Wang, S. Du, M. Driess and Z. Mo, Angew. Chem., Int. Ed., 2022, 61, e202114598.
24. D. Adhikari, J. C. Huffman and D. J. Mindiola, Chem. Commun., 2007, 4489; A. Rossin, M. Peruzzini and F. Zanobini, Dalton Trans., 2011, 40, 4447; S. Chakraborty, J. Zhang, Y. J. Patel, J. A. Krause and H. Guan, Inorg. Chem., 2013, 52, 37.
25. M. R. Espinosa, D. J. Charboneau, A. Garcia de Oliveira and N. Hazari, ACS Catal., 2019, 9, 301.
26. S. J. Geier, C. M. Vogels, J. A. Melanson and S. A. Westcott, Chem. Soc. Rev., 2022, 51, 8877.
27. A. D. Bage, K. Nicholson, T. A. Hunt, T. Langer and S. P. Thomas, ACS Catal., 2020, 10, 13479.

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 2299058 and 2299059. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3cc05455a

‡ These authors have contributed equally.

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