Co(dppbsa)-catalyzed reductive N,N-dimethylation of nitroaromatics with CO₂ and hydrosilane†

Shuang-Shuang Ma‡ ^ab, Rui Sun‡ ^ab, Zi-Heng Zhang‡ ^cd, Peng-Xin Guan ^ab, Jin-Qing Lin ^d, Chun-Shan Li *^ab and Bao-Hua Xu *^ac
aBeijing Key Laboratory of Ionic Liquids Clean Process, Key Laboratory of Green Process and Engineering, State Key Laboratory of Multiphase Complex Systems, Institution of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China. E-mail: csli@ipe.ac.cn
^bCollege of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences, Beijing, 100049, China
^cSchool of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, China. E-mail: bhxu@bit.edu.cn
^dCollege of Materials Science and Engineering, Huaqiao University, Xiamen 361021, China

First published on 23rd September 2023

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Introduction

The emission of CO₂ due to the utilization of fossil energy accounts for the primary environmental issue of the greenhouse effect and global warming nowadays.¹ To alleviate such an issue and further achieve carbon neutrality, a variety of strategies using CO₂ as the sustainable C₁ surrogate in chemical synthesis have been documented. Therein, the reductive coupling of CO₂ and N-based nucleophiles leading to formamide, aminal, and methylamine through adjusting the selective 2e⁻, 4e⁻, and 6e⁻ reduction of CO₂ attracts the most attention.² In this domain, a wide range of homogeneous catalysts³ and heterogeneous catalysts,⁴ coupled with the respective hydrogen source (H source), have been documented (Scheme 1A). Particularly, the N-formylation of amines with CO₂ and hydrosilanes even proceeded without catalysts,⁵ which was promoted by the specific solvent dimethyl sulfoxide. By contrast, nitroaromatics, known as the readily available and cheap aniline precursors, are relatively underutilized in the reductive C–N coupling reactions with CO₂ (Scheme 1B). The pioneering studies by Shi's group⁶ and Liu's group⁷ independently demonstrated that such a transformation is feasible by using Cu-based (CuAlO_x) and Au-based (Au/TiO₂) heterogeneous catalysts of different substrate tolerance. Mechanistic analysis suggested that it proceeds with the initial hydrogenation of nitroarene to aniline, followed by N-methylation with CO₂via the formamide intermediate. Recently, Ma's group⁸ described a straightforward N-formylation of nitroarenes with CO₂ and hydrosilane but in the presence of excess Fe powder. Impressively, they preliminarily supposed that both the amide ion ArNH⁻ and the amide dianion ArN²⁻ (rather than aniline) originated from the reduction of nitroarenes with Fe powder functioning as the reactive species to activate and couple with CO₂.

To the best of our knowledge, no homogeneous catalysts for the reductive coupling of nitroaromatics with CO₂ are currently known. This aspect, coupled with our recent interest in non-noble Co-catalyzed hydrogenation and reductive coupling reactions, encouraged us to explore the potential cobalt complexes enabling the direct N-methylation/formylation of nitroaromatics with CO₂. In fact, a few novel cobalt catalysts or catalytic systems for homogeneous hydrogenation of CO₂ have been disclosed with the implementation of a tailored ligand structure, including a combination of appropriate cobalt salts with multidentate phosphines (tetraphos/triphos/dmpe),⁹ pincer-based cobalt complexes,¹⁰ and half-sandwich pentamethylcyclopentadienyl cobaltic complexes (Cp*Co^III) with proton-responsive ligands.¹¹ Remarkably, the former two systems also witnessed further functionalization through in situ generating intermediates that can be trapped by adding nucleophiles (e.g., alcohols^9d or amines^10a), which delivered the corresponding ethers or amides. Meanwhile, literature surveys demonstrating cobalt complexes that enable the selective reduction of nitroarenes to arylamines are rather few. Several catalytically active systems bearing phthalocyanine Co^II complexes,¹² trinuclear mixed valent Co^II/Co^III/Co^II complexes,¹³ pyrrole carboxamide chelated Co^II complexes,¹⁴ or tris(N-heterocyclic thioamidate) Co^III complexes¹⁵ have been reported to date. Experimental investigations into the possible levels of cobalt-catalyzed hydrogenation of CO₂ and studies on the difference from that of nitroaromatics, with the same catalytic system, would advance the potential strategies of reductive coupling of both reactants.

Following this perspective, Leitner and co-workers demonstrated Co(^NMePNP)Cl₂-catalyzed adaptive and selective hydrogenation of CO₂ with hydrosilanes individually to formic acid, formaldehyde, or methanol.¹⁶ We thereby envisioned that the fabrication of Cp*Co^III complexes with proton/silane-responsive ligands may enable the catalytic reduction of unsaturated CO₂ and nitroarenes using hydrosilanes as the H source and the catalytic level could be modulated by a specific structure and composition. To this end, the phosphinesulfonate ligand was considered due to the tunable coordination modes of the sulfonate moiety therein, which has exhibited unique behaviors by cooperating with the chelated transition metals (e.g., Ru, Ir) in the hydro-functionalization.¹⁷ However, the analogous cobalt complexes have not been accessed and the catalytic performance has been unexplored yet. In this study, the phosphinesulfonate Cp*Co^III complex (Co(dppbsa)) was prepared for the first time, which surprisingly enables the N,N-dimethylation of nitroaromatics with CO₂ and hydrosilanes (Scheme 1C).

Results and Discussion

The half-sandwich phosphinesulfonate cobaltic complex (Co(dppbsa)) was accessed by neutralization of 2-(diphenylphosphanyl)benzenesulfonic acid with tBuOK, followed by a metathesis reaction with the metal precursor Cp*Co(CO)I₂ (Cp* = pentamethylcyclopentadiene anion). It features a typical methyl ¹H NMR (CDCl₃, 298 K) peak at 1.41 ppm, corresponding to the presence of the Cp* moiety. Besides, the phosphine ³¹P{¹H} NMR (243 MHz) peak down-field shifts from 3.8 ppm assigned for 2-(diphenylphosphanyl)benzenesulfonic acid to 33.4 ppm due to the coordination with the Co^III center. The molecular structure of Co(dppbsa) was unambiguously determined by X-ray crystal diffraction analysis. In the solid state (Fig. 1 and Fig. S1†), the cobaltic center is surrounded by one iodo ligand, one pentamethylcyclopentadiene ligand, and one bidentate dppbsa ligand in a pseudo-octahedron geometry, which is reasonable for a d⁶(cobalt) high-spin configuration.

The benchmark reaction of nitrobenzene (1a) with CO₂ catalyzed by Co(dppbsa) was examined in the presence of stoichiometric hydrosilane as the H source (Table 1). To our delight, N,N-dimethylaniline (2a) was obtained with a high yield of 85% when the reaction of 1a (0.2 mmol) with CO₂ (3 MPa) was conducted in the presence of Co(dppbsa) (5 mol%), NaBAr^F₄ (5 mol%), and a combination of PhSiH₃ (6 equiv.) and Et₃SiH (15 equiv.) at 150 °C in toluene for 24 h under argon (entry 1). Such a N,N-dimethylation of 1a with CO₂ did not proceed well without NaBAr^F₄ (entry 2), largely terminating at the amine (3a) and N-monomethylation (4a). By contrast, the reaction did not occur at all without Co(dppbsa) (entry 3). Both the activity and selectivity toward N,N-dimethylation decreased by lowering the reaction temperature to 130 °C (entry 4), but without delivering the N-formylation products. It has not benefited in improving the selectivity toward 2a by increasing the initial pressure of CO₂ to 5 MPa, which instead promoted the reduction of 1a to 3a. Finally, the dppbsa ligand in Co(dppbsa) was found to be indispensable to achieve a high catalytic efficiency since the performance of its precursor (Cp*Co(CO)I₂) was much poor (entry 6). We also noticed that neither PhSiH₃ nor Et₃SiH in a theoretically comparable amount with hydride could promote the desired C–N coupling independently (entries 7 and 8). Particularly, the presence of Et₃SiH alone largely disables the reductive conversion of 1a (entry 8). Further efforts on optimizing the reactivity by screening different hydrosilanes and solvents failed (Table S1†).

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The scope and limitation of nitroaromatic substrates under the newly developed protocol were subsequently investigated (Scheme 2). Evidently, the electronic variation of substituents at the nitroaromatics affects the reaction efficiency. For instance, N,N-dimethyl arylamines substituted with electron-donating groups (EDGs), such as methyl (2b), methoxy (2e), methylthio (2f), ethyl (2g), tertiary butyl (2h), and cyclohexyl (2u), were obtained in higher yields as compared to those with electron-withdrawing groups (EWGs), such as fluoro (2i), chloro (2j), bromo (2k), trifluoromethyl (2l), and trifluoromethoxy (2m). The position of substituents at the nitroaromatics plays a minor role in the desired N,N-dimethylation process as demonstrated by the yield of N,N-dimethylanilines, which decreases in the order of para-(2b) > meta-(2c) > ortho-(2d). Remarkably, dehalogenation under such reduction conditions was not detected for 2i–2k. Besides, the disubstituted, poly-substituted, and polycyclic aromatic nitroarenes proceeded satisfactorily, delivering 2p (83%), 2q (86%), 2r (84%), 2s (89%), 2w (70%), and 2x (79%) by reacting with CO₂. Despite the steric hindrance, both the dimethyl-substituted 1p and the polymethyl-substituted 1q are more reactive than the monomethyl-substituted 1b–1d counterpart probably due to the electron-rich character. Heterocyclic nitroaromatics containing O, S, N atoms are also tolerated, providing 2o (80%), 2s (89%), 2t (81%), and 2v (82%) with excellent yields. The catalytic N,N-dimethylation of 1a with CO₂ on a scale of 5 mmol was finally examined, which delivers 2a in 75% isolated yield (Scheme S1†).

During the investigation of Co(dppbsa)-catalyzed N,N-dimethylation of nitrobenzene (1a) with CO₂, the combination of Co(dppbsa) and NaBAr^F₄ was found to be capable of reducing 1a to 3a with hydrosilanes as the reductant. Further studies demonstrated that such a reduction proceeds moderately without feeding CO₂ (Table 2, entry 1). The presence of PhSiH₃ alone performed similarly (entry 2), largely terminating at the intermediates. In contrast, the reductive formation of 3a was significantly promoted by changing PhSiH₃ to Et₃SiH (entry 3). These results suggested that the coupling of Co(dppbsa) with PhSiH₃ is more reactive than that with Et₃SiH in terms of the reduction of nitroaromatics, thereby inhibiting the activity of Et₃SiH upon using a combination of PhSiH₃ and Et₃SiH. We also observed that the activity of PhSiH₃ toward nitroaromatics has been basically maintained when feeding CO₂ (entry 4 vs. entry 2), while the reduction of 1a with Et₃SiH was highly inhibited in the presence of CO₂ (entry 5 vs. entry 3). It signified the preferential reduction of CO₂ with Et₃SiH rather than PhSiH₃ under the cobalt catalysis. In addition, it allows for reducing the reaction temperature (entry 6) and the dosage of Et₃SiH (entry 7) without remarkably losing the reactivity of reduction with Et₃SiH. The reaction efficiency was thereafter improved by screening different hydrosilanes (entries 8–10). As a result, the best yield of 92% was obtained when the reduction of 1a (0.2 mmol) with diethylsilane (Et₂SiH₂ (3 equiv.)) was conducted with Co(dppbsa) (5 mol%) and NaBAr^F₄ (5 mol%) in toluene at 80 °C for 24 h (entry 9). Noteworthily, the combination of Co(dppbsa) and NaBAr^F₄ was a prerequisite to reach such a high reactivity (entries 11 and 12).

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The functional group tolerance of nitroaromatics in the reductive transformation to amines was also examined (Scheme 3). Notably, nitroaromatics substituted with EDGs, such as methyl (1b), methoxy (1e), methylthio (1f), ethyl (1g), tertiary butyl (1h), and cyclohexyl (1u), performed slightly poorer as compared to that without a substituent (1a). However, the introduction of EWGs, such as fluoro (1i), chloro (1j), bromo (1k), and trifluoromethyl (1l), did not show better results. Particularly, the reduction of 1i, 1j and 1l needs to be conducted at a higher temperature of 120 °C to ensure satisfactory reactivity. This suggested that the rate-limiting step for the Co(dppbsa)-catalyzed reduction of nitroaromatics with Et₂SiH₂ may vary with the different electronic effects of substrates. Again, the dehalogenation was not detected in the formation of 3i–3k. The steric effect is also unfavorable for such a transformation, thereby delivering anilines in the decreasing order of para-(3b) > meta-(3c) > ortho-(3d). Herein, the dimethyl-substituted (1p) and polymethyl-substituted (1p) nitroaromatics perform poorer than the monomethyl-substituted 1b–1d counterpart due to the electron-rich character combined with steric hindrance. At the same time, polycyclic nitroaromatics are well tolerated (3r–3z). Remarkably, nitroaromatics bearing medicinally relevant building blocks also survived with such a system, delivering anilines containing morpholine (3v), pyrrole (3y) and quinoline (3z) scaffolds with excellent chemo-selectivity.

To discern the required number of electron transfer in the reduction of CO₂ for C–N coupling, the catalytic hydrosilylation of CO₂ with hydrosilanes in C₆D₆ was examined. Representatively, the distribution of silylated products after reacting for 4 hours was analyzed (Fig. 2) by quantitative ¹³C{¹H} NMR spectroscopy according to the previous reports,¹⁸ wherein the silyl formate units, the bis(silyl)acetal units, and the methoxy-silane units could be finely identified. As a result, the combination of Co(dppbsa) and NaBAr^F₄ mainly promoted the two-electron reduction of CO₂ to give silyl formate (Fig. 2A), whereas the six-electron reduction leading to methoxysilane also moderately occurred. By contrast, the Co(dppbsa)-catalyzed hydrosilylation of CO₂ did not occur at all without NaBAr^F₄ (Fig. 2B), while the six-electron reduction was significantly preferred with NaBAr^F₄ alone (Fig. 2C). Furthermore, the activity of PhSiH₃ is obviously lower than that of Et₃SiH in the catalytic hydrosilylation of CO₂ regardless of the presence (Fig. 2Dvs.Fig. 2E) or the absence (Fig. 2Fvs.Fig. 2G) of Co(dppbsa). It's consistent with the results of cross-experiments on the catalytic reduction of 1a to 3a (Table 2, entries 2–5). Meanwhile, these independent experiments signified the cooperative effect present not only between Co(dppbsa) and NaBAr^F₄ (Fig. 2Avs.Fig. 2Bvs.Fig. 2C), but also between PhSiH₃ and Et₃SiH (Fig. 2Avs.Fig. 2Dvs.Fig. 2E or Fig. 2Cvs.Fig. 2Fvs.Fig. 2G) in the catalytic hydrosilylation of CO₂, resulting in different reactivities and product distributions.

The performance of C1 surrogates derived from the hydrogenation of CO₂, such as HCOOH, CO, (CH₂O)_n, and MeOH, was further examined in the Co(dppbsa)-catalyzed N-methylation of 1a (Fig. 3). As a result, neither MeOH nor CO allowed for the C–N coupling, which instead moderately delivered the reduction product of 3a. For the reactions with CO feedstock, a diluted CO by coincidently feeding N₂ was introduced to prevent the potential deactivation of the cobalt catalyst. Evidently, the result was almost unchanged by modulating the partial pressure of CO. By contrast, the N,N-dimethylation of 1a with either HCOOH or (CH₂O)_n occurs, despite providing only a moderate yield of 2a. Combined with the catalytic performance of hydrosilylation of CO₂ under the standard conditions (Fig. 2A), we reasonably proposed that the silyl formate or its analogue acts as the carbon-based intermediate while participating in the C–N coupling step.

To approach the reductive C–N coupling, nitroarene (1a) might be parallelly transformed to nitrosoarene (5a), N-aryl hydroxylamine (5b), azoxyarene (5c), azoarene (5d), and even aniline (3a).¹⁹ The reactivity of these N-based substrates (N-Sub) with CO₂ was examined under the optimum conditions (Fig. 4a and b), respectively, to identify the most likely N-based intermediates in the catalytic cycle of N-methylation of nitroaromatics. Evidently, both 3a and 5a led to 2a with an over 70% yield (Fig. 4b). By contrast, the N-methylation of the other nitrogen-based species (5b–5d), documented as the intermediates of the condensation route leading to 3a,¹⁹ performed worse. Furthermore, the time-course plots for the N,N-dimethylation of 1a, 3a, and 5a within the first 8 hours were obtained and compared (Fig. 4c and Fig. S3†). They demonstrated that the rate leading to 2a from 3a is remarkably slower than that from the other two substrates. Besides, the mass-balance for 5a contains a significant amount of 4a (Fig. S3†), which distributed in a pyramid shape along the reaction time, suggesting that the N-methylation of 5a with CO₂ by C–N coupling is too fast to form the hydrodeoxygenation product of 3a. Thus, the possibility of amines as the major intermediate was excluded. Thereafter, it's confirmed by the competitive experiment using an equal amount of nitroaromatic (1e) and aniline (3a) as the N-Sub, which mainly provided 2evia N,N-dimethylation of 1e (Fig. 4d). As such, the nitrosoarene (5a) was the most reactive intermediate in the C–N coupling step.

Based on our results and the related processes, we proposed a tentative route for the Co(dppbsa)-catalyzed reductive N,N-dimethylation of nitroaromatics with CO₂ and hydrosilanes in Scheme 4A. In general, the reduction of nitroarene occurs along with that of CO₂, using different hydrosilanes or their combination as the preferred H source. The structural variation of Co(dppbsa) during the catalysis cannot be specified at present. However, a considerable amount of Cp* was detected by GC-MS analysis in the evaluation of the catalytic performance (Fig. S4†), suggesting that the cobalt center bears at least six coordination vacancies during the catalysis as Ru(dppbsa).¹⁹ Both nitrosoarene (5A) and silyl formate were recognized as the key intermediates participating in the desired C–N coupling, leading to either N-formylation 6⁶ or diaryl urea 7.²⁰ It thus exhibits an apparent cooperative effect between two hydrosilanes (PhSiH₃ and Et₃SiH) since the formation rates of nitrosoarene (5A) and silyl formate are modulated by both the structures and proportions. Despite the ambiguity of multi-step reduction, it's strongly suggested by the experimental results that the silane involved in the catalysis follows an outer-sphere ionic hydrosilylation mechanism.²¹ The schematic illustration of Co(dppbsa)-catalyzed reduction of unsaturated bonds with silane was preliminarily supposed as given in Scheme S2,† taking the reduction of CO₂ to silyl formate as an example. Despite being less reactive, the reductive coupling with anilines (3) cannot be completely excluded from the overall catalytic process due to the readily reduction of 5A either by the direct route via hydroxylamine intermediates (5B) (Pathway I) or by the condensation route (Pathway II) via intermediates 5C, 5D, and E.²² Next, the role of the N-formylation product and diaryl urea as the potential intermediate in the reductive N,N-dimethylation of nitroaromatics with CO₂ was examined (Scheme 4B). The performances of 6a and 7a under the standard conditions are comparable (yield of 2a:6a, 54%; 7a, 45%), indicating that both are possible. However, the yield of 2a obtained from these N-based intermediates containing the carbonyl moiety is apparently lower than that from the nitroaromatic precursor (1a, 92%). It could be attributed to the deregulated reduction rate of N-Sub (6a/7a) and CO₂, thereby negatively affecting the efficiency of consequent C–N coupling. Evidently, NaBAr^F₄ alone, which was previously proved to be active for the catalytic hydrosilylation of CO₂ (Fig. 2C), also enables the full conversion of 6a and 7a but with much worse yields of 2a (27% and 10%). Efforts on isolating and identifying the species for the mass imbalance by basic hydrolysis and GC-MS analysis failed (Fig. S5†), suggesting that high boiling point products had been generated. We therefore supposed that these labile carbonyl groups at the intermediates towards the hydrosilanes account for the unidentified species during the independent examinations by launching a large dosage. It's further confirmed by the comparison between 4a (without the carbonyl group) and 8a (with the carbonyl group), which delivers 2a with yields of 95% and 60%, respectively, under the standard conditions.

Conclusion

In summary, the preparation method and the catalytic performance of a half-sandwich phosphinesulfonate cobaltic complex (Co(dppbsa)) were studied. Remarkably, the combination of Co(dppbsa) and NaBAr^F₄ enables the chemoselective catalytic reduction of not only nitroaromatics, but also CO₂ by taking appropriate hydrosilanes as the reductant. It thus allows for the reductive coupling of nitroaromatics with CO₂ by modulating the catalytic level with a specific structure and composition of hydrosilanes. As a result, the Co(dppbsa)-catalyzed reductive N,N-dimethylation of nitroaromatics with CO₂ was approached with a combination of PhSiH₃ and Et₃SiH of a specific proportion. Mechanistic experiments suggested that the semi-reduction products nitrosoarene and silyl formate are the key intermediates participating in the desired C–N coupling. This study provides an efficient protocol for the high-value utilization of CO₂ catalyzed by non-noble cobalt. The versatile reactivity of phosphinesulfonate chelated transition metal complexes in reductive coupling deserves further exploration.

Author contributions

Shuang-Shuang Ma: investigation, data curation, and formal analysis. Rui Sun: data curation, formal analysis, and validation. Zi-Heng Zhang: formal analysis, visualization and writing – review and editing. Peng-Xin Guan: data curation. Jin-Qing Lin: writing – review. Chun-Shan Li: supervision. Bao-Hua Xu: conceptualization, methodology, writing – original draft, supervision, and project administration.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Financial support from the Excellent Young Scientists Fund (22022815) and the National Natural Science Foundation of China (22178346) is gratefully acknowledged.

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 2149710. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3gc02517a

‡ These authors contributed to this article equally.

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