(NHC-olefin)-nickel(0) nanoparticles as catalysts for the (Z)-selective semi-hydrogenation of alkynes and ynamides

Abstract

Nickel(0) nanoparticles coordinated to NHC ligands bearing N-coordinated cinnamyl moieties were readily prepared by reduction of a [NiCpBr(NHC-cinnamyl)] complex with methyl magnesium bromide. The combination of a strong σ-donor NHC ligand with a π-coordinating appended cinnamyl moiety likely prevents nickel(0) nanoparticle aggregation to larger inactive species, and allows the effective and (Z)-selective semi-hydrogenation of alkynes and ynamides.

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Metal nanoparticles (NPs) are particularly important in catalysis due to their large exposure surface area and because of the unique metal–metal interactions within their core. The major challenge in the application of NPs in catalysis is to design stable NPs under reaction conditions in order to avoid their agglomeration, which ultimately results in the loss of their catalytic activity. To achieve this, different strategies have been explored, including the use of: surfactants and micellar catalysis, various supports (with different metal-support interactions) or organic ligands.¹ Although the latter strategy was initially achieved through the use of ligands such as thiols, amines, cyanides, disulfides, thioethers or phosphines,² N-heterocyclic carbene (NHC) ligands, which are now privileged ligands in organometallic chemistry and homogeneous catalysis, have been established as novel promising stabilisation agents for metal NPs.³ Thus, several studies have demonstrated the versatility of N-heterocyclic carbene ligands in stabilising and structuring NPs of metals like Ru,⁴ Rh,⁵ or Pd⁶ for significant applications in catalysis, such as the hydrogenation of alkenes and alkynes, the Buchwald–Hartwig amination or the oxidation of alcohols. However, NHC ligands have been much less applied to the stabilisation of NPs of Earth-abundant first-row transition metals, whose development is now a priority in catalysis for obvious economic, environmental and societal reasons.⁷

To the best of our knowledge, there are only three reports on the use of NHC ligands to stabilise Ni NPs.^8,9 In the first two, such NHC-Ni NPs proved to be highly active catalysts for Kumada couplings^8a and the semi-hydrogenation of alkynes^8b and could be recycled a few times without significant loss of activity. Furthermore, in a recent report, we have shown that a half-sandwich Ni(II)-NHC-picolyl bromide complex effectively catalysed the hydrosilylation of a broad range of aldehydes without additive and of ketones in the presence of KOtBu at 100 °C.⁹ Dynamic light scattering, scanning electron microscopy in transmission mode and X-ray photoelectron spectroscopy showed evidence of the involvement of NHC-picolyl Ni particles under the catalytic conditions used for the ketones’ reductions.

Besides NHCs, alkenes have also been applied as ligands to low-valent metal species in the last decade and the resulting organometallic complexes and/or NPs displayed interesting catalytic properties.¹⁰ Interestingly, the combination of NHCs with additional alkene ligands allowed the isolation of well-defined organometallics of Ni(0),¹¹ Pd(0)¹² and Pt(0),¹³ which were active in various cross-coupling and reduction reactions.

Herein, we report that the reduction of the [NiCpBr(NHC-cinnamyl)] complex 1 with an excess of MeMgBr leads to coordinated Ni(0) nanoparticles (Fig. 1). The combination of a strong σ-donor NHC ligand with a π-coordinating appended cinnamyl moiety likely prevents Ni(0) particle aggregation into larger inactive species and allows rapid exchanges with reaction substrates. The resulting NHC-olefin-Ni(0) particles effectively catalyse the (Z)-selective semi-hydrogenation of alkynes^8b,14 and ynamides, the latter being a rare example of direct hydrogenation of ynamides without the use of any salt or additive.¹⁵ The catalytic semi-hydrogenation of internal or terminal alkynes is useful in numerous organic syntheses by providing alkene building blocks, both at laboratory and industrial scale.¹⁶ In particular, such hydrogenation represents a critical step in polymerization reactions in order to eliminate alkynes and dienes from alkene raw materials.

Fig. 1 (Z)-Selective semi-hydrogenation of alkyne derivatives catalysed by NHC-olefin-Ni(0) nanoparticles.

Our study started with the synthesis of complex 1 from the corresponding imidazolium bromide and nickelocene in refluxing THF for 48 h (Scheme S1, ESI†). Once isolated in 92% yield and fully characterised by NMR, HRMS, elemental analysis and single-crystal X-ray diffraction analysis (Fig. S1 and Table S1, ESI†), complex 1 was readily reduced by reaction with 3 equiv. MeMgBr in toluene at 100 °C for 2 h (Fig. 1). The reduction was characterised by a striking colour change from pink to yellow then brown (Fig. S2, ESI†).

The resulting species (3 mol%) was then applied to the (Z)-selective semi-hydrogenation of various alkynes and ynamides under 10 bar H₂, in toluene or ethanol and at temperatures ranging from 25 to 80 °C, depending on the substrate. After optimising all the reaction conditions (Tables S2 and S3, ESI†), all hydrogenations led to the corresponding (Z)-alkenes in good to excellent yields (Table 1). The temperature and the solvent were the key parameters to control the activity of these catalyses, ethanol leading to the highest activities due to a better solubility of the substrates and hydrogen. It is worth noting that the substitution of pre-catalyst 1 by nickelocene led to full hydrogenation of alkynes. Regarding terminal alkynes, phenylacetylene 2a, but-3-yn-1-ylbenzene 2b and 1-phenylprop-2-yn-1-ol 2c were readily hydrogenated in 13 h at room temperature in toluene to afford the corresponding alkenes 3a–c with good selectivities (71 to 87%) and yields. By comparison, 1-ethynyl-3-nitrobenzene 2d required 62 h at 40 °C to afford chemoselectively alkene 3d in high yield. Regarding internal alkynes, diphenylacetylene 2e₁ required a 40 °C heat for 13 h to be fully consumed in toluene (Tables S2 and S3, ESI†), but afforded (Z)-stilbene 3e₁ with a high selectivity of 91% and good isolated yield (78%). It was worth to note that the semi-hydrogenation of 2e₁ proceeded also at the gram scale (5.6 mmol) in 80% yield within 30 h at 40 °C in ethanol [3e₁/4e₁/5e₁ 88/2/10]. By comparison, chloro- or methoxy-substituted derivatives 2e₂ and 2e₃ needed 80 °C for 13 or 48 h in toluene to lead selectively to the related (Z)-stilbenes 3e₂ and 3e₃ in good yields. In contrast, the hydrogenation of ethyl 3-phenylpropiolate 2g in ethanol was almost quantitative at room temperature for 13 h leading to (Z)-alkene 3g in a good selectivity (78%) with some alkane 5g (22%). Phenylpropyne 2f, 3-phenylprop-2-yn-1-ol 2h and 3-phenylpropiolonitrile 2i were also quantitatively hydrogenated in ethanol at 25 °C to afford the related (Z)-alkenes 2f,h,i in good isolated yields (81–88%) and selectivities (84–93%) with only minor amounts of the (E)-isomers 4f and 4i (2%) and alkanes 5f,h,i (5–14%). On the whole, in comparison to the “P-2 Ni” catalyst, our Ni catalyst was somehow less (Z)-selective and universal but more chemoselective as the aryl chloride moiety of 2e₂ and the nitro and nitrile functions of 2d and 2i were preserved.¹⁶ In addition, it displayed a modular reactivity for semi- and full hydrogenations (see Table S3, ESI†) and, to the best of our knowledge, allowed for the first time the effective and selective semi-hydrogenation of various ynamides without any additive.¹⁵

Table 1 Catalysis scope – yields and selectivities by GC and ¹H NMR for alkenes 3a–i and by ¹H NMR for enamides 3j–r. Isolated yields in parentheses (average of at least 2 runs on a 0.5 mmol scale)

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Indeed, Ms, Ts, Boc and Moc protected derivatives 2j–m were hydrogenated to the desired (Z)-enamides in high selectivities (85–92%) and good isolated yields (72–80%) within 48 to 62 h in ethanol at 60 °C. Under similar conditions, the hydrogenation of the ynamide 2n bearing isoindolinone and phenylethyl moieties was completed in only 13 h with a good isolated yield (72%) and a high selectivity in 3n (89%) and only minor amounts of 4n and 5n. With the tosyl substituted ynamides 2o and 2p, the phenyl derivative 2o required more time to be reduced than the methyl derivative 2p and afforded the (Z)-enamide 3o with a higher selectivity of 94% (vs. 74% for 3p), the isolated yields remaining good (84% for 3o and 62% for 3p). The ynamide 2q substituted by an indole fragment afforded almost exclusively 3q after 13 h reaction at 60 °C, with a fair isolated yield of 61%. Finally, ynamide 2r led selectively to 3r′, the ester moiety being changed from methyl to ethyl due to concomitant trans-esterification with the solvent.

To gain some insights into the nature of the probable Ni(0) active species, a sample solution was first analysed by DLS and particles with an average size of 110 nm ±21 were observed (Fig. S3, ESI†). Evaporation to dryness of a similar sample afforded a residue that was washed with dry pentane under argon and dried under vacuum. Noteworthily, the remaining solid, i.e., our catalyst, proved still active although with a lower activity than an in situ-generated catalyst (Table S3, ESI†). X-ray diffraction (XRD) in the reflection mode was performed on the isolated catalyst placed on a glass support in open air but no diffraction patterns were observed, most likely due to the poor crystallinity of the sample (Fig. S4, ESI†). Therefore, no Ni(111) plane (i.e. Ni(0) species), nor a NiO one could be observed.^17a As XRD is mainly a bulk technique, we then analysed our isolated catalyst by XPS in order to determine the chemical composition at the topmost surface (i.e., 9–2 nm) of the particles. The survey scan and high-resolution spectra of a freshly prepared sample revealed the core levels of C 1s, Mg 1s, O 1s, N 1s, Ni 2p and Br 3d elements (Fig. S5, ESI†). More specifically, the peak values for Ni 2p_3/2 and 2p_3/2 satellites of the Ni 2p region were found at 854.8 and ca. 861.9 eV, respectively (Fig. S6, ESI†). These values are consistent with the presence of NiO, which likely results from the oxidation of the Ni(0) species during the transfer of the sample in air.¹⁷ Furthermore, XPS provided the % surface ratio of all the elements and confirmed the presence of C, O, N, Si, Br and Ni elements in the catalyst sample (Table S6, ESI†). The observed Ni/N ratio of ca. 1 : 3 suggested the presence of NHC-cinnamyl-coordinated Ni particles. Interestingly, Raman spectroscopy, carried out on a catalyst sample whose exposure to air was avoided, showed no evidence of Ni oxides (150–430 cm⁻¹) and Ni hydroxides (3100–3650 cm⁻¹) (Fig. S7, ESI†). The catalyst was then characterised by SEM, TEM and HR-TEM microscopies. At low magnification, piles with stacked layers of various sizes and thicknesses were observed (Fig. 2a–d, Fig. S8 and S9, ESI†). According to SEM-EDX chemical analysis, they could be attributed to the Ni-based material, as the observed piles were composed of C, N, O, Ni, Br and Mg elements, at least on their surface (Fig. S8, ESI†). At higher magnification, some domains appeared to be partially crystalline but no diffraction patterns were observed (Fig. 2c and Fig. S9, ESI†). The most visible NPs corresponded to a lattice spacing of 0.65 nm, which is rather different from the uniform lattice spacing of 0.203 nm of the pure Ni(111) plane.^17a Other structures appeared to be finer but, due to the presence of organic matter, the contrast was too weak to allow other measurements. The dispersion of the elements was then studied by STEM EDS elemental mapping without quantification and sizing (Fig. 2e–g and Fig. S10, ESI†). A homogeneous dispersion of Ni, N, C and other observed elements was hereby confirmed. Finally, to get a reliable estimation of the number of NHC-cinnamyl ligands that remain bound to the Ni, two samples were then analysed by ICP-AES and CHN elemental analyses. Mass percentages of 7.10 ± 0.1 (Ni) and 3.51 ± 0.2 (N) were measured, which corresponds to a Ni : N molar ratio of 1 : 2, and thus to a Ni : NHC ratio of 1 : 1 as in the precursor complex 1. These results strongly suggest the coordination of NHC ligands and their appended π-accepting cinnamyl moiety to the Ni(0) NPs.

Fig. 2 TEM analyses of reduced complex 1 used as catalyst. (a–d) TEM and HR-TEM images; (e–g) elemental mapping for Ni, N and C by STEM-EDS.

The reusability of the catalyst was next studied for the hydrogenation of alkynes 2b and 2f over 6 runs (Tables S4 and S5, ESI†). Though the terminal alkene 3b was recovered in good yields and selectivities until the 5th run, a gradual deactivation of the catalyst over long reaction times resulted in a decrease of the yields in internal alkene 3f while keeping a high selectivity. SEM and TEM analyses of the spent catalyst revealed features which were similar to the fresh catalyst but without crystalline domains and with lower Ni and N contents (Fig. S11 and S12, ESI†).

In summary, we report here that the half-sandwich Ni(II)-NHC complex 1 bearing an appended cinnamyl moiety can readily be reduced by MeMgBr to lead to the first example of NHC-olefin-coordinated Ni(0) nanoparticles. The resulting catalyst allowed the effective and (Z)-selective semi-hydrogenation of alkynes and, most impressively, of ynamides without any additive. Further developments of such NHC-olefin-Ni nanocatalysts are currently under study in our laboratories and will be reported in the future.

This work has benefitted from the support provided by the University of Strasbourg Institute for Advanced Study (USIAS) for Fellowships, within the French national program “Investment for the future” (IdEx-Unistra) (C. M., C. P.-H. as well as M. G. A. for postdoc). CNRS is warmly acknowledged for his support. The authors thank Dr E. Wasielewski and Mr M. Chessé responsible for the research platforms and facilities (Strasbourg NMR platform and Strasbourg chromatography facilities) of LIMA (UMR7042 CNRS-Unistra-UHA) who contributed, by their valuable technical and scientific support, to the achievement of this research project.

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 2208403. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2cc05302k

‡ This paper is dedicated to Dr Francine Agbossou-Niedercorn on the occasion of her retirement.

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