Introduction

Hydrogenation of unsaturated bonds is one of the most important chemical transformations for the production of commodity and fine chemicals due to the high atom economy and readily available starting materials.¹ High selectivities, including chemo-,² regio-,³ and stereoselectivity,⁴ are essential for fine chemical production and organic synthesis.⁵ Selective hydrogenation reactions are typically catalyzed by precious metal catalysts. For example, the Lindlar catalyst, a poisoned palladium catalyst on calcium carbonate or barium sulfate, has been used for semihydrogenation of alkynes to (Z)-alkenes.⁶ However, due to the low Earth-abundance, high costs, and inherent toxicity of precious metals, it is imperative to develop Earth-abundant metal catalysts for selective hydrogenation reactions.

As a unique class of porous molecular materials, metal–organic frameworks (MOFs),⁷ have been explored for a wide range of applications, including gas storage⁸ and separation,⁹ sensing,¹⁰ and biomedical imaging and cancer therapy.^{10, 11} MOFs are particularly suited for designing reusable porous single-site solid catalysts by taking advantage of their molecular tunability, active site access via large channels, and enhanced catalyst stability.¹² Thus, MOF catalysts can combine the molecular tunability and uniform catalytic sites of homogeneous catalysts and the stability, facile separation, and reusability of heterogeneous catalysts to afford a new class of sustainable catalysts for organic transformations.¹³ In some examples, MOFs allow the stabilization of catalytically active centers via site isolation to design solution-inaccessible catalytic species based on single metal centers.¹⁴

Herein, we report the design and stabilization of novel dinickel active sites in an aluminum MOF for selective hydrogenation of unsaturated compounds (Figure 1). The active catalytic site in MOF-NiH was characterized by multiple spectroscopic methods as a bipyridine (bpy) linker-supported Ni^II₂(μ₂-H)₂ species. MOF-NiH catalyzed semihydrogenation of alkynes to (Z)-alkenes and selective hydrogenation of C=C bonds in α,β-unsaturated aldehydes and ketones. The reaction mechanism was investigated and discussed.

Results and Discussion

MOF-253,¹⁵ a DUT-5-type¹⁶ MOF with 2,2′-bipyridine-5,5′-dicarboxylate (dcbpy) linkers, was synthesized by heating a mixture of Al(NO₃)₃ ⋅ 9H₂O, 2,2′-bipyridine-5,5′-dicarboxylic acid (H₂dcbpy), acetic acid (HOAc), and N,N′-dimethylformamide (DMF) at 120 °C (Figure 2a). Powder X-ray diffraction (PXRD) studies showed that as-synthesized MOF-253 was highly crystalline with peak positions matching well with those of the literature report (Figure 2b).^{15, 17} Transmission electron microscopy (TEM) revealed a needle-like morphology for MOF-253 (Figure S2) whereas N₂ sorption measurements showed type I isotherms with a Brunauer–Emmett–Teller (BET) surface area of 1466 m²/g, which is comparable with the literature value (Figure 2c).¹⁸

The as-synthesized MOF-253 was metalated with NiBr₂(1,2-dimethoxyethane) [NiBr₂(dme)] in acetonitrile to afford the precatalyst MOF-NiBr₂ (Figure 2a). Inductively coupled plasma-mass spectroscopy (ICP-MS) of digested MOF-NiBr₂ showed a Ni : Al ratio of 1.00±0.02, indicating a complete loading of Ni to all bipyridine sites of MOF-253, leading to a formula of Al(OH)(dcbpy)(NiBr₂) for MOF-NiBr₂. TEM imaging and PXRD studies showed that MOF-NiBr₂ maintained the needle-like morphology and crystallinity of MOF-253 (Figure 2b,d). X-ray photoelectron spectroscopy (XPS) spectrum in the Ni 2p region showed two main peaks, corresponding to spin-orbit coupled J=3/2 and 1/2 states, and two satellite peaks that are related to the shakeup of the two main peaks.¹⁹ The +2 oxidation state of Ni centers in MOF-NiBr₂ was evident by the Ni 2p_3/2 peak at ca. 855.0 eV (Figure S5b).²⁰ N₂ adsorption measurements gave a BET surface area of 499 m²/g for MOF-NiBr₂ (Figure 2c), a value that is significantly smaller than that of pristine MOF-253 (1466 m²/g). The reduced porosity of MOF-NiBr₂ can be attributed to the increased molecular weight after metalation and partial filling of the micropores by NiBr₂.

MOF-NiBr₂ was activated with excess NaBHEt₃ to afford the active catalyst MOF-NiH (Figure 2a). The treatment of MOF-NiBr₂ with NaBHEt₃ led to a drastic color change from yellow to dark green and vigorous evolution of H₂ bubbles. TEM and PXRD studies of MOF-NiH indicated the maintenance of needle-like morphology and crystallinity after NaBHEt₃ treatment (Figure 2b, Figure 3a). N₂ adsorption experiments gave a BET surface area of 364 m²/g for MOF-NiH, indicating the maintenance of high porosity (Figure S11). High-resolution TEM (HRTEM) image (Figure 3b) indicated that no Ni nanoparticles were formed during NaBHEt₃ treatment. The generation of H₂ during the treatment of MOF-NiBr₂ with NaBHEt₃ suggested bimetallic reduction elimination of H₂ from the putative MOF-NiH₂ to form MOF-NiH with (bpy)Ni(μ₂-H)₂Ni(bpy) active sites. Extended X-ray adsorption fine structure (EXAFS) spectroscopy confirmed the formation of (bpy)Ni(μ₂-H)₂Ni(bpy) coordination environments. Fitting of EXAFS results gave Ni−H and Ni−N distances of 1.69 and 1.99 Å, respectively. A strong scattering contribution from the neighboring Ni center was found in the secondary coordination sphere, with a Ni−Ni distance of 2.31 Å (Figure 3d).

Treatment of dark green MOF-NiH with excess formic acid changed the color of the solid to light green that is characteristic of Ni^II species, with concomitant formation of 3 equivalents of H₂ for each (bpy)Ni(μ₂-H)₂Ni(bpy) active site (Figure S12). This result is consistent with protonation of both hydrides to form 2 equivalents of H₂ and single-electron transfer from both Ni centers to two protons to form another equivalent of H₂. However, XPS studies of MOF-NiH revealed a Ni oxidation state of +2 with the Ni 2p_3/2 peak at ca. 855.8 eV (Figure 3c), which is close to the 2p_3/2 peak of MOF-NiBr₂ at ca. 855.0 eV. Electron paramagnetic resonance (EPR) spectrum of MOF-NiH (Figure 3e) showed a signal centered at g=2.008, indicative of an S=1/2 species. These results suggest the formation of (bpy⋅⁻)Ni^II(μ₂-H)₂Ni^II(bpy⋅⁻) active sites due to the redox non-innocence of bpy ligands (Figure 3f). The g-value measured for MOF-NiH corresponded to a free electron (g_e=2.002), which supports the ligand-centered radicals delocalized in the bipyridine ligands.²¹ The generation of 3 equivalents of H₂ for each dinickel site upon treatment of MOF-NiH with formic acid is consistent with the assignment of (bpy⋅⁻)Ni^II(μ₂-H)₂Ni^II(bpy⋅⁻) electronic structure. Although homogeneous [(α-diimine)Ni(μ₂-H)]₂ complexes with sterically hindered, redox non-innocent α-diimine ligands have been previously reported,^{21b, 22} a similar homogeneous complex with a bpy ligand is not known. Thus, site isolation in the MOF stabilizes the solution-inaccessible (bpy⋅⁻)Ni^II(μ₂-H)₂Ni^II(bpy⋅⁻) active sites.

MOF-NiH was tested for its catalytic activity in selective hydrogenation reactions. Considering the importance of alkyne semihydrogenation to produce (Z)-alkenes, diphenylacetylene was chosen as a model substrate to optimize the reaction conditions (denoted standard condition) for semihydrogenation of alkynes (Table 1). We chose n-hexane as the solvent based on its chemical inertness and lack of coordination ability.²³ With 0.5 mol % MOF-NiH loading and 10 bar H₂, 16 % of diphenylacetylene was converted after 72 h reaction in n-hexane at room temperature. Increasing the temperature to 50 °C led to the complete conversion of diphenylacetylene to afford (Z)-stilbene in 60 % yield along with the over-hydrogenated product 1,2-diphenylethane in 31 % yield. Decreasing the H₂ pressure to 5 bar significantly increased the semihydrogenation selectivity, resulting in an 86 % conversion and a 73 % yield of (Z)-stilbene. The Z/E ratio of stilbene was determined as 24 by gas chromatography-mass spectrometry (GC-MS). Increasing the catalyst loading to 1 mol % led to a 100 % conversion of diphenylacetylene and an 82 % yield of (Z)-stilbene by GC-MS. Purification by column chromatography gave (Z)-stilbene as a colorless liquid in 80 % isolated yield.

The substrate scope of alkyne semihydrogenation was investigated using the optimized conditions. A variety of diaryl acetylenes were successfully hydrogenated to (Z)-stilbenes with MOF-NiH as the catalyst (Table 2). For substrates with alkyl substituents (−Me, −Et, and −tBu,), the desired (Z)-stilbenes were obtained in 74 %, 86 %, and 70 % at 50 °C. For the substrate with a strong electron-donating substituent (−OMe), the (Z)-stilbene was obtained in an 87 % yield at 35 °C. A lower temperature was needed to avoid overhydrogenation. For substrates with electron-withdrawing substituents (−Br, −Cl, −F, −COOMe, and −CF₃), higher reaction temperatures were needed to produce the targeted (Z)-stilbenes in 89 %, 89 %, 62 %, 84 %, and 73 % yields, respectively. Cyclohexane was used as the solvent in these reactions due to its slightly higher boiling point than n-hexane.

Table 2. MOF-NiH-catalyzed semihydrogenation of diaryl alkynes.^[a]

• [a] Reaction condition: Substrate (0.25 mmol), H₂ (5 bar), MOF-NiH (1 mol %) and solvent (3 mL) for 72 h. [b] Isolated yields. [c] Determined by GC-MS. [d] Solvent: n-hexane. [e] Solvent: cyclohexane. [f] Determined by ¹H NMR spectroscopy using CH₂Br₂ as an internal standard.

Several control experiments were conducted to gain insights into the reaction mechanism of alkyne semihydrogenation. No conversion of the starting material was observed with the unmetalated MOF-253 as catalyst (Table 1, entry 5) or without catalyst (Table 1, entry 6). These results indicate that Ni sites in the metalated MOF are responsible for alkyne semihydrogenation. Only 9 % conversion was observed when in situ-generated Ni nanoparticles were used as the catalyst (Table 1, entry 7), suggesting the importance of the MOF support in catalysis. A mononuclear nickel hydride MOF, MOF-NiH₁, was synthesized by metalation of MOF-253 with 0.12 equivalent of NiBr₂(dme) and activation with NaBHEt₃. The high crystallinity and porosity of MOF-NiH₁ were confirmed by PXRD (Figure S15) and N₂ adsorption (Figure S16) experiments, respectively. Under the standard conditions, dinuclear catalyst MOF-NiH gave 100 % conversion of the starting material, while mononuclear catalyst MOF-NiH₁ with 2-fold loading of active sites gave less than 1 % conversion (Table 1, entry 8). This result shows the importance of the dinuclear Ni sites in the hydrogenation reaction.

Based on these experimental results and literature precedents on dicobalt-catalyzed alkyne semi-reduction,²⁴ we proposed a plausible reaction mechanism for diphenylacetylene semihydrogenation in Figure 4a. First, diphenylacetylene coordinates to both Ni centers via a μ₂,η²-binding mode. Migratory insertion into a Ni−H bond forms a Ni-bound 1,2-dipheylvinyl intermediate with syn stereochemistry. The Ni-vinyl intermediate reacts with H₂ to afford (Z)-stilbene and regenerate the (bpy)Ni(μ₂-H)₂Ni(bpy) catalyst. The syn stereochemistry of the Ni-vinyl intermediate determines the (Z)-selectivity of the stilbene product.

We conducted density functional theory (DFT) calculations to support the proposed mechanism. The energy profile and optimized structures from DFT calculations are presented in Figure 4b and Figure 4c, respectively. Based on DFT calculations, the diphenylacetylene binding step and hydrogen insertion step are both energetically feasible, with ΔG values of −5.3 kcal/mol and 2.7 kcal/mol, respectively. Additionally, the ΔG in the third step, which involves the substitution of the formed diphenylethylene by H₂, is −24.9 kcal/mol.

The recyclability of the MOF-NiH catalyst was also demonstrated. Five consecutive rounds of diphenylacetylene semihydrogenation were performed with an only slight decrease in (Z)-stilbene yields (Figure 5 and Table S2). Consistent with this, <0.01 % of Ni leaching was detected in the supernatant of the hydrogenation reaction. The PXRD pattern (Figure 2b) of the recovered MOF-NiH exhibited no discernible changes relative to the pristine MOF-NiH, suggesting that the MOF-NiH remained stable under catalytic conditions. The stability of MOF-NiH was further supported by our analysis of the recovered MOF-NiH using various techniques, including Fourier-transformed infrared spectroscopy (FTIR) (Figure S19), XPS (Figure S20), and TEM (Figure S21).

MOF-NiH also catalyzed selective hydrogenation of C=C bonds in α,β-unsaturated aldehydes and ketones. With 0.5 mol % MOF-NiH loading and 10 bar H₂ in cyclohexane, trans-cinnamaldehyde was transformed at 90 °C to hydrocinnamaldehyde in 78 % yield over 48 h (Table 3, entry 1). Only the C=C bond was hydrogenated in the reaction, while the carbonyl group remained unchanged. Several α,β-unsaturated ketones were selectively hydrogenated in their C=C bonds in high yields (Table 3, entries 2–3). The electronic effects of the substituents followed a similar trend to alkyne semihydrogenation. Longer reaction times and higher temperature are needed for α,β-unsaturated ketones with the −Cl substituent (Table 3, entries 4 and 6). While 6 b was obtained in 96 % yield at 90 °C over 28 h, 6 d was obtained in 82 % yield over 48 h. A high turnover number of 192 was obtained for the selective hydrogenation of benzylideneacetone to 6 b. Similarly, while hydrogenation of trans-chalcone at 90 °C for 48 h gave 6 e in 79 % yield (Table 3, entry 5), hydrogenation of p-chlorochalcone was carried out at 120 °C to afford 6 f in 61 % yield (Table 3, entry 6).

Table 3. MOF-NiH-catalyzed hydrogenation of C=C bonds in α,β-unsaturated aldehydes and ketones.^[a]

• [a] Reaction condition: substrate (0.50 mmol), H₂ (10 bar), MOF-NiH (0.5 mol %) and cyclohexane (3 mL) for 28 to 48 h. [b] Isolated yields. [c] Determined by ¹H NMR spectroscopy using CH₂Br₂ as an internal standard.

Conclusion

In this work, we have synthesized a novel MOF-based dinuclear Ni^II₂(μ₂-H)₂ catalyst for semihydrogenation of alkynes to (Z)-alkenes and selective hydrogenation of C=C bonds in α,β-unsaturated aldehydes and ketones. Spectroscopic studies identified solution-inaccessible (bpy⋅⁻)Ni^II(μ₂-H)₂Ni^II(bpy⋅⁻) as the catalytically active site. Selective hydrogenation products were obtained in high isolated yields with turnover numbers of up to 192. The present work highlights the potential of generating unusual and solution-inaccessible bimetallic catalysts in MOFs for sustainable catalysis and fine chemical synthesis.

Supporting Information

The authors have cited additional references within the Supporting Information.²⁵

Conflict of interest

The authors declare no conflict of interest.