Encapsulation of Crabtree's Catalyst in Sulfonated MIL‐101(Cr): Enhancement of Stability and Selectivity between Competing Reaction Pathways by the MOF Chemical Microenvironment

Abstract Crabtree's catalyst was encapsulated inside the pores of the sulfonated MIL‐101(Cr) metal–organic framework (MOF) by cation exchange. This hybrid catalyst is active for the heterogeneous hydrogenation of non‐functionalized alkenes either in solution or in the gas phase. Moreover, encapsulation inside a well‐defined hydrophilic microenvironment enhances catalyst stability and selectivity to hydrogenation over isomerization for substrates bearing ligating functionalities. Accordingly, the encapsulated catalyst significantly outperforms its homogeneous counterpart in the hydrogenation of olefinic alcohols in terms of overall conversion and selectivity, with the chemical microenvironment of the MOF host favouring one out of two competing reaction pathways.

Response factors were determined after calibration with the commercially available compounds. All samples were filtered via a 0.2 μm syringe filter (Acrodisc® GHP) before injection. Table S6 lists all retention times for alkenes and olefinic alcohols.
ESI-MS data were collected on a Bruker MicroTOF interfaced with a glovebox. [4]
This was verified by 1 H NMR spectroscopy in solution after digestion ( Figure S10). Characterization of 2@1-SO3Na was carried out with the aid of ICP-OES (Table S3), PXRD (Le Bail fit is shown in Figure S7), N2 adsorption-desorption at 77K ( Figure S9), NMR spectroscopy ( Figure 2c, Figures S14-S15) and SEM imaging ( Figure S12). All analytical data concur that 2 is encapsulated intact inside the pores of

Catalysis
Heterogeneous hydrogenation of alkenes with 2@1-SO3Na: Alkenes were purified as described in materials, methods and instrumentation. Stock solutions were prepared for each substrate in CH2Cl2. Reactions were carried out in J-Young tubes. For hydrogenations at 1000 ppm loading (0.1 mol %), a 50 mL tube was used. For lower loadings, larger J-Young tubes were used with a volume of 100 mL (100 ppm loading) or 300 mL (< 100 ppm loading) in order to increase the amount of available H2. The following procedure describes hydrogenation of 1-octene (4) at 1000 ppm loading. Nominal concentrations and amounts of reagents were adjusted accordingly for other substrates and loadings (see Table 1 in the manuscript). 3 h at 300 rpm after which the tube was depressurized by immersion in liquid N2 for 10 min and slow evacuation of the overhead space for 1 minute to remove any H2 excess. Finally, the tube was left to thaw back to ambient temperature and backfilled with N2.
The product distribution was determined by GC (refer to general methods for more details).
The supernatant was filtered via a syringe filter (0.2 μm, Acrodisc® GHP) and then 200 μL were diluted to 1 mL with CH2Cl2. Retention times are provided in Table S6. Repeated reactions delivered a product distribution that was reproducible to within ± 5 %.
Leaching test: A heterogeneous hydrogenation reaction was set up with 2@1-SO3Na as the catalyst and 1-octene as the substrate (100 ppm loading, [1-octene] = 1.0 M, V = 4.0 mL). After 3 h, the supernatant was filtered off via a filter-cannula and the filtrate was collected in a J-Young tube and transferred inside a N2 purged glove-bag. The supernatant was filtered once more in the glove bag using a 0.2 μm syringe filter (Acrodisc® GHP) and subsequently an S12 aliquot of 100 μL was diluted to 1 mL with CH2Cl2 and analyzed by GC (35% conversion exclusively to n-octane, TON = 3500). The remaining filtrate was then re-pressurized with H2 and left to react overnight. Aliquots (100 μL) were collected after 3 h and 21 h and analyzed by GC. No more conversion was observed, revealing that turnover stopped once the heterogeneous catalyst was separated from the reaction mixture ( Figure S21). (97% conversion selectively to n-octane for the 2 nd cycle). The whole process was repeated once more. Conversion for the 3 rd cycle was 82% ( Figure S22).

Heterogeneous hydrogenation of olefinic alcohols with 2@1-SO3Na:
Olefinic alcohols were purified as described in materials, methods and instrumentation. The According to NMR analysis, all reactions were mass-balanced within ± 7%. For homoallylic alcohol 11a, product distribution and conversion was also determined by GC (Table S6 and Figure S33). Results lie in very good agreement with NMR and reaction is mass-balanced within ± 5%.

Homogeneous hydrogenation with Crabtree's catalyst:
Homogeneous reactions were always run "side by side" with their heterogeneous counterparts.  Table S6 lists the retention times. When olefinic alcohols were employed as substrates, product distribution was determined by 1 H NMR, as described above for the heterogeneous system.  Figure S32a). The same experiment S14 was performed with the heterogeneous catalyst 2@1-SO3Na. Addition of butanal also had a negligible effect on final product distribution ( Figure S32b).

Gas phase hydrogenation of 1-butene:
Gas/solid hydrogenation of 1-butene was carried out according to the literature procedure. [6] A high pressure NMR tube (volume ≈ 1.8 mL) was loaded with finely powdered catalyst (~0.5 mg) inside a glovebox. Subsequently, the high pressure NMR tube was transferred to a schlenk line and the argon atmosphere was replaced with 1-butene (1 atm). The tube was sealed and frozen in liquid nitrogen, hydrogen (1 atm) was introduced at 77 K. As a timer was simultaneously started, the tube was quickly sealed, thawed and transferred to a NMR spectrometer. Experimental results (TONs, TOFs and recycling) are shown in Figure S23.
Gas phase chemical shifts of butenes and butane are as follows:      Figure S3 for 2-PF6, Figure S16    S21 Figure S1. Synthesis of 1-SO3H. [13] Interconnection windows with openings of ~1.1 nm and 1.3 nm, after correcting for van der Waals radii. [17] b) Model for the isostructural sulfonated analogue 1-SO3H. Dimensions of both windows do not change since the sulfonate groups are pointing towards the interior of the pores.
S22 Figure S3. Single crystal structure of 2-PF6 (Crabtree's catalyst) [15] and dimensions of the cation 2 (1.16 x 1.13 x 1.19 nm, V = 1.56 nm 3 ) by enclosure in a rectangular box, defined by tangents to its van der Waals surface (OLEX2 program). [12] The cuboid can be inscribed in a sphere with a diameter of ~ 2.0 nm. Therefore, only 1 cation can fit in the small mesopores (d     Figure S8. TGA graphs of 1-SO3H (green) and 1-SO3Na (red). The latter shows a higher inorganic/organic ratio due to encapsulation of Na + cations. Weight loss at 275 °C corresponds to water trapped within the pores in the as synthesized MOFs and lies in good agreement with elemental analysis (Table S1).  (Table S2).       conversion with only 61% selectivity to hydrogenation and formation of n-pentanol (12b, red squares). Ill-defined condensation products are also detected, particularly after 24 h (magenta triangles). Yield and selectivity (in parenthesis) for each product are shown in the inset.
Mesitylene (*) is used as a standard to measure mass-balance (>95%).  Addition of butanal does not significantly change relative concentration of reagents for either system within experimental error (see also Table S7).
S49 Figure S33. Product distribution based on GC for the hydrogenation of but-3-en-1-ol with 2@1-SO3Na as the catalyst. Total amount of substrate and products remains constant within experimental error of the GC analysis method (± 5%). Calibration curves for but-3-en-1-ol (top left), butanol (top right), crotyl alcohol (bottom left) and butanal (bottom right) are also shown.