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Well-Defined Rh-Ga Catalytic Sites in a Metal-Organic Framework: Promoter-Controlled Selectivity in Alkyne Semi-Hydrogenation to E-Alkenes Sai Puneet Desai, Jingyun Ye, Jian Zheng, Magali S. Ferrandon, Thomas E. Webber, Ana E. Platero-Prats, Jiaxin Duan, Paula Garcia-Holley, Donald M. Camaioni, Karena W Chapman, Massimiliano Delferro, Omar K. Farha, John L. Fulton, Laura Gagliardi, Johannes A Lercher, R. Lee Penn, Andreas Stein, and Connie C Lu J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08550 • Publication Date (Web): 23 Oct 2018 Downloaded from http://pubs.acs.org on October 24, 2018

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Journal of the American Chemical Society

Well-Defined Rh-Ga Catalytic Sites in a Metal-Organic Framework: Promoter-Controlled Selectivity in Alkyne Semi-Hydrogenation to E-Alkenes

Sai Puneet Desai,1 Jingyun Ye,1,2 Jian Zheng,3 Magali S. Ferrandon,4 Thomas E. Webber,1 Ana E. PlateroPrats,5,6 Jiaxin Duan,1 Paula Garcia-Holley,7 Donald Camaioni,3 Karena W. Chapman,5 Massimiliano Delferro,4 Omar K. Farha,7,8 John L. Fulton,3 Laura Gagliardi,1,2 Johannes A. Lercher,3,9 R. Lee Penn,1 Andreas Stein1 and Connie C. Lu1,* 1. Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, United States 2. Supercomputing Institute, and Chemical Theory Center, University of Minnesota, Minneapolis, Minnesota 55455, United States 3. Institute for Integrated Catalysis, and Fundamental and Computational Science Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352, United States 4. Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, IL, USA 5. X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, United States 6. Department of Inorganic Chemistry, Universidad Autónoma de Madrid, Spain 7. Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States 8. Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia 9. Department of Chemistry and Catalysis Research Institute, Technische Universität München, 85748 Garching, Germany

Abstract Promoters are ubiquitous in industrial heterogeneous catalysts. The wider roles of promoters in accelerating catalysis and/or controlling selectivity are, however, not well understood. A model system has been developed where a heterobimetallic active site comprising an active metal (Rh) and a promoter ion (Ga) are preassembled and delivered onto a metal-organic framework (MOF) support, NU-1000. The Rh-Ga sites in NU-1000 selectively catalyze the hydrogenation of acyclic alkynes to E-alkenes. The overall stereoselectivity is complementary to the well-known Lindlar’s catalyst, which generates Zalkenes. The role of the Ga in promoting this unusual selectivity is evidenced by the lack of semihydrogenation selectivity when Ga is absent and only Rh is present in the active site.

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Introduction Heterogeneous transition-metal catalysts are often enhanced by additive elements or ions, which are by themselves not catalytically active. These so-called promoters are ubiquitous in industrial heterogeneous catalysts and ensure the economic viability of most large-scale chemical processes.1-7 However, the fundamental understanding of how promoters function is limited, hindering the progress of rationally designed solid catalysts.8 We have introduced a simple model system, whereby bimetallic active sites are preassembled in a dinucleating ligand scaffold and delivered onto NU-1000,9,10 a metal-organic framework (MOF) consisting of Zr6-oxide nodes and tetrakis(p-benzoate)pyrene linkers.11 The large 31 Å pore diameter of the larger channels in NU-1000 permit easy entry of the bimetallic complexes, which anchor evenly throughout the MOF particle. These functionalized MOFs, if modular, would allow us to draw and test structure-function hypotheses that are potentially relevant to metal-promoter interactions in catalysis. The focus of this work is a bimetallic Rh-Ga active site that is installed post-synthetically onto NU-1000. This catalytic material exhibits key advantages of both homo- and heterogeneous catalysts: tunability, uniformity, site-isolation, and recyclability.12-19 Notably, the single-site nature of the Rh-Ga catalyst has allowed us to elucidate a critical structure-function relationship: the close proximity of Rh and Ga engenders the selective hydrogenation of alkynes to E-alkenes. By comparison, the single Rh site generates alkenes and alkane non-selectively. The chemo- and stereoselective synthesis of alkenes via alkyne hydrogenation has important applications in the pharmaceutical and polymer industries.20,21 To generate a specific alkene cleanly, catalysts must preclude formation of both the alkane and the other alkene isomer. Another challenge for catalyst development is the trade-off between selectivity and activity. For ~50 years, Lindlar’s catalyst concoction of Pd metal, CaCO3, Pb or Ag ions, and quinoline, has been the choice catalyst system for the semi-hydrogenation of alkynes to Z-alkenes.6,22 Despite recent advances in single-atom surface-alloy and intermetallic catalysts for selective alkyne semi-hydrogenations23-31, a similarly versatile catalyst for Eselective alkenes has yet to be fully realized.32-39 To date, the best single heterogeneous catalyst, Rh2Sb/SiO2, hydrogenates diphenylacetylene to E-stilbene in 58% yield, along with the alkane (~38%) and Z-stilbene (~4%).40-42 Hence, the activity/selectivity trade-off, the lack of substrate scope, and catalyst instability all inspire further catalyst development. Here, the MOF-supported Rh-Ga catalyst was tested using a broad swath of substrates and demonstrated excellent alkyne conversion and high selectivity for acyclic (E)-alkenes. Results and Discussion Synthesis and characterization of 1 and 2


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Post-synthetic grafting of RhGa(py3tren) onto NU-1000 was achieved by soaking the MOF in a benzene solution of (py3tren)GaRhX (X = Me or OPh, Fig. 1a) to afford RhGa(py3tren)@NU-1000 (1). Based on a host of physicochemical methods, the Rh-Ga complex is selectively grafted onto the MOF nodes with a nearly ideal stoichiometry of one Rh-Ga complex per Zr6-node. ICP-AES analyses of 1 revealed Rh and Ga metal loadings of 3.3 wt% and 2 wt% respectively, and a Zr:Rh:Ga mole ratio of 6:1.07:1.01. The grafting of (H3py3tren)Rh(C2H4)OPh onto NU-1000 is similarly selective, affording the Rh-only analogue, Rh(C2H4)(κ2-H3py3tren)@NU-1000 (2), where the Rh loading is 3.5 wt% and the Zr:Rh mole ratio is 6:1.20. Powder X-ray diffraction studies and N2 sorption isotherms further confirm the crystallinity and porosity of 1 (1545 cm2/g) and 2 (1637 cm2/g), cf. 1990 cm2/g for NU-1000 (Fig. 1c).9,10 The 1H NMR spectra of D2SO4-digested 1 and 2 each shows D3py3tren in a mole ratio of 1 ligand per 2 linkers, which is equivalent to 1 ligand per node (SI, Fig. S9). The HAADF-STEM images of 1 and 2

reveal that the modified MOF particles retain the overall shape and morphology of pristine NU-1000 particles (Fig. 1e). Additionally, STEM-EDS maps of the individual particles of 1 or 2 display a near conformal distribution of the Rh metal (as well as Ga for 1) throughout the particles.9,10,43 Figure 1. (a) Single-crystal x-ray structures of the RhGa and Rh molecular precursors. (b) DFT-optimized structures of 1 and 2 in the ca. 8.5Å-pore (see SI for details on DFT models A and G, respectively). (c) N2 sorption isotherms of NU-1000, 1, and 2. (d) DED map of 1. (e) HAADF-STEM image of 1 and STEMEDS elemental mapping. Scale bar is 200 nm. 3 ACS Paragon Plus Environment


A combination of synchrotron-based techniques and periodic density functional theory (DFT) calculations were employed to gain detailed structural insight of 1 and 2. A difference envelope density (DED) analysis that compares the synchrotron X-ray powder diffraction data between modified and pristine NU-1000 generates a low-resolution map of the excess electron density introduced into the MOF by the post-synthetic modification. The DED maps of 1 and 2 are both consistent with siting of the Rh complexes between two Zr6-nodes in the ca. 8.5Å-wide pore of NU-1000 (Fig. 1d, SI Fig. S12).9,15,44 To interrogate the local metal coordination environments, Rh and Ga K-edge XANES and EXAFS studies were conducted on 1 and on the model complex, (py3tren)GaRhCl, whose structure is known (SI Fig S2123). Collectively, the synchrotron X-ray data for 1 and 2 rule out agglomeration of Rh metal. More importantly, the coordination geometries and electronic environments of the Rh and Ga centers in 1 are nearly unchanged from their counterparts in (py3tren)GaRhCl. For example, the Rh/Ga XANES spectra of 1 and (py3tren)GaRhCl are strikingly similar (Fig. 2). In silico structural models of 1 were obtained by embedding the “(py3tren)GaRh” species into the ca. 8.5Å-pore of NU-1000, and then using periodic DFT methods to optimize the geometry. The in silico structure shown in Figure 1b was determined to be the best model of 1 because of the close match between its simulated and the experimental EXAFS spectra (SI Figs. S13, S25a, and S25b, model A). The model contains a square pyramidal Rh center that is covalently attached to the Zr6-node via a Rh(μ-OH)Zr linkage. The grafted Rh-Ga species in 1 is nearly isostructural to (py3tren)GaRhCl, with the exception that one N−Ga−N bond angle increases by 15 deg to allow for a weak Ga···(μ-OH)Zr interaction. Refinement of the Rh EXAFS data supports a 5-coordinate Rh atom with 1 Ga and 4 N/O donors (Fig. 2d, Table 1). Importantly, the Rh−Ga bond is intact in 1 with a distance of 2.35(5) Å, which compares well to the Rh−Ga bond in (py3tren)GaRhCl (c.f. 2.3171(2) Å). Multiple scattering contributions at ~4.1 Å further support the ligand remaining intact. This best model is also corroborated by the Ga EXAFS data, wherein scattering from a single Rh atom is evident, and the Rh−Ga bond length refines to 2.336(15) Å (SI, Fig. S27 and Table S9). Another closely related in silico model also aligns well with the Rh/Ga EXAFS data and is predicted to be slightly lower in energy by 4 kcal/mol. In this alternative model, a single proton shifts from the hydroxyl ligand in Zr(μ-OH)···Ga−N to an amide donor, generating a Zr(μ-O)Ga···NH linkage (SI Figs. S14, S25c, and S25d, model B). The Rh XANES spectrum of 2 is also highly similar to that of 1. Importantly, the scattering of a proximal Ga ion is notably absent in the Rh EXAFS spectra of 2 (Fig. 2c). The best structure for 2 is a Rh(ethylene) complex, as shown in Fig. 1b, based on the Rh EXAFS data, DFT energies, as well as complementary experiments that suggest the ethylene ligand remains bound to Rh after grafting (SI Figs. S26g and S26h, model G). Of relevance, the Rh centers in the


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DFT models of 1 and 2 face the 3nm-wide hexagonal channels of NU-1000, allowing good substrate access. Pair distribution function (PDF) studies further support the proposed local structures of 1 and 2 (SI Fig. S28).45,46 A differential analysis that subtracted the PDF measured for pristine NU-1000 from that obtained for 1 or 2 highlights additional (or changed) atom-atom correlations associated with grafting of the Rh-Ga/Rh complexes onto NU-1000. For 2, well defined peaks corresponding to the Rh−N/O bond and 1-3-Rh···C distance are observed at 2.09 Å and 3.01 Å, respectively. Additional features at 3.65 Å (Rh···O), 4.28 Å (Rh···Zr), and 4.88Å (Rh···O/Zr), support the grafting of the Rh compound on the Zr6node. Similar peaks observed for 1 at 3.67, 4.25, and 4.86 Å are attributed to a similar grafting of the RhGa complex on the Zr6-node (Table S10). A further differential analysis, isolating the differences in the PDF data for 1 and 2, was applied to highlight the features associated with the Ga. Features at 1.95 Å and 2.77 Å were assigned to the Ga−N and Ga···(OH)Zr distances, respectively.



Figure 2. X-ray absorption spectroscopy: (a) Normalized XANES spectra at Rh K-edge of 1, 2, (py3tren)GaRhCl, and Rh2O3. (b) Normalized XANES spectra at Ga K-edge of 1, (py3tren)GaRhCl, and Ga2O3. (c) k2-weighted Rh-EXAFS Img[χ(R)] for 1 and 2 showing the similarity in the overall structures of the catalysts barring the Rh-Ga scattering at ~2 Å (no phase corrections). (d) k2-weighted Rh K-edge Magχ(R) and Imgχ(R) spectra of 1 and the corresponding FEFF fit that was obtained from DFT modeling.


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Table 1. The results of Rh and Ga EXAFS from the fit of the experimental spectra of 1 and (py3tren)GaRhCl based on the model derived using FEFF9. The values of bond distances quoted in the table are corrected for phase shift. Energy shift (E0) and reduction factor (S02) are set by reference to the standards. The fit is obtained using k-weighting of 2 in the k range of 2.0 Å−1 < k < 12.0 Å−1 (Rh EXAFS) or 2.0 Å−1 < k < 9.0 Å−1 (Ga EXAFS). Entry

1 (Rh EXAFS) 1 (Ga EXAFS) 2 (Rh EXAFS)

Scattering pair

Na

R/Åb

DWFc

Rh−N/O

3.9 ± 0.7

2.086 ± 0.0012

0.0058 ± 0.0019

Rh−Ga

0.8 ± 0.2

2.387 ± 0.057

0.0035e

Rh−N/O/C (MS) Ga−N (or O)

NA 4.2 ± 0.9

4.178 ± 0.038 1.938 ± 0.023

0.0094e 0.0107 ± 0.0033

Ga−Rh

0.9 ± 0.4

2.336 ± 0.015

0.0035d

Rh−N/O/C

5.1 ± 0.5

2.108 ± 0.001

0.0045 ± 0.001

a N = average coordination number, b R = interatomic distance, c DWF = Debye–Waller Factors, d NA = not available, e Set to value for the (py3tren)GaRhCl complex. Other parameters: amplitude reduction factor (amp) = 0.83 and E0 = −6.2, R-factors are typically about 0.0045.

The homogeneity of the Rh-Ga sites in 1 was assessed by treating 1 with 1.2 equiv PPh3 followed by the standard work-up procedure. The solid-state cross-polarization magic-angle-spinning (CP/MAS) 31P

NMR spectrum of 1-PPh3 showed a single resonance at 43.9 ppm, which is consistent with PPh3

coordination to a single Rh-Ga site (SI Fig. S29). For comparison, free PPh3 displays a 31P peak at −5.5 ppm, which shifts downfield to 50.4 ppm in (κ2-py3tren)GaRh(PPh3)Me (JP−Rh=176 Hz, SI Fig. S31). Catalysis: Selective Semi-Hydrogenation of Alkynes The functionalized MOF materials 1 and 2 were tested for the hydrogenation of diphenylacetylene using standard conditions of 1 mol% catalyst, 5 atm H2, and 100 °C in toluene-d8. Catalyst 1 generated E-stilbene in 93% yield with a high E:Z ratio of 99:1. By contrast, 2 formed bibenzyl in >99% yield. The basis for the different product outcomes was gleaned by 1H-NMR monitoring of the reaction for 13 h. For both 1 and 2, Z-stilbene is observed initially, which suggests syn-hydrogenation of the alkyne, followed by Z-to-E alkene isomerization, and/or hydrogenation of the alkenes to bibenzyl (SI Fig S35). The semi-hydrogenation and E-alkene selectivity of 1 stems from a ~16-fold slower rate for bibenzyl formation compared to Z/E-stilbene production; ultimately, E-stilbene, the thermodynamically favored isomer, prevails. By contrast, 2 shows rapid bibenzyl growth that is competitive with stilbene production. As a control, treating 1 with Z-stilbene under catalytic conditions generates E-stilbene in high yield. Moreover, H2 is critical for the process, because in its absence, the formation of E-stilbene is too slow to be relevant. As negative controls, neither NU-1000 nor the molecular complexes, (PhO)RhGa(py3tren) and (H3py3tren)Rh(C2H4)OPh, are competent for hydrogenation. Upon exposure to H2, (PhO)RhGa(py3tren) and (H3py3tren)Rh(C2H4)OPh, both degrade into an insoluble precipitate, which is presumably an ill-defined oligomer containing Rh(μ−H)Rh (1JRh-H = 27.4 Hz) groups (SI Figs. S36 and 7 ACS Paragon Plus Environment


S37).47,48 Hence, anchoring the Rh complexes onto the MOF-support is necessary to preclude the bimolecular decomposition pathway.49 For the selective hydrogenation of diphenylacetylene to E-stilbene, the best single heterogeneous catalyst, Rh2Sb/SiO2, generates the desired alkene only in 58% yield. The state-of-the-art heterogeneous semi-hydrogenation catalyst system is two-component mixture of Rh2Sb/SiO2 and Pd3Pb/SiO2, which work in tandem to selectively generate E-stilbene with an overall turnover frequency (TOF) of 6 h−1 at 25 °C (per noble metal). Catalyst 1 is equally fast with an identical TOF of 6 h−1 albeit at elevated temperature (100 °C). Notably, catalyst 1 has only a single active site and shows significantly broader substrate scope (vide supra). The substrate scope of 1 was tested with terminal and internal alkynes (Figure 3a). Activated alkynes that are conjugated to aldehyde, ketone and nitrile functionalities are converted into the E-alkenes in a high E:Z ratio >98:2. Additional functional groups such as propargylic/primary alcohols, boronic esters, vinyl cyclopropane, amides, lactones, dithiocarbamates, esters, and even peroxides are also well tolerated. A competitive hydrogenation between PhC≡CH and PhCH=CH2 gave clean alkyne consumption with minimal formation of ethylbenzene (80% yields (Figure 3a). Excitingly, 1 even catalytically hydrogenates internal alkynes within 12 to 14-membered lactone rings (Figure 3b), to the corresponding Z-products in high yields and good selectivities (>95%, E:Z ratio ~95:5). The observed selectivities are remarkable because the E/Z-alkene products are predicted to be nearly isoenergetic (ΔΔG 99%, respectively; Figure 3c), albeit to different major products. In the case of 1, propylene is generated in good yield (82%) and in high selectivity (88%)51-59 (overall TOF = 3.1 min−1), whereas complete hydrogenation was achieved with 2 (>99% propane yield, TOF = 3.6 min−1). By comparison, the propane TOF for 1 is nearly a magnitude lower (0.4 min−1), which is consistent with its selectivity for propylene. These results reinforce the significance of the GaIII “promoter” in altering catalyst selectivity.



Figure 3. Catalytic semi-hydrogenation: For the alkyne semi-hydrogenation catalyst 1 (a) E-selective synthesis of diverse alkenes and Z-selective synthesis of macrocyclic alkenes, and (b) conversion of propyne(g) to propylene(g). Deviations from standard conditions include: i. 10 mol% 1, 323 K, 4.3 atm H2, 15 h; ii. 2 mol% 1, 373 K, 5 atm H2, 12 h; iii. 5 mol% 1, 323 K, 4.3 atm H2, 18 h; iv. 2.5 mol% 1, 373 K, 5 atm H2, 16 h.


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At 1 mol% loading, catalyst 1 was recovered and reused up to 5 times without any significant loss in either activity or selectivity in the semi-hydrogenation of diphenylacetylene (SI Figs. S42 and S43). EXAFS and XANES analysis of the post-catalytic material showed an intact Rh⟶Ga bond (2.400(11) Å, SI Fig. S21). HAADF-STEM images collected after 5 catalytic runs showed no observable nanoparticle formation (SI Fig. S44). Moreover, 1H NMR and ICP-AES analyses of the digested material after 5 catalytic runs confirmed that a nearly similar loading of the RhGa(py3tren) unit is retained with a Rh:Ga:py3tren mole ratio of 1.01:0.89:0.94 per Zr6-node (Table S12). However, the 10% decrease in the Ga mole ratio after five runs suggests minor catalyst decomposition, presumably via undesired hydrolysis of the Ga-amide bonds. Lastly, attribution of activity to the solid catalyst 1, opposed to any soluble Rh species that might have leached into solution, was tested by filtering the catalytic mixture at 50% conversion of the diphenylacetylene. Subjecting the supernatant to the catalytic conditions did not yield any further conversion (SI Figs. S46). Periodic DFT Mechanistic Studies To better understand the effect of Ga in promoting selective semi-hydrogenation, periodic DFT calculations were conducted to compare the catalytic mechanisms of 1 and 2 for hydrogenating the C≡C and C=C bonds of 2-butyne and Z-2-butene, respectively (see Methods and SI). The general framework of their mechanisms comprises three steps: H2 activation to generate a Rh hydride; insertion of the C≡C (or C=C) functionality into the Rh−H bond to form a Rh vinyl (or Rh alkyl) species; and, release of the alkene (or alkane). The full catalytic mechanism and the reaction coordinate diagram for 1 is shown in Figure 4, with enthalpy as the energy axis. Starting from the DFT structure of model B, H2 physisorbs near the Rh center, prior to adding across the Rh−Npyr bond. Hence, metal-ligand cooperativity results in the heterolytic cleavage of H2, where the hydride and proton are simultaneously delivered to Rh and pyridine, respectively. The resulting RhI hydride is also stabilized by two non-classical H-bonds: RhHδ−--Hδ+N, where the N donor represents both the pyridine and amine functionalities on the free ligand arm. Overall, H2 heterolysis is slightly endothermic (1.4 kcal/mol) with an enthalpic barrier of 11.0 kcal/mol. The next step is the favorable physisorption of 2-butyne, followed by the rate-determining insertion of the alkyne to generate a RhI-vinyl intermediate. By comparison, the insertion of 2-butene has an even larger enthalpic barrier (34.1 kcal/mol), which is 14.4 kcal/mol higher than that of 2-butyne (c.f. 19.7 kcal/mol). Hence, these results rationalize the semi-hydrogenation selectivity shown by 1. In the last step, intramolecular proton transfer to the Rh-bound sp2-C (or sp3-C) atom releases the butene (or butane), respectively, with a lower barrier of 5 (or 9) kcal/mol. Notably, the Rh−Ga bond remains intact throughout the catalytic cycle.



The full mechanism for 2 is shown in the SI (Fig. S48). Several notable differences in mechanism emerge (Fig. 4, inset). Catalyst 2 binds 2-butyne strongly prior to H2 activation, which occurs via oxidative addition to generate a RhIII dihydride with no enthalpic barrier. In support, a mixture of 1:1 H2:D2 (4 atm, 298 K, toluene-d8) is rapidly scrambled to give HD when 2 is present (< 10 min); whereas, under similar conditions, H/D-scrambling with 1 requires a longer incubation time of 6 h (SI Figs. S49 and S50). In the next step, chemisorbed 2-butyne undergoes insertion with a barrier of 21.2 kcal/mol, which is rate-limiting. The analogous insertion of Z-2-butene is significantly lower at 13.5 kcal/mol. Hence, the inversion of the energy barriers to favor C=C insertion rationalizes the preference for alkane production by 2. Moreover, the lower insertion barriers for 2 compared to 1 is consistent with the faster hydrogenation rate of 2. In the last step, reductive elimination from the RhIII vinyl hydride to release Z-2butene is also rate-determining with a 20.8 kcal/mol barrier. The corresponding reductive elimination of n-butane has a lower barrier of 12.6 kcal/mol, reinforcing the preference for n-butane production over 2butene.


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Figure 4. For catalyst 1, (a) DFT-calculated mechanism for the hydrogenation of 2-butyne (in red) and 2butene (in blue) and the corresponding (b) reaction energy coordinate. Selected distances (Å) are shown in gray. (c) For pre-catalyst 2, the DFT-calculated alkyne/alkene insertion. 13 ACS Paragon Plus Environment


To achieve chemoselective hydrogenation of alkynes to alkenes, the consensus has been that the free energy barrier for alkene desorption must be lower than that for alkene hydrogenation.23 Indeed, DFT calculations for 1 predict that the enthalpic barrier for (Z)-2-butene desorption is 20 kcal/mol lower than that for alkene insertion into the Rh−H bond. Notably, our catalytic results demonstrate the importance of the GaIII promoter in determining the overall hydrogenation selectivity by altering the mechanism and the corresponding reaction energy profile. It is feasible that the Ga promoter plays both an electronic and a structural role. Since GaIII is a Lewis acid,60 the Rh electron density in 1 should be decreased relative to that in 2. Indeed, a charge analysis indicates that the Rh center is more electropositive in 1 than in 2 (SI, Table S13). This electronic tuning may explain the switchover to a H2 heterolysis mechanism in 1, rather than the classical H2 oxidative addition that operates for more electron-rich metal centers, such as in 2. In support, the H2/D2 scrambling experiments suggest that both H2 activation and/or the nature of the Rh−H intermediate are different for 1 and 2. Also, the GaIII ion may serve as a structural promoter by limiting the number of available coordination sites at Rh. While the Rh center has only two open coordination sites in 1, three binding sites are available in 2, which allow for stabilization of the RhIII(H)2(η2-alkene) intermediate. One consequence is that the enthalpic barrier for (Z)-2-butene desorption is only 4 kcal/mol lower than that for alkene insertion. Hence, alkene hydrogenation (over desorption) is predicted to be 5-fold less disfavored for 1 than for 2, making the latter a significantly better semi-hydrogenation catalyst. Conclusion Using a post-functionalization of NU-1000, well-defined Rh and Rh-Ga active sites were installed uniformly with a stoichiometry of one Rh/Rh-Ga site per Zr6-node. The GaIII promoter ion that is directly bonded to Rh enables high selectivity in the Rh-catalyzed semi-hydrogenation of non-cyclic alkynes to E-alkenes. Impressively, lactone-based macrocycles bearing reactive alkyne groups are converted to the corresponding Z-alkenes in good yields; and, the Rh-Ga active site is a good catalyst for the gas-phase semi-hydrogenation of propyne to propylene. Compared to state-of-the-art heterogeneous systems, the unique aspect of the single Rh-Ga site is that it is responsible for both the chemoselective hydrogenation of alkynes (over alkenes) and the stereoselective alkene isomerization to acyclic E-alkenes. Collectively, this work is a powerful demonstration of tailoring single-site reactivity and selectivity on a solid support by using a covalently bonded, Lewis acidic group 13 promoter. The simplicity of our model system highlights the structure-activity relationship between the metal-promoter interaction and its impact on catalyst selectivity. Future work will focus on a detailed mechanistic analysis of 1. A lingering question regards how/why alkene isomerization is favored over alkene hydrogenation. 14 ACS Paragon Plus Environment

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We will also broadly investigate the impact of the organic scaffold and other promoter elements in tuning these MOF-supported bimetallic active sites.



Experimental Section Additional information is provided in the Supporting Information. Synthesis of (py3tren)GaRhMe. A 1 mL THF solution of MeLi (93 µL, 0.102 mmol, 1.1 M in Et2O) was added to a 50 mL THF slurry of (py3tren)GaRhCl (59 mg, 0.102 mmol) at – 78 ºC. The reaction mixture was allowed to warmup to rt overnight and the volatiles were removed under vacuo. The crude solid was dissolved in 15 mL of benzene, filtered over Celite and layered with pentane to obtain orange crystals of (py3tren)GaRhMe (34 mg, 59% yield). Single crystals for x-ray diffraction were grown from slow evaporation of a concentrated acetonitrile solution of (py3tren)GaRhMe. Anal Synthesis of (H3py3tren)Rh(C2H4)OPh. A 1 mL THF solution of H3py3tren (100 mg, 0.265 mmol) was added to a vial charged with [(C2H4)2RhCl]2 (51.5 mg, 0.132 mmol) at room temperature. The reaction mixture was heated to 40 °C for 2 h and immediately filtered over Celite into a vial containing a 5 mL THF solution of NaOPh (31 mg, 0.265 mmol). After stirring for 12 h, the volatiles were removed under vacuo. The crude solid was dissolved in 10 mL of benzene, filtered over Celite and layered with pentane to obtain single crystals of (H3py3tren)Rh(C2H4)OPh (103 mg, 65% yield). Anal. Calcd. for C29H33N7RhO: C, 58.17; H, 5.55; N, 16.38. Found: C, 58.10; H, 6.11; N, 16.10. Calcd. for C22H24N7GaRh: C, 47.30; H, 4.33; N, 17.56. Found: C, xx; H, xx; N, xx. Synthesis of 1. Microcrystalline NU-1000 (29.2 mg, 0.014 mmol) was added to a 20 mL benzene solution of (py3tren)GaRhMe (19.6 mg, 0.035 mmol) or (py3tren)GaRhOPh (22.3 mg, 0.035 mmol) at room temperature. After stirring for 12 h, the heterogeneous mixture was filtered using a fine porosity glass frit. The resulting solids were washed with copious amounts of benzene (~ 20 mL). The solids were soaked overnight in 20 mL benzene and washed with fresh benzene (4  1 mL) and pentane (5  1 mL). The overnight soaking and rinsing procedure was repeated once more. Finally, the solids were dried under dynamic vacuum (< 100 mTorr) for 5 h. Yield: 36 mg (97%). Synthesis of 2. The synthetic procedure is identical to that of 1 except (H3py3tren)Rh(C2H4)OPh (21.5 mg, 0.035 mmol) was used. The reaction between (H3py3tren)Rh(C2H4)OPh and NU-1000 was monitored by 1H NMR spectroscopy in a sealed J. Young NMR tube. No ethylene was observed, suggesting that ethylene remains bound to the anchored Rh site. Yield: 37 mg (98%). General procedure for semi-hydrogenation catalysis. In a typical catalytic run, a J. Young NMR tube was loaded with 0.6 mL of d8-toluene, 0.0007 mmol of catalyst, 10 µL of mesitylene (0.072 mmol, internal standard) and desired amount of alkyne. The J. Young NMR tube was then subjected to 3 freezepump-thaw cycles. Finally, 1 atm of H2 was admitted into the NMR tube at 77 K and the gas was allowed to expand by warming to rt resulting in 4 atm of H2. After heating the reaction mixture for 15-24 h at 100 °C, the solids were separated from the supernatant by using a fine porosity glass frit. The solids were washed with 5 mL of benzene and the resulting filtrate was combined with the supernatant and dried in vacuo. Percent conversions and yields are reported for the crude reaction mixture based on 1H NMR spectroscopy. All catalytic runs were repeated in triplicate. Semi-hydrogenation of propyne. Semi-hydrogenation of propyne was performed in a 16 fixed bed reactors system (Flowrence, Avantium). Typically, 5 mg of catalyst was diluted with 50 mg high-purity grade silica gel (230-400 mesh, Sigma-Aldrich) and loaded into quartz reactor (ID=2 mm, OD=3 mm, L=300 mm). Catalyst materials were loaded in a nitrogen-filled glovebox (oxygen and moisture less than 0.1 ppm), and capped with paraffin film prior loading them in the Avantium system, which was closed immediately after to avoid contamination with air. Reactions were performed between 50 and 250 °C and between 1 and 7.5 atm. All gases were purchased from Airgas and UHP grade. 2% propyne in argon was used together with either 3% hydrogen in argon or pure hydrogen and helium as the GC standard. Total 16 ACS Paragon Plus Environment

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flowrate was around 15 mL/min per reactor. The effluent of each reactor was analyzed sequentially by a gas chromatograph (7890B, Agilent Technologies), equipped with a thermal conductivity detector (TCD) and 2 flame ionization detectors (FID). Computational Methods. Periodic density functional calculations were carried out using the CP2K code.61 The PBE functional62 with Grimme et al.’s D3 dispersion corrections63 was used to calculate the exchange–correlation energy. The DZVP-MOLOPT basis set was used with Geodecker et al.’s pseudopotentials64 with a plane wave cutoff energy of 360 Ry.

ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 

X-ray crystallographic data for Rh compounds (CIF)



Experimental procedures, characterization and spectroscopic data, computational methods, and calculated structures (PDF)

Corresponding Author [email protected] Notes X-ray crystallographic data have been deposited in the Cambridge Crystallographic Data Centre database (CCDC 1842577 to 1842581). Acknowledgments We acknowledge Dr. Camille Malonzo May (UMN), Dr. David Kaphan (Argonne), Dr. Tata Gopinath (UMN), and Dr. Zhanyong Li (NW) for experimental assistance, and Dr. Dale Pahls for preliminary theoretical calculations. This work was supported as part of the Inorganometallic Catalyst Design Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award No. DE-SC0012702. This research used resources of: (1) the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357; and, (2) the University of Minnesota Characterization Facility, which is supported by the NSF through the MRSEC, ERC, MRI, and NNIN programs. 17 ACS Paragon Plus Environment


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