Article Cite This: Organometallics XXXX, XXX, XXX−XXX
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Mechanistic Study on the Origin of the Trans Selectivity in Alkyne Semihydrogenation by a Heterobimetallic Rhodium−Gallium Catalyst in a Metal−Organic Framework Sai Puneet Desai,† Jingyun Ye,†,‡ Timur Islamoglu,§ Omar K. Farha,§ and Connie C. Lu*,†
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Department of Chemistry, and ‡Chemical Theory Center, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455-0431, United States § International Institute of Nanotechnology and Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States S Supporting Information *
ABSTRACT: A heterobimetallic Rh-Ga active site installed onto the Zr6-oxide nodes of the metal organic framework (MOF) NU1000 was previously shown to catalyze the semihydrogenation of alkynes to alkenes and, of interest, internal alkynes to trans-alkenes with high selectivity. A suite of mechanistic organometallic techniques and periodic density functional theory calculations have been applied to probe the semihydrogenation of diphenylacetylene (DPA) to (E)-stilbene, as a model catalytic reaction. Initial rates confirm that both DPA syn hydrogenation and cis- to trans-stilbene isomerization are faster than (E)-stilbene hydrogenation to bibenzyl by factors of 3 and 4.6, respectively. The semihydrogenation catalysis is first order with respect to catalyst and H2. For diphenylacetylene, the reaction is first order at low concentration but undergoes a sharp switchover to zeroth order when the alkyne concentration exceeds ∼40 equiv per Rh-Ga active site. The kinetic isotope effect for the reaction of diphenylacetylene with H2/D2 is 1.72(7), even though isotopic scrambling between H2 and D2 is facile under catalytic conditions. The Hammett study of p-X(C6H4)CCPh substrates, where X is NH2, OMe, CH3, F, CN, or NO2, yielded a small ρ value of −0.69(3), which is consistent with a concerted transition state in the rate-limiting step. The results collectively indicate that alkyne insertion into the Rh−H bond is rate limiting. An isotope labeling study of the cis- to trans-stilbene isomerization lends strong evidence that H2 is directly involved and is consistent with a β-hydride elimination pathway that sets the overall trans selectivity.
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INTRODUCTION The traditional boundaries between homogeneous and heterogeneous catalysis are blurring as uniform, single-site catalysts are either immobilized onto, or designed into, solidstate materials. Of these, catalysts based on metal−organic frameworks (MOFs) hold incredible promise because of the vast tunability in the active sites by varying the organic linkers and inorganic nodes and/or through postsynthetic modification.1−9 The design of MOF active sites is increasingly reaching the atomic-level precision of homogeneous catalysts, setting them apart from nearly all other heterogeneous catalysts.10−24 An emerging opportunity that blends these traditional boundaries is to study MOF catalysts using the mechanistic toolbox of molecular organometallic and inorganic chemistry. Such mechanistic studies can establish if the MOF active sites are truly uniform and well behaved. More importantly, mechanistic data enable researchers to elucidate the critical structure−activity relationships that underlie rational catalyst design.25 To date, detailed mechanistic studies of MOF catalysts remain limited.26,27 © XXXX American Chemical Society
Previously, postsynthetic incorporation of well-defined heterobimetallic active sites into MOFs was achieved by reacting a metal−metal-bonded organometallic coordination complex and NU-1000, which is a MOF of Zr6-oxide clusters and tetrakis(p-benzoate)pyrene linkers (Figure 1a).28−32 An immobilized heterobimetallic Rh-Ga complex with an intact Rh→Ga bonding interaction (Figure 1b,c) demonstrated excellent chemo- and regioselectivity in catalytic alkyne semihydrogenation to alkenes, where acyclic internal alkynes were transformed to the corresponding alkenes with E:Z ratios >98:2.33 We proposed that the Ga ion acts as both an electronic and structural promoter. Computations had shown that the Rh becomes more electropositive when it is bonded to Ga and also has one fewer open coordination site. Moreover, the observed selectivity of the Rh-Ga bimetallic site was starkly Special Issue: Organometallic Chemistry within Metal-Organic Frameworks Received: May 16, 2019
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DOI: 10.1021/acs.organomet.9b00331 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
support from periodic density functional theory (DFT) calculations of the full catalytic mechanism for 2-butyne hydrogenation, pathway (1) was selected.33 In the current work, we employ organometallic mechanistic tools to investigate the Rh-Ga/NU-1000 catalyst and to further elucidate the origins of its trans selectivity. On the basis of the initial rate method, rate law determination, isotope labeling, and Hammett studies, a detailed mechanistic understanding of the Rh-Ga/NU-1000 catalyst is presented. These results underscore the single-site nature of the Rh-Ga/ NU-1000 catalyst. This study also addresses the current lack of any mechanistic investigation for any heterogeneously catalyzed E-selective alkyne semihydrogenation process.36,43−45
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RESULTS AND DISCUSSION Catalytic Reaction Sequence. Rh-Ga/NU-1000 catalyzes the chemo- and regioselective hydrogenation of diphenylacetylene (DPA) to (E)-stilbene in 93% yield under standard catalytic conditions: 1 mol % catalyst, 0.20 M DPA, 4.84 atm H2, toluene-d8, 100 °C, and 15 h. Bibenzyl was formed as a byproduct in 6% yield, while (Z)-stilbene was observed in
