A heterobimetallic Rh-Ga active site installed onto the Zr6-oxide nodes of the metal organic framework (MOF) NU-1000 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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We thank Sahil Arora and Tom Webber for assistance with GC-MS and TEM characterization, respectively. We also thank Professors Laura Gagliardi and Don Truhlar for helpful discussions. 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.
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