Mechanistic Study on the Origin of the Trans Selectivity in Alkyne Semihydrogenation by a Heterobimetallic Rhodium-Gallium Catalyst in a Metal-Organic Framework

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.