Mechanistic Study on the Origin of the <i>Trans</i> 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)­CCPh 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.