Dynamically Bifurcating Hydride Transfer Mechanism and Origin of Inverse Isotope Effect for Heterodinuclear AgRu-Catalyzed Alkyne Semihydrogenation

Abstract

The mechanism and heterodinuclear cooperative effects for AgRu-catalyzed alkyne semihydrogenation were analyzed with density-functional theory (DFT) and experiment. This combined effort revealed the following: (1) AgRucatalyzed diphenylacetylene hydrogenation initially gives a kinetic mixture of cis-stilbene and trans-stilbene by an ionic Ag−H hydride transfer transition state and post-transition state bifurcation, which was identified by quasiclassical direct dynamics simulations. (2) The hydrogenation reaction exhibits an unexpected inverse kinetic isotope effect (KIE < 1) resulting from an inverse equilibrium isotope effect (EIE) for heterodinuclear H 2 /D 2 activation. (3) The Ag−Ru heterodinuclear cooperative effective is critical for both H 2 activation and vinylsilver protonolysis reaction steps. ( 4) Rate studies and computational analysis show that electron-donating groups accelerate catalysis.

Evidence for Genuine Bimetallic Frustrated Lewis Pair Activation of Dihydrogen with Gold(I)/Platinum(0) Systems

“…Following the above considerations, we calculated the KIE for the reaction from the zero-point Energy differences (ΔΔZPE) between the reactants, 4a + H2/D2, and TS' H-H, FLP 39,48 to find an inverse primary KIE kH/kD = 0.40, in good agreement with the experiments. An (inverse) isotope effect implies the forming/breaking of E-H(D) (E = H(D) or metal atom) bonds in the ratedetermining step.…”

“…Inverse KIEs are rather uncommon in the context of FLPs 37 and bimetallic systems, 38 though a very recent report from Mankad and Ess revealed an akin inverse KIE of 0.6 for the heterobimetallic trans-hydro(deutero)genation of alkynes using a polar ruthenium-silver complex containing a Ru-Ag bond. 39 For that system, formation of a terminal ruthenium hydride with low-energy binding modes that contributes to an inverse equilibrium isotopic effect (EIE) is proposed to be responsible of the measured inverse KIE. In contrast to the Ru/Ag system where an inverse EIE preequilibirum is responsible for an overall inverse KIE, here we propose that our system has an inverse KIE effect in the rate-limiting transition state.…”

“…it is the sum of all vibrational modes involving the forming Au-H and Pt-H and the breaking H-H bonds at the TS, not only stretching modes, that determine the inverse KIE in our case ( Figure 7 and Table S5). 39,50 Figure 7. a) Graphical illustration of the origin of inverse KIE arising from the combination of an inverse equilibrium isotopic effect (EIE) and a normal KIE (left) versus direct one-step inverse KIE as occurs in the reported Au:Pt system (right), and b) difference in selected E-H/D vibrational frequencies (Δν) and vector displacements at TS'HH,FLP.…”

Catalytic Reactions by Heterobimetallic Carbonyl Complexes with Polar Metal–Metal Interactions

“…The [Ag-Ru] catalyst was found to exhibit an unusual inverse H2/D2 isotope effect, which was consistent with computational modeling of the proposed mechanism. 22 Crucially several Ag-only and Ru-only complexes were shown to have little or no catalytic activity as well as poor E/Z selectivity under these conditions, 16 highlighting the importance of the heterobinuclear design. In 2019, Norton and co-workers developed the cooperative heterobimetallic Ti/Cr-catalyzed anti-Markovnikov hydrogenation of epoxides.…”

Pursuit of C–H Borylation Reactions with Non-Precious Heterobimetallic Catalysts: Hypothesis-Driven Variations on a Design Theme

“…Thus, we have recently initiated studies on modifying the Lewis acidic partner more significantly. Most of our published work in this direction has involved pursuing Ag and Au analogues to the Cu catalysts, [62][63][64] but can we get away from group 11 and/or NHC ligands altogether? One foray in this direction has involved the use of Ni(II) pincer complexes.…”