Multireference investigations of ethylene hydrogenation over bimetallic catalysts
Antiferromagnetically coupled bimetallic hydrides are promising catalysts for olefin hydrogenation, but their electronic structure presents a challenging problem for single-reference electronic structure methods. We investigated ethylene hydrogenation by unsupported and silica-supported dinuclear Ni and Fe hydride catalysts using density functional theory (DFT) and GPU-accelerated localized active space (LAS) multireference methods and multiconfiguration pair-density functional theory (MC-PDFT). For the unsupported dinuclear Ni hydride catalyst, different DFT functionals predict different spin-state orderings along the catalytic cycle, ethylene binding energetics, transition-state barriers, and even different rate-determining steps. In contrast, multireference methods provide a consistent description of the spin manifold and reaction energetics across the pathway, with substantially reduced sensitivity to the choice of on-top functional. The calculations further show that the bridging Ni-H-Ni motif remains intact during ethylene coordination and migratory insertion, in contrast to previously proposed mechanisms involving prior bridge opening. Comparison of supported and unsupported systems reveals strongly metal-dependent support effects. Silica coordination substantially lowers the hydride-transfer barrier in the Ni catalyst, whereas the hydrogenolysis barrier is elevated for the Fe systems. These results show that antiferromagnetically coupled bimetallic hydrides can fall outside the reliable regime of single-reference DFT and require multireference treatments to obtain consistent mechanistic predictions.