Insight into Roles\nof Rare-Earth Metals in Heterobimetallic\nNi–Y Bifunctional Catalysis for Alkyne Semihydrogenation

Due to the unique properties of rare-earth (RE) metals,\nRE catalysts\ndemonstrate distinctive catalytic performance in hydrogenation and\nrelated transformations. In typical RE catalytic systems, the roles\nand function modes have been studied and are relevant to ligands.\nIn recent years, heterobimetallic catalytic systems have emerged for\nefficient hydrogenation and related transformations. Among these systems,\nheterobimetallic catalysts with transition metal (TM)-RE combinations\nintegrate the characteristics of TM catalysis and RE catalysis, exhibiting\na TM-RE bifunctional effect with remarkable activity and selectivity.\nHowever, the roles of RE metals in TM-RE bifunctional catalysis remain\nambiguous. This theoretical study takes the Ni–Y system as\na study case, aiming to elucidate the significant roles of the RE\ncenter in the TM-RE bifunctional effect on catalytic alkyne semihydrogenation.\nThe results suggest that dynamic coordination can occur at the Y center\ndue to its large size and coordination ability, which accepts the\nbinding of phosphine groups of the ligand. The dynamic coordination\nof phosphine groups to the large-size RE center assists the Ni center\nin releasing vacant sites for substrate in-cage binding and reduces\nthe steric effect on the Ni center. Meanwhile, the Lewis acidic RE\ncenter can stabilize the bridging hydride, which is crucial for H<sub>2</sub> activation and hydrogenation. The TM-RE bifunctional effect\npromotes the reaction. During the H<sub>2</sub> activation stage,\ndue to the stabilization of nickel hydrides by yttrium, the <i>fac</i>-pathway is more favored. The Ni–H–Y bridging\nstructure is maintained during the initial hydride insertion in the\nsemihydrogenation stage, which is crucial for the reaction. Additionally,\nthe use of the more active terminal hydride makes the terminal hydride\npathway a more plausible mechanism. Benefiting from the capability\nof yttrium to accept the dynamic coordination of phosphine groups,\nthereby releasing steric hindrance and stabilizing the bridging hydride\nconcurrently, (<i>Z</i>)/(<i>E</i>)-isomerization\ncan proceed to achieve (<i>E</i>)-selectivity through the\nH<sub>2</sub>-assisted Ni–Y bifunctional pathway with a relatively\nlow energy barrier. Owing to the RE-bridging hydride stabilization\neffect, the thermodynamic properties of intermediates are closely\nrelated to the size of the RE metal center, thereby influencing the\nactivity and the (<i>Z</i>)/(<i>E</i>)-selectivity.\nThese results underscore the important roles of the RE center in TM-RE\nbifunctional catalysis, offering valuable insights into the future\ndesign of effective bifunctional TM-RE catalysts.