Rational Design of Metallylenes for Hydrogenation Reactions

The hydrogenation of alkenes, alkynes, carbonyls, and imines by H₂-activated metallylenes (H₃C−EH₂−L, CELH₂; E=Si, Ge, Sn and L=NMe₂, PMe₂) was studied using relativistic density functional theory at ZORA-BP86/TZ2P. By means of activation strain and Kohn–Sham molecular orbital analyses, the physical factors underlying the trends in reactivity were identified and quantified. Firstly, the hydrogenation reactivity increases on descending Group 14, that is from silylenes (E=Si) to stannylenes (E=Sn). This reactivity trend originates primarily from a reduced energy required to break the bonds between the Group 14 atom and the hydrogen atoms because the strength of these bonds decreases from Si−H to Ge−H to Sn−H. Secondly, the reactivity decreases as the Group 15 ligand changes from nitrogen to phosphorus (L=NMe₂, PMe₂) which stems from the trigonal pyramidal PMe₂ geometry compared to the trigonal planar NMe₂ geometry. The latter can, therefore, effectively engage in stronger orbital interactions with the substrate than the former, due to a more efficient HOMO–LUMO orbital overlap. By combining our insights, we rationally designed an optimally tuned catalyst (Group 14 atom and ligand) considering the overall catalytic cycle involving H₂ activation of the metallylene and subsequent hydrogenation.