Silylenes and germylenes: The activation of H–H bond in hydrogen molecule

Possible mechanisms of activation reactions of H2 with a variety of acyclic and cyclic silylenes and germylenes have been investigated by using the density functional theory (DFT), the second order Møller–Plesset perturbation theory (MP2), and the complete active space self-consistent field (CASSCF) method. Calculation results demonstrate the facile occurrence of the H2 activation reaction through a concerted mechanism. The relative reactivity of H2 splitting is closely related to the HOMO−LUMO or the singlet–triplet gaps of silylenes and germylenes. The activation energies of H2 split by silylenes are smaller than those by germylenes. For N-heterocyclic silylenes and germylenes with the larger singlet–triplet energy gaps, the higher activation barriers are required to reach the transition states. The cyclopenta-2,4-dienylidene silylenes and germylenes are better candidates for activation reaction of H2 with lower activation barriers. It is also shown that the halogen (F, Cl, Br) substitutions on different ring positions of the cyclopenta-2,4-dienylidene silylenes and germylenes have little influence on the activation energies and the exothermic energies of the insertion reactions with H2.