Addition and elimination reactions of H₂ in ruthenaborane clusters: a computational study

Abstract

Ruthenaborane clusters have been modelled by performing density functional theory calculations using the B3LYP functional. The calculations gain insights into hydrogen storage and the H–H bond activation by ruthenaboranes. To study the nature of the chemical bond of H₂ molecules attached to ruthenaboranes, we carried out structural optimizations for different ruthenaborane clusters and determined transition state structures for their hydrogenation addition/elimination reactions. Calculations of the reaction pathways yielded different transition-state structures involving molecular hydrogen bonded to the cluster or formation of metal hydrides. The H–H bond of H₂ seems to be activated by the ruthenaborane clusters as activation energies of 24–42 kcal/mol were calculated for the H₂ addition reaction. The calculated Gibbs free energy for the H₂ addition reaction is 14–27 kcal/mol. The calculated activation energies and the molecular structures of the [(C₅Me₅)Ru₂B₁₀H₁₆], [(C₅Me₅)Ru₂B₈H₁₄] and [(C₅Me₅)Ru₂B₈H₁₂] clusters with different degree of hydrogenation are compared. The mechanisms of the H₂ addition and elimination reactions of the studied clusters suggest that they might be useful as hydrogen storage materials due to their ability to activate the H–H bond. They also serve as an example of the ability of hypoelectronic metallaboranes to reversibly or irreversibly bind hydrogen.