Dissociation of multiple hydrogen molecules on the three-dimensional aluminium cluster: theoretical study

Abstract

Compared to hydrogen activation on bare transition metal clusters, such reactions involving main group metal clusters have been less studied. Here, dissociative addition of multiple H2 to the group 13 metal cluster Al6 is investigated theoretically to examine the viability of generating Al6H8, the novel alane proposed experimentally by Li et al. Coupled-cluster CCSD(T) calculations with the aug-cc-pVTZ basis set are employed to determine the energetics of these additions, and density functional calculations are used to extensively probe the relevant potential energy surfaces. We find the sequential hydrogenations of Al6 via Al6H2k−2 + H2 → Al6H2k (k = 1–4) exothermic, where the Al6H2k cluster global minima structures exhibit the same ‘H-bridging motif’ with two hydrogens sitting on the neighbouring threefold-bridged sites. Of various H2 dissociation modes probed including octahedral- and trigonal prism-like clusters, we find those involving dissociation transition states with the octahedral-like Al6 cores as kinetically most favoured. A correlation is identified between the H2 dissociation barriers and highest occupied molecular orbital–lowest unoccupied molecular orbital gaps of the Al6H2k−2 clusters versus k. For k = 1, the H2 dissociation is predicted to have no enthalpy barrier at the CCSD(T)/aug-cc-pVTZ level, and a good agreement is found between the coupled cluster and G4-calculated energetics. For k = 2, 3 and 4, the lowest enthalpic hydrogen dissociation barriers are determined to be 12, 14 and 20 kcal/mol, respectively, as measured relative to the Al6H2k−2 cluster global minimum isomer plus H2 reactants. According to our calculations, the entropy contribution (−TS) to the free energy dissociation barrier is 8 kcal/mol per H2.