Theoretical Modeling of the Structure and Stability of Complexes of the Icosahedral Al13− Ion with Borane and Alane Molecules

Resumo

The structures, energies, and spectroscopic characteristics of the \({\rm{A}}{{\rm{l}}_{13}}({\rm{B}}{{\rm{H}}_3})_n^-\) and \({\rm{A}}{{\rm{l}}_{13}}({\rm{Al}}{{\rm{H}}_3})_n^-\) complexes with the centered icosahedral aluminum cluster \({\rm{Al}}_{13}^-\) successively “solvated” with borane and alane molecules with n = 1–16 sequentially “solvated” with borane, alane, diborane, and dialane molecules (L = BH₃, AlH₃, B₂H₆, and Al₂H₆) have been calculated by the density functional theory (DFT) method. According to calculations, in the first half of the borane series \({\rm{A}}{{\rm{l}}_{13}}({\rm{B}}{{\rm{H}}_3})_n^-\), the BH₃ molecules are coordinated to edges of the Al₁₃ cage through two B–Al bonds and the B–H_b–Al hydrogen bridge. With increasing number of ligands n, B–H_b–B′ bridges between the ligands and binuclear B₂H_m moieties with m = 3–5 appear in the surface region of the complexes. The calculations predicted the stability of borane complexes to decomposition with elimination of a diborane molecule and the possibility of their existence in the isolated state. In the early members of the alane series \({\rm{A}}{{\rm{l}}_{13}}({\rm{Al}}{{\rm{H}}_3})_n^-\), the ligands are also bound to the cage through two Al–Al′ bonds and a hydrogen bridge; with increasing n, they are even more prone than boranes to form H-bridges between the ligands and binuclear surface moieties Al₂H_m. At n > 10, the internal cage is severely distorted and opened. At the end of the series, the cage changes its composition due to transfer of several Al atoms to the surface region. The alane complexes are less stable than their borane analogues; however, for the systems of the second half of the \({\rm{A}}{{\rm{l}}_{13}}({\rm{Al}}{{\rm{H}}_3})_n^-\) series, the calculations predict noticeable stability to decomposition with elimination of the dialane molecule. Trends in structural characteristics and energies of the complexes with increasing n are traced. The results can be of interest for modeling the structure and stability of more complicated nanosized aluminoborohydrides and aluminohydrides with high hydrogen content.