Closo-alanes (Al4H4, AlnHn+2, 4≤n≤8):: A new chapter in aluminum hydride chemistry

Anion photoelectron spectroscopy and density functional theory were employed to study aluminum hydride clusters, AlnHm- (4 <= n <= 8, 0 <= m <= 10). Photoelectron spectra revealed that Al4H4, Al4H6, and a family of species with general formula AlnHn+2 (5 <= n <= 8) have small adiabatic electron affinities and large HOMO-LUMO gaps (ranging from 0.5 to 1.9 eV) relative to those of their stoichiometric neighbors, implying their enhanced stabilities. Formulas reminiscent of boranes led us to investigate whether Wade's rules could be applied to alanes as well. DFT calculations showed that the AlnHn+2 (5 <= n <= 8) family adopts n-vertex polyhedral closo-structures with two extra hydrogen atoms occupying opposite bridging positions in agreement with the Wade's (n + 1) rule. These can be viewed as aluminum versions of hypothetical diprotonated closo-borane dianions (BnHn2- + 2H(+)). In addition, Al4H4 assumes a closo-tetrahedral geometry, while Al4H6 takes on a distorted tetrahedral (D-2d) structure with two counter-positioned bridging hydrogen atoms and has the largest HOMO-LUMO gap (1.9 eV) of all the alanes we studied. All of these species can be understood in terms of underlying polyhedral skeletal electron pair theory (PSEPT) concepts. Although, the herein studied alanes do not have direct borane analogues, the ability to understand their structures in terms of the Wade-Mingos rules and the underlying PSEPT concepts suggests that they can be considered as borane analogues, thereby opening a new chapter in aluminum hydride chemistry.