Ligand effects in aluminum cluster-based energetic materials

Abstract

This dissertation examines the electronic structure and thermochemistry of low-valent aluminum clusters that may serve as precursors for new energetic materials. Clusters such as Al₅ₒ0Cp* ₁₂ (Cp*=C₅Me₅) have theoretical heats of combustion more than twice that of common high explosives and potentially faster combustion kinetics than bulk metals. The tetrameric aluminum cluster Al₄Cp* ₄ is a prototypical monovalent aluminum compound, and a potential precursor for these larger metalloid clusters. The synthesis of Al₄R₄ (R=C ₅Me₄Pr, C ₅Me₄iPr), two clusters similar to Al₄Cp*₄, was recently reported and the effect of their increased steric bulk is discussed here. Experimental results and density functional theory (DFT) analysis show that these clusters are enthalpically more stable than the Cp* variant, due primarily to non-covalent interactions (NCIs) across ligand groups. These NCIs show how ligand steric bulk can add stability to tetrameric clusters in addition to low-valent metal bonding. Similar calculations are performed on seven other homoleptic Cp-related clusters with varying levels of steric bulk. DFT results are used to predict monomer/tetramer equilibrium for all clusters and show trends counter to expectations regarding the role of ligand bulk. This equilibrium could be an important component in determining the viability of a cluster as a precursor for larger clusters.