Topology and Equilibrium Analysis of the Monovalent Aluminum Compound Al4Cp*Ph 4 : Topology and Equilibrium Analysis of the Monovalent Aluminum Compound Al4Cp*Ph 4

The theoretical structure and thermochemistry of the tetrameric, low-valence aluminum compound Al4Cp*(Ph)(4) (Cp*(Ph) = C5Me4Ph) is discussed. The first synthesis of this compound was reported in 2005, but the compound failed to crystallize and experimental Al-27 NMR results were inconclusive in regard to the degree of association. Here density functional theory combined with a genetic algorithm is used to predict the expected structure and properties for Al4Cp*(Ph)(4). Synthesis efforts were repeated for this compound, resulting in a product with a Al-27 NMR chemical shift that differed from the previous report by nearly 20 ppm. However, calculated Al-27 NMR chemical shifts for the theoretically predicted structure are within one ppm of these new experimental results, strongly suggesting the tetrameric form has been synthesized. Previous work on five Al4R4 (R = C5H5, C5Me4H, C5Me5, C(5)Me(4)iPr, C5Me4Pr) compounds showed a general trend towards an increased likelihood of disassociation into monomeric species in solution as ligand bulk increased. Analysis of Al4Cp*(Ph)(4), the sixth and bulkiest compound in this series, indicates a departure from this trend. Bonding characteristics for monomer and tetramer forms in this series are examined in detail via topological analysis to understand this trend.