Using effective group potential methodology for predicting organometallic complex properties

The Role of Ligand Steric Bulk in New Monovalent Aluminum Compounds

The tetrameric Al(I) cyclopentadienyl compound Al₄Cp*₄ (Cp* = C₅Me₅) is a prototypical low-valence Al compound, with delocalized bonding between four Al(I) atoms and η⁵ ligands bound to the cluster exterior. The synthesis of new [AlR]₄ (R = C₅Me₄Pr, C₅Me₄iPr) tetramers is presented. Though these systems failed to crystallize, comparison of variable-temperature ²⁷Al NMR data with density functional theory (DFT) calculations indicate that these are Al₄R₄ tetramers analogous to Al₄Cp*₄ but with increased ligand steric bulk. NMR, DFT, and Atoms in Molecules analyses show that these clusters are enthalpically more stable as tetramers than the Cp* variant, due in part to noncovalent interactions across the bulkier ligand groups. Thermochemistry calculations for the low-valence metal interactions were found to be extremely sensitive to the DFT methodology used; the M06-2X functional with a cc-pVTZ basis set is shown to provide very accurate values for the enthalpy of tetramerization and ²⁷Al NMR shifts. This computational method is then used to predict geometrical structures, noncovalent ligand interactions, and monomer/tetramer equilibrium in solution for a series of Al(I) cyclopentadienyl compounds of varying steric bulk.