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 com...