The Role of Ligand Steric Bulk in New Monovalent Aluminum Compounds.

The tetrameric Al(I) cyclopentadienyl compound AlCp* (Cp* = CMe) is a prototypical low-valence Al compound, with delocalized bonding between four Al(I) atoms and η ligands bound to the cluster exterior. The synthesis of new [AlR] (R = CMePr, CMeiPr) tetramers is presented. Though these systems failed to crystallize, comparison of variable-temperature Al NMR data with density functional theory (DFT) calculations indicate that these are AlR tetramers analogous to AlCp* 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 Al 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.