Cation−π Interaction in Al(L)⁺ Complexes (L = C₆H₆, C₅H₅N, C₅H₆, C₄H₄NH, C₄H₄O)

The cation−π interactions between Al + and aromatic systems have been investigated by ab initio molecular orbital and density functional methods. The structures and bond dissociation energies (BDEs) of Al + −L complexes (L = benzene, pyridine, cyclopentadiene, furan, pyrrole) have been calculated using Hartree−Fock (HF), Møller−Plesset perturbation, quadratic configuration interaction, pure density functional theory (DFT), and DFT/HF-hybrid methods. The following 0 K BDE data have been obtained: BDE(Al + −benzene, C 6 v ) = 35.6 kcal/mol, BDE(Al + −pyridine, C 2 v ) = 46.4 kcal/mol, BDE(Al + −cyclopentadiene, C s ) = 33.9 kcal/mol, BDE(Al + −furan, C 2 v ) = 22.2 kcal/mol, BDE(Al + −furan, C s ) = 29.2 kcal/mol, and BDE(Al + −pyrrole, C s ) = 41.6 kcal/mol. As a result from the molecular orbital analysis, the bonding mechanism of the Al + −(π-L) complexes (π-L assigns the ligands L = C 6 H 6, C 5 H 6, C 4 H 4 O, interacting via their π-system with Al + ) is characterized by a π-type electron-donation HOMO(ligand) → LUMO(Al + ). Additionally, a deficiency of the widely applied Lee−Yang−Parr correlation functional is uncovered: As compared to the data obtained from ab initio correlation methods and the results from the Perdew−Wang correlation functional, the BDE(Al + −(π-L)) are underestimated consistently by ca. 5−8 kcal/mol independent of the applied basis set and exchange functional.