Cation-pi interaction in Al(L)(+) complexes (L=C6H6, C5H5N, C5H6, C4H4NH, C4H4O)

The cation-pi interactions between Alf 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), Moller-Plesset perturbation, quadratic configuration interaction, pure density functional theory (DFT), and DFT/HF-hybrid methods. The following OK BDE data have been obtained: BDE(Al+-benzene, C-6v)=35.6 kcal/mol, BDE(Al+-pyridine, C-2v)=46.4 kcal/mol, BDE(Al+-cyclopentadiene, C-s)=33.9 kcal/mol, BDE(Al+-furan, C-2v)=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+-(pi-L) complexes (pi-L assigns the ligands L=C6H6, C5H6, C4H4O, interacting via their pi-system with Al+) is characterized by a pi-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+-(pi-L)) are underestimated consistently by ca. 5-8 kcal/mol independent of the applied basis set and exchange functional.