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

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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, C6v) = 35.6 kcal/mol, BDE(Al+−pyridine, C2v) = 46.4 kcal/mol, BDE(Al+−cyclopentadiene, Cs) = 33.9 kcal/mol, BDE(Al+−furan, C2v) = 22.2 kcal/mol, BDE(Al+−furan, Cs) = 29.2 kcal/mol, and BDE(Al+−pyrrole,Cs) = 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 = C6H6, C5H6, C4H4O, 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.