Contra-Binding Rotation in Al+−L Complexes (L = C6H6, C4H4O, C5H6, C4H4NH): A New Degenerate Rearrangement

A particular part of the potential-energy surface of Al(L) + complexes (L = benzene, furane, cyclopentadiene, pyrrole, pyridine) has been investigated by means of density functional theory and Hartree−Fock calculations including electron correlation. On the basis of these results, a new kind of degenerate rearrangement will be reported: The barrier for the rotation of the ligand L toward the Al + −L binding axis can be energetically below the Al(L) + → Al + + L dissociation limit. Thus, such a rotation can occur prior to dissociation. The term contra-binding rotation is proposed for this process, which is predicted to take place in the systems Al(C 6 H 6 ) +, Al(C 4 H 4 O) +, Al(C 5 H 6 ) +, and Al(C 4 H 4 NH) + . The energies of the corresponding transition states (TS rot ) relative to the dissociation products amount to −5 to −14 kcal/mol at 0 K. Concerning Al(C 5 H 5 N) +, a respective TS rot could not be detected. The aluminum−pyridine complex dissociates instead of a contra-binding rotation. With increasing temperature, the change in free energy of the contra-binding rotation process increases. Under standard conditions (298.15 K and 1013.25 mbar), the G2MP2-calculated Δ G of the contra-binding rotation amount to +0.6 to +0.9 kcal/mol (L = C 4 H 4 O), −2.8 kcal/mol (L = C 6 H 6 ), −1.4 to −4.8 kcal/mol (L = C 5 H 6 ), and −5.5 to −7.3 kcal/mol (L = C 4 H 4 NH). A consequence of the present findings is discussed with regard to planar-chiral metal−arene complexes.