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 investigate d by means of density functional theory and Hartree-Fock calculations inclu ding electron correlation. On the basis of these results, a new kind of deg enerate 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 proc ess, which is predicted to take place in the systems Al(C6H6)(+), Al(C4H4O) (+), Al(C5H6)(+), and Al(C4H4NH)(+). The energies of the corresponding tran sition states (TSrot) relative to the dissociation products amount to -5 to -14 kcal/mol at 0 K. Concerning Al(C5H5N)(+), a respective TSrot could not be detected. The aluminum-pyridine complex dissociates instead of a contra -binding rotation. With increasing temperature, the change in free energy o f the contra-binding rotation process increases. Under standard conditions (298.15 K and 1013.25 mbar), the G2MP2-calculated Delta G of the contra-bin ding rotation amount to +0.6 to +0.9 kcal/mol (L = C4H4O), -2.8 kcal/mol (L = C6H6), -1.4 to -4.8 kcal/mol (L = C5H6), and -5.5 to -7.3 kcal/mol (L = C4H4NH). A consequence of the present findings is discussed with regard to planar-chiral metal-arene complexes.