Aromatic pi-pi interaction mediated by a metal atom: structure and ionization of the bis(eta(6)-benzene)chromium-benzene cluster.

Aromatic pi-pi interaction in the presence of a metal atom has been investigated experimentally and theoretically with the model system of bis(eta(6)-benzene)chromium-benzene cluster (Cr(Bz)(2)-Bz) in which a free solvating benzene is non-covalently attached to the benzene moiety of Cr(Bz)(2). One-photon mass-analyzed threshold ionization (MATI) spectroscopy and first principles calculations are employed to identify the structure of Cr(Bz)(2)-Bz which adopts the parallel-displaced configuration. The decrease in ionization potential for Cr(Bz)(2)-Bz compared with Cr(Bz)(2), resulting from the increase of the cation-pi stabilization energy upon ionization, is consistent with the parallel-displaced structure of the cluster. Theoretical calculations give the detailed cluster structures with associated energetics, thus revealing the nature of pi-pi-metal or pi-pi-cation interactions at the molecular level.