Aromatic π–π interaction mediated by a metal atom: structure and ionization of the bis(η6-benzene)chromium–benzenecluster

Abstract

Aromatic π–π interaction in the presence of a metal atom has been investigated experimentally and theoretically with the model system of bis(η⁶-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–π 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 π–π–metal or π–π–cation interactions at the molecular level.