Ring deformation and pi-electron redistribution of methylbenzenes induced by metal coordination

Scandium (Sc) complexes of p-xylene, mesitylene, and hexamethylbenzene were produced in a laser-vaporization molecular beam source and studied with pulsed-field-ionization zero-electron-kinetic-energy spectroscopy, and density functional theory. In addition, infrared-ultraviolet resonant two-photon ionization spectra were recorded for Sc(hexamethylbenzene) in the C-H stretching region. Adiabatic ionization energies and several vibrational frequencies of these complexes were obtained from the spectroscopic measurements, and electronic transitions were determined by combining the spectra with the theoretical data. The ionization energies of the three complexes decrease with increasing number of the methyl groups, whereas the metal-ligand stretching frequencies of the p-xylene and mesitylene complexes are essentially the same and slightly smaller than that of the hexamethylbenzene species. Unlike benzene, the arene ring of the methylbenzene molecules is bent and the pi-electrons are localized in a 1,4-diene fashion upon Sc coordination. The distortion of the aromatic ring is due to differential metal binding with the ring carbon atoms in the low-spin ground electronic state.