High-Resolution Electron Spectroscopy and Rotational Conformers of Group 6 Metal (Cr, Mo, and W) Bis(mesitylene) Sandwich Complexes

Group 6 metal–bis(mesitylene) sandwich complexes are produced by interactions between the laser-vaporized metal atoms and mesitylene vapor in a pulsed molecular beam source, identified by photoionization time-of-flight mass spectrometry, and studied by pulsed-field ionization zero-electron kinetic energy spectroscopy and density functional theory calculations. Although transition metal–bis(arene) sandwich complexes may adopt eclipsed and staggered conformations, the group 6 metal–bis(mesitylene) complexes are determined to be in the eclipsed form. In this form, rotational conformers with methyl group dihedral angles of 0 and 60° are identified for the Cr complex, whereas the 0° rotamer is observed for the Mo and W species. The 0° rotamer is in a C2v symmetry with the neutral ground state of 1A1 and the singly positive charged ion state of 2A1. The 60° rotamer is in a Ci symmetry with the neutral ground state of 1Ag and the ion state of 2Ag. Partial conversion of the 60 to 0° rotamer is observed from He to He/Ar supersonic expansion for Cr–bis(mesitylene). The unsuccessful observation of the 60° rotamer for the Mo and W complexes is the result of its complete conversion to the 0° rotamer in both He and He/Ar expansions. The adiabatic ionization energies of the 0° rotamers of the three complexes are in the order of Cr–bis(mesitylene) < W–bis(mesitylene) < Mo–bis(mesitylene), which is different from that of the metal atoms. These metal–bis(mesitylene) complexes have lower ionization energies than the corresponding metal–bis(benzene) and −bis(toluene) species.