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 C _2v symmetry with the neutral ground state of ¹A ₁ and the singly positive charged ion state of ²A ₁. The 60 rotamer is in a C_i symmetry with the neutral ground state of ¹A_g and the ion state of ²A_g. 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.