ROTATIONAL CONFORMERS OF GROUP VI (Cr, Mo, and W) METAL BIS(TOLUENE) $SANDWICH$ COMPLEXES

B.S.~Sohnlein, S.~Li, and D.S.~Yang, J.~Chem.~Phys. 123, 214306 (2005); B.S.~Sohnlein and D.S.~Yang, J.~Chem.~Phys. 124, 134305 (2006)S.Y.~Ketkov, H.L.~Sezle, and F.G.N.~Cloke, Angew.~Chem.~Int.~Ed. 46, 7072 (2007) and references therein.Author Institution: Department of Chemistry, University of Kentucky, Lexington, KY 40506-0055Transition metal bis(arene) sandwich complexes may adopt eclipsed or staggered conformations due to the aromatic ring rotations about the metal-arene axis.$^{,}$ In this study, the group VI (Cr, Mo, and W) metal bis(toluene) complexes are synthesized in a laser-ablation molecular beam source, and their rotational conformers are identified by pulsed-field-ionization zero-electron-kinetic-energy (ZEKE) spectroscopy. For Cr-bis(toluene), the ZEKE spectrum shows three distinctive vibrationless (0-0) transitions between the ground electronic states of the neutral and ionic complexes at 42739(5), 42745(5), and 42805(5) cm$^{-1}$, corresponding to ionization energies of $180^irc$, $60^irc$/$120^irc$, and $0^irc$ rotamers. In addition, the spectrum exhibits metal-toluene bending (164, 180, 196, and 223 cm$^{-1}$) and stretching (278 and 291 cm$^{-1}$) frequencies of these rotamers. The ground electronic states of the $0^irc$ and $180^irc$ rotamers are $^{1}$A$_1$ (C$_{2v}$) and $^{1}$A$_g$ (C$_{2h}$) in the neutral form and $^{2}$A$_1$ (C$_{2v}$) and $^{2}$A$_g$ (C$_{2h}$) in the ionized form, respectively. For the $60^irc$ and $120^irc$ rotamers, the ground states of the neutral molecules are $^{1}$A (C$_2$), and those of the corresponding ions are $^{2}$A (C$_2$). Through the variation of the molecular beam conditions, the eclipsed conformer ($0^irc$) is determined to be more stable than the staggered ones ($180^irc$, $120^irc$, and $60^irc$). Similarly, multiple conformers are identified for the Mo and W complexes