Recent Research Developments

Spectroscopic Evidence for the Metal-Mediated Conversion of Ethylene to Benzene: One Nb Atom is Enough!

The spontaneous conversion of simpler to more complex molecules by transition metals plays a crucial role in many industrially and environmentally important catalytic reactions. A combination of experimental and computational studies of the reactivities of metal atoms and clusters in the gas phase can enhance our understanding of the elementary processes occurring in real catalysts, and can ultimately help expedite the rational design of more selective and/or active catalytic materials.

In Professor Doreen Leopold's group, one study of this type focuses on the cyclotrimerization reaction of ethylene (C₂H₄), leading to metal-bound benzene (C₆H₆) upon reaction with three ethylene molecules with H₂ loss. This process is analogous to the well-studied cyclotrimerization of acetylene on bulk metal surfaces. Tantalizing mass-spectrometric evidence for this reaction for small metal clusters and even individual atoms has been reported by other groups in studies of Nb⁺, Nb₂⁺, W⁺, and Fe₄⁺.

In our experiments, atomic niobium and small niobium clusters were produced in a flow tube and reacted with ethylene, and the structures of the mass-selected anionic products were probed using negative ion photoelectron spectroscopy (Figs. 1,2). As is shown in Fig. 3, the anion mass spectrum displays the partially dehydrogenated mononuclear complexes NbC₂H₂^-, NbC₄H₄^-, NbC₆H₆^- and NbC₆H₈^-, as well as completely (e.g., Nb₂C₄^-, Nb₃C_2,4,6^-) or partially (e.g., Nb_2,3C₂H₂^-, Nb_2,3C₄H₄^-) dehydrogenated products incorporating 2 or 3 metal atoms.

The vibrationally-resolved photoelectron spectrum of the NbC₆H₆^- reaction product provides the first direct spectroscopic evidence for the metal-mediated conversion of ethylene to benzene by a single metal atom (or a bare metal cluster). As is shown in Figure 4, obtained by Drs. Tim Marcy and Evan Millam, the spectrum is identical to that observed upon addition of benzene to the flow tube. Comparison of the 4 vibrational frequencies measured in the photoelectron spectrum for the ground and observed excited electronic states of NbC₆H₆ and NbC₆D₆ with vibrational data in the literature for related "sandwich" complexes confirms that the neutral and anionic clusters have the η⁶-benzene structure shown here. (As current Minnesotans, we refer to this as an "open-faced sandwich" complex.

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