EVIDENCE FOR LOW-PRESSURE CATALYSIS IN THE GAS-PHASE BY A NAKED METAL CLUSTER - THE GROWTH OF BENZENE PRECURSORS ON FE4+

Chemical reactions of thermalized bare Fe(n)+ cluster ions (n = 2-13) with ethylene and cyclopropane were studied in a FT-ICR mass spectrometer. The reactivities of Fe(n)+ cluster ions toward both gases show a pronounced size effect. With ethylene, only Fe4+ and Fe5+ react, whereas all the other cluster ions are completely unreactive under the same experimental conditions. In the case of cyclopropane, reactions are observed with Fe3+, Fe4+, and Fe5+. With both gases, Fe4+ forms reaction products of stoichiometry Fe4C6H6+. Low-energy collision-induced dissociation (CID) of these reaction products suggests that the carbon and hydrogen atoms on the Fe4+ nucleus are arranged in such a manner that all 12 atoms are detached simultaneously. Furthermore, comparison with the results from CID experiments on the precursors of the Fe4C6H6+ cluster ions reveals that a dramatic bond weakening of the carbon-cluster bonds occurs when the C6H6 stoichiometry is completed on Fe4+. This leads to the assumption that benzene is detached. Apparently, Fe4+ can act as an active center in a catalytic cycle which yields hydrogen and, by CID, benzene from ethylene and from cyclopropane.