Mechanisms of catalytic dehydrogenation of alkanes by rhodium clusters Rhn+ probed by isotope labelling

The regioselectivity for the dehydrogenation of alkanes by rhodium clusters was investigated by reacting Rhn+, n = 1–20, with the isotopically labelled alkanes ethane-1,1,1-d3 and propane-1,1,1,3,3,3-d6. For Rh+ reacting with propane a clear preference for a 1,2- over a 1,1- and 1,3-mechanism was observed. For larger clusters, hydrogen scrambling is faster than hydrogen elimination, which essentially leads to statistical formation of the neutrals H2, HD, and D2. Isotope scrambling with D2 was also used as a structural probe for the reaction products of rhodium clusters with ethane. The intactness of the CH bonds was demonstrated for (n > 6). The studies are completed with a detailed kinetic analysis for the reaction of Rh7+ with ethane and ethane/hydrogen and ethane/helium mixtures. An over-all picture with efficient CH bond activation and fast and reversible hydrogen rearrangements emerges on the basis of these experiments. Some of the dehydrogenation reactions appear to be reversible.