Reactivity of site-isolated metal clusters:: Propylidyne on γ-Al2O3-supported Ir4

To contrast the reactivity of supported metal clusters with that of extended metal surfaces, we investigated the reactions of tetrairidium clusters supported on porous gamma -Al2O3 (Ir-4/gamma Al2O3) with propene and with H-2. Infrared, C-13 NMR, and extended X-ray absorption fine-structure spectroscopy were used to characterize the ligands formed on the clusters. Propene adsorption onto Ir-4/gamma -Al2O3 at 298 K gave stable, cluster-bound mu (3)-propylidyne. Propene adsorbed onto Ir-4/gamma -Al2O3 at 138 K reacted at approximately 219 K to form a stable, highly dehydrogenated, cluster-bound hydrocarbon species approximated as CxHy (such as, for example, C3H2 or C2H). 112 reacted with Ir-4/gamma -Al2O3 at 298 K, forming ligands (likely hydrides), which prevented subsequent reaction of the clusters with propene to form propylidyne. Propylidyne on Ir-4 was Stable in helium or H-2 as the sample was heated to 523 K, whereupon it reacted with oxygen of the support to give CO. Propylidyne on Ir-4 did not undergo isotopic exchange in the presence of D-2 at 298 K. In contrast, the literature shows that propylidyne chemisorbed on extended metal surfaces is hydrogenated in the presence of H-2 (or D-2) and exchanges hydrogen with gaseous D2 at room temperature; in the absence of Hz, it decomposes thermally to give hydrocarbon fragments at temperatures much less than 523 K. The striking difference in reactivities of propylidyne on clusters and propylidyne on extended metal surfaces implies the requirement of ensembles of more than the three metal surface atoms bonded to propylidyne in the surface reactions. The results highlight the unique reactivity of small site-isolated metal clusters.