Hexairidium clusters supported on gamma-Al2O3: Synthesis, structure, and catalytic activity for toluene hydrogenation

[Ir(CO)(15)](2-) was formed by surface-mediated synthesis on gamma-Al2O3 powder by treatment of adsorbed [Ir(CO)(2)(acac)] in CO at 100 degrees C and 1 atm. The supported clusters were characterized by infrared and extended X-ray absorption fine structure (EXAFS) spectroscopies and by extraction into solution by cation metathesis. Treatment of gamma-Al2O3-supported [Ir-6(CO)(15)](2-) in He at 300 degrees C led to decarbonylation without disruption of the cluster frame, giving Ir-6/gamma-Al2O3, as indicated by EXAFS data determining a first-shell Ir-Ir coordination number of 4.05 +/- 0.07 (where the error bounds represent precision, not accuracy), matching that of the supported [Ir-6(CO)(15)](2-), With a first-shell Ir-Ir coordination number of 4.07 +/- 0.03. The supported Ir-6 clusters were found to be catalytically active for toluene hydrogenation at temperatures in the range of 60-100 degrees C; the turnover frequency at 60 degrees C with a toluene partial pressure of 50 Torr and a H-2 partial pressure of 710 Torr was 1.7 x 10(-3) s(-1). The catalyst was stable in operation in a flow reactor, and consistent with this observation, EXAFS results for the used catalyst indicated that the nuclearity of the supported Ir-6 clusters was essentially unchanged during catalysis. The gamma-Al2O3-supported Ir-6 catalyst is an order of magnitude less active (per total Ir atom) for toluene hydrogenation than a catalyst consisting of small aggregates of iridium (with an average of about 50 atoms each) supported on gamma-Al2O3. Because toluene hydrogenation is known to be a structure-insensitive catalytic reaction, the data suggest that the concept of structure insensitivity in catalysis does not extend to clusters as small as Ir-6. However, the relatively low activity of the Ir-6 clusters may be more an indication of an effect of the support as a ligand changing the electronic properties of the iridium in the cluster than an effect of the cluster size.