Synthesis and Structure of Tetrairidium Clusters on TiO2 Powder:  Characterization by Infrared and Extended X-ray Absorption Fine Structure Spectroscopies

Site-isolated [Ir4(CO)12] on the surface of TiO2 powder (calcined at 200 or 400 °C) was prepared by deposition of [Ir4(CO)12] from n-hexane solution and, alternatively, by reductive carbonylation of TiO2-supported [Ir(CO)2(acac)] in the presence of CO at 1 atm and 100 °C. The preparation of the supported clusters and their subsequent decarbonylation by treatment in He or H2 were characterized by infrared (IR) and extended X-ray absorption fine structure (EXAFS) spectroscopies. The first-shell Ir−Ir coordination number representing the supported iridium carbonyl clusters was found to be 3.0 ± 0.3, with an Ir−Ir bond distance of 2.68 ± 0.03 Å, consistent with X-ray diffraction data characterizing crystalline [Ir4(CO)12]. Decarbonylation in He at 300 °C gave TiO2-supported clusters retaining the tetrahedral Ir4 frame of the precursor [Ir4(CO)12], with an Ir−Ir first-shell coordination number of 3.0 ± 0.3. In contrast, decarbonylation of TiO2-supported [Ir4(CO)12] in H2 at 300 °C led to (nonuniform) aggregated iridium clusters with an Ir−Ir first-shell coordination number of 5.0 ± 1.0, a second-shell Ir−Ir coordination number of 2.0 ± 0.5, and a third-shell Ir−Ir coordination number of 7.0 ± 1.0. The chemistry on the TiO2 surface is consistent with the known solution and surface chemistry of iridium carbonyls. The site-isolated tetrairidium clusters on TiO2 are among the simplest and best-defined supported metals.