TETRAIRIDIUM CLUSTERS SUPPORTED ON GAMMA-AL2O3 - FORMATION FROM [IR4(CO)12] AND CO-INDUCED MORPHOLOGY CHANGES

[Ir4(CO)12] was prepared on the surface of gamma-Al2O3 powder by a surface-mediated synthesis from supported [Ir(CO)2(acac)] in the presence of CO. Supported iridium clusters with nearly unique structures were prepared by decarbonylation of the iridium carbonyl clusters at 300-degrees-C in He. The cluster formation and decarbonylation were characterized by infrared and extended X-ray absorption fine structure spectroscopies. As the sample was heated in He to about 75-degrees-C, the infrared spectrum changed to resemble that of [HIr4(CO)11]-, which could be extracted from the surface by cation metathesis. The decarbonylation of this cluster took place without changes in the frequencies of the infrared bands in the carbonyl region, consistent with the suggestion that no other iridium carbonyls formed. The decarbonylated clusters in the presence of He had an average Ir-Ir coordination number of 2.9 with a bond distance of 2.69 angstrom, indicating that the tetrahedral metal frame of the precursor cluster carbonyl had been largely retained after decarbonylation. Thus, the supported cluster is represented as Ir4 with a tetrahedral structure and is inferred to be one of the simplest and most uniform supported metals. When this sample was recarbonylated at 125-degrees-C, neither [Ir4(CO)12] nor [HIr4(CO)11]- was reformed. Instead, the recarbonylated clusters had an average Ir-Ir coordination number of 4.1 with an average bond distance of 2.70 angstrom, indicating cluster agglomeration.