HEXANUCLEAR IRIDIUM CLUSTERS SUPPORTED ON MAGNESIUM-OXIDE

[Ir4(CO)12] on the surface of MgO reacts to give [HIr4(CO)11]-, which upon treatment with CO at 200-degrees-C gives [Ir6(CO)15]2- on the surface in high yield. Treatment of the latter with a solution of [PPN][Cl] leads to extraction of [Ir6(CO)15]2- by cation metathesis. The surface-bound [Ir6(CO)15]2- was characterized by extended X-ray absorption fine structure (EXAFS) spectroscopy, with the data indicating an average Ir-Ir coordination number of 4.1 and an average Ir-Ir distance of 2.77 angstrom, in close agreement with the published crystallographic data for this anion in the solid state. The reactions of the iridium carbonyls on the basic MgO surface closely parallel those of iridium carbonyl anions in basic solutions. Treatment of the MgO-supported [Ir6(CO)15]2- in He followed by H-2 at 300-degrees-C gives decarbonylated surface species shown by EXAFS spectroscopy to have iridium cluster frames substantially unchanged from those of the carbonyl anion precursor. The decarbonylated iridium clusters are among the most nearly uniform supported metals.