Studies in molten chloroaluminates : I. multipass spectroelectrochemistry II. spectroscopic and electrochemical investigations of iridium carbonyls

The multipass technique is introduced as an optical enhancement method for thin-layer spectroelectrochemistry. In this approach, the light beam is redirected through an optically transparent electrode (OTE) several times by an external mirror assembly. This arrangement is achieved using a low power continuum source which allows simultaneous multiwavelength measurements. The gain in optical sensitivity is directly related to the number of passes through the cell and has a practical limit of three to five. Initial evaluation with an aqueous test system yielded results which agree well with theory. The enhancement is not dependent upon electrode reflectivity and, therefore, the method may be applied to studies in highly corrosive media. Studies of the oxidation of sulfur and the reduction of niobium pentachloride in molten chloroaluminates are presented to demonstrate the utility of this technique for investigations in such media. Because of interest in Fischer-Tropsch catalysis by Ir₄(CO)₁₂ and [IrCl(CO)₃]_{n/ in 63/37 AlCl3-NaCl, an investigation of both compounds in the melt under CO and CO-H (1:3) was performed. The techniques applied to this study include UV-visible spectroscopy, Raman spectroscopy, infrared spectroscopy. X-ray photoelectron spectroscopy, cyclic voltammetry, and normal pulse voltammetry. These results indicate that the same mononuclear iridium chlorocarbonyl complex is ultimately produced from Ir4(CO)12 and [IrCl(CO)3]n/ in the melt under CO. The chloride ligands in this complex are probably coordinated to AICI3 as AlCl4~, forming a metal carbonyl-Lewis acid adduct. Studies of Ir4(CO)12 and [IrCl(CO)3]n/ in the 63/37 melt under CO-H2 (1:3) indicate that only a fraction of the original compounds undergoes the same chemistry as under only (X). Infrared spectra suggest the formation of HIr(CO)4 or similar species under CO-H2 (1:3) from both original carbonyls. This complex may be responsible for catalytic activity in the melt. There is no evidence for a long-lasting iridium carbonyl cluster in the 63/37 melt under either CO or CO-H2 (1:3).}