New types of nonclassical iridium carbonyls formed in Ir-ZSM-5: a Fourier transform infrared spectroscopy investigation.
In this work we report some new nonclassical carbonyls of iridium formed after CO adsorption on Ir-ZSM-5 (Ir-MFI). Mainly Ir+ cations were found on sample activated at 523 K and reduced by CO at the same temperature. With CO they formed Ir+(CO)2 gem-dicarbonyls (2104 and 2033 cm(-1)) that decomposed at 673 K without leaving a measurable fraction of monocarbonyls. The dicarbonyl structure was established by 12CO-13CO coadsortpion experiments. In the presence of CO in the gaseous phase and at ambient temperature the Ir+(CO)2 dicarbonyls were converted into Ir+(CO)3 species (2182, 2099, and 2074 cm(-1)). At 100 K these complexes are able to accommodate a fourth CO molecule thus producing tetracarbonyls (2155, 2145, 2125, and 2105 cm(-1)). The results are explained by the high coordinative unsaturation of the Ir+ cations in the ZSM-5 matrix. This is also the reason for the formation of mixed Ir+(H2O)(CO)2 species after CO-H2O coadsorption (2087 and 2015 cm(-1)). Evacuation of the sample at 673 K, followed by treatment with CO at 523 K, generates Ir2+ cations. With CO these cations form another kind of geminal complex, namely, Ir2+(CO)2 species (2173 and 2129 cm(-1)). Here again, the structure was confirmed by 12CO-13CO coadsortpion experiments. These dicarbonyls are decomposed at 573 K (again without producing monocarbonyls) and are able to accommodate additionally neither CO nor water molecules. The results are explained by the smaller cationic radius of Ir2+ (as compared to Ir+), which is associated with a decrease of the number of ligands required for coordinative saturation.