Unusual Carbonyl−Nitrosyl Complexes of Rh2+ in Rh−ZSM-5: A Combined FTIR Spectroscopy and Computational Study

Abstract

Adsorption of CO on Rh−ZSM-5 leads to the formation of the well-known Rh+(CO)2 gem-dicarbonyls, with vibrational frequencies νs(CO) at 2114 cm-1 and νas(CO) at 2048 cm-1, and the Rh2+(CO)2 species (2176 and 2142 cm-1). The dicarbonyl structures have been proven by 12CO−13CO co-adsorption. The Rh2+(CO)2 species are not able to accommodate a third CO ligand even at 100 K, but in the presence of NO, they form Rh2+(CO)2(NO) complexes. These are characterized by νs(CO) at 2181 cm-1, νas(CO) at 2153 cm-1, and ν(NO) at 1890 cm-1. The composition of the species is established using isotopic labeled molecules 13CO and 15NO. In excellent agreement with the values derived with an approximate force-field model, the Rh2+(12CO)(13CO)(NO) species have been found to display ν(12CO) at 2168 cm-1 and ν(13CO) at 2116 cm-1. Co-adsorption of CO and a 14NO−15NO mixture reveals the existence of only one NO ligand in the complex. Complementary density functional modeling of both supported and isolated model complexes suggests th...