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

• Published in: Journal of physical chemistry. C Vol. 111; no. 28; pp. 10412 - 10418
• Main Authors: Ivanova, Elena, Mihaylov, Mihail, Aleksandrov, Hristiyan A, Daturi, Marco, Thibault-Starzyk, Frederic, Vayssilov, Georgi N, Rösch, Notker, Hadjiivanov, Konstantin I
• Format: Journal Article
• Language: English
• Published: American Chemical Society 19-07-2007
• Subjects: Catalysis; Chemical Sciences
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/jp067531f

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 the dicarbonyl complexes of Rh+ and Rh2+ to be planar and bound close to Al centers of the zeolite framework. While the location of the dinitrosyl complex of Rh+ in the zeolite framework is similar to that of dicarbonyls, the Rh2+(CO)2(NO) complex is considerably distorted in this position, and the simulated N−O frequencies differ strongly from the experimental ones. The vibrational frequencies of the ligands in the mixed complex are determined to be similar to the experimental values only when the complex is bound to one zeolite oxygen center or when the complex is isolated inside the zeolite cavity. On this basis, it is concluded that insertion of a NO molecule in the Rh2+(CO)2 complex is accompanied by the breaking of one or two bonds between the Rh2+ cation and zeolite oxygen atoms.