Unusual carbonyl-nitrosyl complexes of Rh²⁺ in Rh-ZSM-5: A combined FTIR spectroscopy and computational study

Adsorption of CO on Rh-ZSM-5 leads to the formation of the well-known Rh⁺ (CO)₂ gem-dicarbonyls, with vibrational frequencies v_s(CO) at 2114 cm^-1 and v_as(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 v_s(CO) at 2181 cm^-1, v_as(CO) at 2153 cm^-1, and v(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 V(¹²CO) at 2168 cm^-1 and V(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 uggests 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 requencies 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.