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 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.

Synthesis, Modeling, and Catalytic Properties of HY Zeolite-Supported Rhodium Dinitrosyl Complexes

“…This angle represents a borderline region for linear and bent NO forms and indicates that the NO ligands of Rh(NO)2/HY30 significantly deviate from a linear geometry. This result is consistent with previous literature reports demonstrating that the NO ligands of Rh(NO)2/ZSM-5 are bent with respect to the linesdetermined by the Rh ion and the two zeolite oxygen atoms bound to it 33.…”

“…These νNO bands can be assigned to the symmetric and asymmetric vibrations of new NO ligands 61 and are similar to those previously observed for ZSM-5-supported Rh(NO)2 dinitrosyl complexes (i.e., at 1862 and 1785 cm -1 ), the structure of which has been confirmed by 14 NO/ 15 NO exchange and DFT calculations. 33 DFT calculations also predict that NO binds 41 kJ/mol stronger than CO to Rh + cations 18,33 Absorbance (a.u.) in acetone solution with CO yields [Rh(PPH3)2(CO)3]ClO4.…”

“…A surface-mediated chemistry approach allows for the preparation of materials that are more structurally uniform. For example, it has been shown that solid-state ion exchange can be used to introduce Rh 3+ cations to a H-ZSM-5 framework and subsequently form Rh + (CO)2 and Rh 2+ (CO)2 complexes upon exposure to CO. 33 Rh 2+ (CO)2(NO) and/or Rh + (NO)2 complexes can be formed from such rhodium dicarbonyl species, although it remains unclear how well these complexes are distributed over the ZSM-5 framework, since no direct structural data have been provided for these materials.…”

Computational Modelling of Nanoporous Materials

“…79 These corrected assignments were later confirmed experimentally. 80 Ivanova et al 81 reported combined experimental IR and computational study of CO, NO, and mixed CO/NO complexes of Rh þ and Rh 2þ located in ZSM-5 zeolite. The binding energy (BE) of CO to Rh þ (193 kJ/mol per molecule) is more than twice larger than to Rh 2þ (91 kJ/mol).…”

“…Taking into account these values, one can easily rationalize why Rh þ (CO) 2 can be transformed into Rh þ (NO) 2 species, as observed experimentally. 81,82 Following the experimental result for formation of mixed complexes of Rh 2þ with CO and NO ligands, such complexes were modelled with one or two NO ligands: Rh 2þ (NO)(CO) 2 and Rh 2þ (NO) 2 (CO) 2 . However, only the former structure was found stable; in it the Rh 2þ (NO)(CO) 2 moiety is located similarly to the zeolite fragment as the Rh 2þ (CO) 2 with the NO ligand coordinated perpendicularly to the plane of the Rh 2þ (CO) 2 complex.…”