Characterization of CO in Rh₆(CO)₁₆/NaY Clusters and Isolated Rh⁺(CO)₂/NaY by Solid-State ¹³C NMR Spectroscopy

Abstract

Solid-state NMR spectroscopy of 13 C is used to structurally characterize and quantify carbonyl groups in Rh 6 (CO) 16 and Rh + (CO) 2 complexes entrapped in NaY zeolite. MAS NMR line shape simulations show that the dicarbonyl groups present in Rh 6 (CO) 16 and Rh + (CO) 2 have the same shielding parameters (σ iso ∼ 178 ppm, CSA ∼ 400 ppm, and η ∼ 0.2) but exhibit greatly different spin-lattice relaxation rates. Bridged groups exist only in clusters and are bonded to three Rh atoms. Their shielding parameters [σ iso ∼ 251 ppm, CSA ∼ 180 ppm, and negligible anisotropy (η)] differ from bulk Rh 6 (CO) 16 and from those found earlier for CO bridged between two Rh atoms. The Rh 6 :Rh + ratio can be measured directly from the MAS spectra or from the relaxation data. The residual broadening of 150-200 Hz in the MAS spectra is due to distribution of isotropic resonances in the sample. All CO species are immobile at room temperature on the NMR time scale (of the order of a millisecond), whereas slow exchange between dicarbonyls on hexarhodium clusters and isolated rhodium cations was found at 380 K.

Structure and Stability of Rhodium Clusters in NaY Studied by NMR and FTIR

“…Although the linear CO in Rh 6 (CO) 16 and Rh + (CO) 2 are not directly resolved via the isotropic shift, the NMR spin−lattice relaxation times T 1 of these two terminal groups differed by 2 orders of magnitude, which allowed for their quantitative determination. The quantitative analysis yielded the Rh 6 :Rh + ratio of 3:1 for a 4 wt % Rh sample and 1:1 for a 6 wt % Rh sample . The intensity ratio of the linear CO to the bridging CO in the rhodium clusters was found to be 2.9(±0.2):1, in good agreement with the stoichiometry of the cluster molecule.…”

“…Characterization of Rh 6 (CO) 16 /NaY by FTIR and 13 C MAS NMR . Detailed FTIR and 13 C MAS NMR studies of the structure of Rh 6 (CO) 16 /NaY were reported previously. , To facilitate the interpretation of spectroscopic data presented in this work for Rh c /NaY and Rh(imp)/NaY, we first briefly summarize these earlier results.…”

“…Rh loadings of 4 or 6 wt % were used in these samples. A more detailed synthesis procedure was reported elsewhere. , The formation of Rh 6 (CO) 16 was confirmed by FTIR and 13 C MAS NMR.…”

Multiple Modes of Motion: Realizing the Dynamics of CO Adsorbed in M‐MOF‐74 (M = Mg, Zn) by Using Solid‐State NMR Spectroscopy

“…13 C solid‐state nuclear magnetic resonance (SSNMR) experiments have proven to be a valuable probe of CO dynamics, binding strength, and local structure in many compounds. 13 C SSNMR spectroscopy has been used to study the binding affinity and rigidity of CO and CO 2 in copper‐based MOFs,11 along with the local environments, exchange, and binding modes of CO in a rhodium‐exchanged NaY zeolite 12. The adsorption of CO within H‐Y zeolites13 has been probed by 13 C SSNMR spectroscopy, which was also used to understand the role of CO in the carbonylation of alcohols and olefins in the zeolite H‐ZSM‐514 and in the catalytic conversion of methanol to hydrocarbons on unmodified ZSM‐5 15 .…”

Decarbonylation of MgO-Supported PtRh5 Clusters Characterized by Infrared and Extended X-ray Absorption Fine Structure Spectroscopies

“…Because some of the clusters may have been oxidatively fragmented after being adsorbed on the MgO surface (by reaction, e.g., with surface OH groups) to give the rhodium subcarbonyls, , the resultant cationic species would be expected to be in close proximity to each other. Therefore, weak metal−metal interactions might be detected by EXAFS spectroscopy at both the Pt and Rh edges.…”