CHARACTERIZATION OF NAY-ENTRAPPED RH6(CO)16 CLUSTER BY FTIR AND EXAFS SPECTROSCOPIES AND THE CATALYTIC BEHAVIOR IN C-13O ISOTOPIC EXCHANGE-REACTION

NaY zeolite entrapped Rh6(CO)16 and the product of its oxidation followed by reduction, [Rh6]red/NaY, have been structurally characterized by EXAFS and FTIR spectroscopies and CO/H2 chemisorption. Rh6(CO)12(μ3-CO)4, [A], in NaY supercages has been synthesized with minor formation of Rh+(CO)2, [B], by carbonylation of Rh3+/NaY at 373-423 K. The coordination of Rh atoms (coordination number and bond length of Rh-Rh and Rh-CO bonds) and the C-O stretching modes of [A] are similar to those of crystalline Rh6(CO)16. Oxidation and successive reduction of [A] result in [Rh6]red/NaY with high dispersion (less than seven Rh atoms). Admission of CO to [Rh6]red/NaY produces a new Rh carbonyl cluster [C], which resembles [A] in terms of the coordination number and bond length of Rh-Rh and Rh-CO bonds and differs from [A] in the C-O stretching modes ([A], v(CO) = 2098 and 1760 cm-1; [C], v(CO) = 2092 and 1830 cm-1. The new carbonyl cluster is suggested to be an isomer of [A], Rh6(CO)12(μ2-CO)4, which is transformed into Rh6(CO)12(μ3-CO)4 at temperatures higher than 423 K. [Rh6]red/NaY chemisorbs CO with the stoichiometry CO/Rh = 2.6, in accordance with that of the Rh6(CO)16 molecule. The carbonyl activity of [A] in isotopic exchange with 13CO(g) is lower than that of [B]. The carbonyl ligands of [A] are found to take part in an intermolecular scrambling reaction with coexisting [B]. The slow exchange reaction of Rh6(CO)12(μ3-CO)4/NaY with CO(g) takes place through intermolecular scrambling between Rh6(CO)12(μ3-CO)4 and Rh+(CO)2 inside NaY. © 1990 American Chemical Society.