Molecular intracage chemistry: [Rh₆(CO)₁₅]^2- in zeolite NaX

[Rh₆(CO)₁₅]^2- was made by ship-in-a-bottle synthesis in the cages of zeolite NaX by carbonylation of sorbed [Rh(CO)₂(acac)] at 125°C and was identified by ¹³C NMR, infrared, and extended X-ray absorption fine structure (EXAFS) spectra. The ¹³C NMR data indicate yields of [Rh₆(CO)₁₅]^2- up to about 83 % in the cages. The synthesis chemistry is analogous to that in basic solutions. The NMR and infrared results indicate that, at room temperature, cluster-zeolite interactions are strong and CO intraexchange negligible, whereas at 80°C these interactions are weak, allowing rapid intraexchange of CO ligands on the metal frame. Decarbonylation in H₂ at 200 °C gave partially decarbonylated clusters with a Rh-Rh first-shell coordination number match-ing that of [Rh₆(CO)₁₅]^2-, indicating that the metal frame remained nearly intact; decarbonylation at higher temperatures gave aggregated rhodium. The partially decarbonylated clusters formed in H₂ at 200°C (or rhodium subcarbonyl fragments formed by oxidation in air at 25°C) were reversibly recarbonylated to give [Rh₆(CO)₁₅]^2-. Treatment in He at 200-300°C gave fully decarbonylated clusters with a Rh-Rh first-shell coordination number of about 2.6, suggesting that the cluster frames were intact but slightly flattened or fragmented. The decarbonylated rhodium clusters and aggregates catalyze toluene hydrogenation at temperatures of 80-12O°C. All this molecular chemistry is inferred to have taken place in isolated zeolite cages.