(157d) Metal Oxide-like Clusters in MOFs As Supports for Well-Defined Rhodium Complex and Rhodium Cluster Catalysts

The nodes of some metal-organic frameworks (MOFs) closely resemble small pieces of metal oxides, exemplified by the Zr₆ nodes of Nu-1000. Hydroxyl groups on the structural vacancies in Nu-1000 consisting of pairs of terminal OH and OH₂ groups have been evidenced by IR spectroscopy and DFT calculations; they are referred to as site 1. We developed a treatment process to convert site 1 to a new proton topology incorporating hydroxyl sites with pairs of terminal OH groups (site 2). A combination of IR spectroscopy and DFT calculations led to the identification of an intermediate structure with organic ligands coordinated to structural vacancies on the node surface which facilitated the conversion of site 1 to site 2. With Rh(C₂H₄)₂(acac) (acac is acetylacetonate) as a probe of the node surfaces, reaction with the hydroxyl groups of site 1 or site 2 of Nu-1000 gave species identified by IR and EXAFS spectroscopies and DFT as mononuclear Rh(C₂H₄)₂ complexes. Information about the nodes as electron-donor ligands was determined in experiments in which the rhodium diethylene complexes were converted to rhodium gem-dicarbonyls by reaction with Co; the ν_CO values characterize the electron-donor properties of the support sites. The catalytic activity and selectivity of the species initially present as Rh(C₂H₄)₂ for ethylene hydrogenation and dimerization were investigated both experimentally and computationally and shown to be strongly influenced by the node sites. The Nu-1000-supported mononuclear rhodium complexes were treated in H₂ to form small supported rhodium clusters, characterized by changes in the IR and EXAFS spectra; the data suggest the formation of approximately Rh₂ clusters on the nodes, and these were found to be stable and 6 times more active for ethylene hydrogenation than the supported single rhodium sites.