Supported molecular rhodium complexes and clusters: Synthesis, characterization, and catalysis

Site-isolated supported metal catalysts, ranging from mononuclear metal complexes to metal nanoclusters supported on oxides and zeolites, incorporate surface catalytic groups that may be close analogues of molecular complexes in solution. Supported metal complexes and clusters form a class of catalysts that are important in technology and in science.;The goal of fundamental understanding motivated the preparation of supported metal complexes that are structurally unique. The objectives of the work were to synthesize supported metal complexes with a high degree of structural uniformity and to determine the effect of supports, ligands, and cluster nuclearity on the reactivities and catalytic properties. The techniques used to characterize the samples included infrared (IR), nuclear magnetic resonance (NMR), extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge (XANES) spectroscopies, and density functional theory (DFT), with the NMR and DFT work carried out by colleagues in other laboratories.;Using complementary spectroscopic techniques, we have characterized the site-isolated supported rhodium complexes, prepared from organometallic precursor Rh(C2H4)2(acac) and supports with crystalline structure (DAY zeolite), with a focus on the reactivity of the ethylene ligands. The catalyst is active for ethylene hydrogenation at room temperature and is a close analogue for the Wilkinson catalyst for hydrogenation in homogeneous phase chemistry. The supported rhodium catalyst is also active for propene hydrogenation as well as acetylene cyclotrimerization. Evidence of intermediates in the catalytic cycle of cyclotrimerization was observed when the working catalyst was characterized by NMR spectroscopy. The DFT computational results were used to elucidate the catalytic cycle (including transition states), confirmed results observed in the NMR spectra.;Reactivities of the zeolite-supported rhodium complexes in the presence of H2, H2 + N2, C2H4, and CO, were investigated with IR, NMR, and EXAFS spectroscopies. IR spectra of the surface species have been compared with spectra of known compounds to identify these species; the results confirm that ethylene ligands on the rhodium are highly reactive and readily exchange with other ligands, such as isotopically labeled ethylene and CO.;Complementary transient spectroscopic methods were used to investigate the processes of cluster formation and breakup in real time. These methods provide characterization of the changes in the bonds between the metal atoms and the various ligands, including the support.