Magnesium Oxide-supported molecular monometallic and bimetallic clusters: Synthesis, complementary characterization by spectroscopy and high-resolution microscopy and catalytic properties

This dissertation is a report of the synthesis of family of osmium clusters and ruthenium-osmium clusters supported on MgO, their characterization by complementary spectroscopic and high-resolution microscopic methods, and an investigation of initial steps involved in the formation of the supported ruthenium-osmium clusters as well as their testing as catalysts for ethylene hydrogenation.;Supported metal carbonyl clusters approximated as decaosmium carbonyls were prepared by reductive carbonylation of MgO-supported Os3(CO) 12 at 548 K and 1 bar. The samples contained nearly uniform clusters of Os (a heavy metal) on the high-area porous MgO, which consists of light Mg and O atoms. The contrast in the atomic weights of the atoms makes the samples nearly optimum for characterization by scanning transmission electron microscopy (STEM), and the smallness and near uniformity of the clusters make the characterization by extended X-ray absorption fine structure (EXAFS) spectroscopy also nearly optimal. The rms (root-meansquare) radii of the undecarbonylated and partially decarbonylated clusters were found by HAADF-STEM to be 3.11 +/- 0.09 and 3.06 +/- 0.05 A, respectively, and the close agreement between these values is consistent with the inference that the cluster frame was essentially the same in each. The average rms radius of the undecarbonylated clusters (2.94 +/- 0.07 A), agrees closely with rms radii determined from STEM. EXAFS analysis of the data characterizing the partially decarbonylated sample was guided by the STEM data in selection of one of the structural models suggested by EXAFS data.;Supported triosmium clusters were synthesized from Os3(CO) 12 on MgO powder. The structures of supported triosmium clusters were determined by a combination of EXAFS spectroscopy, STEM, aberration-corrected STEM, and conventional STEM. Conventional STEM images demonstrate a high degree of uniformity of the clusters, with average rms radii of 2.03 +/- 0.06 A. The EXAFS coordination number of 2.1 +/- 0.4 confirms the presence of triosmium clusters on average and correspondingly determines an average rms cluster radius of 2.02 +/- 0.04 A. The high-resolution aberration-corrected STEM images show the individual Os atoms in the clusters, confirming the triangular structures of their frames and determining an Os--Os distance of 2.80 +/- 0.14 A, which matches the value of 2.89 +/- 0.06 A determined by EXAFS spectroscopy. The images also demonstrate the individual Os atoms in triosmium clusters atop Mg atoms on the MgO [110] surface, on the basis of which structural models were determined that include the metal--support interface.;The successful approach of adsorption of organometallic cluster precursors to synthesize intact supported clusters was extended to the synthesis of the segregated triosmium and triruthenium clusters on MgO from Os3(CO) 12 and Ru3(CO)12. Timeresolved EXAFS and IR spectra were recorded as the MgO-supported segregated triosmium and triruthenium clusters were treated in flowing hydrogen as the temperature was increased to 423 K. The data demonstrate, first, the decarbonylation of triruthenium clusters starting at 333 K (with the triosmium carbonyl clusters intact). The coordinatively unsaturated reactive ruthenium species at 333 K aggregated substantially so that the average ruthenium cluster was larger than triruthenium (and the EXAFS data provide a basis for an approximate model of the average cluster). When the temperature had been raised to about 358 K, the triosmium clusters underwent decarbonylation, and at approximately 398 K the triosmium clusters lost enough CO ligands to become sufficiently coordinatively unsaturated to allow reaction with Ru atoms of neighboring species. The family of supported bimetallic Ru--Os clusters was tested for ethylene hydrogenation catalysis. The results demonstrate the increase in activity with decarbonylation of supported clusters and increase in size of the Ru--Os clusters.