EXAFS characterization of supported metal-complex and metal-cluster catalysts made from organometallic precursors.

Nearly uniform (nearly molecular) supported metals made from molecular organometallic precursors are ideally suited to characterization by EXAFS spectroscopy at the metal edge. Among the most thoroughly investigated mononuclear metal complexes on metal oxide and zeolite supports are MgO-supported rhenium subcarbonyls, approximately Re(CO)₃{OMg}₃ (where the braces denote groups terminating the bulk of the support). These were made, e.g., from [HRe(CO)⁵] and from [H₃Re₃(CO)₁₂]; the Re–O_surface distance determined by EXAFS spectroscopy is 2.15 ± 0.03 Å the support is a tridentate ligand. The Re–O_surface distances in related supported complexes of Groups 7 and 8 metals are all in the range of 2.1–2.2 Å, matching those in molecular analogues. HTa{OSi}₂, prepared from [Ta(CH₂C(CH₃)₃)₃(=CHCCH₃)₃] on SiO₂, catalyzes a new reaction, propane metathesis. Supported complexes made from [HRe(CO)⁵] catalyze alkene hydrogenation but not cyclopropane hydrogenolysis, whereas catalysts made from [H₃Re₃(CO)₁₂] catalyze both these reactions, and EXAFS data indicate neighboring Re centers on the latter (but not the former), which are implicated in the catalysis. EXAFS data similarly indicate supported Mo and W pair sites as catalysts. Supported metal clusters made by decarbonylation of metal carbonyl clusters, e.g., Ir₄/γ-Al₂O₃ and Ir₆/γ-Al₂O₃ or Rh₆/zeolite NaY, are indicated by EXAFS data to be tetrahedra and octahedra, respectively. Such clusters are the species detected by EXAFS spectroscopy at 298 K in the presence of propene and H₂ undergoing catalytic hydrogenation, and they are identified as the catalytically active species. The catalytic activities of the clusters for toluene hydrogenation and alkene hydrogenation are almost unaffected by changes in metal oxide support composition, but they depend on the cluster size, although the catalytic reaction is structure insensitive. Thus, supported metal clusters offer new catalytic properties. [ABSTRACT FROM AUTHOR]