The structure and bonding of rhodium dicarbonyl bonded to highly dealuminated zeolite Y has been determined by the combined application of extended X-ray absorption fine structure (EXAFS) and infrared spectroscopies and quantum chemical calculations based on density functional theory. The EXAFS and infrared spectra indicate the existence of nearly unique rhodium dicarbonyl species bonded at structurally equivalent positions in the zeolite pores. However, even this anchored structure, one of the simplest known, is not determined fully by the experimental results, and quantum chemical calculations were needed to eliminate the ambiguity. Taken together, the experimental and theoretical results indicate Rh + (CO) 2 located at a four-ring of the faujasite framework; the rhodium center is bonded to two oxygen centers of the framework near an aluminum center with a Rh-O distance of 2.15-2.20 Å. The results show how spectroscopy and theory used in combination can determine the structure and location of a metal complex anchored to a structurally uniform support.

The interactions of Os4, Os5, and Os5C clusters with various sites of a MgO(001) support were investigated theoretically with the aid of a scalar-relativistic density functional cluster model method. Adsorption geometries of C4v clusters centered above a magnesium cation and the Os atoms oriented either to the nearest surface oxygen anions (A) or between them (B) were considered. The influence of surface Vs and Vs 2-defects on the adsorption of the clusters was also investigated. The calculated base Os-Os distances in supported Os5 and Os5C square-pyramidal clusters are at most 0.1 Å longer (2.5-2.6 Å) than the values calculated for the corresponding free osmium clusters but about 0.4 Å (or more) shorter than the values determined by EXAFS spectroscopy for MgO-powder-supported clusters formed by decarbonylation of [Os5C(CO)14] 2-and shown to retain the Os5C frame. The experimental Os-Os distances characterizing the supported clusters are close to the experimental and calculated bond lengths for coordinatively saturated osmium carbonyl clusters; this result favors the suggestion that the supported clusters characterized by EXAFS spectroscopy were not entirely ligand-free. Calculated interaction strengths of the osmium clusters with the MgO(001) support range from nonbonding (defect-free site B when the basal Os atoms are aligned between the nearest O anions) to very weak (0.6 eV for Os5C at defect-free site A when the basal Os atoms are aligned with the nearest O anions) to weak (∼2 eV for pure Os clusters at defect-free site A) to rather strong (∼9 eV for Vs defect site A). The models reported here are inferred to be too simplified to capture all the pertinent structural details of MgO-powder-supported osmium clusters, but they are sufficient to indicate a significant role of defect sites in the adsorption of supported osmium clusters and, we infer, other transition metal clusters.

Rh 6 (CO) 16 ] was prepared on the surface of TiO 2 (calcined at 200 or 400 °C) by deposition from n-hexane solution and by a surface-mediated synthesis from TiO 2 -supported [Rh(CO) 2 (acac)] in the presence of CO at 1 atm and 100 °C. The cluster preparation and subsequent decarbonylation by treatment in He or H 2 were characterized by infrared and extended X-ray absorption fine structure (EXAFS) spectroscopies. Deposition from solution gave aggregated [Rh 6 (CO) 16 ] on TiO 2 ; removal of the carbonyl ligands led to destruction of the Rh 6 frame and sintering to give rhodium aggregates. In contrast, the reductive carbonylation of TiO 2 -supported [Rh(CO) 2 (acac)] gave site-isolated TiO 2 -supported [Rh 6 (CO) 16 ] in high yield, paralleling the chemistry of rhodium carbonyls in neutral solutions and on neutral surfaces. Removal of the carbonyl ligands from the site-isolated clusters by treatment in H 2 at 300 °C led to rhodium aggregates, but decarbonylation in He at 300 °C gave site-isolated Rh 6 clusters on the TiO 2 . The first-shell Rh-Rh coordination number of these clusters was 4.4 ( 0.4 with a bond distance of 2.64 ( 0.03 Å. Thus, the clusters formed by decarbonylation of site-isolated TiO 2 -supported [Rh 6 (CO) 16 ] are represented as octahedral Rh 6 (which has a Rh-Rh firstshell coordination number of 4). EXAFS spectroscopy indicates that the decarbonylated Rh 6 clusters on TiO 2 calcined at 200 °C have a small amount of carbon bonded to them, but no such ligands were indicated in the spectra of the Rh 6 clusters on TiO 2 calcined at 400 °C.

To contrast the reactivity of supported metal clusters with that of extended metal surfaces, we investigated the reactions of tetrairidium clusters supported on porous gamma-Al2O3 (Ir4/gamma-Al2O3) with propene and with H2. Infrared, 13C NMR, and extended X-ray absorption fine-structure spectroscopy were used to characterize the ligands formed on the clusters. Propene adsorption onto Ir4/gamma-Al2O3 at 298 K gave stable, cluster-bound mu3-propylidyne. Propene adsorbed onto Ir4/gamma-Al2O3 at 138 K reacted at approximately 219 K to form a stable, highly dehydrogenated, cluster-bound hydrocarbon species approximated as CxHy (such as, for example, C3H2 or C2H). H2 reacted with Ir4/gamma-Al2O3 at 298 K, forming ligands (likely hydrides), which prevented subsequent reaction of the clusters with propene to form propylidyne. Propylidyne on Ir4 was stable in helium or H2 as the sample was heated to 523 K, whereupon it reacted with oxygen of the support to give CO. Propylidyne on Ir4 did not undergo isotopic exchange in the presence of D2 at 298 K. In contrast, the literature shows that propylidyne chemisorbed on extended metal surfaces is hydrogenated in the presence of H2 (or D2) and exchanges hydrogen with gaseous D2 at room temperature; in the absence of H2, it decomposes thermally to give hydrocarbon fragments at temperatures much less than 523 K. The striking difference in reactivities of propylidyne on clusters and propylidyne on extended metal surfaces implies the requirement of ensembles of more than the three metal surface atoms bonded to propylidyne in the surface reactions. The results highlight the unique reactivity of small site-isolated metal clusters.

Site-isolated [Ir 4 (CO) 12 ] on the surface of TiO 2 powder (calcined at 200 or 400°C) was prepared by deposition of [Ir 4 (CO) 12 ] from n-hexane solution and, alternatively, by reductive carbonylation of TiO 2 -supported [Ir-(CO) 2 (acac)] in the presence of CO at 1 atm and 100°C. The preparation of the supported clusters and their subsequent decarbonylation by treatment in He or H 2 were characterized by infrared (IR) and extended X-ray absorption fine structure (EXAFS) spectroscopies. The first-shell Ir-Ir coordination number representing the supported iridium carbonyl clusters was found to be 3.0 ( 0.3, with an Ir-Ir bond distance of 2.68 ( 0.03 Å, consistent with X-ray diffraction data characterizing crystalline [Ir 4 (CO) 12 ]. Decarbonylation in He at 300°Cgave TiO 2 -supported clusters retaining the tetrahedral Ir 4 frame of the precursor [Ir 4 (CO) 12 ], with an Ir-Ir first-shell coordination number of 3.0 ( 0.3. In contrast, decarbonylation of TiO 2 -supported [Ir 4 (CO) 12 ] in H 2 at 300°C led to (nonuniform) aggregated iridium clusters with an Ir-Ir first-shell coordination number of 5.0 ( 1.0, a second-shell Ir-Ir coordination number of 2.0 ( 0.5, and a third-shell Ir-Ir coordination number of 7.0 ( 1.0. The chemistry on the TiO 2 surface is consistent with the known solution and surface chemistry of iridium carbonyls. The site-isolated tetrairidium clusters on TiO 2 are among the simplest and best-defined supported metals.

Rhodium carbonyl clusters were prepared on the surface of La 2 O 3 powder (calcined at 673 K) by a surfacemediated synthesis from La 2 O 3 -supported Rh(CO) 2 (acac) in the presence of CO at 1 atm and 373 K. The cluster preparation and subsequent decarbonylation by treatment in He were characterized by infrared (IR) and extended X-ray absorption fine structure (EXAFS) spectroscopies. Treatment in He at 573 K removed the carbonyl ligands, giving site-isolated La 2 O 3 -supported clusters that are well approximated as Rh 6 octahedra, being characterized by a first-shell Rh-Rh coordination number of 3.9 ( 0.4 at a distance of 2.64 ( 0.02 Å. The supported clusters were characterized by IR and EXAFS spectroscopies in the presence of ethene and H 2 reacting catalytically to give ethane. The EXAFS first-shell Rh-Rh coordination number was found to be about 4, consistent with the presence of Rh 6 octahedra, which are inferred to be the catalytically active species. IR spectra show that both hydrocarbons and hydride ligands were present on the working cluster catalyst, including π-bonded ethene and others, inferred to be ethyl, ethylidyne, and di-σ-bonded ethene. The concentration of hydride on Rh 6 increased during the initial induction period in a flow reactor as the catalytic activity increased almost proportionately; hydrides are inferred to be reactive intermediates. 1 H NMR spectroscopy showed that hydride remained on the clusters following catalysis. The results suggest that the hydrogenation of ethene on Rh 6 /La 2 O 3 proceeds by insertion of π-bonded ethene into a Rh-H bond to form ethyl, which is subsequently hydrogenated to give ethane. Rh 6 /La 2 O 3 is about 50 times more active for ethene hydrogenation catalysis than Rh 6 /γ-Al 2 O 3 , and we suggest that the difference is related to the electron-donor properties of the supports.

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