Iridium clusters

Reactivity studies of organic ligands with mixed-metal clusters have been utilized in an attempt to shed light on the fundamental steps that occur in heterogeneous catalysis (Table VIII), although the correspondence between cluster chemistry and surface-adsorbate interactions is often poor. While some of these studies have been mentioned in Section ll.D., it is useful to revisit them in the context of the catalytic process for which they are models. Shapley and co-workers have examined the solution chemistry of tungsten-iridium clusters in an effort to understand hydrogenolysis of butane. The reaction of excess diphenylacetylene with... 

From results on interatomic distances derived from analysis of EXAFS data, one can draw some conclusions about the structure of platinum-iridium clusters (13,17). If the clusters were truly homogeneous, the interatomic distance characteristic of the platinum EXAFS should be identical to that characteristic of the iridium EXAFS. When we analyze EXAFS data on the clusters, however, we do not find this simple result. We find in general that the distances are not equal. The data indicate that the clusters are not homogeneous in other words,the environments about the platinum and iridium are different. We conclude that the platinum concentrates at the surface or boundary of the clusters. In the case of very highly dispersed platinum-iridium clusters on alumina, the clusters may well have "raft-like" two dimensional structures, with platinum... 

These authors assumed that the lack of reactivity with the triflate complex is caused by strong binding of the triflate to iridium. Clearly, the tetra-arylborate and the tetra-alkoxyaluminate anions induce the highest rate. When more substrate was added after the reaction had completed and was vented with Ar, the reaction resumed at the same rate with catalysts containing one of the last three anions, whereas the PF6-catalyst had lost all activity. The authors also noted a large difference in the sensitivity to added water. Whereas all catalysts lost some activity upon the addition of 0.05% (v/v) water, the PF6 catalyst completely lost activity. The authors had shown previously that the PF6 catalyst easily forms an inactive trimeric hydride-bridged iridium cluster [88], and it does indeed seem likely that the deactivation proceeds via these clusters. 

The iridium cluster [Ir4(CO)i2], which was used successfully as an olefin silylcarbonylation catalyst, also appeared effective in the formation of nitrogen hetero-... 

The first, made by Ichikawa et al. [29], was the evidence that rhodium or iridium cluster carbonyls, when adsorbed on zinc oxide, titania, lanthanum oxides, zirconia or magnesia, could produce quite selectively ethanol by the Fischer-Tropsch synthesis. This was a timely discovery (metallic catalytic particles produced by traditional methods could not reproduce such selectivity) since it came at a period of geopolitical tension after the Kippur war in 1973, which caused the price of crude oil to increase enormously. Therefore, that period was characterized by intense research into selective Fischer-Tropsch catalysis. 

Ir4(CO)i2 reacts with the surface of MgO to generate surface species in which the tetrahedral metal framework is preserved. The structures obtained after decar-bonylation under H2 at 573 K depend on the degree of hydroxylation of the support The iridium cluster nuclearity of 4 was maintained for a low degree of MgO hydroxylation (MgO pretreated at 973 K), but it increased to 6 when the MgO was highly hydroxylated (MgO pretreated at 573 K) [206, 207]. The activity in propane hydrogenolysis of the tailored catalyst is two orders of magnitude less than that of the conventional catalyst at atmospheric pressure and 200 °C. 

The Liix and Ln edge white-line surface area of 5d metals is generally accepted to be a measure for the d-band occupancy, or better the d-band vacancy (28). Small reduced platinum and iridium clusters were reported to be electron deficient compared to the bulk metal (28). For a given particle size the whiteline surface area provides information on the relative degree of oxidation of the metal the higher the white-line surface area the higher the oxidation state of the metal. 

Homonuclear iridium clusters, reactions, 7, 335 Homonuclear ring compounds, with lead, 3, 900 Homopalladium clusters, with high nuclearity, 8, 207... 

Section V,B. The first silicon-iridium cluster, MeSiIr3(CO)9, results from the reaction between NaIr(CO)4 and MeSiCl3 (445). Also, substituted tricobalt clusters of the type MeSi[Co(CO)3]2[M(CO)3(Cp)] (M = Mo, W) have been prepared by heating H2MeSiM(CO)3(Cp) with Co2(CO)8 (455). 

Panjabi, G., Argo, A. M., and Gates, B. C., Supported iridium cluster catalysts for propene hydrogenation Identification by X-ray absorption spectra measured during catalysis. Chem. Eur. J. 5,2417 (1999). 

Xiao, F.-S., Weber, W. A., Alexeev, O., and Gates, B. C., Probing the limits of structure insensitivity Size-dependent catalytic activity of Al203-supported iridium clusters and particles for toluene hydrogenation. Stud Surf. Sci. Catal. 101,1135 (1996). 

Transition metal ions, within the zeolite framework, may undergo a reductive carbonylation to give mononuclear monovalent carbonyl coumpounds M(I)(CO) and ultimatly to give zerovalent polynuclear carbonyl clusters. The rhodium(I)and iridium(i)carbonyIs were identified using spectroscopic and volumetric methods, the zerovalent rhodium and Iridium clusters M (CO)j were also synthetized in the zeolite matrix and their structure investigated using IR, NMR and spin labelling methods. 

Cobalt forms clusters with 5 to at least 16 metal atoms and rhodium forms them with 5 to at least 22 metal atoms. The number of iridium clusters is more limited. 

Alternatively, reduction can occur with or without the isolation of nitrido-clusters. A comparison of the reactions of the hexanuclear rhodium and iridium clusters is shown 45) ... 

Two polyhydrido iridium clusters with chelating diphosphine ligands are prepared by the reaction of [Ir(dppp)(cod)] [BF4] with hydrogen in a methanol solution. These compounds are formulated as [Ir3(dppp)3H7][BF4]2. 1. attd [Irz(dppp)2H5][BF4], 2. Their structures... 

Li F, Gates BC (2003) Synthesis and structural characterization of iridium clusters formed inside and outside the pores of zeolite NaY. J Phys Chem B 107 11589... 

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