Iridium, pure

Iridium on valve metals is suitable if the consumption rate of platinum is too high at elevated temperatures or critical composition of the medium. Mostly platinum-iridium alloys are used with about 30% Ir, because coating valve metals with pure iridium is somewhat complicated. For the same reason, other noble metals such as rhodium cannot be used [21]. At present there is little price difference between platinum and iridium. 

A mixture of the ketone (4.62 g), iridium tetrachloride (1.23 g), trimethyl phosphite (15 ml), propan-2-oI (200 ml) and water (50 ml) is heated under reflux for 21 hr. Much of the solvent is then distilled off ca. 215 ml) and the organic products remaining are isolated by extraction with ether. If reduction is essentially complete, the product at this stage may be sufficiently pure for most preparative purposes. Pure components can be obtained by chromatography over alumina, a representative experiment (on the above scale) gives unchanged ketone (0.13 g), cw-alcohol (4.36 g) and tmns-2 co o (0.16 g) (eluted in this order by pentane, and then by pentane containing ether). 

A recently discovered reduction procedure provides a convenient route to axial alcohols in cyclohexyl derivatives (5). The detailed mechanism of the reaction remains to be elucidated, but undoubtedly the reducing agent is an iridium species containing one or more phosphate groups as ligands. In any case, it is clear that the steric demands of the reducing agent must be extraordinary since the stereochemical outcome of the reaction is so specific. The procedure below is for the preparation of a pure axial alcohol from the ketone. 

Alloys with iridium Iridium alloys with platinum in all proportions, and alloys containing up to about 40% iridium are workable, although considerably harder than pure platinum. The creep resistance of iridium-platinum alloys is better than that of rhodium-platinum alloys at temperatures below 500°C. Their stability at high temperatures, however, is substantially lower, owing to the higher rate of formation of a volatile iridium oxide. 

Rhodium and iridium have a resistance to anodic corrosion comparable with that of platinum, and are more resistant to the influence of alternating currents. A platinum-iridium alloy, in the form of a coating on titanium, is preferred to pure platinum for the production of chlorine from brine , due to its improved corrosion resistance and lower overvoltage. 

A few of the transition metals, including gold, platinum, and iridium, are found in nature as pure elements, but most of the others are found associated with either sulfur or oxygen. Iron, manganese, and the metals of Groups 3 to 6 (except for Mo) are most often found as oxides less often, they occur as sulfates or carbonates. Molybdenum and the metals of Groups 7 to 12 (e.xcept forMn and Fe) are most often found as sulfides. 

Recently we reported EXAFS results on bimetallic clusters of iridium and rhodium, supported on silica and on alumina (15). The components of this system both possess the fee structure in Efie metallic state, as do the components of the platinum-iridium system. The nearest neighbor interatomic distances in metallic iridium and rhodium are not very different (2.714A vs. 2.690A). From the results of the EXAFS measurements, we concluded that the interatomic distances corresponding to the various atomic pairs (i.e., iridium-iridium, rhodium-rhodium, and iridium-rhodium) in the clusters supported on either silica or alumina were equal within experimental error. Since the Interatomic distances of the pure metals differ by only 0.024A, the conclusion is not surprising. 

The number of protons extracted from the film during coloration depends on the width of the potential step under consideration. As can be seen in the formulation of Fig. 26 an additional valence state change occurs at 1.25 Vsce giving rise to another proton extraction. The second proton exchange may explain the observation by Michell et al. [91] who determined a transfer of two electrons (protons) during coloration. Equation (5) is well supported by XPS measurements of the Ir4/ and Ols levels of thick anodic iridium oxide films emersed at different electrode potentials in the bleached and coloured state. Deconyolution of the Ols level of an AIROF into the contribution of oxide (O2-, 529.6 eV) hydroxide, (OH, 531.2 eV) and probably water (533.1 eV) indicates that oxide species are formed during anodization (coloration) on the expense of hydroxide species. The bleached film appears to be pure hydroxide (Fig. 27). 

Cyclohexane dehydrogenates rather rapidly to benzene. Its rearrangement has not been reported over pure metals until now. Cg Ring opening is negligible over platinum and palladium 48, 5i) slight hexane formation was reported over carbon supported rhodium, iridium, and, especially, osmium and ruthenium (702), as well as over nickel on alumina (99). 

The control of enantioselectivity in the reduction of carbonyl compounds provides an opportunity for obtaining the product alcohols in an enantiomerically enriched form. For transfer hydrogenation, such reactions have been dominated by the use of enantiomerically pure ruthenium complexes [33, 34], although Pfaltz and coworkers had shown by 1991 that high levels of enantioselectivity could be obtained using iridium(I) bis-oxazoline complexes [35]. 

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