Iridium production

Prior to 1982, Crabtree s report of the reaction of cyclopentane with a solvated IrH2(PPh3)2+ species to give a cyclopentadienyl-iridium product stood as the only well characterized example of a reaction of an alkane with a homogeneous transition metal, in contrast to the widespread reactivity of arenes [2]. Based upon the instability of the platinum methyl hydride complex Pt(PPh3)2(CH3)H, it was believed that alkane oxidative addition might not be a thermodynamically feasible process, and consequently few attempts were made to attempt such a reaction [3]. It was not until the discovery of the formation of stable alkane oxidative addition products in 1982 that it was realized that reactions of hydrocarbons were in fact feasible. 

The role of "promoters," such as Ru(CO)Jj, in the Cativa process has been clarified by model studies. Addition of substoichiometric amounts of [Ru(CO)jy2 to the anionic methyliridium complex/ac,cis-[Ir(CH3)(CO)jy" accelerated carbonylation by a factor of 20. Added iodide negated the effect of the promotor (Scheme 17.7) Thus, the promoter is concluded to accept iodide from the anionic iridium product from oxidative addition of Mel to form the neutral tricarbonyl species. This tricarbonyl species, lr(CH3)(CO)3l2, undergoes carbonylation 700 times faster than the anionic triiodide. 

Iridium occurs uncombined in nature with platinum and other metals of this family in alluvial deposits. It is recovered as a by-product from the nickel mining industry. 

Commercial metal anodes for the chlorine industry came about after the late 1960s when a series of worldwide patents were awarded (6—8). These were based not on the use of the platinum-group metals (qv) themselves, but on coatings comprised of platinum-group metal oxides or a mixture of these oxides with valve metal oxides, such as titanium oxide (see Platinum-GROUP metals, compounds Titanium compounds). In the case of chlor-alkaH production, the platinum-group metal oxides that proved most appropriate for use as coatings on anodes were those of mthenium and iridium. 

Cyanuric acid can also be prepared from HNCO (100). Isocyanic acid [75-13-8] can be synthesized directiy by oxidation of HCN over a silver catalyst (101) or by reaction of H2, CO, and NO (60—75% yield) over palladium or iridium catalysts at 280—450°C (102). Ammonium cyanate and urea are by-products of the latter reaction. 

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). 

In 1996, consumption in the western world was 14.2 tonnes of rhodium and 3.8 tonnes of iridium. Unquestionably the main uses of rhodium (over 90%) are now catalytic, e.g. for the control of exhaust emissions in the car (automobile) industry and, in the form of phosphine complexes, in hydrogenation and hydroformylation reactions where it is frequently more efficient than the more commonly used cobalt catalysts. Iridium is used in the coating of anodes in chloralkali plant and as a catalyst in the production of acetic acid. It also finds small-scale applications in specialist hard alloys. 

Cases of the S-coordinated rhodium and iridium are quite scarce. To complete the picture, we next consider the possibilities of S-coordination using complicated derivatives of thiophene. 2,5-[Bis(2-diphenylphosphino)ethyl]thiophene is known to contain three potential donor sites, two phosphorus atoms and the sulfur heteroatom, the latter being a rather nucleophilic center (93IC5652). A more typical situation is coordination via the phosphorus sites. It is also observed in the product of the reaction of 2,5-bis[3-(diphenylphosphino)propyl]thiophene (L) with the species obtained after treatment of [(cod)Rh(acac)] with perchloric acid (95IC365). Carbonylation of [Rh(cod)L][C104]) thus prepared yields 237. Decarbonylation of 237 gives a mixture of 238 and the S-coordinated species 239. Complete decarbonylation gives 240, where the heterocycle is -coordinated. The cycle of carbonylation decarbonylation is reversible. 

IV-Methylpyrrole with (Cp IrH3)2 and 3,3-dimethyl-1-butene gives a couple of unique organometallic products, 86 and 87 (990M134). In 86, the C—H bond in position 2 is activated and a rare tiVC) ti (C=C) coordination mode is realized. Species 87 is a zwitterionic compound containing a triple bond between the iridium atoms. 

Complex [(CXI )Ir(/j,-pz)(/i,-SBu )(/j,-Ph2PCH2PPh2)Ir(CO)] reacts with iodine to form 202 (X = I) as the typical iridium(II)-iridium(II) symmetrical species [90ICA(178)179]. The terminal iodide ligands can be readily displaced in reactions with silversalts. Thus, 202 (X = I), upon reaction with silver nitrate, produces 202 (X = ONO2). Complex [(OC)Ir(/i,-pz )(/z-SBu )(/i-Ph2PCH2PPh2)Ir(CO)] reacts with mercury dichloride to form 203, traditionally interpreted as the product of oxidative addition to one iridium atom and simultaneous Lewis acid-base interaction with the other. The rhodium /i-pyrazolato derivative is prepared in a similar way. Unexpectedly, the iridium /z-pyrazolato analog in similar conditions produces mercury(I) chloride and forms the dinuclear complex 204. 

---