Iridium systems

Iridium has been deposited from chloride-sulphamate and from bromide electrolytes , but coating characteristics have not been fully evaluated. The bromide electrolytes were further developed by Tyrrell for the deposition of a range of binary and some ternary alloys of the platinum metals, but, other than the platinum-iridium system, no commercial exploitation of these processes has yet been made. 

As well as increasing the reaction rate and catalyst stability, at all-important low water concentrations and low CO partial pressures, the iridium system also produces lower levels of by-products. These improvements combine to give the CATIVA process the following advantages ... 

Because of- the similarity in the backscattering properties of platinum and iridium, we were not able to distinguish between neighboring platinum and iridium atoms in the analysis of the EXAFS associated with either component of platinum-iridium alloys or clusters. In this respect, the situation is very different from that for systems like ruthenium-copper, osmium-copper, or rhodium-copper. Therefore, we concentrated on the determination of interatomic distances. To obtain accurate values of interatomic distances, it is necessary to have precise information on phase shifts. For the platinum-iridium system, there is no problem in this regard, since the phase shifts of platinum and iridium are not very different. Hence the uncertainty in the phase shift of a platinum-iridium atom pair is very small. 

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 functional form of this rate expression is consistent with the behavior of the iridium system observed throughout the kinetic investigations. The coordination of nitrile to iridium is anticipated to produce more than a simple inhibitory effect. Being the dominant equilibrium in the mechanism, nitrile coordination may produce the observed first order dependence of the reaction rate with respect to hydrogen. Given Kcn[RCN] is the predominant term in the denominator, the rate expression may be reduced to the form of (8) which is first order with respect to both olefin and [H2]. 

In 1970, the first rhodium-based acetic acid production unit went on stream in Texas City, with an annual capacity of 150 000 tons. Since that time, the Monsanto process has formed the basis for most new capacities such that, in 1991, it was responsible for about 55% of the total acetic acid capacity worldwide. In 1986, B.P. Chemicals acquired the exclusive licensing rights to the Monsanto process, and 10 years later announced its own carbonylation iridium/ruthenium/iodide system [7, 8] (Cativa ). Details of this process, from the viewpoint of its reactivity and mechanism, are provided later in this chapter. A comparison will also be made between the iridium- and rhodium-based processes. Notably, as the iridium system is more stable than its rhodium counterpart, a lower water content can be adopted which, in turn, leads to higher reaction rates, a reduced formation of byproducts, and a better yield on CO. 

Phosphinite pincer iridium systems have also been shown to have a lower tendency to oxidatively add TEE to give (vinyl)(hydride) complexes similar to 3 [18]. While this has been identified as one of the major catalyst deactivation processes in phosphine pincer iridium catalysis, apparently with complexes such as 5, only olefin coordination can occur. However, this is a considerably weaker bonding and is less detrimental to catalyst activity. Eased on steric arguments, product olefin coordination (e.g. COE) is favored over TEE coordination, and therefore at a high TON and high product concentrations the phosphinite catalysts 5 are markedly less active than the phosphine analogues 1. 

The reaction of a 1,10-phenanthroline complex of iridium, [Ir(cod)-(phen)]+, with dioxygen in methanol solution has been studied (38). When the anion for this cationic complex is chloride, no anion-cation interaction occurs, and the iridium system remains four-coordinate. However, when either iodide or thiocyanate is present due to the addition of their sodium salts (or in the presence of added triphenylphos-phine when the anion is chloride), the iridium system becomes five-coordinate because of the interaction between I", SCN", or PPh3 and the iridium center. These five-coordinate systems react more rapidly with dioxygen than did the four-coordinate system at both normal and elevated pressures. An end-on oxidative addition of the dioxygen moiety, with displacement of the , SCN, or PPh3 ligands, was postulated. 

Since 1977 several papers on rhodium and iridium systems relevant to the present work have appeared (10-16). One of these by Vrieze s group (15), which reported partial dehydrogenation of tricyclohexylphosphine coordinated to iridium(I) and rhodium(I), overlapped with some of our studies which were reported almost simultaneously at a conference (17). 

The groups of Felkin, Crabtree" and Tanaka > have dononstrated that alkane ddiydiogenation via oxidative addition is possible (equations 24 and 23). Attack at primary C—41 bonds is fifivored. probably for steric reasons, but the stabilities of the catalysts are not yet sufficient for the reaction to be practically very useful. Tanaka s [RhCl(CO)(PMe3)2]/Av system ako carbonylates alkanes (equations 26 and 27)> has applied the iridium system to more complex alkanes (equation 28). 

Since those initial reports, there have been a number of reports of C-H activation with iridium complexes, induced both thermally andphotochemically. Werner has demonstrated a number of iridium systems with bulky ligands that are capable of adding C-H bonds. [Ir(COD)(PMe3)3]Cl (11) will react with aromatic C-H bonds thermally to yield octahedral iridium(III) complexes of the form mer-(Me3P)3lr(H)(Ar)(Cl) (103).i >i3 ... 

In addition to rhodium-based catalysts, iridium-based eatalysts have also been developed in a process known as the Cativa process. The iridium system follows a cycle similar to the rhodium system in Figure 14-16, beginning with oxidative addition of j CH3I to [Ir(CO)2l2] The first step in the iridium system is much more rapid than in the Monsanto process and the second step is much slower the second step, involving alkyl . migration, is rate determining for the Cativa process. ... 

There is an analogy between the platinum-iridium system and systems such as ruthenium-copper or osmium-copper in the sense that one of the components (platinum or copper) possesses a hydrogenolysis activity that is negligible relative to that of the other (iridium, ruthenium, or osmium). However, the platinum-iridium system differs in the magnitude of the effect of the inactive component on the hydrogenolysis activity of the system. The nature of the interaction between platinum and iridium could reasonably be expected to differ from the interaction between copper and ruthenium or... 

In studies of the platinum-iridium system in the bulk, Raub and Platte (47) have reported a miscibility gap at temperatures lower than about 975°C. At 500°C the gap extends over the composition range from 7 to 99% iridium. Nevertheless, the results presented in Figure 4.23 indicate that it is possible to prepare platinum-iridium catalysts for which X-ray diffraction patterns do not reveal separate lines for platinum-rich and iridium-rich phases, despite the fact that the catalysts have been heated to only 500°C in their preparation. Instead, single diffraction lines are observed. 

In Figure 4.28 phase shift functions are shown for the various possible combinations of absorber and backscattering atoms in the platinum-iridium system (48). For the platinum-iridium pair there are two functions, since there are two different combinations of absorber and backscattering atoms. The two functions are distinguished by using the designation Ptlr when Pt is the absorber atom and IrPt when Ir is the absorber atom. 

A process for the coproduction of acetic anhydride and acetic acid, which has been operated by BP Chemicals since 1988, uses a quaternary ammonium iodide salt in a role similar to that of Lil [8]. Beneficial effects on rhodium-complex-catalyzed methanol carbonylation have also been found for other additives. For example, phosphine oxides such as Ph3PO enable high catalyst rates at low water concentrations without compromising catalyst stability [40—42]. Similarly, iodocarbonyl complexes of ruthenium and osmium (as used to promote iridium systems, Section 3) are found to enhance the activity of a rhodium catalyst at low water concentrations [43,44]. Other compounds reported to have beneficial effects include phosphate salts [45], transition metal halide salts [46], and oxoacids and heteropolyacids and their salts [47]. 

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