Iridium catalyst

Catalysts prepared from iridium neutral binary carbonyl compounds and several supports have been studied extensively. Small Ir (x = 4, 6) clusters supported on several oxides and caged in zeolite, and their characterization by EXAFS, have been prepared [159, 179, 180, 194-196]. The nuclearity of the resulting metallic clusters has been related with their catalytic behavior in olefin hydrogenation reactions [197]. This reaction is structure insensitive, which means that the rate of the reac-hon does not depend on the size of the metallic particle. Usually, the metallic parhcles are larger than 1 nm and consequently they have bulk-like metallic behavior. However, if the size of the particles is small enough to lose their bulk-like metallic behavior, the rate of the catalytic reaction can depend on the size of the metal cluster frame used as catalyst. 

Early studies of the interaction of lr4(CO)i2 with a silica surface indicate that physisorption of the cluster takes place. Although the cluster can sublime during thermal treatments after impregnation [198], the loss of metal carbonyl can be avoided by mild thermal treatments that produce a redispersion of the physisorbed lr4(CO)i2 onto the silica surface [199]. An XPS and FTIR study of the evolution of physisorbed lr4(CO)i2 under different conditions pointed to the formation of metallic particles by mild thermal decomposition under Ar or H2, with the particle size increasing with increasing temperature [200]. 

It was reported early on that thermal decomposition above 110°C of Ir4(CO)i2 adsorbed on partially hydroxylated alumina gives carbonyl decomposition and renders metal parhcles of iridium below 1 nm [201]. 

The difference in reactivity of metal clusters and metal surfaces has also been well illustrated in these iridium-based systems [205]. A lack of reactivity of alkyli-dyne species on Ir4/y-Al203 with H2 is observed meanwhile, the chemisorption of H2 is not hindered. This behavior contrasts with that of metallic surfaces, which allow the reaction between alkylidyne species and H 2. It is inferred that over metallic clusters the reaction of H2 with alkyklidyne is not allowed because of the lack of available adjacent metal sites, which are necessary for the formation of the intermediates [205]. 

An exhaustive characterization by EXAFS of cluster species obtained from the impregnation of Ir4(CO)i2 on MgO made it possible to determine that the size of the clusters obtained not only depends on the characteristics of the carbonylic complex and those of the oxide but also on the degree of hydroxylation of the support, which is determined by the thermal treatment carried out before adsorption of the complex [206]. 

The mechanism of the iridium-catalyzed hydroformylation has been studied by NMR spectroscopy. Species including iridium acyl and alkyl dihydride intermediates were detected. The results confirmed that the CO-deficient atmosphere favors hydrogenation over carbonylation [76]. 

The hydride complexes IrH(CO)2(xantphos) and IrH3(CO)(xantphos), as well as the propionyl complex Ir(COEt)(CO)2(xantphos) were found to be modest catalysts for the hydroformylation of 1-hexene and styrene under mild conditions. Propionyl dihydride species IrH2(COEt)(CO)(xantphos) was detected by addition of para hydrogen to Ir(COEt)(CO)2(xantphos) [77]. 

---

This reaction is rapidly replacing the former ethylene-based acetaldehyde oxidation route to acetic acid. The Monsanto process employs rhodium and methyl iodide, but soluble cobalt and iridium catalysts also have been found to be effective in the presence of iodide promoters. 

Propionic acid is accessible through the Hquid-phase carbonylation of ethylene over a nickel carbonyl catalyst (104), or via ethylene and formic acid over an iridium catalyst (105). Condensation of propionic acid with formaldehyde over a supported cesium catalyst gives MAA directiy with conversions of 30—40% and selectivities of 80—90% (106,107). Catalyst lifetime can be extended by adding low levels (several ppm) of cesium to the feed stream (108). 

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. 

Disiloxane, tetramesityl-, 3,206 Disproportionation iridium catalysts, 4,1159 Dissolution nuclear fuels, 6, 927 Distannene, 3,217 Distannoxane, 1,3-dichloro-, 3,207 Distibine, tetraphenyl-, 2,1008 Distibines, 2,1008 Disulfido ligands metal complexes, 2,531-540, 553 bonding, 2, 539 electron transfer, 2, 537 intramolecular redox reactions, 2,537 reactions, 2, 537... 

The CATIVA process uses an iridium catalyst promoted by ruthenium... 

Hydrogenation of substrates having a polar multiple C-heteroatom bond such as ketones or aldehydes has attracted significant attention because the alcohols obtained by this hydrogenation are important building blocks. Usually ruthenium, rhodium, and iridium catalysts are used in these reactions [32-36]. Nowadays, it is expected that an iron catalyst is becoming an alternative material to these precious-metal catalysts. 

Figure 5. X-ray absorption spectrum of a silica supported platinum-iridium catalyst at 100 K in the region of the L absorption edges of platinum and iridium. Reproduced with permission from Ref. 13. Copyright 1982, American Institute of Physics.

The addition of terminal acetylenes to imines is an important reaction because of the importance of these products as building blocks. Conventionally, the addition reaction shown in Scheme 5.2 is performed with stoichiometric amounts of butyllithium in a step that is, separate from the subsequent nucleophilic addition reaction (see (b)). Carreira has recently developed a procedure that utilizes an iridium catalyst to effect the addition reaction to a wide range of aldimines and ketimines (see (a)). ... 

The mass balance of the processes (Figure 5.4) shows that the catalytic procedure (Scheme 5.2a) is much more resource efficient than the stoichiometric conversion (Scheme 5.2b). As expected, integrating the synthesis of the iridium catalyst results in an increase of the overall waste production (compare (a) II and (a) III, Figure 5.4). 

The iridium catalyst is very expensive (98.1 Euro for 0.25 g), therefore, the overall price of synthesis by means of the iridium catalyst (Figure 5.5a) is much higher than for the classical reaction (Figure 5.5b). 

As expected initial examination of the hydrogenation of this substrate revealed its relatively low activity compared to dehydroamino acids that provide 3-aryl-a-amino acids. By carrying out the hydrogenation at an elevated temperature, however, the inherent low activity could be overcome. A screen of the Dowpharma catalyst collection at S/C 100 revealed that several rhodium catalysts provided good conversion and enantioselectivity while low activity and selectivity was observed with several ruthenium and iridium catalysts. Examination of rate data identified [(l )-PhanePhos Rh (COD)]Bp4 as the most active catalyst with a rate approximately... 

Table 13.2. Binding Energies (eV) of Supported Iridium Catalysts Reduced at High Temperature.

On the other hand, iridium catalysts give very high selectivity for formation of the primary borane.167 Several other catalysts have been described, including, for example, dimethyltitanocene.168... 

The stereochemistry of reduction by homogeneous catalysts is often controlled by functional groups in the reactant. Delivery of hydrogen occurs cis to a polar functional group. This behavior has been found to be particularly characteristic of an iridium-based catalyst that contains cyclooctadiene, pyridine, and tricyclohexylphosphine as ligands, known as the Crabtree catalyst 6 Homogeneous iridium catalysts have been found to be influenced not only by hydroxy groups, but also by amide, ester, and ether substituents.17... 

Presumably, the stereoselectivity in these cases is the result of coordination of iridium by the functional group. The crucial property required for a catalyst to be stereodirective is that it be able to coordinate with both the directive group and the double bond and still accommodate the metal hydride bonds necessary for hydrogenation. In the iridium catalyst illustrated above, the cyclooctadiene ligand (COD) in the catalysts is released by hydrogenation, permitting coordination of the reactant and reaction with hydrogen. 

Fig. 5.5. Suggested basis of enantioselectivity in hydrogenation of a-methylstilbene by a phosphinoaryl oxazoline-iridium catalyst. Reproduced from Chem. Eur. J., 9, 339 (2003), by permission of Wiley-VCH.

The magnitudes of the rate constants for the iridium catalyst were close to those obtained for rhodium 3 and osmium 5 based catalyst systems at similar conditions. However, the unusual dependence on catalyst concentration affects its general utility in comparison to other homogeneous catalysts for the hydrogenation of NBR. 

According to the proposed mechanism, the hydrogenation of olefin by iridium catalyst should conform to the following rate expression,... 

---