Iridium-based catalyst systems

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. 

Because of the importance of olefin metathesis in the industrial production of olefins and polymers, many different catalysts have been developed. Almost all of these are transition metal-derived, some rare exceptions being EtAlCl2 [758], Me4Sn/Al203 [759], and irradiated silica [760]. The majority of catalytic systems are based on tungsten, molybdenum, and rhenium, but titanium-, tantalum-, ruthenium-, osmium-, and iridium-based catalysts have also proven useful for many applications. 

The above-mentioned complexes are the sole iridium derivatives applied to DCR, and the cycloaddition of nitrones to enals or methacrylonitrile, the unique process studied. We think that iridium-based catalysts are underrepresented in 1,3-dipolar cycloaddition chemistry. For example, no iridium (1) systems have been developed to this end. It can be anticipated that the (bidentate ligand)lr(l) fragment could be active (and stereoselective if chiral bidentate ligands are used) in DCR such as those involving azomethine ylides. 

It is worth noting that, contrary to x-olefin polymerisation systems, cycloolefin polymerisation systems are not restricted to dry, oxygen-free reaction conditions and hydrocarbon monomers. For example, iridium-based catalysts are efficient in the polymerisation of exo-bicyclo[2.2.1]hept-5-ene-2-carboxylic acid in aqueous or ethanolic media [48],... 

In addition to rhodium-based catalysts, iridium-based catalysts have also been developed for carbonylation of methanol. The iridium system, known as the Cativa process, follows a cycle similar to the rhodium system in Figure 14.20, beginning with oxidative addition of... 

In the mid-1960s, Paulik and Roth at Monsanto Co discovered that rhodium and an iodide promoter were more efficient than cobalt, with selectivities of 99% and 85%, with regard to methanol and CO, respectively. Moreover, the reaction is operated under significantly milder conditions such as 40-50 bar pressure and around 190 °C [8]. Even though rhodium was 1000 times more costly than cobalt at this time, Monsanto decided to develop the rhodium-based catalyst system mainly for the selectivity concerns, and thus for the reduction of the process cost induced by the acetic acid purification, even if it was necessary to maintain a 14% w/w level of water in the reactor to keep the stability of the rhodium catalyst. In addition, Paulik et al. [9] demonstrated that iridium can also catalyze the carbonylation of methanol although at a lower rate. However, it is noteworthy that the catalytic system is more stable, especially in the low partial pressure zones of the industrial unit. 

It was discovered by Monsanto that methanol carbonylation could be promoted by an iridium/iodide catalyst [1]. However, Monsanto chose to commercialise the rhodium-based process due to its higher activity under the conditions used. Nevertheless, considerable mechanistic studies were conducted into the iridium-catalysed process, revealing a catalytic mechanism with similar key features but some important differences to the rhodium system [60]. 

Monsanto also discovered significant catalytic activity for iridium/iodide catalysts however, they chose to commercialize the rhodium-based process due to its higher activity under conventional high water conditions. Despite this, detailed mechanistic studies by Forster and his colleagues were undertaken at Monsanto and revealed a catalytic mechanism for iridium which is similar to the rhodium system in many respects, but with additional complexity due to participation of both anionic and neutral complexes (see below). 

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. ... 

A range of metals and metal oxides catalyze the reduction of NO. The most successful reducing agent is synthesis gas since the catalytic process is relatively fast 184), unlike the catalyzed decomposition of NO to N2 and O2. The observed nitrogen-containing products depend on the catalytic system used Pd- and Pt-based catalysts convert NO to NH3 184) whereas iridium and ruthenium systems minimize ammonia production and convert nitric oxide to dinitrogen. 

Iridium-based complexes also catalyse reaction 26.12, and the combination of [Ir(CO)2l2] with Ru2(CO)6l2(p-I)2 as a catalyst promoter provides a commercially viable system. 

A recent set of papers describe the use of dpm (bis(diphenylphosphino) methane) ligands to form cluster complexes of rhodium [9,45], iridium [46] and platinum [28,47] that all promote the WGSR under conditions that do not require the addition of acid or base. The earliest report was that of Kubiak and Eisenberg [9] who mentioned that the complex [Rh2(p-H)(p-CO) (CO)2(dpm)2] was an active WGSR catalyst under neutral conditions. In a later paper [45], they report that the catalyst system was most active in the presence of 1 equiv. of toluenesulphonic acid and 2 equiv. of a salt (e.g., LiX, X=C1, Br). Most recently, Sutherland and Cowie [46] have extended the work of Kubiak and Eisenberg through studies of both the original rhodium system and its... 

Methanol carbonylation catalyzed by a combination of iridium-carbonyl compounds and iodide additives was first reported by Monsanto in the 1970s. The mechanism of this process was studied by Forster. ° In the 1990s, BP reported an improved catalyst system based on iridium and iodide that included a "promoter," such as [Ru(CO)jy j. These Ir-based Cativa catalysts are about five times more active than the Rh catalysts, more stable in the presence of low amounts of water (5 wt %), and more soluble. In addition, Lr is usually less expensive than Rh. BP not only built new Cativa plants, but were able to convert existing plants containing rhodium catalysts to plants containing iridium Cativa catalysts because of the similarity of the Ir and Rh systems. 

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