Iridium carbonylation chemistry

Kawi et al. [4, 60] reported on experiments comparing the iridium carbonyl chemistry in NaY zeolite with that in the more basic NaX zeolite (some of whose basicity should perhaps be attributed to the excess NaOH used in the preparation which was ultimately not washed out). The results showed that the supercages in the NaX zeolite were sufficiently basic to provide an effident medium for the synthesis of anionic iridium carbonyl clusters. When [Ir(CO)2(acac)] in NaX zeolite was treated with CO, it was transformed into [HIr4(CO)n] and then into [Irg(CO)t5]. The anionic carbonyl dusters trapped in the cages were characterized by infrared and EXAFS spectroscopies and could not be extracted from the zeolite by ion exchange with bis(triphenylphosphine)iminium chloride, [PPN][Q], in tetrahydrofuran solution. 

Unlike cobalt and rhodium, the chemistry of polynuclear iridium carbonyl derivatives has not been studied in detail (15a). Reduction of Ir4(CO)i2 under carbon monoxide with K2C03 in methanol gives the yellow tetranuclear hydride derivative [Ir4(CO)nH], whereas under nitrogen the brown dianion [Ir8(CO)2o]2- has been isolated as a tetraalkylam-monium salt (97). It has been suggested that the structure of the dianion could result from the linking of two iridium tetrahedra, although its formulation so far is based only on elemental analyses. Clearly such an interesting compound deserves further chemical and structural characterization. 

Zhao, A., and Gates, B. C., Probing metal oxide surface reactivity with adsorbate organometallic chemistry Formation of iridium carbonyl clusters on P-AI2O3, Langmuir 13,4024 (1997). 

In the 1990s, BP re-examined the iridium-catalyzed methanol carbonylation chemistry first discovered by Paulik and Roth and later defined in more detail by Forster [20]. The thrust of this research was to identify an improved methanol carbonylation process using Ir as an alternative to Rh. This re-examination by BP led to the development of a low-water iridium-catalyzed process called Cativa [20]. Several advantages were identified in this process over the Rh-catalyzed high-water Monsanto technology. In particular, the Ir catalyst provides high carbonylation rates at low water concentrations with excellent catalyst stability (less prone to precipitation). The catalyst system does not require high levels of iodide salts to stabilize the catalyst. Fewer by-products are formed, such as propionic acid and acetaldehyde condensation products which can lead to low levels of unsaturated aldehydes and heavy alkyl iodides. Also, CO efficiency is improved. 

The iridium homoscorpionate chemistry continues to be very rich and close to that of rhodium, several papers in the last period reporting analogous [M(Tpx)(L)] (M = Rh or Ir) complexes. Hydride-,433,434 phosphino-,435,436 carbonyl-437,438 and diene-Ir439-442 complexes and their reactivity were widely reported in the first edition also. 

This chapter will not deal with iridium complexes in which the coordination chemistry of the iridium-carbon bond is implicated inasmuch as Leigh and Richards have very recently (1982) provided an excellent detailed review on such compoimds organoiridium and iridium carbonyl complexes were also previously reviewed. Iridium complex chemistry has been reviewed (1980) along with rhodium," and in annual reviews. Additionally, iridium complexes have been treated in Comprehensive Inorganic Chemistry . ... 

The chemistry of anionic Iridium carbonyl clusters in NaX zeolite parallels that in basic solutions and on the basic MgO surface (Fig. 4-9). In basic solutions, the reductive carbonylation of [Ir4(CO)i2] with KOH in methanol under CO initially gives [HIr4(CO) ]-, [61, 67] then [Ir8(CO)22] , [62, 67] and finally [Ir6(CO),5]. ... 

Figure 4-8. Synthesis of iridium carbonyl clusters in neutral solutions and on the nearly neutral surface of amorphous y-AljOs. The chemistry is very similar to that occurring in the cages of NaY zeolite (Fig. 4-7). [3, 5] Whereas the clusters can be readily extracted from the surface of y-AljOj, under the same conditions they cannot be extracted from the zeolite because they are too large to fit through the cage windows and are thus trapped in the supercages.

The chemistry assodated with the recarbonylation of these iridium dusters in zeolite NaY is partially understood. When the decarbonylated clusters are brought in contact with CO at liquid nitrogen temperature, iridium carbonyl dusters of poorly defined structure are formed. As the sample is then heated in the presence of CO, the clusters fragment to give mononudear complexes (iridium subcarbonyls) which are converted into [Ir4(CO)i2] and then into [Ir8(CO)xJ. 

The use of the cobalt triad carbonyls as catalysts continues to provide many papers for this report. Publications cover the silylformylation of 1-Hexyne catalyzed by diodium-cobalt carbonyl clusters the formation of hydroxycarbene cobalt carbonyl derivatives, the use of rhodium cluster carbonyls in the water-gas shift reaction Rh4(CO) 2> and Co3Rh(CO)] 2 catalysts for the hydrosilation of isoprene, cyclohexanone and cyclohexenone catalytic reduction of NO by CO and the carbonylation of unsaturated compounds The chemistry of iridium carbonyl cluster complexes has been extended by making use of capping reactions with HgCl2and Au(PPh3)Q... 

The dimeric complexes [M2(CO)io] (M = Mn or Re) continue to be actively studied. The presence of 17-electron free radicals, [M(CO)s], as intermediates in the thermal substitution reactions, and the chemistry of these and other 17-electron complexes are the questions being addressed. Substitution reactions of cobalt and iridium carbonyl clusters, with an attempt to define and separate electronic and steric effects, has also been an especially active area. This chemistry is discussed in Section 10.1.4. 

The Ti coordination via the carbocycle prevails for indole and carbazole, although the species were also found in organomanganese and -iridium chemistry. Osmium carbonyls tend to produce the species with the bridging indole function. Some illustrations of the ti N) coordination exist. 

The rate of the methanol carbonylation reaction in the presence of iridium catalysts is very similar to that observed in the presence of rhodium catalysts under comparable conditions (29). This is perhaps initially surprising in view of the well-recognized greater nucleophilicity of iridium(I) complexes as compared to their rhodium(I) analogues. It can be seen from the above studies that the difference in the chemistry of the metals at the trivalent stage of the catalytic cycle serves to produce faster rates of alkyl migration with the rhodium system thus, overall the two metal catalysts give comparable rates. 

It is now nearly 40 years since the introduction by Monsanto of a rhodium-catalysed process for the production of acetic acid by carbonylation of methanol [1]. The so-called Monsanto process became the dominant method for manufacture of acetic acid and is one of the most successful examples of the commercial application of homogeneous catalysis. The rhodium-catalysed process was preceded by a cobalt-based system developed by BASF [2,3], which suffered from significantly lower selectivity and the necessity for much harsher conditions of temperature and pressure. Although the rhodium-catalysed system has much better activity and selectivity, the search has continued in recent years for new catalysts which improve efficiency even further. The strategies employed have involved either modifications to the rhodium-based system or the replacement of rhodium by another metal, in particular iridium. This chapter will describe some of the important recent advances in both rhodium- and iridium-catalysed methanol carbonylation. Particular emphasis will be placed on the fundamental organometallic chemistry and mechanistic understanding of these processes. 

Scheme 3.1 Anionic and neutral cycles proposed by Forster for iridium catalysed methanol carbonylation and WGS reaction (adapted from Ref [59] by permission of The Royal Society of Chemistry).

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