Iridium Catalysts

Iridium is a rare precious metal that is expensive on the international market because of its scarce resources. Iridium is one of the densest metals in the known elements and has the highest melting point, high hardness and maximum modulus of elasticity. Moreover, iridium is chemically stable, insoluble in acid, and slightly soluble in aqua regia, chlorine water and molten alkali. In general, noble metal iridium-based catalysts are iridium compounds, noble metal iridium and alloys thereof, and iridium-based supported catalysts. Among them, supported catalysts have the widest application range, that is, noble metal iridium particles are supported on a carrier and fired into a catalyst.

Applications:

As a precious metal, iridium is mainly used in the preparation of hydrazine decomposition catalysts, automobile exhaust gas purification catalysts, and unsaturated hydrocarbon compound hydrogenation catalysts. In recent years, some progress has been made in the study of catalytic oxidation of iridium complexes. Especially in the catalytic oxidation of iridium complexes, many catalytic systems with good catalytic oxidation effects have been found, and they have high stereoselectivity and chemical selectivity. In addition, the use of iridium complexes as catalysts for catalytic cycloaddition has the advantages of high enantioselectivity, short reaction time and high conversion rate. That has important scientific significance and application prospects to synthesis chiral drugs.

• Catalytic alkylation reaction: Some researchers have reported that [Ir(COD)Cl]₂ and chiral bidentate oxazoline can be used as catalysts to catalyze aryl-substituted allyl esters with malonate methyl esters, undergoing an allyl alkylation reaction.

Figure 1. Iridium catalyst catalyzed alkylation reaction.

• Catalytic boronation reaction: The iridium catalyzed aryl C-H bond activation boronization reaction has been rapidly developed, which provides an effective way to synthesize complex target molecules through relatively simple reaction precursors.
• Catalytic arylation reaction: Some researchers used the iridium catalyst system to catalyze arylation reactions. With diaryl high iodide as the aryl source, under the protection of hydrogen, not only the arylation reaction of sp³ carbon-hydrogen bond of nitrogen-containing heterocycle was achieved, but also efficient sp² hydrocarbon bond arylation reaction of different types of substrates was completed. This reaction also has good substrate compatibility and functional group tolerance.
• Catalytic hydrogenation reaction: Some researchers have prepared a supported iridium catalyst by sol-gel method and impregnation method, and studied the catalytic performance of the catalyst for the gas phase catalytic hydrogenation of crotonaldehyde. It was found that the porosity of the catalyst carrier had an effect on the selectivity of the unsaturated alcohol, that was mainly due to the surface modification of the metal compound which enhanced the polarization of the C=O bond.

Production:

The iridium-based catalyst is generally prepared by dipping methods, sol-gel methods, and precipitation deposition methods. In some cases, it is also prepared by ion exchange methods and evaporation methods. The support is immersed in an iridium-containing precursor. The precursor of the noble metal iridium is adsorbed on the surface of the support or in the pores, and then subjected to drying, calcination and reduction treatment to carry the supported iridium catalyst.

References

1. Nguyen P, Blom H P, Westcott S A,(1993). "Synthesis and structures of the first transition-metal tris(boryl)complexes:iridium complexes." Journal of the American Chemical Society, 115(20): 9329-9330.
2. Janssen J P, Helmchen G.(1997). "First enantioselective alkylations of monosubstituted allylic acetates catalyzed by chiral iridium complexes". Tetrahedron letters, 38(46): 8025-8026.