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Organoiridium chemistry is thechemistry oforganometallic compounds containing aniridium-carbonchemical bond.[2] Organoiridium compounds are relevant to many important processes including olefinhydrogenation and the industrial synthesis of acetic acid. They are also of great academic interest because of the diversity of the reactions and their relevance to the synthesis of fine chemicals.[3]
Organoiridium compounds share many characteristics with those of rhodium, but less so with cobalt. Iridium can exist inoxidation states of −3 to +5, but iridium(I) and iridium(III) are the more common. iridium(I) compounds (d8 configuration) usually occur with square planar or trigonal bipyramidal geometries, whereas iridium(III) compounds (d6 configuration) typically have an octahedral geometry.[3]
Iridium(0) complexes are binary carbonyls, the principal member beingtetrairidium dodecacarbonyl, Ir4(CO)12. Unlike the related Rh4(CO)12, all CO ligands are terminal in Ir4(CO)12, analogous to the difference between Fe3(CO)12 and Ru3(CO)12.[4]
A well known example isVaska's complex, bis(triphenylphosphine)iridium carbonyl chloride. Although iridium(I) complexes are often usefulhomogeneous catalysts, Vaska' complex is not. Rather it is iconic in the diversity of its reactions. Other common complexes includeIr2Cl2(cyclooctadiene)2,chlorobis(cyclooctene)iridium dimer, The analogue ofWilkinson's catalyst, IrCl(PPh3)3), undergoes orthometalation:
This difference between RhCl(PPh3)3 and IrCl(PPh3)3 reflects the generally greater tendency of iridium to undergooxidative addition. A similar trend is exhibited by RhCl(CO)(PPh3)2 and IrCl(CO)(PPh3)2, only the latter oxidatively adds O2 and H2.[5] The olefin complexes chlorobis(cyclooctene)iridium dimer and cyclooctadiene iridium chloride dimer are often used as sources of "IrCl", exploiting the lability of the alkene ligands or their susceptibility to removal by hydrogenation.Crabtree's catalyst ([Ir(P(C6H11)3)(pyridine)(cyclooctadiene)]PF6) is a versatile homogeneous catalyst forhydrogenation of alkenes.[6]
(η5-Cp)Ir(CO)2 oxidatively adds C-H bonds upon photolytic dissociation of one CO ligand.
As is the case for rhodium(II), iridium(II) is rarely encountered. One example is iridocene, IrCp2.[7] As withrhodocene, iridocene dimerises at room temperature.[8]
Iridium is usually supplied commercially in the Ir(III) and Ir(IV) oxidation states. Important starting reagents being hydratediridium trichloride andammonium hexachloroiridate. These salts are reduced upon treatment with CO, hydrogen, and alkenes. Illustrative is the carbonylation of the trichloride:IrCl3(H2O)x + 3 CO → [Ir(CO)2Cl2]− + CO2 + 2 H+ + Cl− + (x-1) H2O
Many organoiridium(III) compounds are generated frompentamethylcyclopentadienyl iridium dichloride dimer. Many of derivatives feature kinetically inert cyclometalated ligands.[9] Related half-sandwich complexes were central in the development ofC-H activation.[10][11]
Oxidation states greater than III are more common for iridium than rhodium. They typically feature strong-field ligands. One often cited example is oxotrimesityliridium(V).[12]
The dominant application of organoiridium complexes is ascatalyst in theCativa process forcarbonylation ofmethanol to produceacetic acid.[13]
Iridiumcomplexes such as cyclometallated derived from2-phenylpyridines are used asphosphorescent organic light-emitting diodes.[14] Related complexes arephotoredox catalysts.
Iridium complexes are highly active for hydrogenation both directly and viatransfer hydrogenation. The asymmetric versions of these reactions are widely studied.
Many half-sandwich complexes have been investigated as possible anti-cancer drugs. Related complexes are electrocatalysts for the conversion of carbon dioxide to formate.[9][15] In academic laboratories, iridium complexes are widely studied because its complexes promoteC-H activation, but such reactions are not employed in any commercial process.
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