Iridium aquo complexes

McDaniel ND, Coughlin FJ, Tinker LL, Bernhard S. Cyclometalated iridium(III) aquo complexes efficient and tunable catalysts for the homogeneous oxidation of water. J Am Chem Soc. 2008 130(l) 210-7. 

The chemistry of complexes containing cationic iridium has been studied by Merola and co-workers. Among all the complexes described in the literature, they reported the complex cis,mer-(Me3P)3IrH2Cl, whose aqueous chemistry looks very rich [25]. It resulted from the oxidative addition of H2 to [Ir(cod)(PMe3)3]Cl [26], From XH and 31P NMR measurements, they showed that two different species are formed upon dissolution in water. One species is the cationic aquo complex [mer,cis-(Me3P)3IrH2(OH2)] 17 and the other one is the dinudear cation 18 (Scheme 5). 

Hydrosoluble iridium alkyl complexes may also be prepared by hydrolysis of alkynes and alkenes promoted by water-soluble precursors. The reaction with alkynes follows the well-known mechanism demonstrated by Bianchini et al. for ruthenium complexes.A reasonable mechanism, related to that of hydrolytic breakage, has been proposed by Chin etal. to account for the hydrolysis of ethene promoted by [Cp"lr(TPPMS)Cl2] 344 in the presence of silver salts in water. Scheme 37 describes the Chin s hydrolysis of alkynes, leading to [Cp Ir(TPPMS)(CO)(CH2R)] (69 R=Ph, Bz, Bu, />-Tol) via the aquo complex [Cp Ir(TPPMS)(OH2)OTf 345... 

Chiral amines have been conveniently prepared also by asymmetric reductive amination of ketones using iridium catalysts and intriguing results with up to 96% ee have been obtained by Zhang and co-workers employing a catalytic system based on Ir./-binaphane in the presence of Ti(OPr )4 and iodine (Scheme 61). Water-soluble aquo complexes [Cp lr(H20)3](0Tf)2 494, [CpP Ir(H20)2](0Tf)2 504, and [Cp Ir(bpy)(H20)](0Tf)2 505 have been used to catalyze the reductive amination of hydrosoluble aldehydes and ketones as well as the dehalogenation of alkyl halides. The activity is markedly pH dependent and inactivation of the catalyst takes place reversibly on increasing the solution basicity due to Ir(H20), deprotonation and formation of mono- or dinuclear hydroxo complexes which are catalytically inactive. The structure of one of these compounds, [Cp Ir(bpy)(OH)]OTf 506, which reversibly forms from 494 around pH 6.6, is presented in Figure 42. 

Water-soluble iridium hydrides containing PTA, TPPMS, and TPPTS ligands have been described and a few notes on these compounds have been presented in Section 1.1 of this review." A remarkable hydrosoluble hydride is [Cp Ir(bpy)H] 821 prepared by Ogo and co-workers by controlled protonation of the aquo complex [Cp Ir (bpy)(H20)] 505 with carboxylic acids. ° Compound 821 has been used as a robust and highly active catalyst... 

The search for earth abundant substitutes for ruthenium and iridium is a fundamental target towards WOC sustainability and hydrogen economy. Recently, an increasing interest grew towards cobalt-based electrode coatings and molecular cobalt complexes to promote water oxidation. Cobalt oxide and related aquo or hydroxo complexes have been known as water oxidation catalysts since the early SOs. ... 

Ruthenium, osmium, rhodium, and iridium form octahedral MClj" chloro complexes which, in aqueous solution, usually exist as chloro-aquo mixed species of the general composition [MCl6 ,(H20)J ( c = 1-6). Ruthenium and osmium also form complexes of the type [MCl6 t(H2P),] (jr = 1-6) and oxygen-bridged complexes such as [(RuCl5)20] . In many cases, complexes of the PGM are kinetically inert [4]. 

More recently, systems based on polypyridine coordination compounds of ruthenium(II) [46-49], rhodium(I) [50a] and iridium(I) [50] have been shown to efficiently catalyse the thermal WGSR. An important effect of the substituent ortho to the nitrogen atom of the ligand has been demonstrated in the case of Ir(I) leading to one of the most efficient catalysts known today [50b]. [Ru(bpy)2(CO)Cl] has also been studied and all of the possible intermediates within the catalytic cycle (hydrocarbonyl complex, metal hydride, aquo species) have been isolated and characterized [48]. 

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