Iridium N-heterocyclic carbene complexe

Iridium N-Heterocyclic Carbene Complexes and Their Application as Homogeneous Catalysts... 

The N-heterocyclic carbene complexes of cobalt have been less studied than their heavier analogues, rhodium and iridium, even though NHC-cobalt complexes have been known for more than 3 decades. Despite the fact that their... 

New N-heterocyclic carbene rhodium and iridium complexes derived from 2,2 -diaminobiphenyl were successfully synthesized and their structures unambiguously characterized by X-ray diffraction (XRD) analysis. These are cata-lytically active for the hydrosilylation of ketones with diphenylsilane, although an NHC—rhodium complex was found to be the best among those investigated [45]. 

Iridium(l) precursors [lr(cod)(L)] with bidentate N-heterocyclic carbene ligands L3 appeared slightly less active in the hydrosilylation of acetophenone with diphe-nylsilane than did the similar rhodium complexes, giving respectively yields of 85% of I and 15% of II for the Pr substituent, and 83% of I and 17% of II for the benzyl moiety, after 2 h reaction at room temperature [47]. However, when carbene ligands of type L3 were used a significant increase in the ee-value of the sec-phenethyl alcohol R isomer of up to 60% was observed. 

In a manner similar to platinum ones, rhodium—carbene complexes have been recently tested in the hydrosilylation of alkenes and alkynes (3). Especially, rhodium complexes with N-heterocyclic carbene (NHC) ligands have attracted considerable attention. Their performance is comparable with that of phosphine complexes. The following exemplary ligands were used 1,3-imidazoylidene chelate bis(imidazolinum-carbene) phosphine-functionalized NHC alkylammonium-imidazolium chloride salts NHC pincer complexes, pyridine-functionalized N-heterocyclic carbenes (also with iridium), and bis(dichloroimidazolylidene) (also with iridium). 

Several catalytic systems including iridium hydride, oxo-phosphoranyl, N-heterocyclic carbene, cyclooctadiene, and dimeric cyclopentadienyl complexes have been successfully employed in alkyne hydrosilylation to afford selectively j0-(Z)-alkenylsilanes in high 3delds (4). 

A tetracyclic bis(imidazolidine), containing two fused diazepine rings, was prepared and served as a building block for formation of iridium- and palladium-N-heterocyclic carbene (NHC) complexes (130M6445). The tetracyclic system 95 was formed via condensation of 1,4-diaminobutane 92, glyoxal 93, and formaldehyde 94.The reaction of 95 with [IrCl(COD)]2 in the presence of excess 1,5-cyclooctadiene led directly to the iridium-NHC complex 96.The palladium-NHC complex 97 was accessed via oxidation of bis(imidazolidine) 95 with N-bromosuccinimide and reaction with Pd(OAc)2 in the presence of KL... 

Xu S, Manna K, Ellem A, Sadow AD. Mixed N-heterocyclic carbene-bis(oxazolinyl) borato rhodium and iridium complexes in photochemical and thermal oxidative addition reactions. OrganometaUics. 2014 33 6840-6860. 

Iridium and ruthenium complexes of N-heterocyclic carbenes are particularly sensitive to decomposition by C-H bond activation of the substituents on nitrogen. As described in Section 3.5.2, this reaction can be used productively if reversible. However, it often leads to degradation and loss of catalytic activity. 

Alternatively, they were also found to be good racemization catalysts. Iridium complexes, but also rhodium compounds, catalyzed the racemization step in the enzymatic dynamic kinetic resolution of secondary alcohols, and excellent results were reported for alkyl-aryl as well as dialkyl secondary alcohols. Finally, picolyl and pyridine functionalized N-heterocyclic carbene iridium complexes [(C N)Ir(Cp )Cl]Cl were moderately active catalysts for the polymerization of norbomene in the presence of methylaluminoxane as cocatalyst. ... 

Attempts to coordinate iridium by using the tripodal N-heterocyclic carbene ligand [l,l,l-tris(3-butylimidazolium-l-yl)methyl]ethane trichloride (TlMEHs ") failed as only monocoordinate [(COD)lr(TIMEH )](PF6)2 or dicoordinate [(COD)2lr2(TlME )Cl]Cl 558 complexes were obtained. ... 

Kuo H-Y, Liu Y-H, Peng S-M, Liu S-T. N,N -Dialkylation catalyzed by bimetallic iridium complexes containing a saturated bis-N-heterocyclic carbene (NHC) ligand. Organometallics. 2012 31 7248-7255. 

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