Dinuclear iridium complexes formation

The most fundamental reaction is the alkylation of benzene with ethene.38,38a-38c Arylation of inactivated alkenes with inactivated arenes proceeds with the aid of a binuclear Ir(m) catalyst, [Ir(/x-acac-0,0,C3)(acac-0,0)(acac-C3)]2, to afford anti-Markovnikov hydroarylation products (Equation (33)). The iridium-catalyzed reaction of benzene with ethene at 180 °G for 3 h gives ethylbenzene (TN = 455, TOF = 0.0421 s 1). The reaction of benzene with propene leads to the formation of /z-propylbenzene and isopropylbenzene in 61% and 39% selectivities (TN = 13, TOF = 0.0110s-1). The catalytic reaction of the dinuclear Ir complex is shown to proceed via the formation of a mononuclear bis-acac-0,0 phenyl-Ir(m) species.388 The interesting aspect is the lack of /3-hydride elimination from the aryliridium intermediates giving the olefinic products. The reaction of substituted arenes with olefins provides a mixture of regioisomers. For example, the reaction of toluene with ethene affords m- and />-isomers in 63% and 37% selectivity, respectively. 

It is tempting to speculate that only for the dinuclear complexes of iridium is there initial formation of a dihydrido-complex, which subsequently reacts with an alkene to form an alkyl intermediate, or with an alkyne to form an alkenyl intermediate. If such is the case, the activity of dinuclear rhodium complexes must depend on initial formation of an alkyne or alkene complex, which would then react with hydrogen. There exists some evidence for such a scheme. The successive hydrogenation of alkynes and alkenes " suggests that activation of an alkene is inhibited by an alkyne, probably by preferential coordination of the latter. Further, complexes (VII, X = H) or (IX) do not alone react with hydrogen, but do so after reaction with an alkyne (acetylene or phenylactylene). ... 

The excited states of dinuclear platinum, rhodium, and iridium complexes with a variety of bridging ligands exhibit unusually diverse reactivity. These types of compound in their lowest triplet state engage in oxidative and reductive electron transfer reactions, and exciplex formation [56], They can also engage in atom transfer reactions i.e. they can abstract hydrogen atoms from a wide range of substrates as well as halogen atoms from alkyl and aryl halides. 

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. 

A few model reactions known from the literature also support the idea of an oxametalla-cyclobutane intermediate such as the formation of the irida(III)oxetane 10 modeling the metal-mediated transfer of oxygen to a coordinated olefin (step B). This reaction is explained by autoxidation of the iridium(I)-cyclo-octadiene complex 9 via the plausible dinuclear oxoiridium(III) intermediate. [15]... 

Photolysis of Fe(CO)5 in the presence of olefins leads to formation of olefin complexes which exhibit catalytic properties. The complex [Fe(cyclooctene)2 ( 0)3] catalyzes isomerization of olefins in alkene solutions. The compound Fe(C2H4)(CO)4 which is formed in the gas phase by photolysis of Fe(CO)5 and ethylene catalyzes hydrogenation of 2114. Photochemistry of the following dinuclear complex of iridium(I) is interesting ... 

The stepwise formation of quasi-octahedral macrocyclic complexes of iridium(lll) was observed for the reactions of [ Cp IrCl2 2] with bidentate ligands (L) such as l,4-diisocyano-2,5-dimethylbenzene a, l,4-diisocyano-2,3,5,6-tetra-methylbenzene b, pyrazine c, or 4,4 -dipyridyl d which gave the corresponding dinuclear complexes [Ir2Cl2(Cp )2(L)] 772, which were converted into tetranuclear complexes [( Cp Ir(jli-Cl) 2)2(L)2](OTf)4 773 on treatment with Ag(OTf). ... 

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