Iridium® complexes oxidative addition reactions

The most interesting work on the isocyanide complexes of the elements in this subgroup has been done with rhodium and iridium. For the most part, the work is involved with the oxidative addition reactions of d square-planar metal complexes. 

In order to get a catalytic cycle it is necessary that the metal sulfide intermediate can react with hydrogen to form the reduced metal complex (or compound) and H2S. For highly electropositive metals (non-noble metals) this is not possible for thermodynamic reasons. The co-ordination chemistry and the oxidative addition reactions that were reported mainly involved metals such as ruthenium, iridium, platinum, and rhodium. 

Many related complexes of iridium and rhodium undergo the oxidative addition reaction of alkanes and arenes [1]. Alkane C-H bond oxidative addition and the reverse reaction is supposed to proceed via the intermediacy of c-alkane metal complexes [4], which might involve several bonding modes, as shown in Figure 19.5 (for an arene the favoured bonding mode is r 2 via the K-electrons). 

The reversibility of the rhodium reaction indicates that the reaction is not as energetically favorable as for the iridium analogues. The rate of addition to Rh(I) is more affected by the nature of the phosphine than for iridium. For these rhodium complexes the addition reaction was autocatalytic, the formation of the acetyl product speeded the reaction. Oxidative addition of C2H5I or C4H9I did not occur. An alternate proposal for the preparation of the acyl product has been offered . 

Iridium. The most important of Ir1 complexes are Irfl s-IrCl(CO)(PPh3)2 and its analogs with other phosphines. These compounds have been much studied as they provide some of the clearest examples of oxidative-addition reactions (Chapter 23, page 772), since the equilibria... 

Ir(PMe3)4]Cl (66) is a very reactive iridium(I) species, which undergoes oxidative addition reactions with a number of substrates. Reactions between (66) and ethylene oxide remarkably lead to the formation of formylmethyl iridium (III) complex (67) (equation 18). ... 

The oxidative addition reactions treated in the next section can in principle be interpreted as acid-base reactions. In the oxidative addition of hydrogen to a square-planar d iridium complex (Eq. 2-30), the transition metal complex acts as an electron-providing metal base, and the substance undergoing addition can be regarded as an acid [10] ... 

The solvent effects on oxidative addition reactions to square planar iridium(I) complexes have drawn some comment. The general acceleration of the addition reactions of hydrogen, oxygen and methyl iodide to trans-[lrX(CO)(FFhQ)2] (where Xis a. halogen) by polar solvents, such as dimethylformamide, is taken by Chock and Halpern to be evidence for a polar transition state. ° Perhaps more interesting is the stereochemical result of the addition of alkyl and hydrogen halides to these iridium(I) complexes. "... 

Several examples of oxidative addition reactions of compounds involving group (iv) elements, silicon and germanium, with iridium(i) complexes have been described and reviewed. In the reaction of silicon hydrides (CH3) (C2H50)3 SiH with bis(bis-l, 2-diphenylphosphinoethane)iridium(i), [Ir (diphos)2],... 

The results of molecular-weight measurements on a freshly prepared solution in benzene sug t a dimeric structure (found M = 88 calcd., M = 89S). In the solid state, [IrCI(C Hi4)2]2 decomposes slowly under the influence of atmospheric moisture. The compound is moderately soluble in benzene, chlorofoim, and carbon tetradiloride, but in general, these solutions are unstable for long periods of time. In comparison with the corresponding rhodium complex, this compound is more reactive in oxidative addition reactions. This u demonstrated by tte formation of iridium hydrides during reaction with hydrogen and hydrogen chloride, respectively. 

Chock and Halpern (28) carried out the first kinetic study of oxidative-addition reactions of the remarkable iridium(I) complexes (III). 

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