Phosphine, iridium complex oxide

Henbest and Mitchell [78] have shown that water can be used as hydrogen source with chloroiridic acid (6) as the catalyst through oxidation of phosphorous acid (59) to phosphoric acid (60) in aqueous 2-propanol. Under these conditions, no hydrogen transfer occurs from 2-propanol. However, iridium complexes with sulfoxide or phosphine ligands show the usual transfer from 2-pro-panol [79-81]. 

In contrast to the Pt(0) and Pt(II) complexes and the corresponding Rh(I) and Rh(III) complexes, the iridium complexes have rarely been employed as hydrosilylation catalysts [1-4]. Iridium-phosphine complexes with d metal configura-tion-forexample, [Ir(CO)Cl(PPh3)2] (Vaska s complex) and [Ir(CO)H(PPh3)3]-were first tested some 40 years ago in the hydrosilylation of olefins. Although they underwent oxidative addition with hydrosilanes (simultaneously to Rh(I) com-... 

The oxidative addition has been most extensively studied on iridium complexes, particularly Vaska s complex. The latter is a square planar complex, frans-L2lrCl(CO), with a d8 electron count containing iridium(I). After the oxidative addition we formally obtain iridium(III), an octahedral complex, with a d6 electron configuration i.e. the 16-electron square-planar complex is converted into an octahedral 18-electron complex. In Fig. 4.26 we have depicted the oxidative addition of methyl iodide to Vaska s complex (L = phosphine) [39]. A large... 

Tetrahydrido(i -pentamethylcyclopentadienyl)iridium was synthesized in 1982, and proved to be a useful precursor for several iridium complexes of varied oxidation state.The tetrahydride and its derivatives are among the very few iridium(V) polyhydrides that do not contain phosphine ligands. No alternative preparation of the title complex has been reported. The key feature of the synthesis described here is the oxidation of the iridium(III) intermediate to the iridium(V) product during work-up of the reaction mixture. ... 

Interestingly, Harrod and co-workers (259) also compared the kinetics of triphenylphosphine and hydrosilane addition to a neutral iridium complex (eq. [89]). The kinetic parameters for the phosphine and silane reaction were almost identical AHp - AHSi = 0 kcal/mol and A5p - ASsi = 13 e.u. This striking similarity is not necessarily proof that phosphine and silane addition are mechanistically analogous processes. However, this kinetic behavior is consistent with the above description of oxidative addition. 

Iridium complexes with triphenylphosphine modified with poly(ethylene oxide)s 14 at one of the phosphine groups were active in two-phase hydrogenation of allylbenzene, although the catalyst activity and selectivity with respect to the hydrogenation product, propylbenzene, was substantially lower than in the case of its low molecular analogue and one of the main products was propenylbenzene. Similar results were obtained for an iridium complex with polyethylene oxide modified with pyridyl groups [53],... 

Although oxidation reactions of this type are generally associated with square-planar four-co-ordinate complexes, there are several examples of oxidative addition to five-co-ordinate species. In these systems, however, the mechanism may also be termed as oxidative elimination since the product is six co-ordinate and one of the ligands is displaced. The mechanism of silicon hydride addition to hydridocarbonyltris(triphenyl-phosphine)iridium(i) has been described, the reaction... 

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