Iridium complexes dioxygen

Scheme 15. The reactions of sulfur dioxide with dioxygen complexes of iridium.

Generally, while S02 reacts with dioxygen complexes of iridium, the reverse reaction, in which dioxygen reacts with S02-Ir complexes, is rare (187). However, when the chelating triphos ligand, H3CC(CH2-PPh2)3, is used, as in 73, such a reaction can be observed (189) for both iridium and rhodium. 

The methyl iridium dioxygen complex Ir(CH3)C0(02)[P(p-tolyl)3]2 reacts with added triphenylphosphine to produce triphenylphos-phine oxide (191). That this is a bimolecular reaction was demonstrated both by the complete absence of any oxidation of the tris(para-tolyl)-phosphine and by the lack of any substitution of the bound tris(para-tolyl)phosphine by triphenylphosphine. 

Iridium complexes appear to be less reactive towards carbonyl compounds, although they react with aldehydes " and hexafluoroacetone . This last reaction has been studied kinetically, and the authors find it to be first order in dioxygen complex and in hexafluoroacetone. They conclude the reaction takes place by a direct electrophilic attack on the coordinated dioxygen. This contrasts sharply with the results of Ugo et al. for (Ph3P)2Pt02 the slow substitution reactions of Ir(III) complexes and the absence of vacant coordination sites exclude the coordination of the substrate prior to the attack on coordinated dioxygen, and this may account for the much slower reactions of the iridium complexes. 

The complexes of rhodium and iridium are generally less reactive than those of palladium and platinum no reaction was observed between Rh and Ir dioxygen complexes and CO2, CS2, aldehydes and ketones or between an iridium complex and CO, CO2 and The iridium complexes are not totally inert however, and reactions with... 

Molecular oxygen adducts of transition metal complexes arc of interest and importance to catalytic processes and commercial oxidation processes, as well as being intermediates in oxidation reactions. Vaska " has reviewed the nature of dioxygen bound to transition metal complexes. All known iridium dioxygen complexes possess the peroxo structure (140). Experimental data reveal that the formation of covalent Ir—(O2) bonds on dioxygen addition to IrL, is accompanied by extensive redistribution of electrons, and the electron transfer is from the iridium to dioxygen. SCF-X -SW calculations on [Ir(02)(Ph3)4] and [Ir(Ph3)4] " indicate peroxo -metal bonding. ... 

The vast majority of known dioxygen complexes directly synthesized using 0 contains cobalt, iron, manganese, rhodium or iridium as the... 

Since this discovery, analogous iridium dioxygen complexes have been reported by a number of authors (listed in references 3-7). In all cases the forward reaction was found to follow a bimolecular rate law, equation 20 (see references 5 and 7). 

KINETIC AND THERMODYNAMIC PARAMETERS FOR THE FORMATION OF SOME IRIDIUM DIOXYGEN COMPLEXES ... 

Carbon monoxide is readily oxidized in the coordination sphere of a number of transition metal complexes. In many cases the product of reaction is a carbonate complex which is formed irreversibly, thus precluding the possibility of a catalytic transformation. In Section 5 the reaction between CO and platinum dioxygen complexes was shown to give carbonate complexes. The reaction between iridium, ruthenium and osmium carbonyl complexes and dioxygen to give coordinated carbonate was discussed in Section 6. 

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