Dioxygen complexes of iridium

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 reaction of a 1,10-phenanthroline complex of iridium, [Ir(cod)-(phen)]+, with dioxygen in methanol solution has been studied (38). When the anion for this cationic complex is chloride, no anion-cation interaction occurs, and the iridium system remains four-coordinate. However, when either iodide or thiocyanate is present due to the addition of their sodium salts (or in the presence of added triphenylphos-phine when the anion is chloride), the iridium system becomes five-coordinate because of the interaction between I", SCN", or PPh3 and the iridium center. These five-coordinate systems react more rapidly with dioxygen than did the four-coordinate system at both normal and elevated pressures. An end-on oxidative addition of the dioxygen moiety, with displacement of the , SCN, or PPh3 ligands, was postulated. 

A number of iridium-porphyrin systems, including mononuclear and cofacial diporphyrins, have been adsorbed on pyrolytic edge-plane graphite electrodes and tested for their ability to reduce dioxygen to water (82). The original system investigated, [Ir(oep)H], where oep = 2,3,7,8,12,13,17,18-octaethylprophyrinato, was unique in that, while monomeric, the complex was still active in acidic solutions at potentials of +0.72 V vs NHE at pH 1 (82a). The [Ir(oep)H] did become inactive at potentials less than +0.2 V vs NHE, unlike the cofacial dicobalt diporphyrin systems. In the more recent report of these systems (82b),... 

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. ... 

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