Iridium complexes peroxo

If X-Y bond fission occurs, the product is a 6-coordinate iridium(III) complex (Table 2.6) otherwise a 5-coordinate (or pseudo-5-coordinate) adduct is obtained in which Ir formally retains the (+1) state (Table 2.7). This distinction can be somewhat artificial IrCl(02)C0(PPh3)2 can be regarded as an iridium(III) peroxo complex. 

An unusual peroxo-bridged binuclear iridium complex containing an Ir—Ir bond has been reported (135a, 135b). The reaction of the electron-... 

An X-ray crystal structure of 55, redrawn as Fig. 10, supported the formulation of the complex as that of a peroxo system. Further, the structure demonstrated that no interactions between the [Of-] ligand and the borate moiety were possible because of the relative arrangement of the [Of-] and borate ligands about the iridium center. Such interactions were implicated in the oxygen-initiated decomposition of the iridium complex of 52, while the lack of reactivity of the iridium complexes of 53 and 54 was attributed to steric factors arising from the alkyl chains connecting the sulfur atoms. 

In contrast to inactive iridium(TTI)-Peroxo complexes, Irm-hydroperoxo species have been shown to transfer oxygen to a coordinated alkene, for example in the slightly catalytic oxidation of cyclooctene to cyclooctanone by 02 + H2 mixtures in the presence of IrHCl2(CgH12) (equation 95). 68 Oxygen transfer presumably occurs as for palladium hydroperoxides in equations (89) and (90). 

The well-known rhodium (136) and iridium 137) peroxo complexes (PhgPlaRhCKOa) (40), [(PhgPlaRhCKOalJa (41), and (Ph3P)2(C0)IrCl(02) (42) have been investigated for their reactivity with acetylacetone, acacH 138). Only the former complex, 40, exhibited any reactivity (in the presence of two equivalents of triphenylphosphine), yielding the hydroperoxo complex (43), (see Scheme 8). Complex 43 reacts with PPhg to form triphenylphosphine oxide, but does not react with any active methylene compounds (methyl acetoacetate, diethyl malonate, or acetone) save for cyclopentadiene. In the last instance, a poorly characterized, unstable system tentatively formulated as 44 may have been formed. In refluxing benzene, 43 did react with excess acacH to form the bis(acac) complex 45. 

The catalytic mechanism of the four-electron reduction of oxygen with formic acid is shown in Scheme 4.11. The hydride complex, which is produced by the reaction of lr -OH2 with HCOO, deprotonates to generate the low-valent complex Ir. The Ir complex reacts with O2 to produce an iridium(v)-0x0 complex (B) and water via formation of the iridium(iii)-peroxo complex (A). The formation of an iridium-peroxo complex by the reaction of a low-valent iridium complex with O2 has been well established. The formation of an iridium(v)-oxo complex with cleavage of the 0-0 bond of an iridium(iii)-peroxo complex has also been reported. The 0x0 complex (B) reacts with HCOOH to reproduce Ir -OH2. Each intermediate in the catalytic cycle in Scheme 4.11 has been detected by stopped flow measurements. Because formic acid is used as a reductant for the catalytic four-electron reduction of O2, the water-soluble Ir catalyst can remove dissolved O2 with formic acid completely at an ambient temperature. 

More recently, the iridium complexes [Ir(PMe3)4]Cl, as well as the more electrophilic peroxo derivative [Ir(02)(PMe3)4]Cl, were catalyst precursors for a variety of nitriles RCN (R = Me, /7-NH2C6H4, p-OHCeHj) giving up to 800 turnovers in the hydration reaction realized at 140°C. Various general mechanisms for the hydration of nitriles are possible, two of which involve insertion into a metal-hydroxo bond (Scheme 26a) or nucleophilic attack of water upon coordination of... 

The other main group elements which form peroxo complexes are d6 and d8 systems in group VIII including iridium, palladium and platinum. The ji-peroxo complexes do not generally catalyse the epoxidation of olefins with hydrogen peroxide,95,96 but it has been found that trifluoromethyl-substituted Pd(II) and Pt(II) hydroperoxides will perform such a transformation.97... 

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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