Rhodium complexes peroxo

An almost unique variation on the -peroxo coordination mode is exhibited by a rhodium dimer, [RhCl(02)(PPh3)2]2,60 which is composed of two identical subunits which have the dioxygen moiety coordinated in the -peroxo mode. These subunits are linked, not by a chlorine bridge as in other rhodium complexes such as [RhCl(CO)2]2, but via the coordinated dioxygen group as shown in (3). 

Molecular hydrogen can reduce cyclic peroxo complexes, for example Reaction 30 (111), and Sheldon and Van Doom (33) had envisaged epoxide production via a similar process, Reaction 31. A mechanism based on such reactivity (with subsequent epoxide isomerization) is a possibility for our catalysis using H2/02, although Reaction 31 did not occur under mild conditions (33). Isolation of a related peroxometallocyclic rhodium complex, (Ph3As)2Rh[02C2(CN)4] + (92), allows for a testing of Reaction 31 at a Rh center. 

The remaining results are concerned with redox reactions /t-superoxo-/<-peroxo redox reactions have been reported for rhodium complexes The complex [(NC)sCo( -02)Mo(0)(OH2)(CN)5] decomposes on standing in oxygen to [(>7 02)Mo(0)(CN)4] and isotopic labelling studies show that the dioxygen in this complex does not originate from [(NC)5Co(u-02)Mo(0)(OH2)(CN)5] °). The details of this reaction are not clear but the final product could conceivably arise from the formation of a Mo(IV) complex on breakdown of the rj complex followed by addition of dioxygen. 

The oxidation of HCO2 is induced by the reaction of the Cu(I) complex of phenanthroline with 11202 The reaction has been studied by radiolysis of the Cu(II) complex and by mixing high concentrations of the Cu(I) species with H2O2 and HC02 It is proposed that the peroxo complex, [(phen)2CuH202], oxidized HCO2 with a rate constant of 3 x 10 s" Cationic rhodium complexes... 

The reactions of Scheme 12 have been confirmed independently by the synthesis of cationic rhodium Ill) peroxo complexes [L Rh(02)lA, where... 

Scheme 22. Formation and decomposition of peroxo(catecholato)rhodium complex [84].

Scheme 8. The synthesis of an acetylacetonate complex of rhodium from the peroxo complex.

Some representative spectral data are given in Table VI. The spectroscopic properties of /x-hydroxo-/i-peroxo and -hydroxo/x-superoxo dicobalt(III) complexes have recently been reviewed by Fallab and Mitchell (119) and seem reasonably well rationalized in terms of the molecular orbital (MO) models formulated by Lever and Gray (120). The spectra of -hydroxo -/x-peroxo and /i-hydroxo-/i-superoxo complexes of rhodium(III) show features similar to those of the cobalt(III) species (121-124). 

Preliminary studies seem to indicate that acid hydrolysis of corresponding rhodium(III) complexes leads to peroxo bridge cleavage rather than hydroxo bridge cleavage (124). 

The complexes can be obtained from the reaction of a Rh1 complex with 02 (equation 5),37 from the reaction of a Com-superoxo complex with a reduced metal (equation 6),38 from the acid hydrolysis of a platinum-peroxo complex3 (equation 7) or from the reaction of potassium superoxide with rhodium (equation 8)40 or palladium complexes.41... 

With the notable exception of rhodium, Group VIII metal-peroxo complexes are generally reluctant to react with simple alkenes by nonradical pathways. However, such an oxygen transfer has been shown to occur in the reaction of 180-labeled [(AsPh3)4Rh02]+C104" with terminal alkenes under 02-free, anhydrous conditions, producing lsO-labeled methyl ketone (equation 52).131... 

The decomposition of the peroxometallacycle (72a) or (72b) occurs in a way different from that previously shown to occur in the epoxidation of alkenes by molybdenum-peroxo complexes (equation 26). The three possibilities are shown in equations (58)-(60) and involve (a) a [C-/6, C-a] hydride shift which directly produces the methyl ketone and the rhodium-oxo complex, or the hydroxo species from (72b equation 58) (b) a [C-/3,0-/3] hydride shift which gives enol (equation... 

The decomposition mode is somewhat similar to that previously shown in equation (28) for the Baeyer-Villager oxidation of ketones by Mo-peroxo complexes (c) a /3-hydride abstraction by rhodium followed by a nucleophilic attack at the coordinated alkene by the hydride (equation... 

Complexes containing the Rh—02 moiety are now well known, but there have been a few false starts and misunderstandings along the way. The first claim for an 02 complex of rhodium was made by Wilkinson and co-workers,1141 who reported that [Rh(H)(CN)4(H20)]2 reacted with 02 to give the peroxide-bridged dimer [(H20)(CN)4Rh—02—Rh(CN)4(H20)]4. This was later disputed, and the reaction product identified as the protonated, peroxo monomer of Rh111, [Rh(02H)(CN)4(H20)]2. 1142 The solid is diamagnetic, and displays O—O stretches at 839 and 825 cm-1, characteristic of an unsymmetric monomer no such bands appear in the Co-peroxo dimer.1142. 

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