Reaction of a rhodium(III) alpha-chlorotolyl complex with water and oxygen: Stable rhodium peroxo compounds

The reaction of the rhodium(III) alpha-chlorotolylcomplex [RhCl2(CHClPh>(2,6-(C(Me)=N-i-Pr)(2)C5H3N)] (1) with H2O and O-2 afforded the rhodium(III) chloride [RhCl3(2,6-(C(Me)=N-i-Pr)(2)C5H3N)] (11), benzaldehyde, and H2O2 in 80-90% yield. This reaction proceeds via two reaction sequences. First, when O-2 is absent the hydride complex [Rh(H)Cl-2(2,6-(C(Me>=N-i-Pr)(2)C5H3N)] (2), benzaldehyde, and HCl are formed. Hydrolysis of the metal-bonded CHClPh fragment gives a short-lived CH(OH)Ph moiety, which then by beta-H elimination of the hydroxyl group affords the products. Alternatively, a rhodium carbene type of intermediate might be involved. The subsequent reaction sequence probably proceeds via two separate pathways. In the first one the hydride 2 may insert O-2 to give a rhodium hydroperoxo species which converts with HCl to the Rh(III) complex 11 and H2O2. The second pathway appears to involve first the formation of the Rh(I) complex [RhCl(2,6-(C(Me)=N-i-Pr)(2)C5H3N)] (8) from the hydride 2 in H2O, which subsequently reacts with O-2 to give the peroxo complex [RhCl(O-2)(2,6-(C(Me)=N-i-Pr)(2)C5H3N)] (12). The latter reacts with HCl to give the rhodium(III) chloride 11 and H2O2. Both pathways were investigated separately by employing the novel hydride 2 and peroxo 12 complexes, which have been prepared in high yields from the rhodium(I) complex 8 with HCl(DCl) and O-2, respectively. The hydride 2 has acidic character in H2O and is conducting owing to dissociation to a small extent into HCl and the rhodium(I) complex 8. This solution does react with O-2 in the presence of HCl to from 11 and 80-90% H2O2. The peroxo complex 12, in which O-2 is side-on bonded, dissolves in H2O to give the weakly basic [RhCl(OOH)(OH2)(C5H3N(2,6-(C(Me)=N-i-Pr)(2)C5H3N)](OH-)-O-+ as two equilibrating isomeric forms which can be converted with HC to 11 and about 90% H2O2. The peroxo complex 12 reacted with SO2 to [RhCl(SO4)(2,6-(C(Me>=N-i-Pr)(2)C5H3N)] (15) but not with CO, CO2, and fumaronitrile in dichloromethane. These peroxo complexes are the first examples of rhodium peroxo complexes in which the O-2 is irreversibly bonded to the rhodium atom.