As part of our interest in bimetallic activation processes, the reactions of oxidative addition to the dinuclear (cyclopentadienyldiphenylphosphine)rhodium and -iridium complexes [M(I)(mu-CpPPh2)(CO)]2 (M = Rh (1) Ir (2)) have been studied. The two-electron direct chemical oxidation of 1 with ferrocenium or silver hexafluorophosphate allowed the preparation of a series of Rh(II)-Rh(II) cationic species, [Rh(II)L(mu-CpPPh2)]2(2+ (L = CO (2a), pyridine (2b), acetonitrile (2c), P(OMe)3 (2d)). An X-ray diffraction study of the cationic dinuclear species 2b revealed a drastic configurational change that does authorize a metal-metal bond formation (Rh-Rh distances equal to 4.3029 (6) angstrom in 1 and to 2.7796 (9) angstrom in 2b) with subsequent cisoid disposition of the pyridine ligands. Also, oxidative addition reactions to 1, of the most typical electrophiles, have been studied. With iodomethane, 1 leads to both mono- and biaddition products, [Rh(III)-(CH3CO)(I)(mu-CpPPh2)2Rh(I)(CO)] (3) and [Rh(III)(CH3CO)(I)(mu-CpPPh2)]2 (4), depending on the CH3I:Rh ratio used. The reaction with methyl triflate allowed the formation of the 1:1 electrolyte [(CH3CO)-Rh(II)(mu-CpPPh2)2Rh(II)(CO)]CF3SO3 (5a), from which crystals of the hexafluorophosphate 5a' were obtained. The X-ray diffraction study of 5a' showed that it also adopted the configuration allowing the formation of a metal-metal bond (Rh-Rh = 2.7319 (6) angstrom). Thus, the pathway from 1 to 5 appears as the second example of a metal-promoted alkyl migration in dinuclear complexes. Further transformation of 5 to 3 by nucleophilic attack of iodide on the rhodium atom bearing the acetyl group was also observed and shown to imply the breaking of the metal-metal bond. The reaction of 1 with iodine leads, depending on the stoichiometry, to the formation of both the mono- and bisubstitution products [(I)2Rh(III)(mu-CpPPh2)2Rh(I)(CO)] (6a) and [Rh(III)(I)2(mu-CpPPh2)]2 (7a), respectively; these compounds were fully identified by their spectroscopic data. With chlorine, the precipitation of [(Cl)2Rh(III)(mu-CpPPh2)2Rh(I)(CO)] (6b) is observed together with the formation of a green solution from which the cationic species [ClRh(II)(mu-CpPPh2)2Rh(II)(CO)]+ (8b) was readily precipitated as the hexafluorophosphate. Infrared and NMR spectroscopic arguments allow us to propose for the cationic species 8 a metal-metal-bonded structure analogous to that of 5. Reaction of 8 with nucleophiles, namely iodide or chloride, led to the metal-metal bond breaking, leading to species of type 6 (Rh(III), Rh(I). Starting with 2a, the nucleophilic attack of the halides X- led also to the monocationic intermediates [XRh(mu-CpPPh2)2Rh(CO)]+ (8) then to the formation of dihalo complexes 6. In contrast, the reaction of the solvated cationic species [Rh(II)(solv)(mu-CpPPh2)]2(2+) (2e) with halides allowed the preparation of symmetric dihalo compounds [Rh(II)X(mu-CpPPh2)]2 (X = I (9a), or Cl (9b)). By studying the reactions of the tetraiodo complex (Rh(III)-Rh(III), 7a, and of the diiodo complex (Rh(II)-Rh(II), 9a, with the borohydride LiBHEt3, spectroscopic evidence for the formation of hydrido derivatives was obtained. Reaction of 9a with methyl- or phenyllithium afforded the preparation of the methyl or phenyl derivatives [Rh(II)(R)(mu-CpPPh2)]2 (R = Me (10a); or Ph (10b)). The intermediate species [(CH3)Rh(II)(mu-CpPPh2)2Rh(II)(I)] (11) was also prepared and studied by X-ray diffraction (Rh-Rh = 2.7160 (7) angstrom). A comparison of the molecular structures of the metal-metal-bonded dication 2b, monocationic 5a', and neutral 11 species is developed with reference to the related structure of 1. In addition the general features of the novel [M(mu-CpPR2]2 (R = Ph, Me) bridging unit are summed up and discussed in the context of a short review (presented in the Introduction) on the oxidative addition reactions involving bridged bimetallic complexes.
