Heterobinuclear Methyl Complexes of Rhodium/Iridium:  Reactivity with Nucleophiles and Subsequent Ligand Rearrangements

Reaction of [RhIr(CH3)(CO)3(dppm)2][CF3SO3] (1) (dppm = Ph2PCH2PPh2) with several phosphines and phosphites yields the carbonyl-substitution products [RhIr(CH3)(PR3)(CO)2(dppm)2][CF3SO3] (4), in which the added phosphine and the methyl ligands are coordinated to Ir. At −80 °C an intermediate in this reaction, [RhIr(CH3)(PR3)(CO)3(dppm)2][CF3SO3] (3), is observed in which the monodentate PR3 group is bound to Rh. Similar reactions with the dicarbonyl compound [RhIr(CH3)(CO)2(dppm)2][CF3SO3] (2) suggests that attack occurs directly at Ir in this case. The structure of 2 is subtly different in solution and in the solid state, having the carbonyls both terminally bound (one to each metal) in solution, but having one bridging in the solid. Addition of cyanide and hydride ligands to 2 yields the neutral products [RhIr(CH3)X(CO)2(dppm)2] (X = CN, H), in which both anionic ligands (X and CH3) are bound to Ir. The hydrido species is unstable, eliminating methane at ambient temperature. Addition of iodide ion to 2 yields the related neutral species, which has the methyl group on Rh and the iodide ligand on Ir. The X-ray structures of compounds 2 and 4a (PR3 = PMe3) are reported. Compound 2 has a terminal carbonyl on Rh, essentially opposite the Rh−Ir bond, the methyl group in a similar position on Ir, and a semibridging carbonyl, which is more tightly bound to Ir. The geometry of 4a has a square-planar Rh center in which Ir occupies one of the coordination sites opposite a carbonyl, and an octahedral Ir center in which the PMe3 group is opposite the Ir−Rh bond, and the methyl group is opposite the second carbonyl. The crowding at Ir upon addition of the PMe3 group shows up in a number of distortions within the molecule.