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Heterobinuclear Alkyl Complexes of Rhodium and Iridium. Migratory Insertion or Ir-to-Rh Migration of a Methyl Group in Reactions with Small Molecules

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journal contribution
posted on 06.02.1996, 00:00 by Frederick H. Antwi-Nsiah, Okemona Oke, Martin Cowie
The reaction of [RhIr(CO)3(dppm)2] (dppm = Ph2PCH2PPh2) with methyl triflate yields [RhIr(CH3)(CO)3(dppm)2][CF3SO3] (2), having the methyl group and two terminal carbonyls on Ir and one carbonyl on Rh. An Ir → Rh dative bond is proposed, giving Rh+ a 16e square-planar configuration. Reaction of 2 with hydrogen at ambient temperature yields [RhIrH(CO)2(μ-H)2(dppm)2]+ and methane. At 0 °C the intermediate [RhIrH(CH3)(CO)2(μ-H)(dppm)2][CF3SO3] is observed which under H2 yields the trihydride species and CH4 upon warming. Protonation of 2 with HCl and CF3SO3H yields [RhIr(CH3)(CO)2(μ-H)(μ-Cl)(dppm)2]+ and [RhIr(CH3)(CO)2(μ-H)(μ-CO)(dppm)2]+2, respectively. The reaction of 2 with CO resulted in decomposition to several unidentified products. Reaction of 2 with CNtBu yields the iminoacyl product [RhIr(CO)2(μ-tBuNCMe)(dppm)2]+ via migratory insertion. Two intermediates in the formation of the iminoacyl species were identified at low temperature yielding some information about the subsequent migratory insertion. Although the reaction of 2 with SO2 does not yield the SO2-insertion product, it does yield [RhIr(C(O)CH3)(CO)2(μ-SO2)(dppm)2]+, the product of methyl migration to Rh and migratory insertion of the methyl to a carbonyl group. A mechanism is proposed on the basis of the characterization of low-temperature intermediates. With ethylene, the substitution of one carbonyl in 2 and migration of the methyl group to Rh occurs. The structures of [RhIr(CH3)(CO)3(dppm)2][CF3SO3]·CH2Cl2 (2) and [RhIr(CO)2(μ-tBuNCMe)(dppm)2][CF3SO3] (8) have been determined by X-ray crystallography. Compound 2 has a trigonal bipyramidal ligand arrangement at Ir with the methyl group opposite Rh. In addition the 18e Ir forms a dative bond to Rh giving the latter a square planar geometry. Compound 8 has an “A-frame” geometry in which the bridging iminoacyl group binds to Rh via nitrogen and to Ir via carbon.