Dialkyl and trialkyl heterobinuclear complexes of rhodium and iridium: models for adjacent-metal involvement in bimetallic catalysts
bimetallic catalysts, metals
The heterobinuclear dialkyl complexes [RhIr(R)2(μ-CO)(dppm)2] (dppm = μ-Ph2PCH2PPh2; R = CH3 (2), CH2Ph (3)) have been prepared. Both A-frame-like compounds have one alkyl group terminally bound to each metal and a bridging carbonyl ligand. Some subsequent reactivity studies of 2 are reported. Reaction of 2 with CO yields [RhIr(CO)3(dppm)2] and acetone. If this reaction is monitored at low temperature by NMR spectroscopy, the dicarbonyl species [RhIr(CH3)2(CO)2(dppm)2] (4) is first observed, followed by [RhIr(CH3)2(CO)3(dppm)2] (5). In both products, both methyl groups are bound to Ir. Warming to ambient temperature under CO yields acetone and [RhIr(CO)3(dppm)2]. Crossover experiments suggest that acetone arises primarily from an intramolecular process. We propose that migratory insertion of a CO and a methyl group occurs on Ir; presumably reductive elimination also occurs from this metal. Compound 2 reacts with H2 at −78 °C to yield [RhIrH(CH3)2(μ-H)(μ-CO)(dppm)2] (6), in which one methyl group is bound to Rh while the other, together with a hydride ligand, is terminally bound to Ir. This latter species reacts with CO at 0 °C to yield [RhIrH(C(O)CH3)(CH3)(μ-H)(μ-CO)(dppm)2] (7), in which migratory insertion involving CO and the Rh-bound methyl group has yielded a Rh-bound acetyl group. At ambient temperature, under an atmosphere of CO, [RhIr(CO)3(dppm)2] is formed, together with acetaldehyde and methane. Crossover experiments support a predominantly intramolecular process for acetaldehyde formation but are equivocal on the formation of methane. Compound 2 oxidatively adds CH3I or n-C4H9I, yielding [RhIr(CH3)2(R)(μ-I)(μ-CO)(dppm)2] (R = CH3, n-C4H9), in which the added alkyl group in each case is bound to Ir. The n-butyl product reacts with CO to yield [RhIr(n-C4H9)(CH3)2(μ-CO)2(dppm)2][I], in which the n-butyl group has migrated to Rh. A similar product, [RhIr(CH3)3(μ-CO)2(dppm)2][CF3SO3], is obtained in the reaction of 2 with methyl triflate in the presence of CO.
Hilts, R., Oke, O., Ferguson, M., McDonald, R., & Cowie, M. (2005). Dialkyl and trialkyl heterobinuclear complexes of rhodium and iridium: Models for adjacent-metal involvement in bimetallic catalysts. Organometallics, 24(18), 4393–4405. https://doi.org/10.1021/om058013e
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