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Mechanistic insights into an unprecedented C-C bond activation on a Rh/Ga bimetallic complex: A combined experimental/computational approach

Abstract:

The unusual rearrangement of [RhCp*(GaCp*)(CH3) 2] (1c) to [RhCp*(C5Me4Ga(CH 3)3)] (2) is presented and its mechanism is discussed in detail. 13C MAS NMR spectroscopy revealed that the title reaction proceeds cleanly not only in solution but also in solid state, which supports a unimolecular reaction pathway. On the basis of 1H, 13C, and ROESY NMR spectroscopy as well as isolation and structural elucidation of the hydrolysis product, the compound [RhCp*(endo-η4-C 5Me5GaMe2)] (3a) was identified as a crucial reaction intermediate. DFT calculations on the B3LYP level of theory support this assignment and suggest a concerted C-C bond activation mechanism that topologically takes place at the gallium center. Furthermore, two fluxional processes of the reaction intermediate 3a were studied experimentally as well as by computational methods. First, a mechanism takes place similar to a ring-slipping process that exchanges a GaMe2 group between adjacent ring carbon atoms within the same Cp* ring. This process proceeds at a rate comparable to the NMR time scale and indeed is calculated to be energetically very favorable. Second, a unimolecular exchange process of the GaMe2 group between the two Cp* rings of 3a could be experimentally proven by the introduction of phenyl substituents as a label into the Cp* ligands at both sites, the rhodium as well as the gallium center. A series of experiments including deuteration studies and competition reactions was performed to substantiate the suggested mechanism being in accordance with DFT calculations on possible transition states. © 2005 American Chemical Society.