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)(CH(3))(2)] (1c) to [RhCp(C(5)Me(4)Ga(CH(3))(3))] (2) is presented and its mechanism is discussed in detail. (13)C 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 (1)H, (13)C, and ROESY NMR spectroscopy as well as isolation and structural elucidation of the hydrolysis product, the compound [RhCp(endo-eta(4)-C(5)Me(5)GaMe(2))] (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 GaMe(2) 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 GaMe(2) 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.