Mechanistic Insights into an Unprecedented C−C Bond Activation on a Rh/Ga Bimetallic Complex: A Combined Experimental/Computational Approach

The unusual rearrangement of [RhCp*(GaCp*)(CH3)2] (1c) to [RhCp*(C5Me4Ga(CH3)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-C5Me5GaMe2)] (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.