Mechanistic Insights into an Unprecedented C−C Bond Activation on a Rh/Ga Bimetallic Complex: A Combined Experimental/Computational Approach
摘要:
The unusual rearrangement of [RhCp*(GaCp*)(CH3)(2)] (1c) to [RhCp*(C5Me4Ga(CH3)(3))] (2) is presented and its mechanism is discussed in detail. C-13 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 H-1, C-13, and ROESY NMR spectroscopy as well as isolation and structural elucidation of the hydrolysis product, the compound [RhCp*(endo-eta(4)-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.
Mechanistic Insights into an Unprecedented C−C Bond Activation on a Rh/Ga Bimetallic Complex: A Combined Experimental/Computational Approach
摘要:
The unusual rearrangement of [RhCp*(GaCp*)(CH3)(2)] (1c) to [RhCp*(C5Me4Ga(CH3)(3))] (2) is presented and its mechanism is discussed in detail. C-13 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 H-1, C-13, and ROESY NMR spectroscopy as well as isolation and structural elucidation of the hydrolysis product, the compound [RhCp*(endo-eta(4)-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.