Theoretical studies of inorganic and organometallic reaction mechanisms. 17. Unprecedented C-C bond activation at rhodium(I) and iridium(I)

Activation of C-C and C-H bonds by the Rh(I) and Ir(I) complexes (PCB)MCl ( M = Rh, Ir; PCP = C6H3(CH3)(CH2PH2)(2)) has been studied by density functio nal methodology. C-H activation from either of the three-coordinate interme diates la and Ib has a high barrier (>25 kcal/mol). Direct C-C activation d oes not occur from either la or Ib because the C-C bond is sterically inacc essible. Plausible C-C and C-H activation mechanisms under mild conditions are related to four-coordinate eta(1)-arene intermediates, 3a or 3b, which are formed prior to the activation processes. Activation energies for the d irect C-C oxidative addition at Rh(I) and Ir(I) were calculated to be 14.3 and 12.6 kcal/mol, respectively. The C-C bond activation products form with an exothermicity of -4.4 and -24.1 kcal/mol relative to the eta(1)-arene i ntermediates 3a and 3b, respectively. The unexpected reactivity for C-C act ivation in PCP ligand systems can be ascribed to the formation of the eta(1 )-arene intermediates and the existence of strong Ar-metal bonds in transit ion states for C-C bond activation. The former makes the C-C bond accessibl e to the metal center, and the latter significantly reduces the barrier of the C-C activation process. "Solvent" coordination also stabilizes the C-C activation product more than the C-H activation product.