Theoretical studies of inorganic and organometallic reaction mechanisms. 17. Unprecedented C-C bond activation at rhodium(I) and iridium(I)
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Activation of C-C and C-H bonds by the Rh(I) and Ir(I) complexes (PCP)MCl (M = Rh, Ir; PCP = C6H3(CH3)(CH2PH2)2) has been studied by density functional methodology. C-H activation from either of the three-coordinate intermediates 1a and 1b has a high barrier (> 25 kcal/mol). Direct C-C activation does not occur from either 1a or 1b because the C-C bond is sterically inaccessible. Plausible C-C and C-H activation mechanisms under mild conditions are related to four-coordinate 1-arene intermediates, 3a or 3b, which are formed prior to the activation processes. Activation energies for the direct 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 1-arene intermediates 3a and 3b, respectively. The unexpected reactivity for C-C activation in PCP ligand systems can be ascribed to the formation of the 1-arene intermediates and the existence of strong Ar-metal bonds in transition states for C-C bond activation. The former makes the C-C bond accessible 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.
