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 (PCP)MCl (M = Rh, Ir; PCP = C 6 H 3 (CH 3 )(CH 2 PH 2 ) 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 η 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 η 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 η 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.