Theoretical studies of inorganic and organometallic reaction mechanisms .9. Intermolecular versus intramolecular carbon-hydrogen bond activation in zirconium, rhodium, and iridium complexes

Ab initio quantum mechanical calculations were performed on model reactions to analyze the behavior of intermolecular versus intramolecular C-H bond activation in zirconium, rhodium, and iridium complexes. Intermolecular reactions (inter) were modeled by (Cp)(X)M + CH4 --> (Cp)(X)M(CH3)(H), with (M, X) = (Zr, Cl), (Rh, PH3), and (Ir, PH3) and Cp = C5H5. Intramolecular reactions involving the Cp ring (intra-Cp) were modeled by (X)M(CpR'H) --> (X)M(CpR')(H), while those involving the phosphine (intra-P) (Rh and Ir only) were modeled by (Cp)M(PH(2)R'H) --> (Cp)M(PH(2)R')(H), with R' = CH2 and CH2CH2. It is found that the thermodynamic exothermicity follows the sequence inter > intra-P > intra-Cp with decreasing differences as the ring size increases. The ''strain'' energy for the intra-Cp reactions is less for Zr complexes than it is for the corresponding Rh or Ir complexes. Agostic Ir and Zr intermediates were optimized and are bound by 6.7 and 1.3 kcal/mol, respectively, but the intermolecular reactions have negligible kinetic barriers at the MP2 level. The intra-P (Rh and Ir) reactions beginning with reactants in their singlet states also have virtually no kinetic barrier at the MP2 level, even for systems with significant ''strain'' energy. The barrier for the intra-Cp reaction decreases as the R' fragment increases and is much smaller for the Zr complexes than for the corresponding Rh or Ir complexes.