Ligand Substitution from the (η5-DMP)Mn(CO)2(Solv) [DMP=2,5-dimethylpyrrole, Solv = solvent] Complexes: To Ring Slip or Not to Ring Slip?

The mechanism and energetics of the displacement of solvent from photolytically generated (eta(5)-DMP)Mn(CO)(2)(Solv) complexes has been studied [DMP = 2,5-dimethylpyrrole, Solv = solvent]. Rate enhancement relative to the eta(5)-cyclopentadienyl (Cp) system is not observed in the displacement of weakly bound solvents. The bond dissociation enthalpies obtained from the kinetic analysis are in good agreement with the values obtained by detailed density functional theory (DFT) calculations. The results indicate that for both the Cp and the DMP based systems the displacement of weakly bound solvents proceeds by a dissociative or I-d mechanism. This is in sharp contrast to CO displacement from (eta(5)-DMP)Mn(CO)(3), which is known to proceed by an associative mechanism by way of an eta(3) ring slip intermediate. The associative substitution pathway only becomes competitive with the dissociative channel when the Mn-Solv bond dissociation enthalpy is more than 33 kcal/mol.