Reactivity of the CpMn(CO)2-XR Bond [X = Cl, Br]: A Kinetic Study Using Rapid-Scan FTIR Spectroscopy

The mechanism and energetics of the displacement of dihaloalkanes from the photolytically generated CpMn(CO)(2)-XR (X = Cl, Br) complexes by 2,6-lutidine have been studied using rapid-scan FTIR. The substitution reactions proceed through an Id mechanism, and theoretical calculations indicate that the Mn-XR bond is mostly broken in the transition state. Activation enthalpies of 16-17 kcal/mol for the Mn-CIR and 18-19 kcal/mol for the Mn-BrR complexes are only slightly lower than previous thermodynamic measurements of the Mn-haloalkane bond dissociation enthalpies. The rate of displacement of bromoalkanes was found to be similar to 10 times slower than for the analogous chloroalkanes. Ab initio calculations suggest that this difference in reactivity is primarily due to a more stable interaction between the Mn center and bromoalkanes.