Activation of dioxygen at transition metal centers: Fundamental investigations of the heavy atom isotope effect and its applications

Oxygen (18O) isotope fractionation is used as a mechanistic and structural probe to understand O2 reactivity in chemical and biological oxidation processes. The derived 18O equilibrium isotope effects (18O EIEs) and kinetic isotope effects (18O KIEs) reflect the ground state and transition state structures, respectively, for reactions of 16O- 16O and 16O-18O. Reactions of inorganic compounds for which competitive isotope effect measurements have been performed using natural abundance molecular oxygen and isotope ratio mass spectrometry are discussed.;For group IX transition metal complexes which bind O2 reversibly, the 18O EIEs are shown to differentiate structures where O 2 is bound as a side-on peroxide ligand (18O EIEs > 1.020) and an end-on superoxide ligand (18O EIEs < 1.010). The 18O EIEs for Rh and Co transition metal complexes were found to have opposite temperature dependences; for Rh, 18O EIEs decrease from ∼1.0340 at -50°C to ∼1.0160 at +40°C while those for Co increase from ∼1.0000 at -110°C to ∼1.0070 at +21°C. These opposing trends emphasize that careful attention toward the type of reaction being studied is necessary to accurately interpret 18O IEs. Computational results complement the experiments providing additional insights for understanding fundamental aspects of O2 reactivity. The research that will be presented is the first body of work that systematically analyzes 18O IEs and provides means for creating a fundamental method of applying the results to understanding reactions of O2.