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Colossal kinetic isotope effects in proton-coupled electron transfer
My Hang V Huynh1, Thomas J Meyer
1Dynamic Experimentation Division, Materials Dynamics Group, Los Alamos National Laboratory, Los Alamos, NM 87545, USA.
Summary
Osmium complexes with nitrogen, sulfur, and phosphorus donors exhibit colossal kinetic isotope effects in benzoquinone reduction. These findings illuminate proton-coupled electron transfer mechanisms.
Area of Science:
- Inorganic Chemistry
- Chemical Kinetics
- Electrochemistry
Background:
- Benzoquinone (Q) reduction to hydroquinone (H(2)Q) is a fundamental redox process.
- Osmium(IV) complexes with diverse proton-donor ligands (hydrazido, sulfilimido, phosphoraniminato) offer tunable platforms for studying electron transfer reactions.
Purpose of the Study:
- To investigate the kinetics and mechanisms of benzoquinone reduction by novel Os(IV) complexes.
- To elucidate the role of proton-coupled electron transfer (PCET) and quantify kinetic isotope effects (KIEs).
Main Methods:
- Kinetic studies of Os(IV) complex reduction of benzoquinone in acetonitrile/water mixtures.
- pH-dependent measurements to separate parallel reaction pathways.
- Deuterium kinetic isotope effect measurements using CH(3)CN/D(2)O solvent systems.
Main Results:
- Reactions proceed via parallel pH-independent and pH-dependent pathways.
- Saturation kinetics observed for the pH-independent pathway, suggesting a H-bonded intermediate.
- Colossal H(2)O/D(2)O kinetic isotope effects (455-178) were measured for the Os(IV) complexes.
Conclusions:
- The study demonstrates colossal KIEs in PCET reactions involving N, S, and P proton-donor atoms.
- These results provide significant insights into the fundamental mechanisms of proton-coupled electron transfer.