Pendent Relay Enhances H₂O₂ Selectivity during Dioxygen Reduction Mediated by Bipyridine-Based Co-N₂O₂ Complexes

Area of Science:

• Coordination Chemistry
• Electrocatalysis
• Organic Synthesis

Background:

• Cobalt-N2O2 complexes are commonly used electrocatalysts for dioxygen (O2) reduction.
• These complexes typically exhibit selectivity for hydrogen peroxide (H2O2) production.
• Previous work identified a Co(III)-N2O2 complex producing water (H2O) as the primary product, suggesting unique mechanistic pathways.

Purpose of the Study:

• To investigate the hypothesis that protonation site on a cobalt-hydroperoxide intermediate dictates product selectivity.
• To synthesize and study a new Co(III) complex with pendent methoxy (-OMe) groups designed to influence protonation.
• To elucidate the mechanistic pathways governing O2 reduction selectivity in different solvent and acid conditions.

Main Methods:

• Synthesis of a novel Co(III) complex featuring pendent proton donor groups.
• Electrocatalytic studies in acetonitrile (MeCN) with varying acid strengths and reductants.
• Electron paramagnetic resonance (EPR) spectroscopy to identify key intermediates like cobalt-superoxide species.

Main Results:

• The new Co(III) complex exhibited dual catalytic regimes dependent on acid strength and the presence of the pendent relay.
• In MeCN, the pendent relay promoted primarily H2O2 production, contrasting with previous H2O selectivity.
• EPR studies confirmed the formation of Co(III)-superoxide, with enhanced O2 reactivity observed with the pendent -OMe groups.

Conclusions:

• The proximity and nature of proton donors significantly influence the reaction pathway and product selectivity in cobalt-catalyzed O2 reduction.
• Pendent proton relays can alter the selectivity from water to hydrogen peroxide production.
• Understanding these mechanistic details is crucial for designing advanced electrocatalysts for selective O2 transformations.