Controlling product selectivity during dioxygen reduction with Mn complexes using pendent proton donor relays and...

Area of Science:

• Inorganic Chemistry
• Catalysis
• Electrochemistry

Background:

• The oxygen reduction reaction (ORR) is crucial for energy conversion.
• Homogeneous manganese catalysts for ORR are less explored than iron or cobalt systems.
• Non-porphyrinic ligands offer tunable electronic and steric properties for metal catalysts.

Purpose of the Study:

• To investigate the effect of pendent proton donor relays on manganese-based ORR catalysts.
• To evaluate how ligand modifications influence activity and selectivity for ORR.
• To understand the mechanistic pathways of ORR with different proton sources and manganese complexes.

Main Methods:

• Synthesis of two manganese complexes with bipyridine-based ligands: Mn(p-dhbpy)Cl (1) and Mn(nPrdhbpy)Cl (2).
• Electrocatalytic evaluation of ORR activity and selectivity using an ammonium-based proton source.
• Mechanistic studies involving the addition of conjugate base to probe proton transfer and intermediate species.

Main Results:

• Complex 1 showed 64% selectivity for H2O2, while complex 2 was quantitative for H2O2.
• O2 binding to the reduced Mn(II) center was the rate-determining step for complex 1.
• Addition of a conjugate base shifted selectivity to H2O for both complexes, with enhanced activity for complex 2.
• Base addition altered the rate-determining step to O-O bond cleavage in a Mn-hydroperoxo species for complex 2.

Conclusions:

• Pendent relay moieties can enhance H2O2 selectivity but may decrease reaction rates due to hydrogen bonding.
• Base addition can improve catalytic rates and alter product selectivity by buffering proton activity.
• Manganese catalysts with pendent relays show potential for selective oxygen reduction, but intermediate stabilization can limit maximum rates.