Poster Presentation 21st International Conference on Biological Inorganic Chemistry 2025

Breaking the Scaling Relationship for Oxygen Reduction Reaction Using Molecular Cobalt Complexes (#558)

Avijit Das 1 , Aakash Santra 1 , Ankita Kumari 1 , Dibyajyoti Ghosh 1 , Sayantan Paria 1
  1. IIT Delhi, New Delhi, DELHI, India

Developing catalysts that achieve a higher turnover frequency (TOF)with a lower effective overpotential (ηeff) in electrocatalytic reactions is an emerging focus of research. A promising approach to this end is modifying the secondary coordination sphere (SCS) that can facilitate hydrogen bonding interactions or electrostatic effects, thereby lowering the activation energy barrier in the rate-determining step (rds). Herein, we designed and synthesized a series of CoIII complexes (1−8) featuring a bis-pyridine-dioxime framework, each with a distinctSCS (−C6H5 (1), o-NHMe2+−C6H4− (2), o-OMe−C6H4− (3), o-OH−C6H4−(4), p-NHMe2+−C6H4− (5), p-OMe−C6H4− (6), pyridine (7), and pyrimidine(8)). We investigated their electrocatalytic oxygen reduction reaction (ORR) in acetonitrile, both with CF3COOH and in a 1:1 CF3COOH/CF3COO− buffer solution. All complexes demonstrated selective 4e/4H+ reduction of O2. A linear free energy relationship (LFER) analysis revealed a trend of increasing the TOF with ηeff, aligning with molecular scaling expectations. However, 2 and 3 with the o-NHMe2+−C6H4− and o-OMe−C6H4−substituents diverged from this trend, exhibiting TOF values over 1000 and 250 times higher than predicted based on their positions in the log(TOF)/ηeff correlation within the buffer solution. Kinetic investigations indicate that protonation of the CoIII(O2) adduct is the rds for all catalysts, suggesting that the functional groups in the SCS of 2 and 3 facilitate proton transfer, acting as proton relay sites. We propose that this effect reduces the activation energy barrier in the rds, accounting for the observed deviation from the LFER. This study underscores the critical role of designing an appropriate SCS to enhance catalyst efficiency beyond LFER expectations.