Breaking the Scaling Relationship for Oxygen Reduction Reaction Using Molecular Cobalt Complexes

Abstract

Developing catalysts that achieve a higher turnover frequency (TOF) with a lower effective overpotential (η_eff) 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 Co^III complexes (1–8) featuring a bis-pyridine-dioxime framework, each with a distinct SCS (−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 η_eff, 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)/η_eff correlation within the buffer solution. Kinetic investigations indicate that protonation of the Co^III(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.