Exploring acidity-dependent PCET pathways in imino-bipyridyl cobalt complexes†

Abstract

The electrochemical proton reactivity of transition metal complexes has received intensive attention in catalyst research. The proton-coupled electron transfer (PCET) process, influenced by the coordination geometry, determines the catalytic reaction mechanisms. Additionally, the pK_a value of a proton source, as an external factor, plays a crucial role in regulating the proton transfer step. Understanding the effects of variations in the pK_a values of Brønsted acids on the PCET process is therefore essential. This study compares the PCET pathways of two high-spin cobalt (Co) complexes with contrasting exchange coupling interactions under acidic conditions with high and low pK_a values. These findings reveal how proton reduction reactions in high-spin Co complexes are affected by the internal factor of the spin state, as well as an external factor related to the proton source. The corresponding reaction mechanisms are also proposed based on these observations.