High-spin configuration of asymmetric CoN₁C₂ coordination for boosting d-p orbital hybridization in Fenton-like reactions

Asymmetric single-atom catalysts (ASACs) have attracted much attention owing to their excellent catalytic properties. However, the relationship between asymmetric coordination and the spin states of metal sites remains unclear. Additionally, the modulation of reactive oxygen species in Fenton-like reactions remains challenging. Herein, a novel strategy is reported for the rational design of highly loaded Co ASACs (CoN₁C₂/C₂N) immobilized on three-dimensional flower-like C₂N using an in situ-generated carbon defect method. In particular, the asymmetrically tricoordinated CoN₁C₂/C₂N exhibited excellent catalytic activity for sulfachloropyridazine degradation, with a turnover frequency of 36.8 min^–1. Experimental results and theoretical calculations revealed that the electron spin state of the Co-active sites was transferred from the low-spin configuration (t_2g⁶e_g¹) to the high-spin configuration (t_2g⁵e_g²) owing to asymmetric coordination. The high-spin Co 3d orbital in CoN₁C₂/C₂N possessed more unpaired electrons and therefore, had a strong ability to gain electrons from the O 2p orbitals of HSO₅^–, boosting d-p orbital hybridization. More importantly, the spin-electron filling in the σ* orbital of high-spin Co 3d−O 2p accelerated the desorption of *SO₅^•−, which acted as a rate-limiting step in the reaction, thus facilitating more ¹O₂ generation. This study provides an innovative synthetic route for practical ASACs and clarifies the critical relationship between structure and spin state, paving the way for advancements in environmental remediation and energy conversion applications.