An Axial Chlorine Spin-Collar on Atomic Mn Centers for Oxygen Reduction.

Targeted spin-state programming remains a key yet challenging route to boost transition-metal oxygen reduction reaction (ORR) catalysis. Here, we report an axial chlorine (Cl) spin-collar on Mn single-atom sites (MnNC-Cl), which tunes MnII from medium spin (MS, S = 3/2, dyz 2, dxy 1, dxz 1, dz2 1, dx2-y2 0) to low spin (LS, S = 1/2, dyz 2, dxy 2, dxz 1, dz2 0, dx2-y2 0). This spin transition lowers the Mn─*OH bond order from 1.5 to 1, weakening *OH over-adsorption and steering the reaction toward an efficient four-electron (4e-) pathway. The MnNC-Cl catalyst delivers a high half-wave potential (E1/2) of 0.829 V in acid, with a 2.7× higher turnover frequency (TOF@0.85 V) and 1.2 × greater peak power density in H2-air fuel cells than the pristine counterpart. Through theoretical and experimental investigations, the mechanism of the axial Cl spin-collar is elucidated: symmetry-breaking crystal field distortion induced by axial Cl triggers electron transfer from the spin-up dz2 to the spin-down dxy orbital, locking Mn into a low-spin state and enhancing catalytic activity. This study establishes a spin-collar strategy for precise regulation of the spin state in Mn centers.