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

ABSTRACT 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 Mn II from medium spin (MS, S = 3/2, d yz 2 , d xy 1 , d xz 1 , d z2 1 , d x2−y2 0 ) to low spin (LS, S = 1/2, d yz 2 , d xy 2 , d xz 1 , d z2 0 , d x2−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 (E 1/2 ) 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 H 2 ‐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 d z2 to the spin‐down d xy 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.