# Proximity Engineering of Fe‒N4 Twins for Oriented Generation of Singlet Oxygen for Hospital Wastewater Treatment

## TL;DR

This paper introduces a new method to control the distance between iron-nitrogen sites to efficiently generate singlet oxygen for treating hospital wastewater.

## Contribution

A novel approach to modulate Fe-N4 site distances to regulate spin states and enhance singlet oxygen generation for wastewater treatment.

## Key findings

- Precise modulation of Fe-N4 distances triggers a volcano-shaped Fenton-like activity trend.
- The optimal Fe-Fe distance of 0.43 nm promotes singlet oxygen generation with low peroxymonosulfate dosage.
- The catalyst effectively treats hospital wastewater for over 120 hours with disinfection and pharmaceutical removal.

## Abstract

Precisely tailoring the molecular configurations of single‐atom sites and elucidating their correlation with generated specific reactive species is crucial for advancing Fenton‐like chemistry toward targeted remediation. Herein, we developed a facile approach to precisely modulate the distances between isolated Fe‒N4 sites (dFe–Fe) from nanometer (0.95 nm) to subnanometer (0.43 nm) to construct a family of well‐defined Fe‒N4 twins with manipulated ligand‐field strength and spin states. Different Fe‒N4 twin sites trigger a metal‐loading‐independent volcano‐shaped Fenton‐like activity trend. The optimal configuration, achieved at an Fe‒Fe distance of 0.43 nm (Fed0.43SA), induces an intermediate‐spin (t2g4eg1) configuration that optimizes eg orbital occupancy, thereby promoting peroxymonosulfate (PMS) adsorption to form *HSO5
− and subsequently lowers the energy barrier for coupling with another PMS to selectively generate singlet oxygen (1O2). The robust molecular catalyst with Fe‒N4 twin sites sustains over 120 h of continuous treatment of organic wastewater and demonstrates simultaneous disinfection and pharmaceutical removal of actual hospital wastewater. This work presents an advanced strategy for engineering single‐atom sites with multi‐site cooperativity to regulate Fenton‐like catalysis, enabling rapid and real‐world water purification.

Precise modulation of Fe–N4 site distances constructs spin‐regulated Fe–N4 twins, triggering a volcano‐type Fenton‐like activity and promoting oriented singlet oxygen generation. The twin‐site catalyst enables highly selective and efficient hospital wastewater purification with simultaneous pharmaceutical degradation and disinfection under low peroxymonosulfate dosage.