Oxalate-Bridged Binuclear Fe(II) Drives Concerted Two-Electron Reduction of NO₂ by Substantial Spin Interactions

NO₂ reduction plays a key role in the active nitrogen cycle, influencing atmospheric oxidation capacity and secondary aerosol formation. NO₂ reduction is commonly believed to proceed via a single-electron reduction pathway, yielding NO₂ ^- or HONO as primary products. Here, we report that oxalate-bridged binuclear Fe(II) complex can lead to the two-electron reduction of NO₂ directly to NO, while the citrate-coordinated Fe(II) primarily reduces NO₂ to HONO through the common single-electron reduction pathway. We systematically compare the coordination and magnetic properties between this oxalate-bridged Fe(II) complex with the citrate-coordinated one. Unlike the paramagnetic Fe(II) citrate complex with disordered Fe spin alignments, the oxalate-bridged binuclear Fe(II) complex exhibits antiferromagnetic interactions with antiparallel spin alignments of the unpaired electrons between the two Fe(II) centers. We propose that such antiferromagnetic interactions facilitate the concerted transfer of the two electrons with antiparallel spin alignments from each Fe(II) centers in the oxalate-bridged Fe(II) complex to the empty lowest unoccupied molecular orbital (LUMO) of NO₂, leading to NO formation. Our findings reveal, for the first time, a two-electron reduction pathway of NO₂ by the oxalate-bridged binuclear Fe(II) complex formed from environmentally abundant iron oxides and oxalic acid, providing new insights into the active nitrogen cycling.