Reversible N₂ binding and photoreduction at dinuclear sites in dynamic metal-organic frameworks

Direct structural characterization of reactive intermediates in photocatalytic N₂ reduction is challenging due to their low concentration and fleeting nature, often limiting mechanistic understanding to spectroscopic or computational inference. Herein, we report the crystallographic capture of a coordinatively unsaturated dinuclear intermediate for N₂ activation, generated through a photoinduced single-crystal-to-single-crystal transformation within a dynamic coordination polymer. The initial structure, NJUZ-Zn, is an active photocatalyst for N₂ photoreduction and contains a stable [Zn²⁺-(N≡N)^--Zn²⁺] site with bridging N₂ ^- anions. Upon irradiation, N₂ ^- dissociation was observed by single-crystal X-ray diffraction, yielding the unsaturated [Zn²⁺···Zn⁺] center. Raman spectroscopy and isotope-tracking experiments demonstrated the reversible exchange of ¹⁴N₂ and ¹⁵N₂ at the bimetallic site, confirming dynamic interconversion between saturated and unsaturated states during photocatalysis. Together, these results establish a photocatalytic mechanism in which dissociation of bridging N₂ ^- generates reactive bimetallic intermediates that continuously capture and activate external N₂ to produce ammonia under ambient conditions. This work provides unambiguous structural evidence of a key intermediate in an active photocatalyst for N₂ fixation, offering a blueprint for understanding the reaction pathway and guiding the rational design of cooperative metal sites for sustainable catalysis.