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

Abstract 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, single-crystal X-ray diffraction observed the N2− dissociation, 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.