Surface-Confined Selective Reduction of Nitroarenes via N₂H₃^· Radicals on Dual-Site Titanium-Oxo Clusters

The precise identification of transient active species is fundamental to understanding photocatalytic hydrogen transfer, yet capturing active radical intermediates remains a formidable challenge. Here, we report a mechanistic paradigm shift, demonstrating that the N₂H₃· radical is the dominant hydrogen-transfer intermediate in photocatalytic nitroarene reduction. This discovery is enabled by the design of atomically precise Pb-doped titanium-oxo clusters as programmable molecular platforms that spatially segregate hydrogen activation and substrate binding sites, allowing transient intermediates to be directly captured and identified. By combining electron spin resonance spin trapping, electrospray ionization mass spectrometry, and structural-level site engineering, we directly observe N₂H₃^· and establish a surface-confined concerted proton-electron transfer pathway in which substrate activation, radical generation, and hydrogen transfer all occur within the spatially restricted Pb/Ti dual-site architecture, thereby bypassing conventional hydride, electron, or H^· routes. This mechanism affords high chemo-selectivity (>99%) and broad functional group tolerance under mild conditions. Beyond this specific system, the work establishes a generalizable framework for understanding hydrazine-mediated catalysis and demonstrates the power of molecular clusters as mechanistic microscopes for deconvoluting complex interfacial reaction pathways.