Biradical-mediated synergistic electrocatalysis using metal-free redox molecular catalysts.

Abstract

Nature's redox enzymes achieve remarkable selectivity by organizing active sites with precise spatial control, an ability difficult to replicate in synthetic systems. Inspired by this, we report a class of stable, metal-free bicarbenium-based molecular catalysts that undergo electrochemical two-electron reduction to form biradical intermediates. These biradicals are confined within a rigid xanthene bridge ( ~ 4.3 Å), creating a spatially defined pocket that engages paramagnetic substrates, such as O₂ and NO, resembling nature's synergistic catalysis. This biradical-mediated synergistic catalysis enables highly selective two-electron oxygen reduction (99.3% H₂O₂ selectivity, 96.8% Faradaic efficiency, 2.21 mol g⁻¹_cat h⁻¹ productivity) and three-electron nitric oxide reduction (NH₂OH as major product, 87.2% Faradaic efficiency, 1.68 mol g^-¹_cat h^-¹ productivity). Experimental and computational studies confirm the sustained redox cycling, supported by favorable spin-pairing and substrate binding geometries, which enhance both catalytic selectivity and efficiency. This work presents a blueprint for pathway-specific, radical-mediated catalysis, offering new design principles for metal-free electrocatalytic platforms that exploit open-shell reactivity.