Confining Single-Atom Cobalt Sustains Singlet Oxygen-Driven Aniline Oxidative Coupling toward Azobenzene

Abstract Direct oxidative coupling of anilines represents an atom-economical route to valuable azobenzenes, yet its practical deployment is severely restricted by the uncontrolled generation of chaotic radical species and the rapid degradation of catalysts under harsh oxidative environments. Herein, we demonstrate that a precisely tailored confining microenvironment enables selective control over reactive oxygen species (ROS) generation, overriding the conventional chaotic radical pathway. With a single-atom cobalt catalyst anchored on nanodiamonds and encapsulated within a ZIF-8 derived NC overlayer (Co1/ND@NC) as a model, singlet oxygen (1O2) was identified as the predominant ROS driving the highly selective coupling of aniline with H2O2 as sustainable oxidant. Mechanistic studies reveal that 1O2 effectively accelerates the conversion of the N-phenylhydroxylamine intermediate to nitrosobenzene, a kinetically favored step that promotes subsequent condensation with aniline to yield azobenzene with exceptional selectivity. Moreover, the NC confinement effectively prevents cobalt leaching, ensuring robust structural integrity of the single-atom sites under harsh oxidative conditions. As a result, the Co1/ND@NC catalyst achieves an azobenzene production rate of ∼7.1 mmol g–1 h–1 with 97.0% selectivity and maintains stable performance over seven consecutive cycles. This work not only uncovers the critical role of the nonradical 1O2 pathway in selective aniline coupling but also establishes a new paradigm for engineering confined single-atom microenvironments for sustainable oxidation catalysis.