Ag-S Type Quantum Dots versus Superatom Nanocatalyst: A Single Sulfur Atom Modulated Decarboxylative Radical Cascade Reaction

Abstract

The preparation of high-nuclearity silver nanoclusters in quantitative yield remains exclusive and their potential applications in the catalysis of organic reactions are still undeveloped. Here, we have synthesized a quantum dot (QD)-based catalyst, [Ag₆₂S₁₃(SBu^t)₃₂](PF₆)₄ (denoted as Ag₆₂S₁₂-S) in excellent yield that enables the direct synthesis of pharmaceutically precious 3,4-dihydroquinolinone in 92% via a decarboxylative radical cascade reaction of cinnamamide with α-oxocarboxylic acid under mild reaction conditions. In comparison, a superatom [Ag₆₂S₁₂(SBu^t)₃₂](PF₆)₂ (denoted as Ag₆₂S₁₂) with identical surface anatomy and size, but without a central S^2- atom in the core, gives an improved yield (95%) in a short time and exhibits higher reactivity. Multiple characterization techniques (single-crystal X-ray diffraction, nuclear magnetic resonance (¹H and ³¹P), electrospray ionization mass spectrometry, energy dispersive X-ray spectroscopy, Brunauer-Emmett-Teller (BET), Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, and thermogravimetric analysis) confirm the formation of Ag₆₂S₁₂-S. The BET results expose the total active surface area in supporting a single e^- transfer reaction mechanism. Density functional theory reveals that leaving the central S atom of Ag₆₂S₁₂-S leads to higher charge transfer from Ag₆₂S₁₂ to the reactant, accelerates the decarboxylation process, and correlates the catalytic properties with the structure of the nanocatalyst.