Spatially Confined Photocatalysis: Pyrene-Appended Pd₂L₂ Macrocycle for Selective Visible-Light Aerobic Oxidation of...

The oxidation of sulfides to sulfoxides under visible light is an environmentally benign strategy that uses molecular oxygen (O₂) as a sustainable oxidant. Metal-organic macrocycles (MOCs), constructed via coordination driven self-assembly, offer structurally tunable catalytic environments for such transformations. Herein, we describe the synthesis of a pyrene-functionalized dipyridyl ligand (tBPyrenePy₂, L) that exhibits promising photophysical properties and was subsequently employed to construct a discrete (2 + 2) palladium-based macrocycle, Pd₂L₂. The SC-XRD analysis revealed a Pd₂L₂ composition with a well-defined cocoon-like architecture having an internal cavity measuring 12.65 × 6.33 Å. This confined space was found to play a crucial role in the macrocycle's high photocatalytic efficiency. Pd₂L₂ showed remarkable photocatalytic activity in the aerobic oxidation of thioanisole, achieving over 99% conversion under light irradiation, with no detectable formation of toxic sulfones within the limits of ¹H NMR. Density functional theory (DFT) studies provided significant evidence that the photocatalytic activity is predominantly localized within the internal cavity rather than occurring at the exterior surface of the macrocycle. The study reveals that thioanisole interacts more strongly with the internal cavity of Pd₂L₂, exhibiting an adsorption energy of -0.86 eV, compared to -0.61 eV when bound to the exterior surface. This can be attributed to the unique spatial confinement and electronic environment within the cavity, which likely enhances substrate binding, stabilizes reactive intermediates, and facilitates efficient energy transfer during photocatalysis. This study underscores the potential of structurally confined MOCs in advancing selective and sustainable photocatalytic transformations.