The merger of a metal-organic framework and an electron-proton transfer mediator for photocatalytic selective aerobic oxidation of amines

Abstract

Metal-organic frameworks (MOFs) present a multifaceted avenue for visible light photocatalysis and are candidates for environmental applications, in which electron and proton transfers are crucial. To date, the photocatalytic activity of MOFs has been attempted, but with inherent limitations against formidable redox conditions. This can be addressed by adopting an electron-proton transfer mediator to mediate the redox processes over a MOF photocatalyst. To achieve this goal, an electron-proton transfer mediator, HOOC-TEMPO (4-carboxy-2,2,6,6-tetramethylpiperidine-1-oxyl), is envisioned to steer the selective oxidation of amines over a pyrene-based MOF NU-1000. There are abundant terminal hydroxyl groups of Zr-oxo cluster within the mesoporous channels of NU-1000. Supported by density functional theory calculations, the bidentate chelation of HOOC-TEMPO onto NU-1000 by reacting with the hydroxyl groups is the most feasible mode of adsorption. The optoelectronic properties of NU-1000 can be notably improved by the facile and dynamic adsorption of HOOC-TEMPO. Distinctly, 1 mol% HOOC-TEMPO promotes NU-1000 photocatalysis, allowing for three times of conversions in the aerobic oxidation of amines to imines. Compellingly, the hole transfer between the pyrene ligand of NU-1000 and HOOC-TEMPO is more efficient than other ligands. The merger of an electron-proton transfer mediator and MOFs creates a unique materials avenue for emerging photocatalysis.