Fabrication of a Cu(i)-carboxylate metal–organic framework by reduction of metal nodes for an azide–alkyne “click” reaction

Abstract

Metal–organic frameworks (MOFs) permit significant structural diversity and functional design flexibility due to the various combinations of metal centers and different organic linkers. However, their combinations generally adhere to the classic hard and soft acid and base (HSAB) theory. This makes it impossible to directly synthesize desired MOFs with converse Lewis types of metal ions and ligands. Herein, we present a novel Cu(II) ion cleavage reduction strategy that circumvents the limitations of HSAB theory. We demonstrate this approach by utilizing a metastable CuH₂DOBDC MOF, whose inherent structural instability facilitates structural transformations and phase transitions in response to external stimuli. By leveraging the unique structure of ascorbic acid to reduce Cu(II) ions and cleave the H₂DOBDC²⁻ linkers, we successfully fabricate a new Cu₂H₂DOBDC structure composed of a soft acid Cu(I) and a hard base (H₄DOBDC). The resultant Cu₂H₂DOBDC integrates the characteristics of mesoporosity and hierarchical nanostructures, resulting in excellent mass transfer and abundant accessible Cu(I) active sites, which endows the material with outstanding catalytic activity in the azide–alkyne cycloaddition reaction. This is the first example of fabricating stable single-phase Cu(I)-carboxylic acid MOFs with nearly all Cu(I) ions, and this work offers a new perspective on the creation of a new category of MOFs with soft-acid and hard-base type coordination bonds.