Conversion of Imine to Oxazole and Thiazole Linkages in Covalent Organic Frameworks

Imine-linked ILCOF-1 based on 1,4-phenylenediamine and 1,3,6,8-tetrakis(4-formylphenyl)pyrene was converted through consecutive linker substitution and oxidative cyclization to two isostructural covalent organic frameworks (COFs), having thiazole and oxazole linkages. The completeness of the conversion was assessed by infrared and solid-state NMR spectroscopies, and the crystallinity of the COFs was confirmed by powder X-ray diffraction. Furthermore, the azole-linked COFs remain porous, as shown by nitrogen sorption experiments. The materials derived in this way demonstrate increased chemical stability, relative to the imine-linked starting material. This constitutes a facile method for accessing COFs and linkages that are otherwise difficult to crystallize due to their inherently limited microscopic reversibility.