Abstract
Covalent organic frameworks have emerged as a powerful synthetic platform for installing and interconverting dedicated molecular functions on a crystalline polymeric backbone with atomic precision. Here, we present a novel strategy to directly access amine-linked covalent organic frameworks, which serve as a scaffold enabling pore-wall modification and linkage-interconversion by new synthetic methods based on Leuckart–Wallach reduction with formic acid and ammonium formate. Frameworks connected entirely by secondary amine linkages, mixed amine/imine bonds, and partially formylated amine linkages are obtained in a single step from imine-linked frameworks or directly from corresponding linkers in a one-pot crystallization-reduction approach. The new, 2D amine-linked covalent organic frameworks, rPI-3-COF, rTTI-COF, and rPy1P-COF, are obtained with high crystallinity and large surface areas. Secondary amines, installed as reactive sites on the pore wall, enable further postsynthetic functionalization to access tailored covalent organic frameworks, with increased hydrolytic stability, as potential heterogeneous catalysts.
Highlights
In recent years, covalent organic frameworks (COFs) have emerged as a versatile class of crystalline porous polymers, which have been pushing the frontiers of single-site heterogeneous catalysis ever since
We found that a reduction of imine-linkages would both increase the hydrolytic stability of the framework and introduce secondary amine-linkages as reactive centers for further functionalization of the pore wall
Amine-linked frameworks were introduced as a hydrolytically stable and tailorable system for further postsynthetic modification, which can be accessed from imine-linked frameworks or directly from their corresponding amine and aldehyde building blocks (Figure 4)
Summary
Covalent organic frameworks (COFs) have emerged as a versatile class of crystalline porous polymers, which have been pushing the frontiers of single-site heterogeneous catalysis ever since. The dichotomy of dynamic covalent chemistry in COF synthesis implies that, while reversible bond formation is critical for crystallization, the reversibility of imine bond formation causes its limited stability against hydrolysis To address this issue, several postsynthetic locking strategies have been developed in the past, e.g., converting labile imine-linked COFs into stable benzothiazole-,5,6 amide-,7 or quinoline-linked frameworks.[4,8−13] these methods significantly increase the material’s hydrolytic stability, most do not activate but rather deactivate potential reactivity of the linkages for further pore-wall modification. These findings enable us to identify unique pH-dependent amorphization pathways and expand our fundamental understanding of amine-linked covalent organic frameworks as an important yet underexplored class of heterogeneous catalysts
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