While metal-organic frameworks (MOFs) boast a vast structural diversity—originating from the large variety of metals in the periodic system and their ability to form clusters of different shape and size—covalent organic frameworks (COFs) are predominantly made by connecting preformed carbon-based building units, thus severely limiting their structure space.<br/>We here present two strategies to target new structure types in COFs: conformational design of building units to selectively crystallize 3D structures and an isoelectronic strategy to implement higher valency in COFs.<br/>In the first, we show how conformational preorganization of organic building units has enabled us to selectively crystallize one specific topology out of seven possible structures. In the second, we show how to achieve higher valency in COFs by designing molecules capable of forming cubic and infinite linkages through clustering. Specifically, we designed organic molecules based on the main group elements boron (B) and phosphorus (P), which self-condense into polycubic COFs of valency 8. Upon addition of acid, these polycubic frameworks rearrange into layered structures with an unusual infinite rods-within layer arrangement.<br/>This chemistry breaks with previously pursued synthetic strategies for COFs and opens the doors to new solid-state organic chemistry and its application in materials science.

Higher valency and constitutional dynamics in covalent organic frameworks

ACS Fall 2021

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