Molecular cubism: Design of higher valency in covalent organic frameworks

The chemistry of crystalline, porous metal-organic frameworks and covalent organic frameworks (MOFs and COFs), collectively referred to as reticular chemistry, has extended zero-dimensional metal complexes and organic molecules into ‘infinite’ two- and three-dimensions (2D and 3D). While MOFs boast a huge diversity of organizational structures — originating from the large variety of metals in the periodic system and their ability to form clusters of different shape and size — COFs are largely made by connecting preformed trigonal-planar, square-planar, and tetrahedral carbon-based building units, thus limiting their structural space.
We here present an isoelectronic strategy to implement 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-condensed into polycubane COFs of valency 8. Upon addition of acid, the polycubic framework rearranged into a layered structure with an unusual infinite rods-within layer arrangement.
The ability of B and P to cluster and rearrange into building units of high valency promises exciting new directions for COF chemistry.

Single organic linkers containing main group elements B and P (A) self-condense into polycubane BP-COFs (B and C). Upon addition of acid these rearrange into layered BP-COFs (D).