Higher-valency ligands for COFs
Metal-organic frameworks (MOFs) have exhibited more extensive connectivity (valency) and topological diversity than covalent organic frameworks (COFs), mainly because MOF linkers can connect from 3 to 24 discrete units or even infinity for one-dimensional rods. For COFs, linkers generally have a valency of 3 or 4 that reflect the valency of organic carbon. Gropp et al. created cubane-like linkers from 1,4-boronophenylphosphonic acid that could condense to make COFs with a valency of 8 or, by adding acid, could form large, single crystals with an infinite-rod topology.
Science, this issue p. eabd6406
Valency is the connectivity of building units in reticular frameworks. Although metal-organic frameworks (MOFs) are known to have valencies of 3 to 24, covalent organic frameworks (COFs) are limited to the lower valencies of 3 and 4, principally owing to the heavy reliance of organic chemistry on sp2 and sp3 hybridization. We show that the diversity of COFs can be increased by finding new ways of linking simple organic molecules into building units of higher and even infinite valencies.
Now, COFs are made by connecting preformed trigonal-planar, square-planar, and tetrahedral organic building units with linkages of low valency. Our strategy is to implement higher valency in COFs by designing molecules capable of forming higher-valency linkages through clustering. This is difficult to achieve by conventional organic methods. We therefore relied on the isoelectronic replacement of carbon-group elements by boron and phosphorus and demonstrated the feasibility of this chemistry by using borophosphonate for the silicate cube motif. We designed simple organic BPA linkers (BPA-1 to 5) based on boron and phosphorus, which self-condensed into cubic units and polycubane COFs of valency 8. The versatility of this chemistry was further exploited by cleaving the cubes in the polycubane COFs, leading to structures with rod units of infinite valency.
The BPA-1 linker combines a boronic acid and a phosphonic acid functionality in a single molecule, which was converged into the boron-phosphorus (BP) cube of a reticulated polycubane BP-COF-1. Specifically, eight BPA-1 linkers self-condensed to form the BP cube with the elimination of eight water molecules per cube. The crystallization of BP-COF-1 was realized through microscopic reversibility: The polarized B–O–P linkage can dynamically form and break at the B–O bond. This constituted new chemistry whereby simple linkers converged into frameworks composed of higher-valency clusters. The versatility of this strategy was demonstrated by the successful crystallization of isoreticular polycubane structures, BP-COF-2 to 5 from the BPA-2 to 5 linkers. This series of functionalized and expanded polycubane structures exhibited permanent porosity. We found that upon addition of acid to BP-COF-1, eight B–O bonds per cube were cleaved and rearranged into BP-COF-6, which exhibits rods of infinite valency. The structure of BP-COF-6 was obtained from single-crystal x-ray diffraction, making it one of the few COFs grown as large crystals. It revealed infinite B–O–P rods linked by phenyl units to form layers, which in the crystal were joined by interstitial water molecules acting to stabilize an otherwise unusual rod-within-layer arrangement.
The chemistry of BPA linkers and their demonstrated ability to form cubes and higher-valency structures expand the scope of COFs. In essence, BPA linkers were indispensable in making linkages in the form of cubes and rods of higher valency and their corresponding extended BP-COF structures, thereby opening opportunities for the diversification of COFs through clustering of organic molecules based on sp2 and sp3-hybridized atoms. The extension of this approach to COFs containing combinations of different shapes promises to uncover what we anticipate is a large, untapped structure space.
(A) BPA linkers (BPA-1 to 5) combine a boronic and a phosphonic acid functionality in a single molecule. (B and C) Self-condensation of BPA-1 (B) afforded BP cubes of valency 8 (C) and their reticulated polycubane BP-COF-1. Isoreticular forms (BP-COF-2 to 5) demonstrated the versatility of this chemistry. (D) Upon addition of acid, BP-COF-1 rearranged into the layered BP-COF-6, which exhibits rods of infinite valency.
The valency (connectivity) of building units in covalent organic frameworks (COFs) has been primarily 3 and 4, corresponding to triangles and squares or tetrahedrons, respectively. We report a strategy for making COFs with valency 8 (cubes) and “infinity” (rods). The linker 1,4-boronophenylphosphonic acid—designed to have boron and phosphorus as an isoelectronic combination of carbon-group elements—was condensed into a porous, polycubane structure (BP-COF-1) formulated as (–B4P4O12–)(–C6H4–)4. It was characterized by x-ray powder diffraction techniques, which revealed cubes linked with phenyls. The isoreticular forms (BP-COF-2 to 5) were similarly prepared and characterized. Large single crystals of a constitutionally isomeric COF (BP-COF-6), composed of rod units, were also synthesized using the same strategy, thus propelling COF chemistry into a new valency regime.