Design of higher valency in covalent organic frameworks

• Published in: Science (American Association for the Advancement of Science) Vol. 370; no. 6515
• Main Authors: Gropp, Cornelius, Ma, Tianqiong, Hanikel, Nikita, Yaghi, Omar M.
• Format: Journal Article
• Language: English
• Published: United States American Association for the Advancement of Science (AAAS) 23.10.2020
• ISSN: 0036-8075; 1095-9203; 1095-9203
• DOI: 10.1126/science.abd6406

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 A polycube linker formed from 1,4-boronophenylphosphonic acid affords cubic and infinite-rod covalent organic frameworks. 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 (–B 4 P 4 O 12 –)(–C 6 H 4 –) 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.