COF-5: structure, synthesis and properties

COF-5 is built from two organic molecules. HHTP (2,3,6,7,10,11-hexahydroxytriphenylene) is a flat, triangular molecule with three catechol (1,2-dihydroxy) pairs at its corners. BDBA (benzene-1,4-diboronic acid) is a straight rod with a boronic acid at each end. Each boronic acid reacts with one catechol, losing two water molecules, to close a five-membered boronate ester ring (C₂O₂B).

Every HHTP joins three BDBA and every BDBA joins two HHTP, so each sheet is a honeycomb (hcb): HHTP at the corners of large hexagons, BDBA along their edges. The sheets are flat and made only of boron, carbon, oxygen and hydrogen.

The sheets stack directly on top of one another, eclipsed (AA), in space group P6/mmm, with a hexagonal cell edge of about 30.0 Å and about 3.40 Å between layers (from its crystal file). Because the hexagons line up, they form open one-dimensional channels 27 Å across, which makes COF-5 a mesoporous material.

1. Condensation. BDBA and HHTP are heated together in an organic solvent mixture in a sealed tube (solvothermal synthesis; Côté et al. 2005).
2. Self-correction. The B–O bonds form and break while the solid grows, so misplaced building blocks can come off again and the product is crystalline rather than amorphous.
3. Activation. The powder is washed and the solvent removed from the pores before gas sorption. Boronate esters react with water (DeBlase et al. 2013), so the material is best kept dry.

Before 2005, organic polymers made of strong covalent bonds were amorphous: errors made during growth stayed in place. COF-1 and COF-5 showed that reversible condensation reactions can give crystalline, permanently porous frameworks “entirely held by strong bonds between B, C, and O atoms” (Côté et al. 2005), extending reticular chemistry from metal–organic frameworks to purely organic ones.

COF-5 in particular showed that the pore size follows from the building blocks: a larger node (HHTP) and an eclipsed stack give 27 Å channels, compared with about 7 Å in COF-1 (Côté et al. 2005). Its hexagonal boronate ester layer became a template for many later 2D COFs with longer linkers.

• Gas storage. COF-5 is one of the reference COFs for hydrogen, methane and carbon dioxide uptake; at 35 bar it takes up more of each than COF-1, but less than the 3D COF-102 (Furukawa & Yaghi 2009).
• Thin films and electronics. Boronate ester COFs from HHTP have been grown as oriented thin films on single-layer graphene, with improved crystallinity compared with powders (Colson et al. 2011).
• Teaching and design. Its simple eclipsed honeycomb is the standard picture of a 2D COF and of how the building blocks set the pore size.
• Limitation. Boronate esters hydrolyse; imine and β-ketoenamine COFs such as TpPa-1 were developed partly to overcome this (DeBlase et al. 2013; Kandambeth et al. 2012).

For a paper, slide or poster, show three levels: the two building blocks (HHTP and BDBA), the boronate ester ring they form, and the layer or its eclipsed stack with a channel marked. The figure above shows all three and opens as an editable LabCanvas figure. The crystal structure drawing is a high-resolution SVG from the COF-5 crystal file and opens in the LabCanvas figure editor, where you can change the style, the number of cells and the view. The 3D model opens in the LabCanvas 3D Studio, where you can change the number of layers, show the channels and export a high-resolution image.