COF-300: the first imine-linked 3D covalent organic framework

COF-300 takes covalent organic frameworks into three dimensions with a tetrahedral building block. Tetra-(4-anilyl)methane is a central carbon atom carrying four aminophenyl arms that point to the corners of a tetrahedron. Terephthaldehyde is a straight rod with an aldehyde at each end. Each amine condenses with an aldehyde, losing water, to form an imine (C=N) bond.

Every tetrahedral node joins four linkers and every linker joins two nodes, so the framework follows the diamond (dia) net, the same arrangement as the carbon atoms in diamond, with the tetrahedral carbons in place of the atoms and the long linkers in place of the C–C bonds. Its smallest cage has the shape of adamantane.

The cages are so open that further nets grow through them: the X-ray structure shows five independent diamond nets interwoven (interpenetrated) with one another. Even so, pores 7.2 Å across remain (Uribe-Romo et al. 2009).

1. Imine condensation. Tetra-(4-anilyl)methane and terephthaldehyde are heated together under solvothermal conditions (Uribe-Romo et al. 2009).
2. Self-correction. Imine formation is reversible, so bonds can break and re-form while the solid grows, which lets a crystalline framework form.
3. Activation. The solvent is removed from the pores, after which COF-300 shows permanent porosity, 1,360 m² g⁻¹, and stability up to 490 °C (Uribe-Romo et al. 2009).

Later, a general procedure for growing large single crystals of 3D imine COFs gave COF-300 and its hydrated form as crystals solved by single-crystal X-ray diffraction, revealing the degree of interpenetration and the arrangement of the water guests (Ma et al. 2018).

The first 3D COFs, COF-102 and COF-103, were boroxine-linked (El-Kaderi et al. 2007). COF-300 was the first 3D COF linked by imines, made by condensing amines with aldehydes rather than boronic acids. It showed that the geometry of the building blocks, a tetrahedron plus a rod, sets the net, the same design logic that reticular chemistry uses for MOFs.

It also made interpenetration a central topic in 3D COFs: a large, open net tends to fill itself with copies, shrinking the pores. In 2018 COF-300 was among the first COFs solved by single-crystal X-ray diffraction, which let the degree of interpenetration be determined with atomic precision (Ma et al. 2018).

• Model 3D COF. COF-300 is the classic example of how a tetrahedral and a linear building block assemble into a 3D COF.
• Crystallography. COF-300 and its hydrated form were among the first COFs solved from single crystals, at up to 0.83 Å resolution across the series studied (Ma et al. 2018).
• Adsorption screening. It is included in the CURATED COFs database, whose optimized structures were used to compute CO₂ and N₂ isotherms (Ongari et al. 2019).
• Limitation. The five-fold interweaving leaves pores of only 7.2 Å, much smaller than a single diamond net of the same linkers would have.

For a paper, slide or poster, show the two building blocks, one adamantane-shaped cage of the diamond net with imine links, and a sketch of the five interwoven nets in different colours. The figure above has all three and opens as an editable LabCanvas figure. The crystal structure drawing is a high-resolution SVG from the CURATED COFs 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 show the cage atom by atom, change the atom and bond sizes and export a high-resolution image.