TpPa-1: the acid-stable β-ketoenamine COF

TpPa-1 is built from two molecules. Tp (1,3,5-triformylphloroglucinol) is a benzene ring carrying three aldehyde groups, with a hydroxyl group between each pair. Pa-1 (p-phenylenediamine) is a benzene ring with an amine at each end. Each aldehyde condenses with an amine, losing water, in a Schiff base reaction.

Every Tp joins three Pa-1 and every Pa-1 joins two Tp, so each sheet is a honeycomb (hcb) of Tp corners and Pa-1 edges, like COF-5 but with nitrogen-containing links.

The key is the link itself. The expected product is an enol-imine: a C=N imine bond with the neighbouring O–H hydrogen-bonded to it. Instead, the proton moves from oxygen to nitrogen and only the keto-enamine form (C=O and N–H, hydrogen-bonded) is observed (Kandambeth et al. 2012). This tautomerism is irreversible, and with no imine left there is no bond for water or acid to hydrolyse.

1. Schiff base condensation. Tp and p-phenylenediamine react in a 1:1 mixture of mesitylene and dioxane under solvothermal conditions (Kandambeth et al. 2012). This step is reversible, which lets the layers order and crystallize.
2. Tautomerism. Each enol-imine link converts irreversibly to the β-ketoenamine, locking the framework in place.
3. Washing and activation. The powder is washed and its pores emptied before gas sorption.

Using 2,5-dimethyl-p-phenylenediamine (Pa-2) instead gives TpPa-2, whose methyl groups make it stable in strong base as well.

The first COFs, built on boroxine and boronate ester links, are sensitive to water, and imine COFs can be hydrolysed. TpPa-1 and TpPa-2 showed a way out: combine a reversible step, which gives crystallinity, with an irreversible one, which gives stability. Both COFs resist 9 N hydrochloric acid and boiling water, and TpPa-2 also 9 N sodium hydroxide (Kandambeth et al. 2012).

The same idea works with other aromatic amines in place of Pa-1, giving a family of Tp-based β-ketoenamine COFs (Geng et al. 2020). DeBlase et al. (2013) used this chemistry to make a redox-active β-ketoenamine COF that is stable in a strongly acidic electrolyte.

• Work in water and acid. Its chemical stability makes TpPa-1 a candidate for catalysis and separations in conditions that would destroy boronate ester COFs.
• Energy storage. A β-ketoenamine COF electrode kept a higher capacitance than a non-redox-active COF even after 5,000 charge–discharge cycles in acidic electrolyte (DeBlase et al. 2013).
• Gas adsorption screening. TpPa-1 is in the CURATED COFs database, whose structures were optimized and used to compute CO₂ and N₂ isotherms for carbon capture (Ongari et al. 2019).
• Stability testing. Its soaking experiments (acid, base and boiling water, then powder X-ray diffraction) show how the chemical stability of a COF can be checked.

For a paper, slide or poster, show the two monomers, the enol-imine and keto-enamine forms side by side with an arrow for the proton shift, and a honeycomb layer with a β-ketoenamine at every link. The figure above has all of these, plus the soaking tests and powder patterns, 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 change the layers and guests and export a high-resolution image.