# COF linkages: from reversible to robust

> A COF crystallizes only if its bonds can break and re-form while it grows, so that mistakes heal. Boron–oxygen rings (boroxines, boronate esters) do this easily but fall apart in water; imines are sturdier; hydrazones and azines add hydrogen bonds; Tp-based imines turn irreversibly into β-ketoenamines that survive acid and base; and triazine rings made from nitriles in molten salt are tougher still.

- Page: https://www.labcanvas.com/templates/covalent-organic-frameworks/cof-linkages-boroxine-imine-ketoenamine-triazine
- Open in the LabCanvas editor (LabCanvas account): https://www.labcanvas.com/editor.html?template=cof-linkage-chemistry
- Updated: 2026-10-14
- Category: Covalent organic frameworks
- Method: Linkage chemistry, bottom-up
- Starting materials: Boronic acids, catechols, aldehydes, amines, hydrazides, hydrazine, aromatic nitriles
- Product: The bonds between COF building blocks and how robust each is
- Key result: Reversibility gives crystals; irreversible or very strong links give stability to water, acid and base
- Figure: 1920 × 1080 px, 54 editable layers (6 generated objects with settings)

## Steps

1. **Boroxine**: Three boronic acids close a B₃O₃ ring, releasing three water (COF-1; Côté et al. 2005)
2. **Boronate ester**: Boronic acid + catechol: a five-membered C₂O₂B ring (COF-5)
3. **Imine**: Aldehyde + amine: C=N, reversible and sturdier than B–O links (COF-LZU1, COF-300, COF-366)
4. **Hydrazone and azine**: Aldehyde + hydrazide or hydrazine; hydrogen bonds hold the layers flat (TFPT-COF; N₃-COF)
5. **β-Ketoenamine**: Tp aldehyde + amine: the enol-imine tautomerizes irreversibly to the keto-enamine (TpPa-1; Kandambeth et al. 2012)
6. **Triazine**: Three nitriles trimerize in a ZnCl₂ melt at 400 °C (CTF-1; Kuhn et al. 2008)

## Strengths

- One design language for many linkages
- Linkage choice sets stability and electronic properties

## Limitations

- Robust linkages are harder to crystallize
- Boron–oxygen COFs hydrolyse in moist air

## Uses

- Choosing a linkage for catalysis, separation or electrochemistry in water

## References

1. Waller, Gándara & Yaghi, “Chemistry of Covalent Organic Frameworks”, Acc. Chem. Res. 2015. https://doi.org/10.1021/acs.accounts.5b00369 (review)
2. Geng et al., “Covalent Organic Frameworks: Design, Synthesis, and Functions”, Chem. Rev. 2020. https://doi.org/10.1021/acs.chemrev.9b00550 (review)
3. Côté et al., “Porous, Crystalline, Covalent Organic Frameworks”, Science 2005. https://doi.org/10.1126/science.1120411 (primary)
4. Kandambeth et al., “Construction of Crystalline 2D Covalent Organic Frameworks with Remarkable Chemical (Acid/Base) Stability via a Combined Reversible and Irreversible Route”, J. Am. Chem. Soc. 2012. https://doi.org/10.1021/ja308278w (primary)
5. Kuhn, Antonietti & Thomas, “Porous, Covalent Triazine-Based Frameworks Prepared by Ionothermal Synthesis”, Angew. Chem. Int. Ed. 2008. https://doi.org/10.1002/anie.200705710 (primary)
6. Stegbauer et al., “A hydrazone-based covalent organic framework for photocatalytic hydrogen production”, Chem. Sci. 2014. https://doi.org/10.1039/C4SC00016A (further reading)

Credit: After Waller, Gándara & Yaghi, Acc. Chem. Res. 2015 (doi:10.1021/acs.accounts.5b00369) and Geng et al., Chem. Rev. 2020 (doi:10.1021/acs.chemrev.9b00550), with the primary papers in the references. Fragments drawn from bond lengths.

## Questions

**What does the “COF linkages: from reversible to robust” template show?** A COF crystallizes only if its bonds can break and re-form while it grows, so that mistakes heal. Boron–oxygen rings (boroxines, boronate esters) do this easily but fall apart in water; imines are sturdier; hydrazones and azines add hydrogen bonds; Tp-based imines turn irreversibly into β-ketoenamines that survive acid and base; and triazine rings made from nitriles in molten salt are tougher still.

**What are the steps shown in “COF linkages: from reversible to robust”?** 1. Boroxine: Three boronic acids close a B₃O₃ ring, releasing three water (COF-1; Côté et al. 2005). 2. Boronate ester: Boronic acid + catechol: a five-membered C₂O₂B ring (COF-5). 3. Imine: Aldehyde + amine: C=N, reversible and sturdier than B–O links (COF-LZU1, COF-300, COF-366). 4. Hydrazone and azine: Aldehyde + hydrazide or hydrazine; hydrogen bonds hold the layers flat (TFPT-COF; N₃-COF). 5. β-Ketoenamine: Tp aldehyde + amine: the enol-imine tautomerizes irreversibly to the keto-enamine (TpPa-1; Kandambeth et al. 2012). 6. Triazine: Three nitriles trimerize in a ZnCl₂ melt at 400 °C (CTF-1; Kuhn et al. 2008).

**What are the starting materials and the product?** Starting materials: Boronic acids, catechols, aldehydes, amines, hydrazides, hydrazine, aromatic nitriles. Product: The bonds between COF building blocks and how robust each is.

**Which papers is “COF linkages: from reversible to robust” based on?** Waller, Gándara & Yaghi, “Chemistry of Covalent Organic Frameworks”, Acc. Chem. Res. 2015 (doi:10.1021/acs.accounts.5b00369); Geng et al., “Covalent Organic Frameworks: Design, Synthesis, and Functions”, Chem. Rev. 2020 (doi:10.1021/acs.chemrev.9b00550); Côté et al., “Porous, Crystalline, Covalent Organic Frameworks”, Science 2005 (doi:10.1126/science.1120411); Kandambeth et al., “Construction of Crystalline 2D Covalent Organic Frameworks with Remarkable Chemical (Acid/Base) Stability via a Combined Reversible and Irreversible Route”, J. Am. Chem. Soc. 2012 (doi:10.1021/ja308278w); Kuhn, Antonietti & Thomas, “Porous, Covalent Triazine-Based Frameworks Prepared by Ionothermal Synthesis”, Angew. Chem. Int. Ed. 2008 (doi:10.1002/anie.200705710); Stegbauer et al., “A hydrazone-based covalent organic framework for photocatalytic hydrogen production”, Chem. Sci. 2014 (doi:10.1039/C4SC00016A).

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