Diurea and Triurea Derivatives for the Synthesis of Highly Crystalline COFs

Covalent organic frameworks (COFs) are crystalline porous materials synthesized by the covalent linkage of organic linkers, and are anticipated to be used as molecular separation or storage, or as photocatalysts, and electronics materials.¹⁾ Diurea derivatives such as 1,1'-(1,4-phenylene)diurea (1), are employed in the synthesis of COFs composed of flexible urea linkages.²⁾ In addition, Zhu and Cooper et al. have reported a method for synthesizing β-ketoenamine-type COFs using urea-type COFs as precursors.³⁾ Initially, urea-linked COFs were synthesized by the condensation of urea derivatives including 1 and 1,1'-([1,1'-biphenyl]-4,4'-diyl)diurea (2) with 2,4,6-triformylphloroglucinol. Since the crystallinity of COFs is improved via error-correction process during synthesis, the highly reversible urea bonds contribute to improving crystallinity. Subsequently, heating in a solvent containing a small quantity of water led to the hydrolysis of the urea bonds, followed by imine condensation, resulting in β-ketoenamine-type COFs. These “reconstructed” COFs were shown to have higher crystallinity, porosity and photocatalytic performance compared to the corresponding directly synthesized COFs.

References

• 1) The Atom, the Molecule, and the Covalent Organic Framework
  • C. S. Diercks, O. M. Yaghi, Science 2017, 355, 923.
• 2) Urea-Linked Covalent Organic Frameworks
  • C. Zhao, C. S. Diercks, C. Zhu, N. Hanikel, X. Pei, O. M. Yaghi, J. Am. Chem. Soc. 2018, 140, 16438.
• 3) Reconstructed covalent organic frameworks
  • W. Zhang, L. Chen, S. Dai, C. Zhao, C. Ma, L. Wei, M. Zhu, S. Y. Chong, H. Yang, L. Liu, Y. Bai, M. Yu, Y. Xu, X.-W. Zhu, Q. Zhu, S. An, R. S. Sprick, M. A. Little, X. Wu, S. Jiang, Y. Wu, Y.-B. Zhang, H. Tian, W.-H. Zhu, A. I. Cooper, Nature 2022, 604, 72.

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