Mechanistic investigations into the cyclization and crystallization of benzobisoxazole-linked two-dimensional covalent organic frameworks

Abstract

Although many diverse covalent organic frameworks (COFs) have been synthesised over the past decade, our fundamental understanding of their nucleation and growth during the crystallization process has progressed slowly for many systems. In this work, we report the first in-depth mechanistic investigation detailing the role of nucleophilic catalysts during the formation of two distinct benzobisoxazole (BBO)-linked COFs. The BBO-COFs were constructed by reacting 1,3,5-tris(4-formylphenyl)benzene (TFPB) and 1,3,5-tris(4-formylphenyl)triazine (TFPT) C ₃-symmetric monomers with a C ₂-symmetric o-aminophenol substituted precursor using different nucleophiles (e.g. NaCN, NaN₃, and NaSCH₃). Our experimental and computational results demonstrate that the nucleophiles help initiate an oxidative dehydrogenation pathway by producing radical intermediates that are stabilized by a captodative effect. We also demonstrate that the electron deficient TFPT monomer not only aids in enhancing the crystallinity of the BBO-COFs but also participates in the delocalization of the radicals generated to help stabilize the intermediates.

Scheme 1. Cyanide-catalyzed synthesis of BBO-COF 2 and 3.

Fig. 1. Nitrogen isotherm at 77 K (a) and NLDFT pore size distribution (b) for BBO-COF 3.

Fig. 2. Nitrogen isotherms at 77 K (a) and NLDFT (b) pore size distributions for BBO-COF 3 over a four-day reaction period.

Fig. 3. IR spectra of BBO-COF 3 highlighting the disappearance of the OH stretch from phenolic imine-linked intermediate and aldehyde stretch from the TFPT linker over a four-day reaction period.

Scheme 2. Proposed mechanism for the stepwise oxidative dehydrogenation pathway to form the BBO-linkage using NaCN as a catalyst.

Fig. 4. Nitrogen gas isotherms (a and d), IR spectra (b and e), and PXRD patterns (c and f) of BBO-COF 2 (top) and BBO-COF 3 (bottom). Each reaction was catalyzed by adding 1 eq. of NaCN (red), NaSCH₃ (blue), and NaN₃ (green). Reactions in which no catalyst was used are shown in orange.