Covalent Organic Frameworks (COFs) are highly porous crystalline materials comprised of light elements (C, H, N, O, and B), which are possible candidates for gas transport and storage. However, all boronate based COFs are not stable in humid air because of the electron-deficient boron sites that are susceptible to nucleophilic attack.A pyridine-assisted post-synthesis modification was demonstrated to significantly enhance the stability of COF-5 and COF-10 in air, when a trace amount of pyridine (< 1 wt%) was added in as-synthesized COFs in dry acetone suspension. To elucidate the structure associated with this enhanced stability, 15N and 11B solid-state NMR techniques were applied. Two conformations of pyridine and boron interactions were observed, corresponding to N:->B Lewis interaction and pyridinium cations (pyH+) resulting from the neutralization of Br�nsted acid, respectively. In addition, DFT calculations indicate the binding energy of pyridine N with B of the boronate ester ring increases significantly when hydroxyl functionalities are present. Based on the experimental and modeling results, we postulate that at low pyridine concentrations, pyridine is preferentially coordinated with defect sites, which prevent the attack by water molecules in humid air.Alternatively, COF-1 was still unstable with post-synthesis modification. Only Lewis interactions between pyridine and boron were observed by NMR in COF-1. These results suggested that Br�nsted type interactions were critical for stabilizing COFs. The carbon heteroatoms in boronate ester ring-based COF (e.g., COF-10 and COF-5) generate B-(H)O-C acid sites which are neutralized by pyridine. The capped Br�nsted defect sites enhanced the stability of COFs. In order to improve the hydrostability of COF-1, we propose to design new agent which can introduce heteroatoms to generate Br�nsted defect sites as well as capping these defect sites.

Air stable covalent organic frameworks: Post-synthesis modification and stabilization mechanism study

248th National Meeting (2014)

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