Secondary amine-functionalised metal–organic frameworks: direct syntheses versus tandem post-synthetic modifications

Abstract

[Zn₄O(bdc-NH₂)₃], IRMOF-3 (bdc-NH₂ = 2-amino-1,4-benzenedicarboxylate), and [Cr₃O(H₂O)₂F(bdc-NH₂)₃], MIL-101(Cr)-NH₂, undergo tandem post-synthetic modification reactions with aldehydes and NaCNBH₃ to form secondary amine-functionalised metal–organic frameworks (MOFs). The degree of conversion ranges from 17–74% for IRMOF-3 (2a–i) and from 35–51% for MIL-101(Cr)-NH₂ (4a–d), and alkene, sulfide, ferrocenyl and pyridyl substituents can be successfully introduced into the zinc MOFs. For both the zinc and chromium MOFs, an increase in the steric bulk of the aldehyde leads to a reduction in the degree of conversion. Low conversion with bulky aldehydes can be exploited to generate products containing two different secondary amine substituents, such as [Zn₄O(bdc-NH₂)_1.93(bdc-NHCH₂CH₂CH₂SMe)_0.47(bdc-NHEt)_0.59] 2k in which sequential tandem modifications have taken place. N₂ adsorption experiments reveal that the post-synthetically modified MOFs display lower than anticipated BET surface areas together with hysteresis. This is consistent with some degradation of crystallinity occuring on treatment with NaCNBH₃, as verified by control experiments, which leads to the formation of mesopores. Reactions of H₂bdc-NH₂ with aldehydes and NaCNBH₃ afforded a series of secondary amine functionalised dicarboxylic acids H₂L^1–8 after acid work up. These acids were reacted with Zn(NO₃)₂·6H₂O to form the secondary amine-functionalised MOFs 5a–h through direct synthesis. ¹H NMR analysis of 5a–g showed the presence of some bdc-NH₂ linkers in addition to the secondary amine-containing linkers, and indeed the presence of H₂bdc-NH₂ was required for the reaction with H₂bdc-NHCH₂Fc (Fc = ferrocenyl) to form a crystalline MOF. In the case of the zinc MOFs, N₂ adsorption experiments generally showed greater BET surface areas for the products of the direct synthesis reactions than for those from post-synthetic modifications suggesting that, in this case, the former is the best approach to these functionalised MOFs. In contrast, post-synthetic modification is the optimal approach to form crystalline derivatives of MIL-101(Cr)-NH₂ as direct synthesis gave amorphous products.