Triazolium-containing metal-organic frameworks: Control of catenation in 2-D Copper(II) paddlewheel structures

Abstract

One approach to exploit MOFs as heterogeneous catalyst platforms requires the development of materials containing groups that can be utilised to anchor a catalytic moiety into the links within the structure. Here we report the synthesis of the first integrated triazolium-containing MOF linker and the first MOFs containing linkers of this type. 1,4-Bis(4-benzoic acid)-1-methyl-1H-1,2,3-triazolium chloride, H₂L1ᴹᵉ, was synthesised in three steps by a 'Click' reaction of methyl 4-ethynylbenzoate with methyl 4-azidobenzoate, methylation using methyl triflate, followed by ester hydrolysis in overall 74% yield. The equivalent neutral triazole precursor, 1,4-bis(4-benzoic acid)-1H-1,2,3-triazole hydrochloride, H₂L1(HCl), was also prepared and a comparison of the chemistry with Zn(NO₃)2·6H₂O and Cu(NO₃)₂·3H₂O is presented. [Zn(L1)₂(H₂O)₂] is a 2-D MOF with infinite chains of zinc carboxylates bridged by L1, while an equivalent structure is not observed for L1ᴹᵉ. In turn, two catenation isomers of [Cu₂(DMF)2(L1ᴹᵉ)2](NO3)₂ were isolated from a single reaction of L1ᴹᵉ and Cu(NO₃)₂·3H₂O. The α-form, a close-packed 3-fold interpenetrated structure, was obtained from reactions undertaken in the presence of nitric acid or at lower temperatures, while undertaking the reaction at higher temperatures leads to a predominance of the 2-fold interpenetrated and potentially porous β-form of the structure. The work presented provides further support for the use of reaction conditions to control interpenetration and additional evidence that charge on structurally similar ligands can drastically alter the types of structures that are accessible due to the requirements for charge balance in the final product.