Site-specific metal and ligand substitutions in a microporous Mn2+-based metal–organic framework

Abstract

The precise tuning of the structural and chemical features of microporous metal–organic frameworks (MOFs) is a crucial endeavour for developing materials with properties that are suitable for specific applications. In recent times, techniques for preparing frameworks consisting of mixed-metal or ligand compositions have emerged. However, controlled spatial organisation of the components within these structures at the molecular scale is a difficult challenge, particularly when species possessing similar geometries or chemical properties are used. Here, we describe the synthesis of mixed-metal and ligand variants possessing the Mn₃L₃ (Mn-MOF-1; H₂L = bis(4-(4′-carboxyphenyl)-3,5-dimethylpyrazolyl)methane) structure type. In the case of mixed-ligand synthesis using a mixture of L and its trifluoromethyl-functionalised derivative (H₂L′ = bis(4-(4′-carboxyphenyl)-3,5-di(trifluoromethyl)pyrazolyl)methane), a mixed-ligand product in which the L′ species predominanantly occupies the pillar sites lining the pores is obtained. Meanwhile, post-synthetic metal exchange of the parent Mn₃L₃ compound using Fe²⁺ or Fe³⁺ ions results in a degree of cation exchange at the trinuclear carboxylate-based clusters and metalation at the pillar bispyrazolate sites. The results demonstrate the versatility of the Mn₃L₃ structure type toward both metal and ligand substitutions, and the potential utility of site-specific functionalisations in achieving even greater precision in the tuning of MOFs.