Using geometric simulation software ‘GASP’ to model conformational flexibility in a family of zinc metal–organic frameworks

Here, a new tripodal tricarboxylic acid ligand, 4,4′-(4′-(4′-carboxy-[1,1′-biphenyl]-4-yl)-[2,2′:6′,2′′-terpyridine]-5,5′′-diyl)-dibenzoic acid (H3cbt), was synthesised using a three-step convergent strategy. Subsequent reactions with zinc(ii) nitrate hexahydrate yielded three metal-organic frameworks (MOFs). The three MOFs, [Zn(Hcbt)]·4DMF (1), [Zn(Hcbt)]·4DMSO·1.5H2O·DMF (2), and [Zn(Hcbt)]·2DMF·3H2O (3), each adopt flexible interdigitated 2D net topologies. Framework 1 has DMF-filled channels that retain porosity upon desolvation, with a measured BET surface area of 248 m2 g-1. Framework 2 possesses larger DMSO-containing channels that collapse upon desolvation, resulting in near-equivalent porosity values to framework 1. In silico calculations and topological considerations determined using the geometric simulation software GASP dictate that framework 2 can feasibly alter conformation to approximate 1, but cannot perfectly replicate the interdigitated motif. Framework 3 formed when wet solvents were used to synthesise 1. Interestingly, the interdigitated structure of 3 contains a unique carboxylate binding mode that precludes its subsequent adoption by either 1 or 2 upon their exposure to water. This diverse array of structural considerations recommends this MOF family for modelling using GASP. Interrogating frameworks 1-3 using this software provided insights that justified experimentally observed conformational trends, as well as barriers to interconversion between members of this MOF family. In a broader sense, this work demonstrates the wider applicability of GASP software to modelling structural changes within flexible MOF materials.