The Last Piece of the Puzzle: Access to 7-Connected Zirconium Metal-Organic Frameworks for Hexane Separation

Abstract

Zirconium metal-organic frameworks (Zr-MOFs) exhibit a wide range of Zr6 cluster connectivities, from 3-connected to 12-connected, enabling diverse structural designs. However, odd-numbered connectivities, especially the 7-connected Zr-MOFs, are exceptionally rare due to geometric symmetry challenges. To address this, we developed a geometric pre-assembly strategy to achieve targeted cluster connectivity. Using this approach, we synthesized NU-5001 {[Zr6O4(OH)4(H4-L)2]}, a Zr-MOF featuring 7-connected Zr6 clusters, derived from the precursor 6-connected Zr-MOF, NU-5000 {[Zr6O4(OH)4(H3-L)2]}. NU-5000, constructed with a tritopic chiral linker H3-L {(R)-4,4',4''-[6'-chloro-2,2'-diethoxy-(1,1'-binaphthalene)-4,4',6-triyl]tribenzoic acid}, forms a (3,6)-connected ant topology. Introducing a tetratopic chiral linker, 1,1'-bi-2-naphthol tetracarboxylic acid H4-L {(R)-4,4',4'',4'''-[2,2'-diethoxy-(1,1'-binaphthalene)-4,4',6,6'-tetrayl]tetrabenzoic acid}, enabled an additional connection, resulting in NU-5001 with a (3,4,7)-connected topology. The H4-L linker acts as either fully 4-connected or 3-connected with one arm uncoordinated. Additionally, NU-5001M {[Zr6O4(OH)4(H4-L) (H3-L)]}, an isostructural variant, was synthesized using a mix of H3-L and H4-L linkers. NU-5001’s unique pore structure and slight flexibility enable distinct adsorption behaviors for hexane isomers, highlighting its potential for kinetic separations. This work completes the Zr6 cluster connectivity landscape and demonstrates the feasibility of tuning Zr-MOF connectivities through rational linker design, paving the way for rare, low-symmetry frameworks with tailored properties.