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.