Fine Tuning the Hydrophobicity of a New Three-Dimensional Cu²⁺ MOF through Single Crystal Coordinating Ligand Exchange Transformations

Abstract

The synthesis, characterization, and single–crystal–to–single–crystal (SCSC) exchange reactions of a new 3D Cu2+ MOF based on 5-aminoisophthalic acid (H2AIP), [Cu6(μ3-ΟΗ)3(ΑΙΡ)4(HΑΙΡ)] n ·6nDMF·nH2O - UCY-16·6nDMF·nH2O, are reported. It exhibits a 3D structure based on two [Cu4(μ3–OH)2]6+ butterfly–like secondary building units, differing in their peripheral ligation, bridged through HAIP–/AIP2– ligands. This compound displays the capability to exchange the coordinating ligand(s) and/or guest solvent molecules through SCSC reactions. Interestingly, heterogeneous reactions of single crystals of UCY-16·6nDMF·nH2O with primary alcohols resulted not only in the removal of the lattice DMF molecules but also in an unprecedented structural alteration that involved the complete or partial replacement of the monoatomic bridging μ3–OH– anion(s) of the [Cu4(μ3–OH)2]6+ butterfly structural core by various alkoxy groups. Similar crystal-to-crystal exchange reactions of UCY-16·6nDMF·nH2O with long-chain aliphatic alcohols (C x H2x+1OH, x = 8–10, 12, 14, and 16) led to analogues containing fatty alcohols. Notably, the exchanged products with the bulkier alcohols UCY-16/ n-C x H2x+1OH·S′ (x = 6–10, 12, 14, and 16) do not mix with H2O being quite stable in this solvent, in contrast to the pristine MOF, and exhibit a hydrophobic/superhydrophobic surface as confirmed from the investigation of their water contact angles and capability to remove hydrophobic pollutants from aqueous media.