Embedding alkenes within an icosahedral inorganic fullerene {(NH4)42[Mo132O372(L)30(H2O)72]} for trapping volatile organics

Abstract

Eight alkene-functionalized molybdenum-based spherical Keplerate-type (inorganic fullerene) structures have been obtained via both direct and multistep synthetic approaches. Driven by the opportunity to design unique host-guest interactions within hydrophobic, π-electron rich confined environments, we have synthesised {(NH4)42[Mo132O372(L)30(H2O)72]}, where L = (1) acrylic acid, (2) crotonic acid, (3) methacrylic acid, (4) tiglic acid, (5) 3-butenoic acid, (6) 4-pentenoic acid, (7) 5-hexenoic acid, and (8) sorbic acid. The compounds, which are obtained in good yield (10-40%), contain 30 carboxylate-coordinated alkene ligands which create a central cavity with hydrophobic character. Extensive Nuclear Magnetic Resonance (NMR) spectroscopy studies contribute significantly to the complete characterisation of the structures obtained, including both 1D and 2D measurements. In addition, single-crystal X-ray crystallography and subsequently-generated electron density maps are employed to highlight the distribution in ligand tail positions. These alkene-containing structures are shown to effectively encapsulate small alkyl thiols (1-propanethiol (A), 2-propanethiol (B), 1-butanethiol (C), 2-butanethiol (D) and 2-methyl-1-propanethiol (E)) as guests within the central cavity in aqueous solution. The hydrophobically driven clustering of up to 6 equivalents of volatile thiol guests within the central cavity of the Keplerate-type structure results in effective thermal protection, preventing evaporation at elevated temperatures (ΔT ≈ 25 K).