Metallogels and Molybdate Arrays Formed by Self-Assembly of Carbohydrate Molybdenum Complexes

Abstract N-Alkyl-D-galactonamides are amphiphilic galactose derivatives that form supramolecular hydrogels through self-assembly. To enable the processing of these hydrogels at low temperatures in water only, we investigated the reversible complexation of N-heptyl and N-nonyl-D-galactonamides with molybdenum. These molecules readily react with ammonium molybdate to form well-defined dinuclear molybdenum complexes in water. Acidification of the complex solution triggers the formation of transparent, reversible, and thixotropic supramolecular gels, which are markedly distinct from the hydrogels derived from the parent N-alkyl-d-galactonamides. The gelation process is reversible and arises from the self-assembly of N-alkyl-d-galactonamide-molybdenum complexes, driven by the segregation of fatty chains in an aqueous environment and the neutralization of molybdate charges at low pH. In addition, at low pH, polyoxometalates are known to form large aggregates that can also play a role in the self-assembly. The complexes and resulting gels were characterized using 1H, 13C, and 95Mo NMR spectroscopy, mass spectrometry, tensiometry, rheology, small-angle X-ray scattering, transmission electron microscopy, and energy-dispersive X-ray spectroscopy. The N-nonyl-D-galactonamide complex self-organizes into core–shell cylinders that self-assemble further in hexagonal packing, while the N-heptyl-D-galactonamide complex self-organizes in ellipsoidal flat bicelles. In both cases, this organization yields highly ordered molybdate arrays with regular spacings of 3.9 or 2.8 nm, respectively. An alternative gelation method via wet-spinning is also demonstrated. Injection of the complex solution into an acidic bath produces gel membranes and threads. The unique properties of these metallogels, namely, organization of molybdate arrays, in 2D or in 3D, transparency, reversibility, and thixotropy, open promising avenues for shaping composite materials in electrochemistry, electronics, and catalysis.