Free-Standing Supramolecular Pyridine N-Oxide-Silver(I) Metallogels

Abstract

Twenty-seven pyridine N-oxides (PyNOs) are investigated to evaluate the gelation of their silver(I) trifluoroacetate (AgTFA) complexes across eight solvents. Gelation occurs selectively with PyNOs featuring electron-donating groups, while those with electron-withdrawing or mixed groups do not form gels. A combination of two different PyNOs, one with an electron-donating group and the second with an electron-withdrawing group, form a gel, suggesting that gel-forming PyNO-AgTFA can override the non-gelling tendency of PyNOs comprising electron-withdrawing groups. Pyridine-AgTFA complexes lacking the N–O group fail to gel, underscoring the crucial role of the N–O functionality and its coordination with silver(I) in facilitating gelation. The resulting PyNO-AgTFA gels demonstrate remarkable mechanical strengths, enabling the fabrication of free-standing and load-bearing gel shapes, such as rods and horseshoes, with sizes up to several centimeters. The analysis of 65 X-ray crystal structures reveals that PyNO-AgTFA complexes manifest four distinct structural motifs, even when crystallized under different solvents and ligand-to-metal ratios, demonstrating a strong preference for a specific set of silver(I) complexes. X-ray crystallography and powder X-ray diffraction studies predict gel structures without single crystals. Density functional theory calculations of recurring non-covalent interactions in the crystal structures show interaction energies ranging from −1 to −92 kJ mol−1.