Exploring the reversible host-guest chemistry of a crystalline octanuclear Ag(i) metallosupramolecular macrocycle formed from a simple pyrazinylpyridine ligand.

Here we report the synthesis of two new 2-(2'-pyrazinyl)pyridine ligands, and explore their coordination chemistry with Cu(ii) and Ag(i) ions, leading to the discovery of metallosupramolecular architectures of surprising complexity. Differing only in the substitution pattern of a nitrophenyl substituent attached to the 4-pyridyl position, ligands L1 and L2 both form sulfate-bridged dinuclear complexes on reaction with copper sulfate, without coordination of the nitrogen atom at the pyrazine 4-position. However, on reaction with Ag(i), both ligands adopt bridging coordination modes which lead to dramatically different outcomes. Ligand L2 reacts with AgCF3SO3 to give a densely packed coordination polymer 3 of unusually low symmetry (Z' = 7), while the structural isomer L1 instead gives an octanuclear macrocycle 4 in the crystalline state. Macrocycle 4, containing crystallographically-defined solvent molecules within its central cavity as well as in the interstitial spaces, readily undergoes solvent exchange in a single-crystal-to-single-crystal transformation. This allows a direct comparison of guest solvent affinity from single component and mixed-component exchange solutions using a combination of X-ray diffraction, NMR and TGA techniques, revealing the fully reversible uptake and exchange preferences of this material.