The effect of anion–ligand π-stacking interaction on silver(I) extraction by thiaoxa-macrocycles: A search for the evidences by X-ray and NMR approaches

S2Ox macrocyclic ligands with (L1: x = 3 and L2: x = 4) and without (L3: x = 3 and L4: x = 4) dibenzo-subunit were employed as extractants to clarify the anion–ligand interaction on the extraction efficiency of soft metal picrates. All of the ligands shows silver(I) selectivity and the silver picrate was more extractable with L1 and L2 when compared with L3 and L4. From the X-ray and NMR data, the enhanced extractability for the silver picrate by L1 and L2 was revealed due to the additional stabilization of the complexes, [Ag(L)picrate], by anion–ligand π–π interaction between the picrate ion and one benzo-subunit in the ligand. Such anion–ligand interaction would give a valuable hint for the molecular design of new extractants for the soft metal ions including silver(I).