Ternary Complexes in Solution, X X X [1] Increased Stability Through Intramolecular Stacking in Mixed-Ligand Cu2+ and Zn

Mixed-Ligand Complexes, Stacking Interactions, 2,2'-Bipyridyl, 3-Phenylpropionate, Carboxymethyl Aryl Sulfides The stability constants of ternary Cu2+ and Zn2+ complexes, each of which contains 2,2'-bipyridyl and a carboxymethyl aryl sulfide, were determined in 50% aqueous dioxane. A comparison of the stability of these ternary complexes with those formed with simple carboxylates demonstrates an enhanced stability of the carboxymethyl aryl sulfide containing mixed ligand complexes. This enhanced stability is not due to a thioether-metal ion interaction, but due to an intramolecular aromatic stacking interaction between the aryl moiety of the carboxymethyl aryl sulfide and 2,2'-bipyridyl. Indeed, by UV difference spectra and by PMR measurements it is possible to show that a binary (metal ion-free) stacked adduct between the aromatic moieties of the two mentioned ligands is formed. Furthermore, by studying the binary and ternary systems of Zn2+ or Cu2+, 2,2'-bipyridyl and hydrocinnamate, i.e. 3-phenylpropionate ( S of carboxymethyl phenyl sulfide is replaced by -CH2-), it becomes obvious that the thioether moiety is not essential for the observation of an enhanced stability of the ternary complexes. PMR shift studies of 2,2'-bipyridyl/Zn2+/carboxymethyl aryl sulfide systems confirm the presence of stacking in the corresponding ternary complexes. Depending on the kind of the ternary metal ion complex the stability enhancement, due to the intramolecular stacking between the aromatic parts of the coordinated ligands, is between about 0.2 to 0.5 log unit.