8-Quinoline based ligands and their metallic derivatives: A structural and statistical investigation of quinoline π–π stacking interactions

Abstract

The new ligand 8-sulfonyl-(1-pyrazolyl)-quinoline, (C₉H₆N–SO₂-pz, L), which combines two different aromatic rings with donor properties in one molecule (a quinoline moiety functionalized with a pyrazolyl ring through a sulfonyl bridge) has been synthesized and characterized in both solution and solid state. L reacts with AgO₃SCF₃ (AgOTf) and AgBF₄ to form discrete compounds (C₉H₆N–SO₂-pz)₂AgO₃SCF₃ (1) and (C₉H₆N–SO₂-pz)₂AgBF₄ (2). Supramolecular associations were observed in all cases, based on a similar association algorithm, involving the π–π stacking of the quinoline tectons. The presence of the pyrazolyl ring in an appropriate position with respect to the quinoline moiety makes possible the occurrence of additional C–H⋯π interactions between the aromatic rings. These interactions generate dimers in all cases, and in two cases (L and compound 1) the dimers undergo further self-assembly into higher dimension architectures, based either on π–π stacking interactions alone (L) or on combined π–π stacking/C–H⋯π interactions of the quinoline and pyrazolyl groups (1). A geometrical and statistical analysis has also been performed on π–π stacking in metal complexes of quinoline based ligands, by carrying out a Cambridge Structural Database search. The search showed that this interaction is relatively common for metal complexes of functionalized quinoline based ligands and occurs in 69% of all cases. At its simplest level, such an interaction produces discrete dimeric species. Higher dimensionality networks are obtained either by the extension of the π–π stacking interactions in one or more dimensions, or by bringing into play other supramolecular synthons, such as hydrogen bonding or C–H⋯π interactions.