Towards understanding π-stacking interactions between non-aromatic rings

Abstract

The first systematic study of π interactions between non-aromatic rings, based on the authors' own results from an experimental X-ray charge-density analysis assisted by quantum chemical calculations, is presented. The landmark (non-aromatic) examples include quinoid rings, planar radicals and metal-chelate rings. The results can be summarized as: (i) non-aromatic planar polyenic rings can be stacked, (ii) interactions are more pronounced between systems or rings with little or no π-electron delocalization (e.g. quinones) than those involving delocalized systems (e.g. aromatics), and (iii) the main component of the interaction is electrostatic/multipolar between closed-shell rings, whereas (iv) interactions between radicals involve a significant covalent contribution (multicentric bonding). Thus, stacking covers a wide range of interactions and energies, ranging from weak dispersion to unlocalized two-electron multicentric covalent bonding (`pancake bonding'), allowing a face-to-face stacking arrangement in some chemical species (quinone anions). The predominant interaction in a particular stacked system modulates the physical properties and defines a strategy for crystal engineering of functional materials.

Supramolecular Architecture of Chloranilate Salts with Organic Cations

“…Besides hydrogen bonding, π-stacking is also present in all structures except in compound 2 (Table 4); planar chloranilate moiety and aromatic rings are commonly involved in stacking interactions, [31] while non-planar piperazinediium cations of 2 do not stack. In compound 1 the main hydrogen-bonded units Hpy + and CA 2- (Figure 3 a) form stacks which extend in the direction [100] (Figure 4 a).…”

“…[9] Hydrogen chloranilate monoanions in their simple alkali salts often form a rare type of face-to-face π-stacks with extremely short interplanar separations of about C 3.3 Å; [10,11] this specific arrangement is a result of unique electronic structure of the anion, with alternating areas of positive and negative electrostatic potential. [12] This type of π-interaction has an energy exceeding 10 kcal mol -1 , which is by an order of a magnitude stronger than common π-interactions between aromatic rings, [12,31] and was also found in salts of closely related hydrogen bromanilate dianion [32] and unsubstituted 2,5-dihydroxyquinonate. [33] This face-to-face stacking is the dominant interaction in the crystals, even where hydrogen bonding is present.…”

Natural Blues: Structure Meets Function in Anthocyanins

“…π–π interactions are not limited to arene moieties; cyclical aliphatic molecules have also been demonstrated to take part in these interactions, suggesting that the requirement for electron delocalisation (as seen in aromatics) is not strict [ 54 ]. This is particularly noteworthy within non-aromatic heterocycles, such as glucose, where the presence of an oxygen atom introduces asymmetry within the heterocycle, resulting in polarisation.…”

Pharmaceutical co‐crystal: An alternative strategy for enhanced physicochemical properties and drug synergy

“…He also described these weak interactions as what keeps the organic world together. [49,50] Interactions such as hydrogen bonding; strong (2.2-2.5Å), intermediate (2.5-3.2Å) and weak (3.2-4.0 Å) with angle cutoffs > 90 degree, [51] π−π stacking (either in a flat configuration with face-to-face arrangement or edge configuration with edge-to-face arrangements), [52][53][54] electrostatic, hydrophobic, charge transfer, metal coordination, halogen bonding, and metallophilic interactions [55] interconnect molecules in the lattice via different supramolecular synthons discussed in the previous paragraphs. Molecular recognition (which is a necessary complementarity between molecules forming an aggregate) is compulsory in this formation and stabilization of supramolecular systems.…”