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.

On supramolecular chemistry grounds proton donors (chloranilic acid) and acceptors (organic bases) were combined to generate multi-component hydrogen bonded systems. Two steps of deprotonation of chloranilic acid, leading to the monoanion and dianion, are coupled by a single or double electron delocalization, respectively. The dissociation of chloranilic acid is accompanied by proton transfer from the acid to an organic base if their ΔpK_a value is large enough to support salt formation. A series of salts with organic bases such as dimethylamine, 2-methylpyridine, 2,6-dimethyl-4-amino-pyrimidine, and R,S-tryptophane were prepared and their crystal structures determined. The co-crystals of neutral chloranilic acid with 5,6-phenantroline were also obtained. In the crystal packing of these multicomponent systems, the predominant interactions are hydrogen bonds. According to the topology of the hydrogen bonds, six distinctive motifs are recognized. The basic structural units are either dimeric monoanions of chloranilic acid or monomeric dianions, and monomers of the neutral acid. Due to the presence of protonated bases, the dominant interaction is the N⁺ –H⋯O hydrogen bond whereas a negative charge of an acceptor from the chloranilate anion is delocalised. The stereochemical parameters of the bases influence the topology leading to the formation of discrete motifs or chains.

The multicomponent crystals of chloranilic acid (H₂CA) and its alkali salts of monochloranilate trihydrate [RbHCA·0.5H₂CA·3H₂O and CsHCA·0.5H₂CA·3H₂O] and cocrystals with urea [NaHCA·2(NH₂)₂CO and Cs₂CA·(NH₂)₂CO] were prepared and their crystal structures determined. In the crystal structures of RbHCA·0.5H₂CA·3H₂O and CsHCA·0.5H₂CA·3H₂O, molecules of neutral chloranilic acid and its monoanion form stacks of quinoid rings in a face-to-face fashion (the sequence ⋯HCA⁻⋯H₂CA⋯HCA⁻⋯); within the triplet very short centroid–centroid separation distances (<3.30 Å) with the offset of 0.27 Å are observed. In an infinite stack of triplets, interactions between two monoanions are characterised by the centroid–centroid separation distances of ∼3.50 Å and offset of 1.5 Å. The two novel cocrystals with urea reveal anion stacks similar to those of aromatic rings. The Cambridge Structural Database search was used to spot preferences of chloranilic acid and its anions for π-stacking.

The anion radicals with nonaromatic π-systems, such as semiquinones, are interesting in the design of functional materials. Magnetic properties of solvates of alkali salts of tetrachlorosemiquinone (chloranil) radical anion are tuned by crystal engineering using different solvents (2-butanone and acetonitrile) and/or cations (potassium and ammonium) in crystals. The structural and magnetic characteristics of two salts were studied by variable-temperature single crystal X-ray diffraction and magnetic susceptibility measurements. DFT and CAS-MP2 calculations were used to correlate magnetic and structural properties of radical anions. The solid-state structure of potassium tetrachlorosemiquinone 2-butanone solvate, influenced by temperature, switches between a paramagnetic-like monomeric radical form at high temperature (200 K) and diamagnetic dimeric one at low temperature (100 K). The reversible transition is controlled by solvent and cation types which affect intrastack distance and magnetic properties.

The alkali diaqua complexes of hydrogen chloranilic acid of [KHCA(H₂O)₂], [RbHCA(H₂O)₂] and [CsHCA(H₂O)₂], and the salt of NH₄HCA·2H₂O were prepared and their crystal structures determined. The homologues of lithium and sodium are not stable, but their analogues [LiHCAEtOH] and [Na₅HCA(CA)₂(H₂O)₁₀] were obtained. All crystal structures studied, with exception of [LiHCAEtOH], reveal 3D-hydrogen bond networks with pronounced ππ interactions between anions supported by attractive polar interactions; the shortest separation distance between the centroids [3.229(2) Å] of the hydrogen chloranilate rings in a face-to-face stack is observed in the structure of [KHCA(H₂O)₂]. The analysis presented is focused on noncovalent interactions, in particular, the influence of cations and hydrogen bonds on the π-stacking of quinoid rings.