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.

X-ray charge density was determined and analyzed for two polymorphs of the Nmethylpyridinium salt of the tetrachlorosemiquinone radical anion and its analogous closed-shell relatives, tetrachloroquinone (chloranil) and tetrachlorohydroquinone. The study, which was combined with calculations of electron delocalization, electrostatic potentials, and aromaticity, presents details of electronic structure of the semiquinoid ring. This comparative study reveals that the negative charge is delocalized over the entire semiquinone radical, and that the chlorine substituents play a crucial role in its stabilization through induction effect. In general, the semiquinoid ring has partially delocalized π-electrons and is approximately halfway between a quinoid and an aromatic ring. In the orthorhombic polymorph with stacks of equidistant radicals electron density between the rings of almost 0.05 e Å-3 and four (3,-1) saddle points between the contiguous rings were found. In the diamagnetic triclinic polymorph, comprising strongly bound radical dimers (with significant covalent character-'pancake bond'), maximum electron density between the rings exceeds 0.095 e Å-3 and multiple (3,-1) critical points are found. However, only negligible electron density is observed between the dimers. Thus, in the radical anion stacks spin coupling, along with dispersive and polarization effects, defines interplanar distance and magnetic behaviour, whereas intermolecular electrostatic potential determines the ring offset.

The present review is aimed to compare crystal packing interactions contributing to stacking arrangements of primarily nonaromatic systems referring only briefly to classical aromatic stacking. The classical aromatic stacking is mainly based on weak dispersion interactions (E ≤ 1 kcal mol–1) whereas heteroaromatics reveal more electrostatic (or specifically dipolar) contributions (E = 5–10 kcal mol–1). Based mainly on our charge density studies and DFT calculations, the results show that (i) all planar rings stack, regardless of aromaticity (or delocalization of π electrons) and (ii) stacking interactions cover a wide continuum ranging from weak, mainly dispersion interactions (E < 5 kcal mol–1) to unlocalized two-electron multicentric (2e/mc) covalent bonds (“pancake bonds”, E > 15 kcal mol–1). Our recent studies showed that quinones form face-to-face stacks and the energies of interactions exceed 10 kcal mol–1; ours and other authors’ results indicate that interactions between planar radicals involve a significant contribution of covalent bonding. Thus, π-interactions cover a broad range of energies, ranging from ≤1 to ≥20 kcal mol–1, and the interactions span from weak dispersion to multicentric covalent bonding. Therefore, development of a universal model of stacking is needed. In this respect, stacking can be compared to hydrogen bonding, which also ranges between dispersion (weakest hydrogen bonds, such as C–H···S and C–H···Cl) and two-electron/three-centric covalent bonding (the strongest “symmetrical” hydrogen bonds).

The studied heterometallic [CuFe] compounds, based on an [Fe(C2O4)3]3− building block and containing a 3D network or 1D ladder-like chains, were synthesized depending on whether the test tube with the same reaction layers was exposed to daylight or not.

A crystal engineering approach is used to stabilize a radical anion in the crystalline state and to modulate the separation distance within π-stacks of anion radicals. Alkali metal salts of 2,3-dicyano-5,6-dichlorosemiquinone (C8Cl2N2O2, DDQ∙- radical anions were prepared and their crystal structures determined: LiDDQ·2H2O·(CH3)2CO, RbDDQ·2H2O and CsDDQ·2H2O. In these structures, stacked dimers of radical anions are formed within π-stacked columns. Within the stacked dimers, interplanar separation distances are significantly shorter than the sum of the van der Waals radii for two C atoms; the shortest is 2.812 Å for the Li salt and the longest is 2.925 Å for the Cs salt. Diamagnetic character, observed by electron paramagnetic resonance spectroscopy, indicates spin-coupling of the unpaired electrons within the radical anion dimer. The electron-rich cyano substituents on DDQ∙- influence the electron redistribution within the ring skeleton. The crystalline compounds are also characterized by IR spectroscopy, complemented by quantum-chemical calculations based on both isolated and periodic models.

The covalent nature of strong NÀBr···N halogen bonds in ac ocrystal (2)o fN -bromosuccinimide (NBS)w ith 3,5-dimethylpyridine (lut)w as determined from X-rayc harge density studies and compared to aw eak N À Br···O halogen bond in pure crystalline NBS (1)and acovalent bond in bis(3methylpyridine)bromonium cation (in its perchlorate salt (3). In 2,the donor NÀBr bond is elongated by 0.0954 ,while the Br···acceptor distance of 2.3194(4) is 1.08 shorter than the sum of the van der Waals radii. Amaximum electron density of 0.38 e À3 along the Br···N halogen bond indicates ac onsiderable covalent contribution to the total interaction. This value is intermediate to 0.067 e À3 for the Br···O contact in 1,a nd approximately 0.7 e À3 in both N À Br bonds of the bromonium cation in 3.Acalculation of the natural bond order charges of the contact atoms,and the s*(N1ÀBr) population of NBS as af unction of distance between NBS and lut,h ave shown that charge transfer becomes significant at aB r···N distance belowabout 3 .

A novel oxalate-based complex of the formula {Ba(2)(H(2)O)(5)[NbO(C(2)O(4))(3)]HC(2)O(4)}·H(2)O (1) was prepared from an aqueous solution containing the [NbO(C(2)O(4))(3)](3-) and Ba(2+) entities in the molar ratio 1:2, and characterized by X-ray single-crystal diffraction, IR spectroscopy, and thermal analysis. The crystal packing of 1 reveals a complex three-dimensional (3D) network: the Nb polyhedron is connected to eight neighboring Ba polyhedra through the oxalate ligands and the oxo-oxygen group, whereas the Ba polyhedra share edges and vertices. The ability of compound 1 to act as a single-source precursor for the formation of bimetallic oxides was investigated by the thermal analysis (TGA and DSC) and X-ray powder diffraction. Thermal processing of 1 resulted in the formation of mixed-metal oxide phases, Ba(4)Nb(2)O(9) and Ba(5)Nb(4)O(15). Three stable polymorphs of Ba(4)Nb(2)O(9) were isolated: the known, hexagonal α- and orthorhombic γ-Ba(4)Nb(2)O(9), and another one, not previously reported, hexagonal δ-Ba(4)Nb(2)O(9) polymorph. The new, δ-Ba(4)Nb(2)O(9) polymorph has the 6H-perovskite structure (space group P6(3)/m), in which the Nb(2)O(9)(8-) face-sharing octahedral dimers are interconnected via corners to the regular BaO(6)(10-) octahedra. Formation of the mixed-metal oxides takes place at different temperatures: the Ba(5)Nb(4)O(15) oxide occurred at ∼700 °C, as the major crystalline oxide phase; by heating the sample up to 1135 °C, the α-Ba(4)Nb(2)O(9) form was obtained, whereas the heating at 1175 °C caused the crystallization of two polymorphs, γ-Ba(4)Nb(2)O(9) and δ-Ba(4)Nb(2)O(9). Special focus was set on the electrical properties of the prepared mixed Ba(II)-Nb(V) oxides obtained by this molecular pathway in a single-step preparation.

The heterodimetallic [CuFe] compounds [CuII 4(terpy)4Cl5]­[FeIII(C2O4)3]·10H2O (1;terpy = 2,2′:6′,2′′-terpyridine), [CuII 2­(H2O)2(terpy)2(C2O4)]­[CuIIFeIII(CH3OH)­(terpy)­(C2O4)3]2 (2), and {[Cu2 IIFeIII(H2O)­(terpy)2(C2O4)7/2]·6H2O} n (3) were obtained using building block approach, from reaction of aqueous solution of [Fe­(C2O4)3]3– and a methanol solution containing Cu2+ ions and terpy by the layering technique. Interestingly, by changing only the anion of the starting salt of copper­(II), Cu­(NO3)2·3H2O instead of CuCl2·2H2O, an unexpected change in the type of bridge, oxalate (2 and 3) versus chloride (1), was achieved, thus affecting the overall structural architecture. Two polymorphs of 3D coordination polymer [CuIIFeII 2(H2O)­(terpy)­(C2O4)3] n (4), crystallizing in the triclinic (a) and monoclinic (b) space groups, were formed hydrothermally, depending on whether CuCl2·2H2O or Cu­(NO3)2·3H2O was added to the water, besides K3[Fe­(C2O4)3]·3H2O and terpy, respectively. Under hydrothermal conditions iron­(III) from initial building block is reduced to the divalent state, creating 2D honeycomb [FeII 2(C2O4)3] n 2n– layers, which are bridged by [Cu­(H2O)­(terpy)]2+ cations. Compounds were investigated by single-crystal X-ray diffraction, IR, and impedance spectroscopies, magnetization measurements, and density functional theory (DFT) calculations. In compounds 1 and 2, 0D magnetism is observed, with 1 having a ground-state spin of 1 due to different interactions through chloride bridges of Cu2+ ions in tetramer [CuII 4(terpy)4Cl5]3+ and 2 showing strong antiferromagnetic coupling of Cu2+ ions mediated by oxalate ligand in [CuII 2(H2O)2(terpy)2(C2O4)]2+ and weak ones between Cu2+ and Fe3+ ions through oxalate bridge in [CuIIFeIII(CH3OH)­(terpy)­(C2O4)3]−. Polymer 4 exhibits antiferromagnetic phase transition at 25 K: The [FeII 2(C2O4)3] n 2n– layers are antiferromagnetically ordered, and a small amount of interlayer interaction is transferred through [Cu­(H2O)­(terpy)]2+ cations via Oox–Cu–Oox bridges. Additionally, compounds 1 and 2 are electrical insulators, while 4a and 4b show proton conductivity.