Exploring the structure-property schemes in anion-π systems of d-block metalates

Anion-π based compounds, materials, and processes have gained significant interest due to the diversity of their aesthetic non-covalent synthons, and thanks to their significance in biological systems, catalytic processes, anion binding and sensing, or the supramolecular organization of hierarchical architectures. While systems based on typical inorganic anions or organic residues have been widely reviewed in recent years, those involving anionic d metal comlexes as the main components have been treated with a rather secondary interest. However, actively exploring the new systems of the latter type we have recognized systematic advances in the field. As a result, in the current review we describe the landscape that has recently emerged. Focusing on the established groups of π-acidic species, i.e. polycarbonitirles, polyazines, polyazine N-oxides, diimide derivatives, fluoroarenes, and nitroarenes, we explore and discuss anion-π crystal engineering together with the structure-property schemes important from the standpoint of charge transfer (CT) and electron transfer (ET), magnetism, luminescence, reactivity and catalysis, and the construction of core-shell crystalline composites.

Crystal structures of N6-modified-amino acid nucleobase analogs(iii): adenine–valeric acid, adenine–hexanoic acid and adenine–gabapentine

H-bonding networks, anion–π and π–π interactions in the crystal structures of N⁶-modified-amino acid adenine analogs are investigated by means of DFT calculations and X-ray crystallography analysis.

Crystal structures of N6-modified-aminoacid/peptide nucleobase analogs: hybrid adenine–glycine and adenine–glycylglycine molecules

Anion–π interactions in crystal structures of N⁶-modified-aminoacid and dipeptide adenine analogs are investigated using X-ray crystallography and DFT calculations.

Anion Complexes with Tetrazine-Based Ligands: Formation of Strong Anion-π Interactions in Solution and in the Solid State

Ligands L1 and L2, consisting of a tetrazine ring decorated with two morpholine pendants of different lengths, show peculiar anion-binding behaviors. In several cases, even the neutral ligands, in addition to their protonated HL(+) and H2L(2+) (L = L1 and L2) forms, bind anions such as F(-), NO3(-), PF6(-), ClO4(-), and SO4(2-) to form stable complexes in water. The crystal structures of H2L1(PF6)2·2H2O, H2L1(ClO4)2·2H2O, H2L2(NO3)2, H2L2(PF6)2·H2O, and H2L2(ClO4)2·H2O show that anion-π interactions are pivotal for the formation of these complexes, although other weak forces may contribute to their stability. Complex stability constants were determined by means of potentiometric titration in aqueous solution at 298.1 K, while dissection of the free-energy change of association (ΔG°) into its enthalpic (ΔH°) and entropic (TΔS°) components was accomplished by means of isothermal titration calorimetry measurements. Stability constants are poorly regulated by anion-ligand charge-charge attraction. Thermodynamic data show that the formation of complexes with neutral ligands, which are principally stabilized by anion-π interactions, is enthalpically favorable (-ΔG°, 11.1-17.5 kJ/mol; ΔH°, -2.3 to -0.5 kJ/mol; TΔS°, 9.0-17.0 kJ/mol), while for charged ligands, enthalpy changes are mostly unfavorable. Complexation reactions are invariably promoted by large and favorable entropic contributions. The importance of desolvation phenomena manifested by such thermodynamic data was confirmed by the hydrodynamic results obtained by means of diffusion NMR spectroscopy. In the case of L2, complexation equilibria were also studied in a 80:20 (v/v) water/ethanol mixture. In this mixed solvent of lower dielectric constant than water, the stability of anion complexes decreases, relative to water. Solvation effects, mostly involving the ligand, are thought to be responsible for this peculiar behavior.

Towards design strategies for anion–π interactions in crystal engineering

This highlight article summarizes some of the fundamental aspects of the anion–π interaction leading to several design strategies for generating it in solids. In the main body we highlight some relevant examples that illustrate the viability of these strategies and the importance of anion–π interactions in crystal engineering.

Synthesis, X-ray characterization and DFT studies of N-benzimidazolyl-pyrimidine–M(ii) complexes (M = Cu, Co and Ni): the prominent role of π-hole and anion–π interactions

Five Cu(ii), Co(ii) and Ni(ii) complexes with 2-(N-benzimidazolyl)-pyrimidine and nitrate co-ligands have been synthesized and characterized by X-ray diffraction.

Syntheses, crystal structures and density functional theory investigations of copper(ii) complexes bearing tridentate Schiff base ligands derived from 8-aminoquinoline

Three copper(ii) Schiff base complexes have been synthesized and characterized. Supramolecular assemblies in the solid state are analyzed by DFT calculations.