Crystal engineering of high explosives through lone pair-π interactions: Insights for improving thermal safety.

In this high-risk/high-reward study, we prepared complexes of a high explosive anion (picrate) with potentially explosive s-tetrazine-based ligands with the sole purpose of advancing the understanding of one of the weakest supramolecular forces: the lone pair-π interaction. This is a proof-of-concept study showing how lone pair-π contacts can be effectively used in crystal engineering, even of high explosives, and how the supramolecular architecture of the resulting crystalline phases influences their experimental thermokinetic properties. Herein we present XRD structures of 4 novel detonating compounds, all showcasing lone pair-π interactions, their thermal characterization (DSC, TGA), including the correlation of experimental thermokinetic parameters with crystal packing, and in silico explosion properties. This last aspect is relevant for improving the safety of high-energy materials.

Solid State and Solution Study on the Formation of Inorganic Anion Complexes with a Series of Tetrazine-Based Ligands.

Four molecules (L1-L4) constituted by an s-tetrazine ring appended with two identical aliphatic chains of increasing length bearing terminal morpholine groups were studied as anion receptors in water. The basicity properties of these molecules were also investigated. Speciation of the anion complexes formed in solution and determination of their stability constants were performed by means of potentiometric (pH-metric) titrations, while further information was obtained by NMR and isothermal titration calorimetry (ITC) measurements. The crystal structures of two neutral ligands (L3, L4) and of their H2L3(ClO4)2∙2H2O, H2L4(ClO4)2∙2H2O, H2L3(PF6)2, and H2L3(PF6)2∙2H2O anion complexes were determined by single crystal X-ray diffraction. The formation of anion-π interactions is the leitmotiv of these complexes, both in solution and in the solid state, although hydrogen bonding and/or formation of salt-bridges can contribute to their stability. Evidence of the ability of these ligands to form anion-π interactions is given by the observation that even the neutral (not-protonated) molecules bind anions in water to form complexes of significant stability, including elusive OH- anions.

Iodide and triiodide anion complexes involving anion-π interactions with a tetrazine-based receptor.

Protonated forms of the tetrazine ligand L2 (3,6-bis(morpholin-4-ylethyl)-1,2,4,5-tetrazine) interact with iodide in aqueous solution forming relatively stable complexes (ΔG° = -11.6(4) kJ mol-1 for HL2+ + I- = (HL2)I and ΔG° = -13.4(2) kJ mol-1 for H2L22+ + I- = [(H2L2)I]+). When solutions of [(H2L2)I]+ are left in contact with air, crystals of the oxidation product (H2L2)2(I3)3I·4H2O are formed. Unfortunately, the low solubility of I3- complexes prevents the determination of their stability constants. The crystal structures of H2L2I2·H2O (1), H2L2(I3)2·2H2O (2) and (H2L2)2(I3)3I·4H2O (3) were determined by means of X-ray diffraction analyses. In all crystal structures, it was found that the interaction between I- and I3- with H2L22+ is dominated by anion interactions with the π electron density of the receptor. Only in the case of 1, the iodide anions involved in close anion-π interactions with the ligand tetrazine ring form an additional H-bond with the protonated morpholine nitrogen of an adjacent ligand molecule. Conversely, in crystals of 2 and 3 there are alternate segregated planes which contain only protonated ligands hydrogen-bonded to cocrystallized water molecules or I3- and I- forming infinite two-dimensional networks established through short interhalogen contacts, making these crystalline products good candidates to behave as solid conductors. In the solid complexes, the triiodide anion displays both end-on and side-on interaction modes with the tetrazine ring, in agreement with density functional theory calculations indicating a preference for the alignment of the I3- molecular axis with the molecular axis of the ligand. Further information about geometries and structures of triiodide anions in 2 and 3 was acquired by the analysis of their Raman spectra.

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.