Inorganic anion induced supramolecular architectures of flexible unsymmetrical bis(pyridyl) ionic salts mediated by various non-covalent interactions

Self-assembly of three flexible bis(pyridyl) molecules with an unsymmetrical –CH2–NH– spacer, N-(pyridin-2-ylmethyl)pyridin-2-amine (M1), N-(pyridin-3-ylmethyl)pyridin-2-amine (M2), and N-(pyridin-4-ylmethyl)pyridin-2-amine (M3), and different inorganic acids (HCl, HNO3, HClO4 and HPF6) leads to the formation of twelve salts, H2M12+·2Cl− (1), H2M22+·2Cl− (2), H2M32+·2Cl− (3) H2M12+·2NO3− (4), H2M22+·2NO3− (5), H2M32+·2NO3− (6), H2M12+·2ClO4− (7), H2M22+·2ClO4− (8), H2M32+·2ClO4− (9), HM1+·PF6− (10), HM2+·PF6− (11), and H2M32+·HM3+·3PF6− (12), which have been characterized by elemental analysis, IR, TG, PL, powder and single-crystal X-ray diffraction. Structural analyses indicate that the nature of the four inorganic anions and the conformation of the three flexible unsymmetrical bis(pyridyl) molecules can effectively influence the final structures of the salts through diverse non-covalent bonding interactions, such as hydrogen bonds, π⋯π stacking, C–H⋯π and anion⋯π interactions. This leads to the formation of diverse architectures, in which salts 1–4, 6–9 and 11 contain either left- and right-handed helical chains or a single helical chain. Interestingly, all the four types of inorganic anions in the twelve salts form anion⋯π interactions with the H2M2+ or HM+ cations, which also modulate the variation of the 3-D supramolecular network. Thermogravimetric analyses indicate that the thermal stability of the twelve salts presents a regular decrease with the sequence of perchlorates, chlorates, hexafluorophosphates, and nitrates. The emission intensity of the perchlorates is much stronger than those of the hexafluorophosphates, nitrates, chlorates and their corresponding organic molecules in the solid-state at room temperature.