Supramolecular assemblies involving anion–π and lone pair–π interactions: experimental observation and theoretical analysis

Abstract

Two nickel(II) malonate coordination compounds, namely (C₅H₇N₂)₄[Ni(C₃H₂O₄)₂(H₂O)₂](ClO₄)₂ (1) and (C₅H₇N₂)₄[Ni(C₃H₂O₄)₂(H₂O)₂](PF₆)₂ (2) [where C₅H₇N₂ = protonated 2-aminopyridine, C₃H₄O₄ = malonic acid, ClO₄⁻ = perchlorate, PF₆⁻ = hexafluoridophosphate], have been synthesized from purely aqueous media, and their single-crystal structures have been determined by X-ray diffraction. As anticipated, various weak forces, i.e. lone pair–π, π–π and anion–π interactions, play a key role in stabilizing the self-assembly process observed for both compounds. Intricate combinations of lone pair–π, π–π and anion–π interactions of the type lone pair–π/π–π/π–anion–π/π–lone pair and lone pair–π/π–π/π–anion are observed that are fully described, including by computational methods. The theoretical calculations allow estimating the strength of these π contacts.