Different cationic forms of (-)-cytisine in the crystal structures of its simple inorganic salts

Abstract

The crystal structures of 13 simple salts of cytisine, an alkaloid isolated from the seeds of Laburnum anagyroides, have been determined, namely cytisinium (6-oxo-7,11-diazatricyclo[7.3.1.0^2,7]trideca-2,4-dien-11-ium) bromide, C₁₁H₁₅N₂O⁺·Br^-, cytisinium iodide, C₁₁H₁₅N₂O⁺·I^-, cytisinium perchlorate, C₁₁H₁₅N₂O⁺·ClO₄^-, cytisinium iodide triiodide, C₁₁H₁₅N₂O⁺·I^-·I₃^-, cytisinium chloride monohydrate, C₁₁H₁₅N₂O⁺·Cl^-·H₂O, cytisinium iodide monohydrate, C₁₁H₁₅N₂O⁺·I^-·H₂O, cytisinium nitrate monohydrate, C₁₁H₁₅N₂O⁺·NO₃^-·H₂O, hydrogen dicytisinium tribromide, C₂₂H₃₁N₄O₂³⁺·3Br^-, hydrogen dicytisinium triiodide, C₂₂H₃₁N₄O₂³⁺·3I^-, hydrogen dicytisinium triiodide diiodide, C₂₂H₃₁N₄O₂³⁺·I₃^-·2I^-, hydrogen dicytisinium bis(triiodide) iodide, C₂₂H₃₁N₄O₂³⁺·2I₃^-·I^-, cytisinediium (6-oxidaniumylidene-7,11-diazatricyclo[7.3.1.0^2,7]trideca-2,4-dien-11-ium) bis(perchlorate), C₁₁H₁₆N₂O²⁺·2ClO₄^-, and cytisinediium dichloride trihydrate, C₁₁H₁₆N₂O²⁺·2Cl^-·3H₂O. Cytisine has two potential protonation sites, i.e. the N atom of the piperidine ring and the carbonyl O atom of the pyridone ring. Three forms of the cytisinium cation were identified, namely the monocation, which is always protonated at the N atom, the dication, which utilizes both protonation sites, and the third form, which contains two cytisine moieties connected by very short and linear O...H...O hydrogen bonds, with an O...O distance of approximately 2.4 Å. This third form may therefore be regarded as a 3+ species, or sesqui-cation, and is observed solely in the salts with bromide, iodide or triiodide (heavier halogen) anions. The cation is quite rigid and all 19 cytisinium fragments in the studied series have very similar conformations. The crystal structures are determined mainly by Coulombic interactions and hydrogen bonds, and the latter form is determined by different networks. Additionally, some anion-π and lone-pair...π secondary interactions are identified in almost all of the crystal structures. Hirshfeld surface analysis generally confirms the role of different interactions in the determination of the crystal architecture.

Keywords: (–)-cytisine; Hirshfeld surfaces; cationic forms; crystal structure; intermolecular interactions; sesqui-cation.