Co-Crystallization of Achiral Components into Chiral Network by Supramolecular Interactions: Coordination Complexes – Organic Radical

Co-crystallization is an important aspect of crystal engineering that has found major applications in the development and manufacture of pharmaceutical drugs but is rarely invoked in the development of magnetic materials. Co-crystals have been serendipitously obtained instead of the usual coordination polymers from the organic radical, 2,2-pentamethylene-4,4,5,5-tetramethylimidazolidine-l-oxyl, and coordination complexes, cis-M^II(hfac)₂(H₂O)₂, where M = Co or Mn and hfac = hexafluoroacetylacetonato. The extensive intramolecular H-bond is the cause for the segregation of the two entities but the supramolecular interactions between these two neutral building blocks resulted in a rare chiral co-crystal system. The directional properties of the supramolecular interactions, N–O···OH₂, N–H···OH₂, N–O···CH₃, N–O···CH₂, F···CH₃, and F···CH₂, have been identified as working in tandem to generate the chirality from achiral components in achiral solvents. The absence of charge or proton transfer as suggested by X-ray structural analyses (bond lengths and angles), values of the magnetic moments, and lack of magnetic exchange between the metal spin and that of the organic radical are characteristics to define these two solids as genuine co-crystals. We therefore proposed that these solids be more effective as biomarkers than the pure radical. A comparison of their properties to related radicals is provided.