Synthesis, Crystal Structure, and Magnetic Properties of 4-(2-Methyl-1-azaspiro[4.5]deca-1-oxyl-2-yl)phenol

Abstract

The two polymorphic forms (α and β) of the racemate of the title chiral nitroxide radical were separately obtained by changing the composition ratio of the recrystallization solvent (hexane/Et 2 O). Both polymorphs existed as a racemic compound; the thermally less stable α-form was monoclinic (space group, C2/c), and the stable β-form was orthorhombic (Pbca). The origin of the formation of the racemic compound crystals for the α- and β-forms was interpreted in terms of the formation of a strong intermolecular head-to-tail type of hydrogen bond between a nitroxyl group and the nearest hydroxy group in a pair of R and S enantiomers giving a heterochiral cyclic dimer and a heterochiral ID chain, respectively, by X-ray crystallographic analysis. Furthermore, magnetic susceptibility measurements of the two polymorphs indicated the presence of weak antiferromagnetic interactions, which were rationalized in terms of the formation of homochiral and heterociral ID chains in the crystal structures of the α- and β-forms, respectively, by DFT calculations.