Magneto-Structural Correlation in Two Isomeric Series of Nitronyl Nitroxide Molecular Magnets: Intermolecular Interactions Relevant to Ferromagnetic Exchange in Naphthyl and Quinolyl Derivatives

Two isomeric series of stable organic free radicals, 1- and 2-naphthyl nitronyl nitroxide (1- and 2-NAPNN) and 2-, 3-, and 4-quinolyl nitronyl nitroxide (2-, 3-, and 4-QNNN), were synthesized and their crystal structures were determined by X-ray diffraction. The crystal structure of 1-NAPNN consists of a three-dimensional (3D) molecular network formed by several atomic contacts between the O atom of the ONCNO group and the aromatic ring as well as between the O atom and the methyl groups. Since 1-NAPNN shows intra- and interlayer ferromagnetic (FM) intermolecular interactions with exchange coupling constants J/k=0.09 K and Jʹ/k=0.03 K ( =-2JSiSj), respectively, these contacts are relavant to the FM intermolecular interaction. In contrast, 4-QNNN, having a molecular structure close to that of 1-NAPNN, contains molecular chains with alternating antiferromagnetic (AFM) intermolecular interaction with J/k=-7.8 K and =0.5 [ =-2J(S2i-1S2i+ S2iS2i+1)]. The crystal structures of 2-NAPNN, 2-QNNN, and 3-QNNN consist of 2D molecular layers formed through contacts similar to those observed in 1-NAPNN. The radical molecules are oriented in the same direction forming a herringbone-like arrangement in the layers. 2-NAPNN exhibits interlayer AFM interaction with Jʹ/k=-0.31 K in addition to FM interaction with J/k=0.14 K in the layer, whereas 2- and 3-QNNN show 3D FM intermolecular interactions with J/k= 0.018 K, Jʹ/k=0.01 K (2-QNNN) and J/k=0.12 K, Jʹ/k=0.03 K (3-QNNN). Each layer in 2-NAPNN is related to adjacent layers by twofold screw symmetry to yield atomic contacts between the naphthyl rings standing opposite across the interface of the layers. Since the contacts between the aromatic rings of these nitronyl nitroxides favor AFM intermolecular interaction while the contacts within the 2D layer favor FM interaction, 2-NAPNN shows overall AFM behavior. In contrast, the crystals of 2- and 3-QNNN have no such twofold axis and hence the 2D FM layers are connected through interlayer contacts, which result overall in FM behavior. Possible origins of the characteristic molecular arrangements that give rise to FM intermolecular interaction in the 2D layer are discussed in terms of hydrogen-bond formation between the O atom of the ONCNO group and the methyl groups of the nitronyl nitroxide moieties.