A Crystallographic Study of a Novel Tetrazolyl-Substituted Nitronyl Nitroxide Radical

Spin-labelled compounds are widely used in chemistry, physics, biology, and material sciences, but the directed synthesis of some functionalized organic radicals is still a challenge. We succeeded in the preparation of a tetrazolyl-substituted nitronyl nitroxide radical in pure crystalline form. According to the single-crystal X-ray data, intra-(NH ... O, 2.43 angstrom) and inter-molecular hydrogen bonds (NH center dot center dot center dot O, 1.91 angstrom) are formed between NH groups of the tetrazole cycles and 0 atoms of the paramagnetic moieties. The intermolecular H-bonds connect the molecules forming chains along the a-axis. Moreover, there are short intermolecular contacts between the O atoms (3.096 angstrom) and between the 0 and C atoms (3.096 angstrom) of the nitronyl nitroxide moieties within the chain. The spin-unrestricted broken-symmetry calculations performed at the BS-UB3LYP/def2-TZVP level of theory predicted a sufficient ferromagnetic interaction (J approximate to 20 cm(-1)) between the adjacent radicals inside the chain, but a weak antiferromagnetic interaction (-J <= 0.2 cm(-1)) between the nearest radicals belonging to the different chains. Thus, a rare case when stable radicals, the tetrazolyl-substituted nitronyl nitroxides, are ordered into ferromagnetic chains was revealed; an investigation of the magneto-structural correlations inherent in the nitroxide radical will demand a special experiment in the sub-Kelvin regime.