Polyfluorophenyl-Substituted Blatter Radicals: Synthesis and Structure–Property Correlations

Blatter radicals are extremely stable organic electron-deficient paramagnets having good potential as a spin-bearing building block in diverse applications. The packing mode and molecular conformation of such radicals are appropriate for attaining distinctive magnetic and electronic properties. Herein, within the framework of systematic exploration of the "structure–property" correlations inherent in fluorinated Blatter radicals, 1-(2,3,4-trifluorophenyl)-(2a) and 1-(2,3,5,6-tetrafluorophenyl)-3-phenyl-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl (2b) were synthesized and completely characterized. The crystal structure of both radicals 2a and 2b was found to consist of two different centrosymmetric dimers with relatively short intermolecular distances between atoms of the benzotriazinyl moieties. SQUID magnetometry in the range of 2–300 K revealed that crystals of the two radicals, just as crystals of previously synthesized 1-(pentafluorophenyl)-3-phenyl-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl (2c), are dominated by rather strong antiferromagnetic interactions. For the two dimer types, spin-unrestricted broken-symmetry DFT calculations predicted similar parameters J differing by less than 20%. Subsequent fitting of χT vs T dependences using theoretically predicted magnetic motifs resulted in equal values of J for two types of dimers and yielded the following best-fit J/kB parameters: −156.4 ± 0.8 K for 2a, −230 ± 3 K for 2b, and −58.3 ± 0.6 K for 2c. For comparison, in the corresponding nonfluorinated radical (1a), only weak intermolecular interactions were observed (J/kB = −2.2 ± 0.2 K). By contrast, previously studied radicals 1b and 1c (containing two fluorine atoms) are dominated by strong antiferromagnetic interactions (J/kB = −292 ± 10 and −222 ± 17 K, respectively). The reason for the difference in magnetic properties of 1a and fluorinated radicals is that the introduction of fluorine atoms into the phenyl substituent in 1a has a considerable effect on its electrostatic potential, thereby leading to changes in the crystal structure and in bulk magnetic properties.