Magnetic Interactions through a Nonconjugated Framework Observed in Back-to-Back Connected Triazinyl-Nitroxyl Biradical Derivatives.

Three hetero-biradical derivatives, with the structure of a back-to-back connected benzotriazinyl and tetramethyl or tetraethylisoindoline N-oxyl sharing a common benzo ring, 3-tert-butyl-1-phenyl-1,4,6,8-tetrahydro-6,6,8,8-tetramethyl-pyrrolo[4,5-g]-1,2,4-benzotriazin-4-yl-7-oxyl (1-tBu), 1,3-diphenyl-1,4,6,8-tetrahydro-6,6,8,8-tetramethyl-pyrrolo[4,5-g]-1,2,4-benzotriazin-4-yl-7-oxyl (1-Ph), and 3-tert-butyl-1-phenyl-1,4,6,8-tetrahydro-6,6,8,8-tetraethyl-pyrrolo[4,5-g]-1,2,4-benzotriazin-4-yl-7-oxyl (2-tBu), were synthesized and characterized by single-crystal X-ray analyses, variable-temperature magnetic susceptibility studies, and DFT calculations. Temperature dependences of the magnetic susceptibilities of 1-tBu, 1-Ph, and 2-tBu exhibit broad maxima at 70, 71, and 43 K, respectively. Although these radical derivatives form a columnar or chained assembly in the solid state, magnetic measurements of diluted samples in the polymer matrices and computational results imply that the magnetic properties of the polycrystalline sample can be explained by a two-spin system with an intramolecular antiferromagnetic interaction. The magnetic behavior can be reproduced by using the Bleaney-Bowers model, with 2J=-80.0 cm-1 for 1-tBu, 2J=-77.1 cm-1 for 1-Ph, and 2J=-48.9 cm-1 for 2-tBu. The moderately strong intramolecular antiferromagnetic interactions can be interpreted by a through-bond interaction through the nonconjugated framework and/or through-space interactions based on molecular orbital theory. The strong distance dependency between the N-O spin site and vinylic carbon atoms indicates that the orbital interaction plays an important role in the intramolecular magnetic interaction. The reduced magnetic interaction in 2-tBu relative to those of 1-tBu and 1-Ph can be attributed to restricted rotation of the tetraethyl group.