Organic Ferrimagnetics Based on Charged Open-Shell Molecules

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Construction of genuinely organic molecule-based ferrimagnets as an exotic magnetic material has been a challenging issue in materials science and spin chemistry during last decades. A practical difficulty in constructing organic molecule-based ferrimagnetics is co-crystallization of two kinds of molecules with different spin quantum numbers S's, e.g., S=1 and S=1/2. We propose a supramolecular approach to the co-crystallization. Noncovalent bonding such as hydrogen bonding and coulombic interaction between ionic charges can be a driving force for crystallization of open-shell molecules with differing S's, e.g., biradical (S=1) and monoradical (S=1/2).We have designed and synthesized nitronyl-and imino-nitroxide biradicals with pyridine and aniline substituents as S=1 components for the supramolecular ferrimagnets. The neutral biradicals of 2,6- and 3,5-substituted pyridines and anilines were synthesized. The cationic biradicals were prepared by N-methylation of the neutral precursors. The molecular ground states of of the neutral biradicals and the salts with closed-shell anions were found to be triplet (S=1) from magnetic susceptibility measurements. Therefore, both the neutral biradicals and the cations serve as building blocks for organic supramolecular ferrimagnets.One of the the neutral biradicals, the 3,5-pyridine derivative of nitronyl nitroxide was found to co-crystallize with p-benzoic acid substituted monoradical. The hydrogen-bonded complex undergoes an antiferromagnetic phase transition at 5 K. The nature and the origin of the phase transition have been clrified on the basis the X-ray crystal structure and magnetic susceptibility measurements on aligned single crystals.