Proposed Mechanism of Ferromagnetic Interaction of Organic Ferromagnets: 4-(Arylmethyleneamino)-2,2,6,6-tetramethylpiperidin-1-oxyls and Related Compounds

A possible mechanism of ferromagnetic coupling of a series of organic radical ferromagnets is presented, based on their crystal structures and molecular-orbital calculations. The investigated radicals are 4-(arylmethyleneamino)-2,2,6,6-tetramethylpiperidin-1-oxyls (abbreviated as Ar–CH=N–TEMPO, where Ar = phenyl, 4-methylthiophenyl, 4-chlorophenyl, 4-biphenylyl, and 4-phenoxyphenyl), and 4-hydroxyimino-TEMPO. All of these radical crystals have characteristic features that oxygen atoms of N–O radical sites of TEMPO moieties always locate near methyl- and/or methylene-hydrogens at β-positions of N–O in the TEMPO moieties of adjacent molecules. A positive spin on the N–O radical induces negative spins on the β-hydrogen atoms due to an intramolecular spin-polarization, ON(↑)–C(↓)–C(↑)–H(↓), which in turn induces a positive spin on the N–O sites of the adjacent molecules. This spin-alternation mechanism of ferromagnetic coupling among N–O sites via hydrogen bridges is also applicable to the case of an organic ...