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 beta-positions of N-O in the TEMPO moieties of adjacent molecules. A positive spin on the N-O radical induces negative spins on the beta-hydrogen atoms due to an intramolecular spin-polarization, ON(dagger)-C(down arrow)-C(up arrow)-H(down arrow), 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 metamagnet, bis(2,2,6,6-tetramethylpiperidin-1-oxyl-4-yl) 1,6-hexanedicarboxylate. In the case of an organic ferromagnet, 2,4,6,8-tetramethyl-1,5-diazaadamantane-1,5-dioxyl, both the spin-alternation and direct exchange mechanisms seem to be operative.