Hydrogen-bonded organic ferromagnet

Nitronyl nitroxide derivatives carrying a hydroquinone or a resorcinol moiety 2-(2',5'-dihydroxyphenyl)-4,4,5, 5-tetramethyl-4,5-dihydro-1H-imidazolyl-1-oxyl-3-oxide (HQNN) and 2-(3',5'-dihydroxyphenyl)-4,4,5,5-tetramethyl-4,5-dihydro- 1H-imidazolyl-1-oxyl 3-oxide (RSNN)) have been designed and prepared. These compounds were found to afford :hydrogen-bonded crystals. In the case of HQNN, two phases of crystals (alpha- and beta-HQNN) were obtained. Of these, the crystal structure of alpha-HQNN is characterized by the intramolecular hydrogen bond between the o-hydroxy group and the nitronyl nitroxide and by the intermolecular one-dimensional hydrogen-bonded chain between the o- and p-hydroxy groups of the neighboring molecules, respectively. Two of the hydrogen-bonded chains run in parallel, and they are connected by the bifurcated hydrogen bonds formed between the o-hydroxy groups of the facing molecules in the individual chain. The chi T value of this crystal increased monotonously with lowering temperature (ST model, J = +0.93 K, theta = +0.46 K) and turned out to exhibit a ferromagnetic phase transition at 0.5 K. On the basis of the heat capacity data, alpha-HQNN was found to be a three-dimensional ferromagnet. On the other hand, the crystal of beta-HQNN was characterized by the intermolecular hydrogen-bonded chain formed between the hydroxy group at the meta position and the nitronyl nitroxide and by the interchain pi-pi stacking. The magnetic interaction of beta-HQNN was interpreted by the ferromagnetic ST model, and this ferromagnetic interaction (J = +5.0 K) was accompanied by a weak antiferromagnetic interaction (theta = -0.32 K) at lower temperatures. Although the structural feature of the crystal of RSNN resembles that of the beta-HQNN, it exhibited the antiferromagnetic interaction predominantly (ST model, J = +10.0 K, theta = -4.0 K). These magnetic behaviors are consistent with McConnell's theory, when the spin densities at the interacting sites connected by hydrogen bonds are taken into account. Thus, it may be concluded that the hydrogen bond plays a role not only in constructing hydrogen-bonded crystals but also in transmitting spin polarization along the hydrogen bond.