Syntheses, crystal structures, and magnetic properties of nitronyl nitroxide triradicals composed of ground-state singlet biradicals and monoradicals: Molecular spin clusters in the crystal

We report X-ray crystal structures and magnetic properties of novel nitronyl nitroxide thradicals, p-triNN (1) and m-triNN (2), in which a ground-state singlet (S = 0) biradical with a pi-conjugated phenol substituent and an S = 1/2 carboxyl-substituted monoradicals are united by a-bonds of an ester bridge. It is found for both I and 2 that the exchange interactions Act) between the biradical and the monoradical moieties through the a-bonds of the ester bridge are much smaller than those through the T-conjugation, J(pi), within the biradical moieties. Thus, the inequivalent magnetic degrees of freedom, one from the biradical and the other from the monoradical moiety, are retained in a single molecule. X-ray crystallography and magnetic susceptibility of 1 indicate that an intermolecular antiferromagnetic exchange interaction between the biradical and the monoradical moieties gives a three-spin cluster. On the other hand, the triradical 2 is found to form a six-spin cluster, consisting of two sets of monoradical-biradical pairs in the crystal. The formation of the molecular spin clusters in 1 and 2 gives us the way of developing exotic spin coupling systems from sigma-bonded pi-oligoradicals carrying multiple magnetic degrees of freedom.