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 triradicals, p-triNN (1) and m-triNN (2), in which a ground-state singlet (S = 0) biradical with a π-conjugated phenol substituent and an S = ¹/₂ carboxyl-substituted monoradicals are united by σ-bonds of an ester bridge. It is found for both 1 and 2 that the exchange interactions J(σ) between the biradical and the monoradical moieties through the σ-bonds of the ester bridge are much smaller than those through the π-conjugation, J(π), 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 σ-bonded π-oligoradicals carrying multiple magnetic degrees of freedom.