Masato Kuratsu

Nitroxide-substituted nitronyl nitroxide and iminonitroxide.

We report on the highly compact nitroxide-substituted nitronyl nitroxide 1 and iminonitroxide 2; they have isoelectronic structures with trimethylenemethane. These diradicals are stable under aerated conditions at room temperature and have large positive exchange interactions: J/k(B) = +390 K (H = -2JS(1)(/)(2)·S(1/2)) for 1 and J/k(B) ≈ +550 K for 2.

A New Ferrimagnet Based on a Radical-Substituted Radical Cation Salt

Radical-substituted radical cations are attractive spin building blocks of molecule-based magnets. The introduction of an additional spin as a counteranion provides a unique three-spin system wherein the magnetic interactions between the spins of the radical substituent and the radical cation (J(intra)) and those between the spins of the radical cation and the anion (J(inter)) play decisive roles in determining the magnetic properties of the system. We report the first demonstration of a ferrimagnet by utilizing a large-J(intra) system, nitronyl nitroxide-substituted dihydrophenazine radical cation (NNDPP(*+)) in combination with tetrabromoferrate (FeBr(4)(-)) as the counteranion. On the basis of measurements of dc and ac magnetic susceptibilities and heat capacity, the magnetic properties of NNDPP(*+) x FeBr(4)(-) are elucidated to be those of a three-dimensional long-range-ordered ferrimagnet with T(c) = 6.7 K.

(Nitronyl Nitroxide)-Substituted Trioxytriphenylamine Radical Cation Tetrachlorogallate Salt: A 2p-Electron-Based Weak Ferromagnet Composed of a Triplet Diradical Cation

The synthesis and the solid state magnetic properties of (nitronyl nitroxide)-substituted trioxytriphenylamine radical cation tetrachlorogallate, NNTOT(+)⋅GaCl(4)(-), are reported. In the temperature region between 300 and 3 K, the magnetic behavior is characterized by the strong intramolecular ferromagnetic interaction (J/k(B)=+400 K) between the radical (NN) and the radical cation (TOT(+)) and the weak intermolecular antiferromagnetic interaction (J/k(B)=-1.9 K) between NNTOT(+) ions. Below 3 K, a 3D-type long-range magnetic ordering into a weak ferromagnet was observed (T(N)=2.65 K). The magnetic entropy (S(mag)=8.97 J K(-1)  mol(-1)) obtained by the heat capacity measurement is in good agreement with the theoretical value of R ln3=9.13 J K(-1)  mol(-1) based on the S=1 state.

Pyrene-Dihydrophenazine Bis(Radical Cation) in a Singlet Ground State

A new pyrene-dihydrophenazine dyad was prepared. Oxidation of the neutral species produced a bis(radical cation) species, which was characterized by the absorptions of their component radical cations in the visible region. A thermally accessible triplet state was observed in the ESR measurement in frozen n-PrCN. The energy gap between the singlet and triplet states was determined to be 2J/k(B) = -36 +/- 3 K.

Radical-substituted dihydrophenazine radical cation salts: Molecular packing structure and bulk magnetic property*

Radical-substituted radical cation salts are exotic species that are potentially applicable as spin building blocks for molecular magnets. We recently found that these species, which are derived from a diphenyldihydrophenazine (DPP) framework, are stable under aerated conditions at room temperature. Of these species, nitronyl nitroxide (NN•)-substituted DPP•+ tetrachloroferrate (NNDPP••+·FeCl4–) showed antiferromagnetic interaction at low temperature (<150 K), whereas the tetrabromoferrate salt NNDPP••+·FeBr4– exhibited a magnetic phase transition at 6.7 K to produce a bulk ferrimagnet. Both salts had very similar molecular structures. The difference in the magnetic properties was ascribed to the difference in molecular packing structures. A significant difference in these two salts was observed at the (NNDPP••+)–(NNDPP••+) intermolecular contact, including the oxygen atom of the nitroxide moiety; NNDPP••+·FeCl4– had a serious antiferromagnetic O–O (nitroxide oxygen, 3.02 Å) intermolecular contact, whereas NNDPP••+·FeBr4– had a ferromagnetic O–HC (2.53 Å) intermolecular contact.