Masanori Yasui

Magnetic properties of pyrimidine-bridged copper(II) complexes

Abstract The magnetic measurements of [PM2·Cu(NO3)]n (PM = pyrimidine) showed ferromagnetic interaction between the Cu(II) spins with J/kB = +0.89 K. On the other hand, those of PM2·Cu(NO3)2·(H2O)2 and [PM·CU(NO3)2·(H2]n showed para- and antiferromagnetic behaviors respectively. The magnetic properties of pyrimidine-bridged Cu(hfac)2 and Cu(ClO4)2 complexes are also reported.

Dipyrimidine-copper(II) dinitrate complexes showing magnetic interactions.

Crystals of Cu(II)(NO(3))(2)(pm)(3) (1), and two crystalline forms of Cu(II)(NO(3))(2)(H(2)O)(2)(pm)(2), (2) and (3), showed ferromagnetic, antiferromagnetic and paramagnetic interactions at extremely low temperatures, respectively. Crystal structure analyses revealed that the complexes were catena-dinitrato[mu-pyrimidine-kappaN(1):kappaN(3)]-(pyrimidine-N(1))copper(II), [Cu(NO(3))(2)(pm)(2)](n), catena-diaquadinitrato[mu-pyrimidine-kappaN(1):kappaN(3)]copper(II), [Cu(NO(3))(2)(H(2)O)(2)(pm)](n), and diaquadinitratodipyrimidinecopper(II), Cu(NO(3))(2)(H(2)O)(2)(pm)(2) for (1), (2) and (3), respectively. In (1) the Cu atom is coordinated by the two nitrates and N atoms of the non-bridging pyrimidine and bridging pyrimidine to form a one-dimensional coordination polymer. The complex is a five-coordinated square pyramid and can be regarded as a pseudo-seven-coordinated complex, since other short non-bonding Cu.O contacts are observed. In the crystals of (2) the pyrimidine bridges the Cu atoms to form a one-dimensional coordination chain. On the other hand, complex (3) is not a coordination polymer. It is important to form a coordination polymer for the appearance of the magnetic interactions. Types of coordination of the bridging organic moieties should also play an important role in magnetic properties. Magnetic measurements of (1) and (2) show that they are good examples of uniform S = 1/2 ferro- and antiferromagnetic Heisenberg chains with exchange parameters 2J/kB = +1.8 and -36 K, respectively.

Ferromagnetism of Organic Radical Crystals of Tempo Derivatives

Abstract Several TEMPO-based radicals were found to be bulk ferromagnets with T C of 0.2–0.4 K, as indicated by the measurements of ac magnetic susceptibilities and magnetization curves. On the basis of the X-ray crystal structure analysis, a possible mechanism of ferromagnetic interactions between neighboring N-O sites is proposed, in which the methyl or methylene groups play a role of ferromagnetic exchange couplers. New organic ferromagnets, 4-(p-methylthiobenzylideneamino)-TEMPO, 4-(benzylamino)-TEMPO, and 4-(hydroxyimino)-TEMPO as well as several metamagnets are reported.

An organic metamagnet: 4-(2-naphtylmethyleneamino)-2,2,6,6-tetramethylpiperidin-1-oxyl

Abstract Magnetic measurements on the organic radical crystal of 4-(2-naphtylmethyleneamino)-2,2,6,6-tetramethylpiperidin-1-oxyl exhibited a Neel transition at about 0.12 K ( T N ) and a metamagnetic transition below T N (the coercive field was around 180 Oe at 40 mK). X-ray crystallographic analysis revealed that the molecules were arranged to give an almost columnar structure with the nearest intermolecular O…O distance of 5.70 A. The magnetic property was analyzed on the basis of the crystal structure.

Magnetic Interaction Via β-Hydrogen Atoms in Tempo Derivatives

Abstract Theoretical studies of the ferromagnetic property of phenyl-CH=N-TEMPO were carried out. The effective exchange integrals (J ab) were calculated for the nearest neighbour molecules in the crystal and the ferromagnetic interaction was revealed. To study the role of the hydrogen atoms, modified pair models were considered. It was concluded that the bridged β-hydrogen atoms dominantly contribute to the ferromagnetic interaction in the phenyl-CH=N-TEMPO crystal.

New organic ferromagnets: 4-arylmethyleneamino-2,2,6,6-tetramethylpiperidin-1-oxyl (aryl = phenyl, 4-biphenylyl, 4-chlorophenyl, and 4-phenoxyphenyl)

Abstract The title radicals were found to be bulk ferromagnets, as indicated by the sharp divergence of the susceptibility. Their transition temperatures were found in the range of 0.2–0.4 K. The X-ray crystal structure analysis of the phenyl, biphenylyl, and chlorophenyl derivatives at room temperature revealed that the crystal consisted of an almost two-dimensional network of the N-O sites. In the case of aryl = 2-naphthyl, metamagnetic behavior was observed below about 0.3 K.

Self-assembled meso-helicates of linear trinuclear nickel(II)-radical complexes with triple pyrazolate bridges.

Complexation of 3-nitronyl-nitroxide-substituted pyrazolate (pzNN) and 3-imino-nitroxide-substituted pyrazolate (pzIN) with nickel(II) gave [Ni3(pzNN)6] and [Ni3(pzIN)6], respectively. They were practically isomorphous and characterized as a linear trinuclear structure with neighboring nickel ions triply bridged by pyrazolate moieties. The space groups were P2(1)/n but the molecules have a pseudo three-fold axis. We found polymorphs in the crystals of [Ni3(pzIN)6] depending on the solvate molecules; another space group was a cubic Pa3[combining macron]. The opposite chirality around the inversion center at the central nickel(II) ion leads to a meso-helical symmetry in the whole molecule. The radical oxygen atoms participate in the 6-membered chelation at the terminal nickel(II) ions. Antiferromagnetic couplings were observed in both complexes, which are ascribable to interactions between the nickel and radical spins and between the nickel spins across the pyrazolate bridges.

Intermolecular ferromagnetic interaction in the crystal of a diphenyl nitroxide derivative. The role of spin-polarized hydrogen atoms located near a neighboring N–O site

Abstract The crystal of 9,9-dipropyl-9,10-dihydroacridin-10-yloxyl exhibited a positive Weiss constant (+0.77 K) ascribable to a three-dimensional network of ferromagnetic interactions. The orbital orthogonality between the antibonding N–O π∗ and H 1s orbitals was found in a nearly isosceles triangle geometry of neighboring H atoms and the N–O site. The following spin polarization scheme is proposed: N(↑)–C(↓)Co(↑)–Cm(↓)–Hm(↑)⋯ ⋅ O (↑)–N, where o- and m-positions are defined with respect to the N–O group.

ISOLATION AND STEREOCHEMISTRY OF OPTICALLY ACTIVE SELENONIUM IMIDES

Optical resolution of racemic diaryl selenonium-N-toluene-4‘-sulfonimides (rac-1a−d) by liquid chromatography using an optically active column yielded optically pure selenonium imides. The absolute configuration around the selenium atom of (−)-2,4,6-tri-tert-butyldiphenylselenonium N-toluene-4‘-sulfonimide [(−)-1a] was determined to be S by X-ray crystallographic analysis, and those of the other optically active selenonium imides were determined on the basis of their specific rotations and CD spectra. The kinetics of racemization by pyramidal inversion of the optically active selenonium imides (+)- and (−)-1b and 1d were studied. The results indicated that the activation energy for the racemization of optically active selenonium imides was greater than those for sulfonium imides.

CRYSTAL STRUCTURES OF TEMPO RADICALS SHOWING FERROMAGNETIC INTERACTION

Abstract Some of the 4-arylmethyleneamino-TEMPO (2,2,6,6-tetramethyl-piperidin-1-oxyl) radical crystals showed an intermolecular ferromagnetic interaction. The X-ray structure analyses were performed on 4-arylmethyleneamino-TEMPO crystals (aryl = phenyl, p-chlorophenyl, p-biphenyl, p-phenoxyphenyl, p-bromophenyl, 3,5-dichlorophenyl, 3-pyridyl, naphthyl and p-fluorophenyl) at room temperature to investigate whether any systematic intermolecular interactions were demonstrated among these crystals. No special interactions except van der Waals interaction were observed in these crystals. As for the arrangements of N-O radicals in the crystals, two-dimensional O…O networks are constructed, of which forms are various. A sheet-like arrangement of the N-O radicals is considered to be essentially important for the magnetic interactions, although the O…O distances are long for the direct interaction between radicals.