<i>tert</i>-Butyl 2-Pyridyl Nitroxide Available as a Paramagnetic Chelate Ligand for Strongly Exchange-Coupled Metal−Radical Compounds

tert-Butyl 2-Pyridyl Nitroxide Available as a Paramagnetic Chelate Ligand for Strongly Exchange-Coupled Metal−Radical Compound

Polymorphic Alternating HNN−Cobalt(II) Chains Both Behaving as Single-Chain Magnets (HNN = 4,4,5,5-Tetramethylimidazolin-1-oxyl 3-Oxide)

Metal-radical alternating chains [Co(hfac)2HNN]n crystallized in two morphs containing repeating all-cis (α) and cis,cis,trans (β) configurations with respect to the OHNN−Co−OHNN geometry (HNN = 4,4,5,5-tetramethylimidazolin-1-oxyl 3-oxide). Both phases showed magnetization jumps with hysteresis at 2 K. The α phase has a relatively high activation energy of magnetization reorientation (193 K)

Polymorphic Alternating HNN−Cobalt(II) Chains Both Behaving as Single-Chain Magnets (HNN = 4,4,5,5-Tetramethylimidazolin-1-oxyl 3-Oxide)

Metal-radical alternating chains [Co(hfac)2HNN]n crystallized in two morphs containing repeating all-cis (α) and cis,cis,trans (β) configurations with respect to the OHNN−Co−OHNN geometry (HNN = 4,4,5,5-tetramethylimidazolin-1-oxyl 3-oxide). Both phases showed magnetization jumps with hysteresis at 2 K. The α phase has a relatively high activation energy of magnetization reorientation (193 K)

<i>tert</i>-Butyl 2-Pyridyl Nitroxide Available as a Paramagnetic Chelate Ligand for Strongly Exchange-Coupled Metal−Radical Compounds

tert-Butyl 2-Pyridyl Nitroxide Available as a Paramagnetic Chelate Ligand for Strongly Exchange-Coupled Metal−Radical Compound

Ferromagnetic Superexchange Coupling through a Diamagnetic Iron(II) Ion in a Mixed-Valent Iron(III, II, III) <i>meso</i>-Helicate

meso-Helicates [Ni3(pzNTR)6]·4CH3CN and [Fe3(pzNTR)6](ClO4)2·CH3OH were prepared (HpzNTR stands for N-tert-butyl-α-3-pyrazolylnitrone). The former showed antiferromagnetic coupling between neighboring ions with J/kB = −28.2(2) K, which can be reasonably interpreted in terms of the superexchange mechanism. On the other hand, the latter exhibited ferromagnetic coupling with J/kB = +0.292(8) K between the two terminal S = 5/2 Fe3+ ions through the intervening diamagnetic Fe2+ ion. The admixture of low-lying excited states with the ground state stabilizes the ferromagnetically coupled state, like the Prussian blue magnet

Molecular Metamagnet [Ni(4ImNNH)₂(NO₃)₂] (4ImNNH = 4-Imidazolyl Nitronyl Nitroxide) and the Related Compounds Showing Supramolecular H-Bonding Interactions

A new chelating radical ligand 4ImNNH (2-(4-imidazolyl)-4,4,5,5-tetramethylimidazolin-1-oxyl 3-oxide) was prepared, and complexation with divalent transition metal salts gave complexes, [M(4ImNNH)2X2], which showed intermolecular ferromagnetic interaction in high probability (7 out of 10 paramagnetic compounds investigated here). The nitrate complexes (X = NO3; M = Mn (1), Co (2), Ni (3), Cu (4)) crystallize isomorphously in monoclinic space group P21/a. The equatorial positions are occupied with two 4ImNNH chelates and the nitrate oxygen atoms are located at the axial positions. Magnetic measurements revealed that the intramolecular exchange couplings in 1, 2, and 4 were antiferromagnetic, while that in 3 was ferromagnetic with 2J/kB = +85 K, where the spin Hamiltonian is defined as H = −2J(S1·S2 + S2·S3) based on the molecular structures determined as the linear radical−metal−radical triads. The intramolecular ferromagnetic interaction in 3 is interpreted in terms of orthogonality between the radical π* and metal dσ orbitals. Compounds 1−3 exhibited intermolecular ferromagnetic interaction ascribable to a two-dimensional hydrogen bond network parallel to the crystallographic ab plane. Complex 3 became an antiferromagnet below 3.4 K and exhibited a metamagnetic transition on applying a magnetic field of 5.5 kOe at 1.8 K. The complexes prepared from metal halides, [M(4ImNNH)2X2] (X = Cl, Br; M = Mn, Co, Ni, Cu), showed intramolecular antiferromagnetic interactions, which are successfully analyzed based on the radical−metal−radical system. The crystal structures determined here on 1−4, [Mn(4ImNNH)2Cl2], and [Cu(4ImNNH)2Br2] always have intermolecular hydrogen bonds of H(imidazole)···X(axial ligand)−M, where X = NO3, Cl, Br. This interaction seems to play an important role in molecular packing and presumably also in magnetic coupling

Ferromagnetic Superexchange Coupling through a Diamagnetic Iron(II) Ion in a Mixed-Valent Iron(III, II, III) <i>meso</i>-Helicate

meso-Helicates [Ni3(pzNTR)6]·4CH3CN and [Fe3(pzNTR)6](ClO4)2·CH3OH were prepared (HpzNTR stands for N-tert-butyl-α-3-pyrazolylnitrone). The former showed antiferromagnetic coupling between neighboring ions with J/kB = −28.2(2) K, which can be reasonably interpreted in terms of the superexchange mechanism. On the other hand, the latter exhibited ferromagnetic coupling with J/kB = +0.292(8) K between the two terminal S = 5/2 Fe3+ ions through the intervening diamagnetic Fe2+ ion. The admixture of low-lying excited states with the ground state stabilizes the ferromagnetically coupled state, like the Prussian blue magnet

Polymorphic Alternating HNN−Cobalt(II) Chains Both Behaving as Single-Chain Magnets (HNN = 4,4,5,5-Tetramethylimidazolin-1-oxyl 3-Oxide)

Metal-radical alternating chains [Co(hfac)2HNN]n crystallized in two morphs containing repeating all-cis (α) and cis,cis,trans (β) configurations with respect to the OHNN−Co−OHNN geometry (HNN = 4,4,5,5-tetramethylimidazolin-1-oxyl 3-oxide). Both phases showed magnetization jumps with hysteresis at 2 K. The α phase has a relatively high activation energy of magnetization reorientation (193 K)

Molecular Metamagnet [Ni(4ImNNH)₂(NO₃)₂] (4ImNNH = 4-Imidazolyl Nitronyl Nitroxide) and the Related Compounds Showing Supramolecular H-Bonding Interactions

A new chelating radical ligand 4ImNNH (2-(4-imidazolyl)-4,4,5,5-tetramethylimidazolin-1-oxyl 3-oxide) was prepared, and complexation with divalent transition metal salts gave complexes, [M(4ImNNH)2X2], which showed intermolecular ferromagnetic interaction in high probability (7 out of 10 paramagnetic compounds investigated here). The nitrate complexes (X = NO3; M = Mn (1), Co (2), Ni (3), Cu (4)) crystallize isomorphously in monoclinic space group P21/a. The equatorial positions are occupied with two 4ImNNH chelates and the nitrate oxygen atoms are located at the axial positions. Magnetic measurements revealed that the intramolecular exchange couplings in 1, 2, and 4 were antiferromagnetic, while that in 3 was ferromagnetic with 2J/kB = +85 K, where the spin Hamiltonian is defined as H = −2J(S1·S2 + S2·S3) based on the molecular structures determined as the linear radical−metal−radical triads. The intramolecular ferromagnetic interaction in 3 is interpreted in terms of orthogonality between the radical π* and metal dσ orbitals. Compounds 1−3 exhibited intermolecular ferromagnetic interaction ascribable to a two-dimensional hydrogen bond network parallel to the crystallographic ab plane. Complex 3 became an antiferromagnet below 3.4 K and exhibited a metamagnetic transition on applying a magnetic field of 5.5 kOe at 1.8 K. The complexes prepared from metal halides, [M(4ImNNH)2X2] (X = Cl, Br; M = Mn, Co, Ni, Cu), showed intramolecular antiferromagnetic interactions, which are successfully analyzed based on the radical−metal−radical system. The crystal structures determined here on 1−4, [Mn(4ImNNH)2Cl2], and [Cu(4ImNNH)2Br2] always have intermolecular hydrogen bonds of H(imidazole)···X(axial ligand)−M, where X = NO3, Cl, Br. This interaction seems to play an important role in molecular packing and presumably also in magnetic coupling

Trans-Fused Macrobicycle “Betweenanene” around a Planar Cu₂(μ-CH₃O)₂ Core Showing Remarkable Antiferromagnetic Interaction

Complexation of 4,6-bis{3-(2-pyridyl)-1H-pyrazol-1-yl}pyrimidine (bppp) and copper(II) tetrafluoroborate in methanol gave a self-assembled [Cu2(CH3O)2(bppp)2]2+ ion having a betweenanene-type structure. Each bppp spans across the common Cu2O2 plane in a trans fashion to ligate each copper center. The antiferromagnetic interaction observed in the Cu2O2 core is ascribable to the superexchange along the Cu−O−Cu linkage and found to be one of the largest among the analogous compounds, owing to the large Cu−O−Cu angle perturbed with the two bppp molecular tweezers