Satoru Karasawa

Cyclic single-molecule magnet in heterospin system.

The 2:2 complex of Co-carbene is generated by photolysis of [Co(hfpip)2(D2py2(TBA))]2, 1, having a cyclic structure functioning as a single-molecule magnet with Ueff = 96 K and Hc = 10 kOe at 1.9 K after annealing at 70 K.

Crystal design of monometallic single-molecule magnets consisting of cobalt-aminoxyl heterospins

Five N-aryl-N-pyridylaminoxyls, which have no substituent (PhNOpy), one substituent (MeOPhNOpy and tert-BuPhNOpy) at the 4-position, and three substituents (TPPNOpy and TBPNOpy) at the 2, 4, and 6-positions of the phenyl ring, were prepared as new ligands for cobalt-aminoxyl heterospin systems. The 1:4 complexes, [Co(NCS)2(PhNOpy)4] (1), [Co(NCS)2(MeOPhNOpy)4] (2), [Co(NCS)2(tertBuPhNOpy)4] (3), [Co(NCS)2(TPPNOpy)4] (4), [Co(NCS)2(TBPNOpy)4] (5a), and [Co(NCO)2(TBPNOpy)4] (5b), were obtained as single crystals. The molecular geometry revealed by X-ray crystallography for all complexes except 4 is a compressed octahedron. In the crystal structure of 1, 2, and 3, the organic spin centers have various short contacts within 4 A with the neighboring molecules to form 3D and 2D spin networks. On the other hand, complexes 5a and 5b have no significant short intermolecular contacts, indicating that they are magnetically isolated. 1 and 2 behaved as a 3D antiferromagnet with a Neel temperature, T(N), of 22 K and as a weak 3D antiferromagnet with a T(N) of 2.9 K and a spin-flop field at 1.9 K, Hsp(1.9), of 0.7 kOe, respectively. 3 was a canted 2D antiferromagnet (a weak ferromagnet) with T(N) = 4.8 K and showed a hysteresis loop with a coercive force, Hc, of 1.3 kOe at 1.9 K. On the other hand, the trisubstituted complexes 4, 5a, and 5b functioned as single-molecule magnets (SMMs). 5b had an effective activation barrier, U(eff), value of 28 K in a microcrystalline state and 48 K in a frozen solution.

Crystal Structures and Emitting Properties of Trifluoromethylaminoquinoline Derivatives: Thermal Single-Crystal-to-Single-Crystal Transformation of Polymorphic Crystals That Emit Different Colors

2,4-Trifluoromethylquinoline (TFMAQ) derivatives that have amine (1), methylamine (2), phenylamine (3), and dimethylamine (4) substituents at the 7-position of the quinoline ring were prepared and crystallized. Six crystals including the crystal polymorphs of 2 (crystal GB and YG) and 3 (crystal B and G) were obtained and characterized by X-ray crystallography. In solution, TFMAQ derivatives emitted relatively strong fluorescence (lambda(max)(f)=418-469 nm and Φ(f)(s)=0.23-0.60) depending on the solvent polarity. From Lippert-Mataga plots, Δμ values in the range of 7.8-14 D were obtained. In the crystalline state, TFMAQ derivatives emitted at longer wavelengths (lambda(max)(f)=464-530 nm) with lower intensity (Φ(f)(c)=0.01-0.28) than those in n-hexane solution. The polymorphous crystals of 2 and 3 emitted different colors: 2, lambda(max)(f)=470 and 530 nm with Φ(f)(c)=0.04 and approximately 0.01 for crystal GB and YG, respectively; and 3, lambda(max)(f)=464 and 506 nm with Φ(f)(c)=0.28 and approximately 0.28 for crystal B and G, respectively. In both crystal polymorphs of 2 and 3, crystals GB and G showed emission color changes by heating/melting/cooling cycles that were representative. By following the color changes in heating at the temperature below the melting point with X-ray diffraction measurements and X-ray crystallography, the single-crystal-to-single-crystal transformations from crystal GB to YG for 2 and from crystal B to G for 3 were revealed.

Polymorphic equilibrium responsive thermal and mechanical stimuli in light-emitting crystals of N-methylaminonaphthyridine.

Crystal polymorphs of 1,8-naphthyridine derivative, being anti and syn conformers, show a reversible transformation from anti to syn by heating and from syn to anti by grinding with the alteration of emittance intensity, and notably, thermal transformation from anti to syn conformer took place in single-crystal-to-single-crystal (SC-to-SC) form, which was confirmed by a single crystal X-ray crystallography.

Formation of monometallic single-molecule magnets with an Stotal value of 3/2 in diluted frozen solution

A combination of cobalt(II) complexes and pyridine ligands carrying organic spins, aminoxyl and carbene, provided single-molecule magnets with Stotal = 3/2 in diluted frozen solutions.

Stable π Radical from a Contracted Doubly N‐Confused Hexaphyrin by Double Palladium Metalation

A contracted doubly N-confused dioxohexaphyrin derivative served as a dinucleating metal ligand for unsymmetrical coordination. The complexation of two palladium(II) cations led to the formation of π-radical species that were persistent in atmospheric air in the presence of moisture. Effective delocalization of an unpaired electron over the hexaphyrin backbone could contribute to the distinct chemical stability.

Magnetic properties after irradiation of 1:4 complexes consisting of CoX2, X=NCS⁻, Cl⁻, and NCO⁻, and phenylpyridyldiazomethane in dilute frozen solutions: axial ligand effect in heterospin single-molecule magnets.

The solutions of 1:4 complexes of Co(X)(2)(D1py)(4), X = Cl(-), and NCO(-) and D1py = phenylpyridyldiazomethane, were photolyzed under cryogenic conditions, and their magnetic properties were investigated by direct current (DC) and alternating current (AC) magneto/susceptometries. After irradiation, the resulting cobalt-carbene complexes, Co(X)(2)(C1py)(4), exhibited the behaviors of heterospin single-molecule magnets (SMMs) strongly depending on the axial ligands. In Co(X)(2)(C1py)(4): X = Cl(-) and NCO(-), the effective activation barriers, U(eff), for the reorientation of the magnetic moment and the resonant quantum tunneling time, τ(Q), characteristic to SMM properties were estimated to be 91 and 130 K, and 4 × 10(3) and 2 × 10(5) s, respectively. The τ(Q) of Co(NCS)(2)(C1py)(4) with U(eff) = 89 K was found to be 6 × 10(2) s. In Co(X)(2)(C1py)(4): X = Cl(-) and NCO(-), temperature-dependent hysteresis loops were also observed below the blocking temperature (T(B) = 3.2 and 4.8 K, respectively) and the coercive forces, H(c), of 7.0 and 20 kOe at 1.9 K, respectively, were obtained. In a series of 1:4 complexes of Co(X)(2)(C1py)(4), X = NCS(-), Cl(-), and NCO(-), the axial ligands strongly affected the heterospin SMM properties, and the NCO(-) ion having the large magnitude of the ligand-field splitting in a spectrochemical series, gave the largest U(eff) and H(c) and the longest τ(Q).

Magnetic behavior of tetrakis[4-(N-tert-butyl-N-oxylamino)pyridine]bis(isocyanato-N)cobalt(II) in frozen solution.

A complex of cobalt(II)(OCN)(2) coordinated with four pyridines having a stable tert-butyl aminoxyl exhibited in frozen MTHF a slow magnetic relaxation for the reorientation of magnetization with activation barrier, Delta/k(B) = 50 K, and a hysteresis loop having a fast relaxation at 0 Oe below 2.5 K.

Magnetic properties of 1 : 4 complexes of CoCl2 and pyridines carrying carbenes (S0 = 4/2, 6/2, and 8/2) in diluted frozen solution; influence of carbene multiplicity on heterospin single-molecule magnets

The microcrystalline sample of a parent complex, [CoCl(2)(py)(4)], showed a single-molecule magnet (SMM) behavior with an effective activation barrier, U(eff)/k(B), of 16 K for reversal of the magnetism in the presence of a dc field of 3 kOe. Pyridine ligands having 2-4 diazo moieties, DYpy; Y = 2, 3l, 3b, and 4, were prepared and confirmed to be quintet, septet, septet, and nonet in the ground state, respectively, after irradiation. The 1 : 4 complexes, CoCl(2)(DYpy)(4); Y = 2, 3l, 3b, and 4 in frozen solutions after irradiation showed the magnetic behaviors of SMMs with total spin multiplicity, S(total) = 17/2, 25/2, 25/2, and 33/2, respectively. Hysteresis loops depending on the temperature were observed and the values of coercive force, H(c), at 1.9 K were 12, 8.4, 11, and 8.1 kOe for CoCl(2)(CYpy)(4); Y = 2, 3l, 3b, and 4, respectively. In dynamic magnetic susceptibility experiments, ac magnetic susceptibility data obeyed the Arrhenius law to give U(eff)/k(B) values of 94, 92, 93, and 87 K for CoCl(2)(CYpy)(4); Y = 2, 3l, 3b, and 4, respectively, while the relaxation times for CoCl(2)(CYpy)(4); Y = 2 and 3l, obtained by dc magnetization decay in the range of 3.5-1.9 K slightly deviated downward from Arrhenius plots on cooling. The dynamic magnetic behaviors for CoCl(2)(CYpy)(4) including [CoCl(2)(py)(4)] and CoCl(2)(C1py)(4) suggested that the generated carbenes interacted with the cobalt ion to increase the relaxation time, τ(q), due to the spin quantum tunneling magnetization, which became larger with increasing S(total) of the complex.

Magnetic properties of 1: 2 mixed cobalt(II) salicylaldehyde schiff-base complexes with pyridine ligands carrying high-spin carbenes (S car = 2/2, 4/2, 6/2, and 8/2) in dilute frozen solutions: Role of organic spin in heterospin single-molecule magnets

The 1:2 mixtures of Co(p-tolsal)2, p-tolsal = N-p-tolylsalicylideniminato, and diazo-pyridine ligands, DXpy; X = 1, 2, 3l, 3b, and 4, in MTHF solutions were irradiated at cryogenic temperature to form the corresponding 1:2 cobalt-carbene complexes Co(p-tolsal)2(CXpy)2, with Stotal = 5/2, 9/2, 13/2, 13/2, and 17/2, respectively. The resulting Co(p-tolsal)2(CXpy)2, X = 1, 2, 3l, 3b, and 4, showed magnetic behaviors characteristic of heterospin single-molecule magnets with effective activation barriers, Ueff/kB, of 40, 65, 73, 72, and 74 K, for reorientation of the magnetic moment and temperature-dependent hysteresis loops with a coercive force, Hc, of ∼0, 6.2, 10, 6.5, and 9.0 kOe at 1.9 K, respectively. The relaxation times, τQ, due to a quantum tunneling of magnetization (QTM) were estimated to be 1.6 s for Co(p-tolsal)2(C1py)2, ∼2.0 × 10(3) s for Co(p-tolsal)2(C2py)2, and >10(5) s for Co(p-tolsal)2(CXpy)2; X = 3b, 3l, and 4. In heterospin complexes, organic spins, carbenes interacted with the cobalt ion to suppress the QTM pathway, and the τQ value increased with increasing the Stotal values.