Showing papers by "Osamu Sato published in 2012"

Photoswitchable Dynamic Magnetic Relaxation in a Well-Isolated {Fe2Co} Double-Zigzag Chain

Abstract: The design and synthesis of new molecular compounds whose physical properties can be controlled by external stimuli have recently attracted much attention. Until now, various switchable compounds have been developed. An important challenge in this field is controlling the magnetic properties of molecular nanomagnets, that is, single-molecule magnets (SMMs) and single-chain magnets (SCMs). Molecular nanomagnets that exhibit slow magnetic relaxation can retain spin information over long time periods at a nanometer scale, and thus have potential applications in highdensity information storage, quantum computation, and spintronics. Although several examples of switchable nanomagnets have been reported, the number thereof, especially that of SCMs, is very limited. Guest-tuned SCMs with microporous metal–organic frameworks are an important development, and pressure control of nanomagnets has been reported. However, photocontrol of the magnetic properties of SCMs has not yet been disclosed. To observe light-induced SCM behavior, the developed compounds should have both SCM and photoswitching properties. Because simultaneously fulfilling these requirements is difficult, light-induced SCMs have not yet been successfully fabricated. In this study on developing a phototunable SCM, we designed an Fe2Co bimetallic one-dimensional system with an N6 coordination environment around Co ions with the expectation of charge-transfer-induced spin transition between the Fe and Co sites. Furthermore, to prevent through-bond magnetic interactions between chains, we used bulky [Fe(pzTp)(CN)3] (pzTp = tetrakis(pyrazolyl)borate) building blocks to increase the metal–metal distances between neighboring chains and adopted the monodentate 4-styrylpyridine ligand. Note that it has recently been revealed that some of the reported SCMs, including ours, which show slow magnetic relaxation are not actual SCMs but are their antiferromagnetic phases. Hence, we have carefully designed a one-dimensional (1D) system, in which 1D chains are sufficiently separated by bulky ligands. With the above strategy, we synthesized novel switchable complex {[Fe(pzTp)(CN)3]2Co(4-styrylpyridine)2}·2H2O·2CH3OH (1), which is composed of well-isolated {Fe2Co} chains. Complex 1 exhibits thermally induced charge transfer, wherein all Co ions and half of the Fe ions are involved in the metal-tometal charge-transfer (MMCT) process. At low temperatures, Fe and Fe are regularly arranged, in contrast to the typical randomly arranged examples. Furthermore, when 1 was irradiated, it showed SCM behavior with no antiferromagnetic ordering. This means that 1 is the first example of a photoswitchable SCM. The target compound was synthesized by a diffusion method in a test tube. A solution of Bu4N[Fe(pzTp)(CN)3] and 4-styrylpyridine in methanol was slowly layered over an aqueous solution of Co(ClO4)2·6H2O. [13] Crystallization required several weeks. Single-crystal X-ray diffraction analysis revealed that 1 crystallizes in space group P1̄. The crystal structure comprises neutral bimetallic double-zigzag [Fe(pzTp)(CN)3]2Co(4-styrylpyridine)2 chains with uncoordinated water and methanol molecules located between them (Figure 1). In the neutral chain, the [Fe(pzTp)(CN)3] unit bridges two Co ions through two of its three cyanide ligands in the cis position, and each Co ion is coordinated to four nitrogen atoms from the CN bridges. Dihedral angles of 38.78 are formed between the planes of the triangular Fe2Co units owing to the steric effect of the bulky [Fe(pzTp)(CN)3] building blocks. In contrast, in other reported Fe2Co complexes, four metal ions of the Fe2Co2 units are nearly aligned in a plane and the mean planes of the square units are parallel to each other. The crystal structure contains two unique iron and cobalt centers. Each iron center is located in an octahedral environment with three nitrogen atoms from the pzTp units and three cyanide carbon atoms. The equatorial plane of the cobalt center is occupied by four cyanide nitrogen atoms, and two nitrogen atoms from 4-styrylpyridine occupy its apical positions, providing an N6 coordination sphere. At 260 K, the Co Nequatorial bond lengths are 2.092–2.115 and 2.087– 2.112 , and the Co Napical distances are 2.150 and 2.162 for Co1 and Co2, respectively. The elongated N6 octahedral environment of cobalt suggests a negative anisotropy constant D, which is essential for SCM behavior. However, the apical axis of the neighboring Co ion shows an angle of 47.08, which is expected to counteract part of the Co ion [*] Dr. D.-P. Dong, Prof. T. Liu, Prof. C. He, Prof. C.-Y. Duan State Key Laboratory of Fine Chemicals Dalian University of Technology 2 Linggong Rd., 116024 Dalian (China) E-mail: liutao@dlut.edu.cn cyduan@dlut.edu.cn

Reversible Electron Transfer in a Linear {Fe2Co} Trinuclear Complex Induced by Thermal Treatment and Photoirraditaion

Abstract: Bistable materials possess two close-lying states, which can be reversibly interchanged by external stimuli such as temperature, light, pressure, and guest molecules. These materials offer attractive opportunities for the realization of energyefficient, switchable, molecule-based data storage in electronic devices. A current topic for research in this field is the preparation of switchable multifunctional molecules in which more than two properties coexist or interact synergistically. An important multifunctional compound shows a significant change in both magnetic properties and charge distribution (polarization) at the molecular level. Tunable magnetic molecules, such as spin-crossover complexes, are important for magnetic recording devices. Furthermore, switchable polarity is an essential feature for regulating nonlinear optical and ferroelectric properties. In particular, the study of electronic ferroelectricity, in which an electronic charge order without inversion symmetry is responsible for the electric polarization, has recently attracted significant attention. Thus, it is important to design new compounds in which the spin state and charge distribution can be reversibly controlled by external stimuli. To this end, the development of compounds that consist of paramagnetic donors and acceptors is attracting considerable interest because lattice distortion and charge-transfer processes in such compounds involve not only concomitant spin-state changes but also changes in their dielectric properties. 9] Significant changes in the magnetic susceptibility and the dielectric constant were observed near the neutral–ionic phase transition temperature of chargetransfer complexes. Furthermore, dimerization of the donor and the acceptor induces the formation of a polar structure from a nonpolar structure because of the breaking of the inversion center as a result of molecular charge transfer. Moreover, it has been reported that a dinuclear cobalt complex with a dioxolene bridging ligand exhibits charge transfer between the bridging ligand (donor/acceptor) and cobalt (acceptor/donor) induced by a temperature change and light irradiation. This transfer is accompanied by magnetization change and the formation of a polar structure in the cluster, which is a molecular-level representation of the interconversion of both magnetization and electric polarization similar to that in the aforementioned charge-transfer complex. With a rational design, various discrete multinuclear complexes have been synthesized by using different building blocks such as cyanometallates. However, until now, only the cobalt-dioxolene system has been reported to show such magnetization changes and polar–nonpolar transformation through charge transfer in response to both thermal and optical stimuli. Therefore, the preparation of new compounds with such properties remains a challenge. In this work, we aimed at synthesizing linear bimetallic trinuclear clusters with centrosymmetrical structures that are capable of charge transfer between the metal in the center and a metal ion on the edge. The charge-transfer process was expected to induce a change in magnetization because of the change in spin multiplicity. Furthermore, charge transfer in a cluster with an inversion center also induces the formation of a polar structure from a nonpolar one. To synthesize such a centrosymmetric bimetallic trinuclear cluster, we choose [FeTp(CN)3] (Tp = hydrotris(pyrazolyl)borate) as the building block to treat with [Co(Meim)4] 2+ (Meim = N-methylimidazole). One cyanide bridge of the [FeTp(CN)3] unit is thought to coordinate with the Co ion, which tunes the redox potential required for charge transfer. Moreover, the terminal cyanide ligands are thought to form potential hydrogen-bonding interactions with noncoordinated solvent molecules, stabilizing the bistable state through intermolecular cooperative interactions. In fact, we recently synthesized an Fe2Co trinuclear cluster {[FeTp(CN)3]2Co(Meim)4}·6 H2O (1), in which the cobalt ion is sandwiched between two iron building blocks (Scheme 1). Compound 1 exhibited thermally induced, reversible electron transfer with a thermal hysteresis and photoinduced electron transfer by excitation of the charge-transfer band. Single-crystal X-ray diffraction (XRD) analysis revealed that 1 crystallizes in a P 1 space group. The crystal structure comprises neutral {[FeTp(CN)3]2Co(Meim)4} trinuclear clusters (Figure 1 a) with noncoordinated water molecules located between the clusters (Figure 1b). Within the neutral trinu[*] Prof. T. Liu, Dr. D.-P. Dong, Prof. S. Wu, Prof. C. He, Prof. C.-Y. Duan State Key Laboratory of Fine Chemicals, Dalian University of Technology, 2 Linggong Rd., 116024 Dalian (China) E-mail: liutao@dlut.edu.cn cyduan@dlut.edu.cn

Switchable molecular magnets.

Abstract: Various molecular magnetic compounds whose magnetic properties can be controlled by external stimuli have been developed, including electrochemically, photochemically, and chemically tunable bulk magnets as well as a phototunable antiferromagnetic phase of single chain magnet In addition, we present tunable paramagnetic mononuclear complexes ranging from spin crossover complexes and valence tautomeric complexes to Co complexes in which orbital angular momentum can be switched Furthermore, we recently developed several switchable clusters and one-dimensional coordination polymers The switching of magnetic properties can be achieved by modulating metals, ligands, and molecules/ions in the second sphere of the complexes

Supercritical Water Gasification of Organosolv Lignin over a Graphite-supported Ruthenium Metal Catalyst

Abstract: A graphite-supported ruthenium metal catalyst was highly active for catalytic gasification of organosolv lignin in supercritical water at 673 K because of the easy accessibility of the reaction int...

Cooperative spin transition and thermally quenched high-spin state in new polymorph of [Fe(qsal) 2]I 3

Abstract: Novel polymorph of the ferric spin crossover complex, β-[Fe(qsal)2]I3 [qsalH = N-(8-quinolyl)salicylaldimine], has been prepared and characterized by magnetic susceptibilities and Mossbauer spectra. β polymorph exhibited a cooperative complete spin transition with a thermal hysteresis of 25 K and a tendency to be quenched in the high-spin (HS) state, which is contrastive to a gradual incomplete spin conversion in α polymorph.

Slow magnetic relaxation in a 4,2-ribbon like FeIII2CoII heterobimetallic chain

Abstract: The reaction of Co(NO3)2·6H2O and 3-benzoylpyridine with the low-spin iron(III) complex, H[FeIII(phen)(CN)4], in methanol gives rise to a cyanide-bridged heterobimetallic chain [{FeIII(phen)(CN)4}2CoII(3-bpe)2] (1) that exhibits intrachain ferromagnetic coupling and slow double magnetic relaxation.