Shinji Kanegawa

Photoinduced Metal-to-Metal Charge Transfer toward Single-Chain Magnet

Single-chain magnets (SCMs) that exhibit slow relaxation of their magnetization are attracting considerable attention. To tune the properties of such materials with external stimuli such as light, heat, and pressure is a challenge. Through the exploitation of light and heat induced transformation between diamagnetic Fe(II)(LS)(mu-CN)Co(III)(LS) (LS = low spin) units and paramagnetic Fe(III)(LS)(mu-CN)Co(II)(HS) (HS = high spin) units, we show the photoswitched transformation from a paramagnetic state to an antiferromagnetic ordered SCM state and the thermally induced reverse transformation, thus providing an effective way to control the spin topology of the SCM via light or a thermally induced metal-to-metal charge transfer.

Reversible single-crystal-to-single-crystal transformation from achiral antiferromagnetic hexanuclears to a chiral ferrimagnetic double zigzag chain

A reversible single-crystal-to-single-crystal transformation from hexanuclear clusters to a one-dimensional double-zigzag chain was established. With the reversible polymerization, the chirality and magnetic interactions are switched between achiral and chiral and between antiferromagnetic in hexanuclear clusters and ferrimagnetic in chains, respectively.

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

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

Interconversion between a Nonporous Nanocluster and a Microporous Coordination Polymer Showing Selective Gas Adsorption

Using reversible polymerization and depolymerization reactions in a single crystal state, we achieved a reversible transformation from a nanocluster to a coordination polymer. During the interconversion, the structural frameworks switched between nonporous hexanuclear clusters and porous double-zigzag chains; the magnetic behaviors switched between paramagnetism and metamagnetism, respectively. The microporous framework, which had 1D channels 1.9 A x 3.6 A in size, exhibited selective gas adsorption of H(2) and CO(2) over N(2).

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.

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

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

A ferromagnetically coupled Fe 42 cyanide-bridged nanocage

Self-assembly of artificial nanoscale units into superstructures is a prevalent topic in science. In biomimicry, scientists attempt to develop artificial self-assembled nanoarchitectures. However, despite extensive efforts, the preparation of nanoarchitectures with superior physical properties remains a challenge. For example, one of the major topics in the field of molecular magnetism is the development of high-spin (HS) molecules. Here, we report a cyanide-bridged magnetic nanocage composed of 18 HS iron(III) ions and 24 low-spin iron(II) ions. The magnetic iron(III) centres are ferromagnetically coupled, yielding the highest ground-state spin number (S=45) of any molecule reported to date.

Multi-step spin crossover accompanied by symmetry breaking in an Fe III complex: Crystallographic evidence and DFT studies

Spin doctor: A mononuclear ferric complex [Fe(H-5-Br-thsa)(5-Br-thsa)]⋅H2O (1) (H2-5-Br-thsa = 5-bromo-2-hydroxybenzylidene)hydrazinecarbothioamide) was synthesized and its magnetic properties and structure were investigated by DFT calculations. This complex shows unprecedented reversible, six/five-step spin-crossover behavior accompanied by symmetry breaking. More importantly, each step in the multi-step transition was successfully characterized by single-crystal X-ray diffraction.

Molecular motor-driven abrupt anisotropic shape change in a single crystal of a Ni complex

Many molecular machines with controllable molecular-scale motors have been developed. However, transmitting molecular movement to the macroscopic scale remains a formidable challenge. Here we report a single crystal of a Ni complex whose shape changes abruptly and reversibly in response to thermal changes at around room temperature. Variable-temperature single-crystal X-ray diffraction studies show that the crystalline shape change is induced by an unusual 90° rotation of uniaxially aligned oxalate molecules. The oxalate dianions behave as molecular-scale rotors, with their movement propagated through the entire crystalline material via intermolecular hydrogen bonding. Consequently, the subnanometre-scale changes in the oxalate molecules are instantly amplified to a micrometre-scale contraction or expansion of the crystal, accompanied by a thermal hysteresis loop. The shape change in the crystal was clearly detected under an optical microscope. The large directional deformation and prompt response suggest a role for this material in microscale or nanoscale thermal actuators.

Bistability of magnetization without spin-transition in a high-spin cobalt(II) complex due to angular momentum quenching

[Co(NO(3))(2)L] (L: 2,6-di(pyrazol-1-yl)pyrazine) (1) exhibits an abrupt transition with hysteresis in magnetic susceptibility between 228 and 240 K. The results of spectroscopic and XRD measurements showed that 1 is in the high spin state in the whole temperature range. Therefore the observed hysteresis is not due to a spin transition but corresponds to a partial quenching of the angular momentum contribution to the magnetic susceptibility. Crystallographic measurements on the low- and high-temperature form of 1, combined with DFT calculations, showed that a symmetric twisting of the coordinating nitrate ions upon the transition is the most important factor in the orbital quenching mechanism. Utilizing such quenching to control magnetic properties can be a new approach to engineer transition metal complexes with magnetic functionalities without changing their spin or oxidation state.