Ryuta Ishikawa

Single-chain magnets: beyond the Glauber model

Single-chain magnets (SCMs) are one-dimensional (1D) slow-relaxing magnetic chains with interesting fundamental properties and applications, such as information storage. The mechanism underlying the slow relaxation of SCMs is Glauber dynamics, in which R. J. Glauber proposed about 50 years ago that the 1D correlation of ferromagnetically coupled Ising spins causes very slow magnetic relaxation at a finite temperature. Since the first experimental observation of SCM dynamics of a cobalt(II)–organic radical alternating chain in 2001, several SCM systems have been reported, supporting and expanding the Glauber model. In this review, we present the recent advances concerning SCMs with the main focus on SCM systems beyond the Ising limit, SCMs constructed with non-collinear or canted antiferromagnetic intrachain interactions, the relation between SCM dynamics and interchain interactions, and some SCMs with chirality, porosity, spin-crossover, photo-switchable states, and so on.

MnIII(tetra-biphenyl-porphyrin)–TCNE Single-Chain Magnet via Suppression of the Interchain Interactions

A single-chain magnet (SCM) of [Mn(TBPP)(TCNE)]·4m-PhCl(2) (1), where TBPP(2-) = meso-tetra(4-biphenyl)porphyrinate; TCNE(•-) = tetracyanoethenide radical anion; m-PhCl(2) = meta-dichlorobenzene, was prepared via suppression of interchain interactions. 1 has a one-dimensional alternating Mn(III)(porphrin)-TCNE(•-)chain structure similar to those of a family of complexes reported by Miller and co-workers. From a comparison of the static magnetic properties of 1 with other Mn(III)(porphyrin)-TCNE(•-) chains, a magneto-structural correlation between the intrachain magnetic exchange and both the dihedral angle between the mean plane on [Mn(TBPP)(TCNE)] and Mn-N≡C was observed. The ac magnetic susceptibility data of 1 could be fit with the Arrhenius law, indicating that slow magnetic relaxation and ruling out three-dimensional long-range ordering and spin-glass-like behavior. The Cole-Cole plot for 1 was semicircular, verifying that it is an SCM. Therefore, 1 is an ideal single-chain magnet with significantly strong intrachain magnetic exchange interactions beyond the Ising limit.

Construction of a novel topological frustrated system: a frustrated metal cluster in a helical space.

A novel topologically frustrated pentanuclear cluster helicate [{Cu(II)(μ-L)(3)}(2)Cu(II) (3)(μ(3)-OH)](3+) (L(-)=3,5-bis(2-pyridyl)pyrazolate) has been synthesized and characterized. This cluster has a helical arrangement of ligands around the central metal core. Dzyaloshinsky-Moriya interactions are essential components to observe a gradual magnetization and forbidden transitions of high-field/multi-frequency (HF/MF)-ESR. The origin of the magnetic anisotropy of this compound is influenced by its helical spin structure, and consequently, the Cu(5)-cluster helicate introduces a unique magnetic anisotropy. This observation is a direct evidence of the topological part of the new spin phase in a magnetic system.

Solid state conversion of a double helix thallium(I) coordination polymer to a corrugated tape silver(I) polymer

We observed the solid state conversion of a nanostructured TlI coordination polymer with a double helix chain structure, prepared by a sonochemical procedure, to a nanostructured corrugated tape silver(I) polymer via the mechanochemical reaction of [Tl(μ2-dcpa)]n (1) [Hdcpa = 2,4-dichlorophenoxyacetic acid] with AgNO3. The internal packing of these two compounds looked similar and low-energy structural changes allowed the conversion to occur smoothly. The transformation was irreversible as a result of the formation of stronger Ag–O bonds (in 2) compared with the initial Tl–O bonds (in 1). There are weak interaction planes in the crystal packing of 1, so the structure is not mechanically rigid, which allows Ag ions to penetrate the lattice and form stronger bonds.

Reversible solid-state hydration and dehydration process involving anion transfer in a self-assembled Cu2 system

Supramolecular assembly [CuII2(μ-bpypz)2Br1.25(H2O)0.75]Br0.75·2.25H2O (1) was prepared by using 3,5-di(2-pyridyl)pyrazole (Hbpypz). Assembly 1 consists of supramolecular dimers of dinuclear species based on doubly bpypz− bridged dinuclear building blocks. The supramolecular dimer is flexible as a result of π⋯π interactions. Compound 1 undergoes reversible structural transformation in the solid state involving reversible apical-ligand substitution at the copper centers upon hydration–dehydration.

Proton conduction via lattice water molecules in oxalato-bridged lanthanide porous coordination polymers.

The proton conducting properties of two different structural types of porous coordination polymers [La2(ox)3(H2O)6]·4H2O (1) and [Er2(ox)3(H2O)6]·12H2O (2), where ox2- = oxalate, were investigated. 1 has a two-dimensional layered structure, whereas 2 has a three-dimensional structure. Both 1 and 2 have hydrophilic one-dimensional channels filled by lattice water molecules with hydrogen-bonding networks. The coordinated H2O molecules are Lewis acidic due to the lanthanoid ions donating protons to lattice-filling H2O molecules, thereby forming efficient proton conduction pathways. Alternating-current impedance analyses of 1 and 2 indicated significant proton conduction (σ = 3.35 × 10-7 S cm-1 at 368 K for 1, 1.79 × 10-6 S cm-1 at 363 K for 2 under RH = 100%, with Ea = 0.35 eV for 1 and 0.47 eV for 2), which was attributed to the Grotthuss mechanism via the lattice H2O molecules.

The impact of halogen ions on the guest dependent spin crossover behaviour and porosity of Co(II) one-dimensional coordination polymers [CoX2(4′-(4-pyridyl)-2,2′:6′,2′′-terpyridine)] (X = Cl and Br)

Discrete Co(II) spin crossover (SCO) compounds have attracted much attention for constructing highly sensitive molecular devices; however, the use of Co(II) SCO-coordination polymers (CPs) in such applications is extremely limited. The syntheses of the one-dimensional Co(II) SCO-CPs, [CoX2(pyterpy)] (pyterpy = 4′-(4-pyridyl)-2,2′:6′,2′′-terpyridine; X = Cl (1), Br (2)), are presented. The structure of 1 shows a ‘closed’ packing arrangement that exhibits gate-type water adsorption at room temperature while 2 has an ‘open’ packing arrangement that shows type I water adsorption behaviour at room temperature. In addition, both 1 and 2 adsorb CO2 at 201 K. Following water adsorption, 1·H2O and 2·H2O showed cooperative SCO at 226 K and 299 K, respectively.