Takuya Shiga

Three-way switching in a cyanide-bridged [CoFe] chain

Bistable compounds that exist in two interchangeable phases under identical conditions can act as switches under external stimuli. Among such switchable materials, coordination complexes have energy levels (or phases) that are determined by the electronic states of their constituent metal ions and ligands. They can exhibit multiple bistabilities and hold promise in the search for multifaceted materials that display different properties in different phases, accessible through the application of contrasting external stimuli. Molecular systems that exhibit both thermo- and photoinduced magnetic bistabilities are excellent candidates for such systems. Here we describe a cyanide-bridged [CoFe] one-dimensional chiral coordination polymer that displays both magnetic and electric bistabilities in the same temperature range. Both the electric and magnetic switching probably arise from the same electron-transfer coupled spin-transition phenomenon, which enables the reversible conversion between an insulating diamagnetic phase and either a semiconducting paramagnetic (thermoinduced) or a type of ferromagnetic single-chain magnet (photoinduced) state.

A Light-Induced Phase Exhibiting Slow Magnetic Relaxation in a Cyanide-Bridged [Fe4Co2] Complex†

Single-molecule magnets: A cyanide-bridged hexanuclear complex showed a thermal electron-transfer-coupled spin transition centered at 220 K. Light irradiation at low temperature (LT; HT = high temperature) generated a metastable state showing slow magnetic relaxation in measurements of the alternating-current magnetic susceptibility (χ(m); see picture).

A series of trinuclear Cu(II)Ln(III)Cu(II) complexes derived from 2,6-Di(acetoacetyl)pyridine: synthesis, structure, and magnetism.

A series of trinuclear Cu(II)Ln(III)Cu(II) complexes with the bridging ligand 2,6-di(acetoacetyl)pyridine have been prepared by one-pot reaction with Cu(NO(3))(2).3H(2)O and Ln(NO(3))(3).nH(2)O in methanol. X-ray crystallographic studies for all the complexes indicate that two L(2)(-) ligands selectively sandwich two Cu(II) ions with the 1,3-diketonate entities and one Ln(III) ion with the 2,6-acetylpyridine entity to form a trinuclear CuLnCu core bridged by the enolate oxygen atoms. Cryomagnetic properties of the complexes are studied with respect to the electronic structure of the Ln ion.

Programmable spin-state switching in a mixed-valence spin-crossover iron grid

Photo-switchable systems, such as discrete spin-crossover complexes and bulk iron-cobalt Prussian blue analogues, exhibit, at a given temperature, a bistability between low- and high-spin states, allowing the storage of binary data. Grouping different bistable chromophores in a molecular framework was postulated to generate a complex that could be site-selectively excited to access multiple electronic states under identical conditions. Here we report the synthesis and the thermal and light-induced phase transitions of a tetranuclear iron(II) grid-like complex and its two-electron oxidized equivalent. The heterovalent grid is thermally inactive but the spin states of its constituent metal ions are selectively switched using different laser stimuli, allowing the molecule to exist in three discrete phases. Site-selective photo-excitation, herein enabling one molecule to process ternary data, may have major ramifications in the development of future molecular memory storage technologies.

Undecanuclear mixed-valence 3d–4f bimetallic clusters

Two undecanuclear 3d-4f clusters with the general formula {Mn(III)(4)Mn(IV)Ln(III)(6)}, where Ln = Gd or Tb, were synthesized, with both showing large spin ground states, and the Tb species acting as a single molecule magnet.

Copper(II)-terbium(III) Single-Molecule Magnets linked by photochromic ligands

Two assemblies composed of single-molecule magnets (SMMs) linked by photochromic ligands, [Cu(II)(2)Tb(III)(2)(L)(2)(NO(3))(2)(dae-o)(2)]·2(n-BuOH) (1) and {[Cu(II)Tb(III)(L)(n-BuOH)(0.5)](2)(dae-c)(3)}·5(DMF)·4(n-BuOH)·2(H(2)O) (2), were synthesized by reacting the SMM [Cu(II)Tb(III)(L)(NO(3))(3)] (H(2)L = 1,3-bis((3-methoxysalicylidene)amino)propane) and photochromic molecules, H(2)dae-o and H(2)dae-c, which are open- and closed-ring isomers of 1,2-bis(5-carboxyl-2-methyl-3-thienyl)perfluoropentene (H(2)dae), respectively. 1 has a tetranuclear ring-like structure comprised of two [CuTb] units and two dae-o(2-) ligands. On the other hand, 2 has a one-dimensional ladder-type structure involving the [CuTb] and dae-c(2-) units in a 3 : 2 ratio. Magnetic studies revealed that 1 and 2 had ferromagnetic interactions between the Cu(II) and Tb(III) ions and that both compounds exhibited frequency dependence of ac susceptibilities owing to freezing the magnetization of the [CuTb] SMM. Upon irradiation with ultraviolet light and visible light, an absorption band at ∼600 nm changed, indicating that photochromic reactions involving the dae(2-) ligands occurred. After irradiation, the magnetic behaviour of 1 did not change, whereas magnetic behaviour of 2 changed, due to the modification of intermolecular environment.

Synthesis, stability, and complexation behavior of isolable salen-type N2S2 and N2SO ligands based on thiol and oxime functionalities.

The new salen-type N(2)S(2) tetradentate ligands, H(2)L(1) and H(2)L(2), which have a donor set comprising oxime and thiol groups, were synthesized. These ligands are obtained as isolable colorless crystals, whereas the imine analogues are too unstable to be isolated. The unsymmetrical N(2)SO ligands, H(2)L(3) and H(2)L(4), were also obtained as stable compounds. When ligands H(2)L(1)-H(2)L(4) are heated above the melting points, they mainly decompose via cleavage of the N-O bonds of a thiosalicylaldoxime moiety to give 1,2-benzisothiazole derivatives. The complexation of the N(2)S(2) ligands (H(2)L(1) and H(2)L(2)) with nickel(II) acetate afforded square-planar mononuclear complexes [Ni(L(1))] and [Ni(L(2))], respectively. In contrast, the complexation of the N(2)SO ligand H(2)L(3) with nickel(II) acetate resulted in cleavage of the N-O bond, giving a tetranuclear complex having a cubane-type Ni(4)O(4) core. The N-O bonds of H(2)L(1)-H(2)L(4) are more readily cleaved when the ligands are allowed to react with copper(II) acetate. In these cases, the alkoxo-bridged dinuclear complexes having a Cu-O-Cu-O four-membered ring are obtained. On the other hand, mononuclear complexes can be obtained by complexation of the ligands (H(2)L(1) or H(2)L(3)) with palladium(II) acetate without N-O bond cleavage.

Cyanide‐Bridged [Fe8M6] Clusters Displaying Single‐Molecule Magnetism (M=Ni) and Electron‐Transfer‐Coupled Spin Transitions (M=Co)

Cyanide-bridged metal complexes of [Fe(8)M(6)(μ-CN)(14)(CN)(10)(tp)(8)(HL)(10)(CH(3)CN)(2)][PF(6)](4)⋅n CH(3)CN⋅m H(2)O (HL=3-(2-pyridyl)-5-[4-(diphenylamino)phenyl]-1H-pyrazole), tp(-) =hydrotris(pyrazolylborate), 1: M=Ni with n=11 and m=7, and 2: M=Co with n=14 and m=5) were prepared. Complexes 1 and 2 are isomorphous, and crystallized in the monoclinic space group P2(1)/n. They have tetradecanuclear cores composed of eight low-spin (LS) Fe(III) and six high-spin (HS) M(II) ions (M=Ni and Co), all of which are bridged by cyanide ions, to form a crown-like core structure. Magnetic susceptibility measurements revealed that intramolecular ferro- and antiferromagnetic interactions are operative in 1 and in a fresh sample of 2, respectively. Ac magnetic susceptibility measurements of 1 showed frequency-dependent in- and out-of-phase signals, characteristic of single-molecule magnetism (SMM), while desolvated samples of 2 showed thermal- and photoinduced intramolecular electron-transfer-coupled spin transition (ETCST) between the [(LS-Fe(II))(3) (LS-Fe(III))(5)(HS-Co(II))(3)(LS-Co(III))(3)] and the [(LS-Fe(III))(8)(HS-Co(II))(6)] states.

Ferromagnetically coupled chiral cyanide-bridged {Ni6Fe4} cages

Enantiomeric, ferromagnetically coupled decanuclear {Ni₆Fe₄} cages with adamantane-like cores were synthesized around templating tetraethylammonium cations, as shown by crystallographic analysis and CSI-MS, and their homochiral nature was confirmed by circular dichroism measurements.

Redox‐Controlled Magnetic {Mn13} Keggin Systems

Polyoxometalates (POMs) have been widely reported in recent years. These molecular metal oxides, or polyanions, are most commonly constructed of tungsten, molybdenum, or vanadium ions in their highest oxidation state, bridged by oxide ligands to form clusters which can range in size from low-nuclearity building blocks to large-scale protein-like superstructures. An archetypical POM structural motif is the {XM12O40} n species (X = P, Si...) known as the Keggin anion and Keggin structures have been successfully shown to act as catalysts among other potential applications. POMs are inorganic materials that can be functionalized through their combination with organic ligands and/or the introduction of paramagnetic heterometal ions which leads to magnetic heterometallic POMs. In addition, there are a few studies of related species consisting exclusively of late first-row transition-metal ions such as some mixed-valence manganese Keggin-related clusters described by Lampropoulos et al, the uncapped {Fe13} cluster reported by Bino et al., and the reverse-Keggin structures presented by Baskar et al. To the best of our knowledge, there are no other examples of POM-type complexes consisting exclusively of open-shell transition metals, and so far their physical properties have been barely investigated. In contrast, transition-metal oxide materials are widely used and their properties such as magnetic ordering, semiand superconductivity, giant magnetoresistance, and ferroelectricity are much studied. Their electronic properties can be understood by their band structures and changed to show the desired characteristics. 11] Replication or improvement of metal oxide properties in discrete molecules can be extremely difficult. However, molecular metal clusters can possess wellseparated energy levels and their characteristic electronic structures can be altered to show, for example, valence tautomerism, multi-bistability with spin crossover, and singlemolecule-magnetic (SMM) behavior by tuning the frontier orbitals and the electronic interactions between the metal centers. The band filling in solids is readily controlled in their syntheses by altering the ratio of the constituent elements which drastically changes the physical properties. The question arises whether the chemist can synthesize metal oxide clusters displaying controllable redox states which can perturb the physical properties. Herein, a polyoxometalate-type cluster was synthesized by using exclusively first-row transition-metal ions in combination with organic capping ligands. In the resultant system the spin state and magnetic properties were tuned without substantial change to the molecular structure, and its SMM behavior was perturbed through manipulation of the cluster oxidation state. Herein, the synthesis, magnetic properties, and redox behavior of three mixed-valence {Mn13} Keggintype clusters are reported. The one-pot reaction of Mn(NO3)2·6 H2O with 2,6-bis[N(2-hydroxyethyl)iminomethyl]-4-methylphenol (H3bemp) [16] in methanol yielded a tridecanuclear cluster [Mn12Mn O6(OH)2(OMe)4(bemp)6](NO3)4·10MeOH·6H2O (1(NO3)4) (Figure 1 and Figure S1 in the Supporting Information). Subsequent high-yielding crystallization led to the formation of dark brown square blocks of 1(NO3)4, the counterions of which were exchanged to yield 1(PF6)4 from a solution of 1(NO3)4 and NH4PF6 in methanol. The core structure and physical behavior of 1(PF6)4 were identical to 1(NO3)4. Dark brown platelets of 1(PF6)4 were then dissolved in methanol with one or two equivalents of [Fe(bpy)3](PF6)3 (bpy = 2,2’-bipyridine) to yield dark brown rhombic crystals of oxidized 1(PF6)5 and 1(PF6)6, respectively. Cyclic voltammetry (CV) measurements conducted on 1(NO3)4 (1 mm) in N,N-dimethylformamide (DMF) showed four quasireversible waves at 0.02, 0.18, 0.50, and 0.73 V versus the saturated calomel electrode (SCE), corresponding to four one-electron redox processes of 1/1, 1/1, 1/ 1, and 1/1, respectively (Figure 2). Approximating the complex as a trinuclear redox-active system, we calculated comproportionation constants of 560, 2.6 10, and 5.5 10 for the reduced 1 (MnMn11Mn ), the native species 1 (Mn12Mn ), and the oxidation product 1 (Mn11Mn IV 2), [*] Dr. G. N. Newton, S. Yamashita, K. Hasumi, J. Matsuno, N. Yoshida, Dr. M. Nihei, Dr. T. Shiga, Prof. Dr. H. Oshio Graduate School of Pure and Applied Sciences University of Tsukuba Tennodai 1-1-1, Tsukuba 305-8571 (Japan) Fax: (+ 81)29-853-4238 E-mail: oshio@chem.tsukuba.ac.jp