Masayuki Nihei

Multiple Bistability and Tristability with Dual Spin-State Conversions in [Fe(dpp)2][Ni(mnt)2]2.MeNO2

The multicomponent system of [Fe(dpp)(2)][Ni(mnt)(2)](2).MeNO(2) (1; dpp = 2,6-bis(pyrazol-1-yl)pyridine and mnt = maleonitriledithiolate) was prepared by the reaction of [Fe(dpp)(2)](BF(4))(2) with (Bu(4)N)[Ni(mnt)(2)] in MeNO(2). Variable-temperature X-ray structural analyses, magnetic susceptibility, and heat capacity measurements confirmed that 1 undergoes multiple spin-state conversions in both the cationic and anionic components. The asymmetric unit in the crystal contains one [Fe(dpp)(2)](2+) cation, two [Ni(mnt)(2)](-) anions ([Ni1](-) and [Ni2](-)), and one solvent molecule. Magnetic susceptibility measurements revealed that a paramagnetic state in the high-temperature region (HT phase) was abruptly converted to a diamagnetic low-temperature (LT) phase below 180 K as the temperature was lowered from 270 K. As the temperature was raised from 125 to 270 K, successive phase transitions occurred to the HT phase via intermediate phases (IM1, IM2, and IM3) at 175.5, 186.5, 194.0, and 244.0 K, respectively. In the HT phase [Fe(dpp)(2)](2+) is in the high-spin state, and each [Ni1](-) and [Ni2](-) moiety is arranged in monomeric form with an S = (1)/(2) spin ground state. In the LT phase [Fe(dpp)(2)](2+) is in the low-spin state and the nickel moieties are dimerized and diamagnetic. In the IM1 and IM2 phases the iron(II) sites are partially in the HS state and both [Ni](-) moieties are dimeric, as suggested by (57)Fe Mossbauer measurements. In the IM3 phase, [Fe(dpp)(2)](2+) is in the HS state and the anions exist in both their monomeric ([Ni1](-)) and dimeric ([Ni2](-)) forms. Rapid thermal quenching from 300 to 5 K yielded a metastable HS phase, which relaxed to the LT phase via the IM1 phase as the temperature was raised to 150 K. A partial light induced spin transition on the iron site was observed at 5 K.

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).

Tuning the Spin States of Two Apical Iron(II) Ions in the Trigonal‐Bipyramidal [{FeII(μ‐bpt)3}2FeII3(μ3‐O)]2+ Cations Through the Choice of Anions

When located in octahedral environment, the iron(II) ion with a d electronic configuration may adopt two different stable electronic states, namely, a diamagnetic low-spin (LS) state and a paramagnetic high-spin (HS) state, which both give rise to different magnetic, optical and electronic properties. So tuning the spin state of the iron(II) ion is significant and contributes to the development of amazing materials that can be used as molecular switches, sensors and display devices. As is well established, the spin state depends on relative strength of spin paring energy (P) and splitting energy (D0). If the former is much stronger than the latter, the HS state will be stabilised, and if D0 is stronger then the LS state will be the ground state. If P and D0 are comparable a spin crossover (SCO) may occur between the HS and LS state by external perturbations such as temperature, pressure or light irradiation. So the main task is to make a judicious choice of ligand, which can impose a proper ligand-field strength. However, in reality, the spin state of the iron(II) ion is quite sensitive to even more subtle changes such as solvent molecules, polymorphism and counterions. As one of the best representatives of switchable molecules, SCO materials have attracted considerable interest in the chemistry and materials fields. Current work mainly focuses on the enhancement of cooperativity, which results in an abrupt transition and a wider thermal hysteresis. Another promising research area, but with few examples, is the combination of magnetic-exchange and spin-transition (ST) phenomena in the same molecule or polymeric network, which could eventually afford new switching materials with considerable amplification of the response signal. Much more work should be done to expand our knowledge of spin electronics. To induce a ST, the most common method is by varying the temperature. However, light-and pressure-induced SCOs play an increasingly important role owing to their potential applications as optical and pressure switches, for example. Moreover, the latter two methods are not restricted to thermal SCO compounds: LIESST (light-induced excited spinstate trapping) may also be observed in LS compounds, whereas HS compounds may experience STs with the application of external hydrostatic pressure. Although there are many examples that exhibit thermal STs, rare spin-crossover clusters of iron(II) have been found to exhibit a mixed-spin structure and synergy between ST and magnetic interaction. Fortunately, by introducing counterions, we have successfully tuned the spin states of two apical iron(II) ions in the pentanuclear [{Fe ACHTUNGTRENNUNG(m-bpt)3}2FeII3ACHTUNGTRENNUNG(m3-O)]2+ (Hbpt=3,5-bis(pyridin-2-yl)-1,2,4-triazole) cations through anions. Both apical ions are of LS states in [{Fe ACHTUNGTRENNUNG(mbpt)3}2Fe II 3ACHTUNGTRENNUNG(m3-O)] ACHTUNGTRENNUNG(NCS)2·10H2O (1), [{Fe ACHTUNGTRENNUNG(m-bpt)3}2FeII3ACHTUNGTRENNUNG(m3O)] ACHTUNGTRENNUNG(ClO4)2·3H2O (2) and [{FeIIACHTUNGTRENNUNG(m-bpt)3}2FeII3ACHTUNGTRENNUNG(m3-O)]I2· 4MeCN (3), and are of HS states in [{Fe ACHTUNGTRENNUNG(m-bpt)3}2FeII3ACHTUNGTRENNUNG(m3O)] ACHTUNGTRENNUNG[FeIII2ACHTUNGTRENNUNG(m-O)Cl6]·1/2H2O (4). In this new {Fe5} family, two new developments have been achieved: 1) The ligand 4amino-3,5-bis(pyridine-2-yl)-1,2,4-triazole (abpt) has been, for the first time, used to produce the oxo-centred polynuACHTUNGTRENNUNGcle ACHTUNGTRENNUNGar iron(II) complexes; 2) Two types of spin topology have been trapped in the [{Fe ACHTUNGTRENNUNG(m-bpt)3}2FeII3ACHTUNGTRENNUNG(m3-O)]2+ cluster that are controlled by counterions. Note that only one [a] X. Bao, J.-D. Leng, Z.-S. Meng, Dr. Z.-J. Lin, Prof. Dr. M.-L. Tong Key Laboratory of Bioinorganic and Synthetic Chemistry of Ministry of Education State Key Laboratory of Optoelectronic Materials and Technologies School of Chemistry & Chemical Engineering Sun Yat-Sen University, Guangzhou 510275 (P.R. China) Fax: (+86) 20-8411-2245 E-mail : tongml@mail.sysu.edu.cn [b] Dr. M. Nihei, 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 Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201000526.

Spin Crossover versus Low‐Spin Behaviour Exhibited in 2D and 3D Supramolecular Isomers of [FeII(2,4‐bpt)2]⋅Guest

The bistable properties (magnetic, structure and colour) of spin crossover (SCO) materials have resulted in a bright future for their applications as molecular memory, molecular switches, molecular sensors, data storage and display devices. Spin transitions (STs) between high(HS) and lowspin (LS) states can be achieved by external stimuli, such as temperature, pressure, light, magnetic or electrical switching. From practical aspects, the design and the synthesis of SCO compounds with high transition temperatures, wide thermal hysteresis and thermochroism are of crucial importance. Two strategies have been universally acknowledged to improve the cooperatives: 1) to construct covalently bridged coordination polymers; 2) to enrich intermolecular interactions, such as hydrogen bonding and p–p stacking. However, in reality, it is hard to predict theoretically the SCO property of a material until it has been characterised. The sensitivity of the SCO property to subtle changes does cause problems, but from another perspective, it may also be viewed as an opportunity to deepen our knowledge of the nature of SCO. To this end, the rich structural diversity of supramolecular isomers provided us a useful and unique perspective to understand structure–property relationships. As a branch of isomerism, polymorphism-dependent SCO compounds are not uncommon, however, most of which are mononuclear. To the best of our knowledge, only two examples focus on structural isomerism and another shows catenane isomerism-dependent SCO properties. Herein, we report the crystal structures and physical properties of four supramolecular isomers based on [FeACHTUNGTRENNUNG(2,4bpt)2]·guest (2,4-Hbpt= 3-(2-pyridyl)-5-(4-pyridyl)-1,2,4-triazole): a 2D SCO polymer [FeACHTUNGTRENNUNG(2,4-bpt)2] (1·Fe), two LS twofold interpenetrated 3D coordination polymers with NbO topology, [Fe ACHTUNGTRENNUNG(2,4-bpt)2]·2.17 H2O (2 a) and [Fe ACHTUNGTRENNUNG(2,4bpt)2]·2.5 H2O (2 b) and a LS non-interpenetrated 3D coordination polymer with NbO topology, [FeACHTUNGTRENNUNG(2,4bpt)2]·4 dioxane·4 H2O (3). It should be mentioned that these complexes include all kinds of isomers except for optical isomers, that is, structural, conformational and catenane supramolecular isomerism. Moreover, a cobalt analogue and a Fe Co solid solution species: [Co ACHTUNGTRENNUNG(2,4-bpt)2]·0.5 H2O (1·Co) and [FexCo1 x ACHTUNGTRENNUNG(2,4-bpt)2] (x=0.93) (1·Fe–Co) have also been synthesised and characterised. The latter compound allows us to investigate SCO properties in a doped system. Our work provides a good example to study magneto–structural relationships. We choose the 2,4-Hbpt ligand mainly for the following reasons: 1) as a variant of the well-studied ligand 2,2-Rbpt (2,2-Rbpt= 4-substituted 3,5-di(2-pyridyl)-1,2,4-triazole), it should also provide an appropriate ligand field favouring the occurrence of SCO; 2) it is a good candidate to construct a coordination polymer by using the 4-position N atom, thus enhancing cooperation between SCO sites; 3) the ligand can take on abundant conformations due to its rotatable feature, thus resulting in a variety of isomers and fits our need to study structure-dependent SCO properties. All products are synthesised in solvothermal conditions at 160 8C for 3 days. However, different solvent media have an important influence on the final products. By hydrothermal treatment, five products are captured, namely, two mononuclear polymorphs based on [FeACHTUNGTRENNUNG(2,4-bpt)2ACHTUNGTRENNUNG(H2O)2],[10] 1·Fe, 2 a and 2 b. However, there are problems with low yields for all products and also poor reproducibility. So it was laborious work to collect pure 1·Fe to study its magnetic behaviour. [a] X. Bao, J.-L. Liu, J.-D. Leng, Dr. Z. Lin, Prof. Dr. M.-L. Tong Key Laboratory of Bioinorganic and Synthetic Chemistry of Ministry of Education State Key Laboratory of Optoelectronic Materials and Technologies School of Chemistry & Chemical Engineering Sun Yat-Sen University Guangzhou 510275 (P.R.China) Fax: (+86) 20-8411-2245 E-mail : tongml@mail.sysu.edu.cn [b] Dr. M. Nihei, Prof. Dr. H. Oshio Graduate School of Pure and Applied Sciences University of Tsukuba, Tennodai 1-1-1, Tsukuba 305-8571 (Japan) [**] 2,4-Hbpt=3-(2-pyridyl)-5-(4-pyridyl)-1,2,4-triazole Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201001179.

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.

Cyanide-bridged iron(II,III) cube with multistepped redox behavior.

A building unit of Prussian blue was isolated as a cyanide-bridged iron cube of [Fe(II)4Fe(III)4(CN)12(tp)8] x 12 DMF x 2 Et2O x 4 H2O [tp(-) = hydrotris(pyrazolyl)borate]. A cyclic voltammogram showed quasi-reversible four-stepped redox waves, which correspond to [Fe(III)4Fe(II)4]/[Fe(III)5Fe(II)3](+), [Fe(III)5Fe(II)3](+)/[Fe(III)6Fe(II)2](2+), [Fe(III)6Fe(II)2](2+)/[Fe(III)7Fe(II)1](3+), and [Fe(III)7Fe(II)1](3+)/[Fe(III)8](4+) processes. Controlled potential absorption spectral measurements revealed two intervalence charge-transfer bands at 816 and 1000 nm, which were assigned to charge transfers from Fe(II) ions to adjacent and remote Fe(III) ions, respectively, in the cube.

Syntheses, structures and magnetic properties of multinuculear manganese complexes with Schiff base ligands

Abstract Multinulear manganese complexes with Schiff base ligands, [{MnIII4(μ3-O)(sae)4(μ-N3)(CH3OH)}2(μ-N3)]N3 ([1]N3), [MnIII6(μ3-O)2(sae)6(NCS)2] (2) and [MnII4MnIII2(sae)6(CH3OH)2Cl2] (3) (H2sae=2-salicylideneaminoethanol), were prepared and the crystal structures and magnetic properties were studied.

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

Structures and magnetic properties of metal cubes

Abstract Structures and magnetic properties of copper(II), nickel(II) and manganese(II) cubes are presented. In the cubes, four metal ions are assembled into the cubes by tridentate Schiff base ligands. Magnetic succeptibility measurements revealed the copper and nickel cubes have high-spin ground state, while the manganese cube has a S =0 spin ground state.

Single chain magnet of a cyanide bridged FeII/FeIII complex

A 1D compound composed of cyanide bridged iron(II,III) ions was prepared and magnetic measurements revealed it to be a single chain magnet.