A metallic (EDT-DSDTFVSDS)2.FeBr4 salt: antiferromagnetic ordering of d spins of FeBr4- ions and anomalous magnetoresistance due to preferential pi-d interaction

The 2:1 salt of a new donor molecule, EDT-DSDTFVSDS with FeBr4- ion, (EDT-DSDTFVSDS)2.FeBr4 showed an essentially metallic behavior despite a small upturn in the electrical resistance below ca. 30 K (electrical conductivities at 290 and 4.2 K are 200 and 170 S cm-1, respectively). The Fe(III) d spins of the FeBr4- ions in this salt were subject to antiferromagnetic ordering at 3.3 K by virtue of a strong pi-d interaction (Jpid) which is comparable to that in a molecular metallic conductor, lambda-(BETS)2.FeCl4, and of a very weak d-d interaction (Jdd). This strong pi-d interaction was evidenced by a large and negative magnetoresistance effect (ca. 20% at 5 T) as well as by the appearance of a large dip in the resistance at the magnetic field (ca. 2.0 T) parallel to the easy axis for the spin-flop transition of the Fe(III) d spins.     

Crystal structure and physical properties of conducting molecular antiferromagnets with a halogen-substituted donor: (EDO-TTFBr2)2FeX4 (X = Cl, Br)

The crystal structure and physical properties of radical ion salts (EDO-TTFBr2)2FeX4 (X = Cl, Br) based on halogen-substituted organic donor and magnetic anions are investigated, including the comparison with the isomorphous compounds (EDO-TTFBr2)2GaX4 with nonmagnetic anions. The crystal structure of these four salts consists of uniformly stacked donor molecules and tetrahedral counter anions, and the Br substituents of the donor molecules are connected to halide ligands of anions with remarkably short intermolecular atomic distances. These salts show metallic behavior around room temperature and undergo a spin-density-wave transition in the low-temperature range, as confirmed with the divergence of the electron spin resonance (ESR) line width. Although close anion-anion contacts are absent in these salts, the FeCl4 salt undergoes an antiferromagnetic transition at TN = 4.2 K, and the FeBr4 salt shows successive magnetic transitions at TN = 13.5 K and TC2 = 8.5 K with a helical spin structure as a candidate for the ground state of the d-electron spins. The magnetoresistance of the FeCl4 salt shows stepwise anomalies, which are explained qualitatively using a pi-d interaction-based frustrated spin system model composed of the donor pi-electron and the anion d-electron spins. Although on the ESR spectra of the FeX4 salts signals from the pi- and d-electron spins are separately observed, the line width of the pi-electron spins broadens under the temperature where the susceptibility deviates from the Curie-Weiss behavior, showing the presence of the pi-d interaction.     

Ferrimagnetic ordering due to Fe(III) d and donor pi spins in (ethylenedithiotetrathiafulvalenoquinone-1,3-dithiolemethide)2.FeBr4

Magnetization and heat capacity were measured down to 0.4 K in a 2:1 charge-transfer (CT) salt of a new donor molecule, ethylenedithiotetrathiafulvalenoquinone-1,3-dithiolemethide (1) with a magnetic FeBr(4)(-) ion (1(2).FeBr(4)). The Fe(III) d spins of FeBr(4)(-) ions were subject to apparently ferromagnetic interaction with each other through the interaction with the pi spins developed by localization of the conducting pi electrons on the donor columns, eventually giving rise to ferrimagnetic ordering (FI) near 1 K, which provides the first example in a molecular pi-d system.     

Weak ferromagnetism in a semiconducting (ethylenedithiodiselena- dithiafulvalenoquinone-1,3-diselenolemethide)2.FeBr4 salt

The 2:1 salts of a new bent donor molecule, ethylenedithiodiselenadithiafulvalenoquinone-1,3-diselenolemethide (EDT-DSDTFVODS) and either an FeBr(4)- or a GaBr(4)- ion exhibit semiconducting properties and had small activation energies. The Fe(III) d spins of the FeBr(4)- salt are initially subject to a strong antiferromagnetic interaction and afterward exhibited a weak ferromagnetism at 3.8 K with a very small remanent magnetization of ca. 4 x 10(-2) mu(B) and a spin-flop near 25 kOe along the intercolumnar direction.     

Electrical conducting and magnetic properties of (ethylenedithiotetrathiafulvalenothioquinone-1,3-diselenolemethide)2.FeBr4 (GaBr4) crystals with two different interlayer arrangements of donor molecules

Two donor molecules newly synthesized, dimethylthio- and ethylenedithio-tetrathiafulvalenothioquinone-1,3-diselenolemethides (1 and 2), were used to prepare their charge-transfer (CT) salts with a magnetic FeBr(4)(-) counteranion. For 1, a low electrical conducting 1:1 salt (1.FeBr(4)) was obtained, in which molecules of 1 are tightly dimerized in a one-dimensional (1D) stacking column. On the other hand, 2 gave a 2:1 salt (2(2).FeBr(4)) as two different kinds of plate crystals (I and II). Both I and II possess similar stacking structures of molecules of 2 in each 1D column with a half-cut pipelike structure along the c axis. However, for I, the stacking columns are aligned in the same direction along the a and b axes, while for II they are in the same direction along the a axis, but in the reverse direction along the b axis, resulting in the difference in the relative arrangement of molecules of 2 and FeBr(4)(-) ions between the two crystals. The room-temperature electrical conductivities of the single crystals of I and II were 13.6 and 12.7 S cm(-)(1), respectively. The electrical conducting behavior in I was metallic above 170 K but changed to be semiconducting with a very small activation energy of 7.0 meV in the temperature range 4-170 K. In contrast, II showed the semiconducting behavior in the whole temperature range 77-285 K. The corresponding nonmagnetic GaBr(4)(-) salts with almost the same crystal structure as I and II showed definitively different electrical conducting properties in the metal to semiconductor transition temperature in I as well as in the magnitude of activation energy in the semiconducting region of I and II. The interaction between the d spins of FeBr(4)(-) ions was weak and antiferromagnetic in both I and II, but the magnitude of the spin interaction was unexpectedly larger compared with that in the FeBr(4)(-) salt of the corresponding sulfur derivative of 2 with closer contact between the neighboring FeBr(4)(-) ions. These electrical conducting and magnetic results suggest a significant interaction between the conducting pi electrons and the d spins of FeBr(4)(-) ions located near the columns or layers.     

Metallic conductivity and ferromagnetic interaction of iron(III) d spins in the needle crystals of (ethylenedithiotetrathiafulvalenoquinone-1,3-dithiolemethide)(2).FeBr(4) salt

The 2:1 charge-transfer (CT) salts (1(2).FeBr(4) and 1(2).GaBr(4)) of ethylenedithiotetrathiafulvalenoquinone-1,3-dithiolemethide (1) with FeBr(4)(-) and GaBr(4)(-) counteranions were obtained as needle crystals, whose structures are almost the same as each other. The 1 molecules form a one dimensionally stacked column with alternation of their molecular axis direction, while the counteranions are aligned in parallel with the 1-stacked columns with the direction of their distorted-tetrahedral geometry maintained. The room-temperature electrical conductivities measured on the single crystals of 1(2).FeBr(4) and 1(2).GaBr(4) were 4.6 and 2.1 S cm(-1), respectively. From the temperature dependences of their electrical conductivities in both cases the electrical conducting properties were metallic between ca. 170 and 300 K, but below ca. 170 K converted to be semiconducting and continued till 5 K, although the activation energies are very small (4-10 meV). For 1(2).FeBr(4) very weak and antiferromagnetic interaction occurred between the d spins of FeBr(4)(-) ions in the temperature range of ca. 1-300 K. However, below ca. 15 K the ferromagnetic interaction was reversely preferential possibly by participation of the pi spin of 1.     

Stable metallic behavior and antiferromagnetic ordering of Fe(III) d spins in (EDO-TTFVO)2.FeCl4

We report the crystal structure and physical properties of the 2:1 FeCl4- salt of a new donor molecule, EDO-TTFVO. Crystal structure analysis of this salt revealed that the donor molecules formed a beta' '-type two-dimensional conducting layer, and there is a short S...Cl contact between the donor molecules and the FeCl4- ions, which is expected to mediate a strong pi-d interaction. This salt showed a stable metallic conducting behavior down to 0.3 K and an antiferromagnetic ordering at TN approximately 3.0 K, indicating that this salt becomes a new antiferromagnetic molecular metal at ambient pressure. The appearance of the magnetic ordering is considered to originate from the strong pi-d interactions between the donor molecules and the FeCl4- ions because the field dependence of magnetoresistances was remarkably affected below the antiferromagnetic transition temperature.     

An indication of magnetic-field-induced superconductivity in a bifunctional layered organic conductor,kappa-(BETS)(2)FeBr(4)

Hybrid systems consisting of the conducting layers of organic donor molecules and the magnetic layers of inorganic anions have been focused on as possible bifunctional materials, whose conducting properties can be tuned by controlling the magnetic state of the anion layers on an application of magnetic field. Here we report the magnetoresistance of the antiferromagnetic organic superconductor, kappa-(BETS)2FeBr4 [BETS = bis(ethylenedithio)tetraselenafulvalene], consisting of the two-dimensional superconducting layers of the BETS semications and the insulating layers of the FeBr4- anions. Due to the metamagnetic nature of the Fe3+ spin system, characteristic resistivity decrease was observed just below the antiferromagnetic superconductor-to-ferromagnetic metal transition at 1.6 T. Furthermore, an indication of the onsets of the magnetic-field-induced superconductivity was discovered around 12.5 T.     

A novel antiferromagnetic organic superconductor kappa-(BETS)(2)FeBr(4) [where BETS = bis(ethylenedithio)tetraselenafulvalene].

The electrical and magnetic properties of kappa-(BETS)(2)FeBr(4) salt [where BETS = bis(ethylenedithio)tetraselenafulvalene] showed that this system is the first antiferromagnetic organic metal at ambient pressure (T(N) = 2.5 K). The characteristic field dependence of the magnetization at 2.0 K indicates a clear metamagnetic behavior. The small resistivity drop observed at T(N) clearly shows the existence of the interaction between pi metal electrons and localized magnetic moments of Fe(3+) ions. In addition, this system underwent a superconducting transition at 1.1 K. That is, kappa-(BETS)(2)FeBr(4) is the first antiferromagnetic organic metal exhibiting a superconducting transition below Néel temperature. The magnetic field dependence of the superconducting critical temperature indicated that the superconductivity in this system is strongly anisotropic also in the conduction plane because of the existence of the metamagnetically induced internal field based on the antiferromagnetic ordering of the Fe(3+) 3d spins in contrast to the cases of the other conventional organic superconductors. Furthermore, the specific heat measurement exhibited a lambda-type large peak of zero-field specific heat corresponding to the three-dimensional antiferromagnetic ordering of high-spin Fe(3+) ions. The lack of distinct anomaly in the C(p) vs T curve at T(c) suggests the coexistence of the superconductivity and the antiferromagnetic order below T(c).     

π- d Interaction-Based Molecular Magnets: Role of Sulfur-to-Selenium Substitution

Abstract

The crystal structure and physical properties of the three conducting molecular magnets are discussed. (DMET)₂FeBr₄ is composed of alternating stacks of quasi-one-dimensional donor sheets and square lattice magnetic anion sheets. This salt undergoes an spin density wave (SDW) transition of the donor layer at 40 K and an antiferromagnetic transition of Fe³⁺ spins on the anion layer at 3.7 K. The one-to-one correspondence of the anomalies appearing on the magnetization curves with those on the magnetoresistance supports the presence of the π-d interaction. The all-sulfur analog (EDTDM)₂FeBr₄ shows, besides similar behaviors as the DMET salt, insulator-to-metal transition of the ground state by applying the pressure, accompanied with a large negative magnetoresistance. (EDS-TTF)₂FeBr₄ shows little π-d interaction despite the presence of close Se–Br contacts, showing the importance of the intermolecular orbital overlap between the π- and d-components.     

Antiferromagnetic or canted antiferromagnetic orderings of Fe(III) d spins of FeX4- ions in BEDT-TTFVO(S).FeX4 (X = Cl, Br) [BEDT-TTFVO(S) = bis(ethylenedithio)tetrathiafulvalenoquinone(-thioquinone)-1,3-dithiolemethide

By the reaction of new donor molecules, bis(ethylenedithio)tetrathiafulvalenoquinone(-thioquinone)-1,3-dithiolemethides [BEDT-TTFVO (1) and BEDT-TTFVS (2)] with FeX3 (X = Cl, Br) in CS2/CH3CN, 1:1 salts of 1 or 2 with an FeX4- ion (1.FeX4 and 2.FeX4) were obtained as black needle crystals. Their crystal structures are very similar to each other, in which the donor molecules are strongly dimerized and the dimers construct a one-dimensional uniform chain along the a axis, while the FeX4- ions are located at an open space surrounded by the neighboring donor molecules and also construct a one-dimensional uniform chain along the a axis. There are close contacts between the donor molecules and the FeX4- ions and significant differences in the contact distances among the four salts. All of the salts are semiconductors with room-temperature electrical conductivities of 10-4-10-2 S cm-1. The Fe(III) d spins of the FeX4- ions are subject to dominant ferromagnetic interaction through the participation of one of the singlet pi spins to form a short-range ferromagnetic d-spin chain. Such neighboring chains interact antiferromagnetically with each other through the singlet pi spins and are ordered at 1.0, 2.4, and 0.8 K for 1.FeCl4, 1.FeBr4, and 2.FeCl4, respectively. On the other hand, the antiferromagnetic ordering occurred with some canted angle at 1.9 K to leave a small magnetization for 2.FeBr4.     

Ferromagnetic ordering of Fe(III) d spins of FeBr4- ions in (ethylenedithiotetrathiafulvalenothioquinone-ethylenedithio-1,3-dithiolemethide) x FeBr4

The 1:1 salt of a new donor molecule, ethylenedithiotetrathiafulvalenothioquinone-ethylenedithio-1,3-dithiolemethide (1), with FeBr4- ion, 1 x FeBr4, was prepared and found to exhibit a room-temperature electrical conductivity of 4 x 10(-2) S cm(-1) and semiconducting behavior with an activation energy of 170 meV. The paramagnetic susceptibility obeyed the Curie-Weiss law with a Curie constant of 4.42 emu K mol(-1) and a Weiss temperature of +3.4 K, and below 15 K, this short-range ferromagnetic interaction increasingly extended to two- and/or three-dimensional interactions, eventually giving rise to a ferromagnetic ordering, whose temperature (TC) was determined to be 1.8 +/- 0.2 K using a resonant circuit method. The magnetic field dependence of magnetization showed that the saturation of magnetization was accomplished at ca. 60 kOe and the saturated value was ca. 5 microB, which is very close to the value obtained only due to Fe(III) (S = 5/2) d spins of one FeBr4- ion.     

Magnetic molecular conductors based on BETS molecules and divalent magnetic anions [BETS = bis(ethylenedithio)tetraselenafulvalene

Several conducting salts based on BETS [where BETS = bis(ethylenedithio)tetraselenafulvalene] molecules and divalent magnetic anions such as the (CoCl(4))(2-), (CoBr(4))(2-), and (MnBr(4))(2-) were prepared. Electrocrystallization by using the (CoCl(4))(2-) anion gave two kinds of crystals. Block-shaped crystals were cleared to be (BETS)(2)CoCl(4), which is an insulator with the high-spin state of cobalt 3d spin. On the other hand, the X-ray crystal structure analysis of a plate-shaped crystal of the (CoCl(4))(2-) salt revealed the system to be kappa-(BETS)(4)CoCl(4)(EtOH), which is metallic down to 0.7 K. The electronic band structure calculation gave a typical two-dimensional cylindrical Fermi surface. However, there is only very weak antiferromagnetic interaction between the S = 3/2 cobalt 3d spins because of its anion-solvent-intermingled layer structure. On the other hand, the electrocrystallization by using the (MnBr(4))(2-) anion yielded the plate-shaped black crystals of the (MnBr(4))(2-) salt. Crystal structure analysis of the (MnBr(4))(2-) salt showed that the salt is theta;-(BETS)(4)MnBr(4)(EtOH)(2) with alternating donor and anion-solvent mixed layers. The stacking direction in one donor layer is perpendicular to those of the neighboring layers. The electrical and magnetic properties of the theta;-(BETS)(4)MnBr(4)(EtOH)(2) salt showed the metallic behavior down to approximately 30 K and the paramagnetism of the high-spin manganese 3d spins. Band structure calculation of this salt gave an elliptical cylindrical Fermi surface. Because the Fermi surfaces of the adjacent donor layers are rotated to each other by 90 degrees, the theta-(BETS)(4)MnBr(4)(EtOH)(2) salt becomes a two-dimensionally isotropic metal.     

Conducting Molecular Magnets Based on TTF-Derivatives

The crystal structures, electronic and magnetic properties of conducting molecular magnets developed in our group are reviewed. (DMET)₂FeBr₄ is composed of alternating stacks of quasi-one-dimensional (1D) donor sheets and square lattice magnetic anion sheets. This salt undergoes an SDW transition of the donor layer at 40 K and an antiferromagnetic transition of Fe³⁺ spins on the anion layer at 3.7 K. The one-to-one correspondence of the anomalies appearing on the magnetization curves with those on the magnetoresistance supports the presence of the π-d interaction. (EDO-TTFI₂)₂[M(mnt)₂] (M=Ni,Pt) consists of 1D chains of conducting donors and magnetic anions aligned in parallel. These salts show metallic conductivity accompanied with a metal-insulator transition around 90 K. Localized spins on the anions behave as a 1D ferromagnet, whose origin is explained by McConnell's first model. The properties of related materials, (EDTDM)₂FeBr₄, (EDS-TTF)₂FeBr₄ and (EDO-TTFBr₂)₂FeBr₄, are also presented.     

Magnetoresistance effects evidencing the pi-d interaction in metallic organic conductors, (EDT-DSDTFVO)2*MX4 (M = Fe, Ga; X = Cl, Br)

The 2:1 salts of a new donor molecule, EDT-DSDTFVO with MX4- (M = Fe, Ga; X = Cl, Br) ions, were prepared. The crystal structures of the donor molecules had a beta-type packing motif. All the salts essentially exhibited metallic behaviors despite the small upturns in the resistances below 30-70 K. A large negative magnetoresistance (MR) effect [-14.7% (rho(perpendicular)) at 4.0 K and 5 T] was observed in the FeCl4- salt, while a positive MR effect [+4.0% (rho(perpendicular)) at 4.0 K and 5 T] was observed in the GaCl4- salt, suggesting that there is a pi-d interaction in the FeCl4- salt. The pressure application suppressed the resistivity upturns, increased the negative MR effect (-17.7% at 9.5 kbar) in the FeCl4- salt, and decreased the positive MR effect (+3.3% at 15 kbar) in the GaCl4- salt.     

Field-induced ferrimagnetic state in a molecule-based magnet consisting of a Co(II) ion and a chiral triplet Bis(nitroxide) radical

We present the synthesis, crystal structure, and temperature and field dependence of the magnetic properties of a new molecule-based magnet, [Co(hfac)2].BNO* (1), where hfac = 1,1,1,5,5,5-hexafluoroacetylacetonato and BNO* is the chiral triplet bis(nitroxide), 1,3-bis(N-tert-butyl-N-oxylamino)-5-{1'-methyl-1'-[2' '-(S)-methylbutoxy]ethyl}benzene. The presence of enantiomer-pure BNO induces the formation of chiral one-dimensional chains that are packed parallel to each other in the noncentrosymmetric P1 space group. 1 exhibits four magnetic ground states: paramagnetic; antiferromagnetic; forced ferrimagnetic; field-induced metastable ferrimagnetic. In the paramagnetic state (T > 20 K), it presents short-range antiferromagnetic interaction between Co ion and nitroxide radical and has a minimum of chimT value at 220 K. The Weiss temperature estimated in the temperature range 220-300 K is found to be -89.9 K. At 20 K (TN), an antiferromagnetic long-range ordering is established. In the temperature range 4 K < T < 20 K, the isothermal magnetization curve show a spin-flip transition to the forced ferrimagnetic state at around 850 Oe. Below 4 K, this compound enters into a field-induced ferrimagnetic state, which is metastable and stabilized by the Ising character of the Co ion. In the low-temperature phase, the material becomes a very hard magnet with wide hysteresis loop whose coercive field reaches 25 kOe at 2 K. The magnetic phase diagram based on these magnetic data is presented.     

Dual-action molecular superconductors with magnetic anions

Dual-action organic superconductors, whose conducting properties can be sharply controlled by an external magnetic field, have been discovered in systems consisting of organic conduction layers based on bis(ethylenedithio)tetraselenafulvalene (BETS) molecules and magnetic anions. Owing to the metamagnetic nature of the anion layers, the superconducting state of kappa-BETS2FeBr4 can be switched on or off by applying the external field. In lambda-BETS2Fe0.4Ga0.6Cl4, exhibiting a field-induced superconducting transition for the field parallel to the conduction plane, the insulating, metallic, and superconducting states can be realized in a stepwise manner by slightly tuning the external magnetic field.     

A spin crossover ferrous complex with ordered magnetic ferric anions

The first tetrahaloferrate spin crossover compound, [Fe(Metz)(6)](FeBr(4))(2) (Metz = 1-methyltetrazole), is reported. The FeBr(4)(-) ions form ferromagnetically coupled 1D stacks and exhibit an antiferromagnetic order at 2.2 K, which coexists with the gradual spin crossover centred at 165 K.     

Structural and magnetic evidence concerning spin crossover in formamidinate compounds with ru2 5+ cores

The magnetic and structural properties of two Ru2(DArF)4Cl compounds, where DArF is the anion of a diaryl formamidine, are presented here. The compounds with Ar = p-anisyl and m-anisyl both show temperature dependence of chiT (chi = molar magnetic susceptibility), but for different reasons. For the para compound, there is a Boltzmann distribution between a pi*3 ground state and a delta*pi*2 upper state, and this is confirmed by a temperature dependence of the Ru-Ru bond length: 2.4471(5) A at 23 K and 2.3968(5) A at 300 K. For the meta compound, a delta*pi*2 configuration persists over the range of 23-300 K as shown by an invariant Ru-Ru bond length, but the chiT drops with decreasing temperature owing to zero-field splitting of a 4B2u ground state.     

Hexaazaphenalenyl anion revisited: a highly symmetric planar pi system with multiple-networking ability for self-assembled metal complexation

The hexaazaphenalenyl anion (HAP, 3-), a highly symmetric heterocyclic pi system, has been synthesized and characterized. The crystal structures of its potassium salt K+.3- and copper complex Cu2+.(NH3)4.(3-)2 show pi-pi stacking and radially extended hydrogen bonds as well as coordination bonds constructed by the HAP anion. These in-plane and out-of-plane strong interactions demonstrate the multiple-networking ability of the HAP anion.