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).     

Two metal-organic frameworks constructed from one-dimensional cobalt(II) ferrimagnetic chains with alternating antiferromagnetic/ferromagnetic and AF/AF/FM interaction: synthesis, structures, and magnetic properties

Here, we report two three-dimensional metal-organic frameworks of formula [Co(2)(4-ptz)(2)(bpp)(N(3))(2)](n) (1) and [Co(3)(OH)(2)(bdt)(2)(bpp)(2)(H(2)O)(2)](n) (2), which were synthesized by hydrothermal reaction from the respective tetrazole ligand (5-(4-pyridyl)tetrazole (4-H-ptz) for 1 and 5,5'-(1,4-phenylene)bis(1-H-tetrazole) (H(2)bdt) for 2), long and flexible pyridyl-containing ligand 1,3-bi(4-pyridyl)propane (bpp), NaN(3), and CoCl(2). Both 1 and 2 consist of well-isolated one-dimensional cobalt(II) alternating chains further linked by the bpp and/or the tetrazole ligand, while their chain structures are totally different. The chains of 1 are formed by Co(2+) ions bridged by single μ-EE-N(3) and triple (μ-EO-N(3))(μ-tetrazole)(2) alternately, whereas the Co(2+) ions are bridged by μ(3)-OH to form Co(3)(OH)(2) chains in compound 2. Magnetic measurements demonstrate that compound 1 contains an alternating antiferromagnetic (AF)/ferromagnetic (FM) ferrimagnetic chain, while compound 2 exhibits the coexistence of spin canting, slow magnetic dynamics, and finite-size effect, with alternating AF/AF/FM ferrimagnetic chains.     

Co4(OH)2(C10H16O4)3 metal-organic framework: slow magnetic relaxation in the ordered phase of magnetic chains

Reported here are the synthesis and structural and topological analysis as well as a magnetic investigation of the new Co(4)(OH)(2)(C(10)H(16)O(4))(3) metal-organic framework. The structural analysis reveals a one-dimensional inorganic subnetwork based on complex chains of cobalt(II) ions in two different oxygen environments. Long alkane dioic acid molecules bridge these inorganic chains together to afford large distances and poor magnetic media between dense spin chains. The thermal dependence of the χT product provides evidence for uncompensated antiferromagnetic interactions within the cobaltous chains. In zero-field, dynamic magnetic susceptibility measurements show slow magnetic relaxation below 5.4 K while both neutron diffraction and heat capacity measurements give evidence of long-range order (LRO) below this temperature. The slow dynamics may originate from the motion of broad domain walls and is characterized by an Arrhenius law with a single energy barrier Δ(τ)/k(B) = 67(1) K for the [10-5000 Hz] frequency range. Moreover, in nonzero dc fields the ac susceptibility signal splits into a low-temperature frequency-dependent peak and a high-temperature frequency-independent peak which strongly shifts to higher temperature upon increasing the bias dc field. Heat capacity measurements have been carried out for various applied field values, and the recorded C(P)(T) data are used for the calculation of the thermal variations of both the adiabatic temperature change ΔT(ad) and magnetic entropy change ΔS(m). The deduced data show a modest magnetocaloric effect at low temperature. Its maximum moves up to higher temperature upon increasing the field variation, in relation with the field-sensibility of the intrachain magnetic correlation length.     

Inter-string arrays of bimetallic assemblies with alternative Cu2+-Cl-Cu2+ and Cu-NC-M (M = Co3+, Fe+3, Cr+3) bridges: syntheses, crystal structure, and magnetic properties

Three bimetallic assemblies with alternate homometallic bridges through chloride ligands and heterometallic bridges through cyanide ligands of formula [(323)(2)Cu(2)(Cl)M(CN)(6)](n).2n(H(2)O), where 323 = N,N'-bis(3-aminopropyl)ethylenediamine and M = Co(3+) for 1, Fe(3+) for 2, and Cr(3+) for 3, were synthesized. They have been characterized structurally, analytically, spectroscopically, and magnetically. All three assemblies crystallize in the monoclinic system in the same space group P2(1)/n, with a = 11.642(2) A, b = 10.285(3) A, c = 13.622(2) A, beta = 95.69(3) degrees, V = 1623.1(6) A(3), and Z = 4 for 1; a = 11.681(4) A, b = 10.315(3) A, c = 13.567(5) A, beta = 95.62(3) degrees, V = 1626.8(9) A(3), Z = 4 for 2, and a = 11.782(4) A, b = 10.386(2) A, c = 13.755(4) A, beta = 95.51(3) degrees, V = 1657.4(8) A(3), Z = 4 for 3. Crystal structure analyses reveal that one-dimensional zigzag chains propagate in two different crystallographic directions (a and b) which are held together during the course of their propagation. All three assemblies have a homometallic Cu-Cl-Cu core in common. Assembly 1 exhibits metamagnetic behavior and shows weak antiferromagnetic interactions between Cu(2+) paramagnetic centers, through the chloride bridges. The Neel temperature, T(N), is 3.5 K, and the critical field is 4 T. In the presence of a magnetic field larger than 4 T, the local spin doublets of Cu(2+) in the assembly 1 remain in parallel arrangements. Assemblies 2 and 3 may be described as an alternative repetition of the antiferromagnetically coupled Cu-Cl-Cu fragment and ferromagnetically coupled Cu-CN-Fe(3+)/Cr(3+)fragment. The overall magnetic character of the strings in assemblies 2 and 3 are antiferromagnetic. Ferromagnetic interaction evidenced by the (Cu-CN-Fe(3+)/Cr(3+)) fragment was masked by the antiferromagnetic interaction between the Cu(2+) centers through the chloride bridge. The magnetic properties agree well with those expected for two [323 Cu(2+)] and a [Fe(CN)](3+) unit with spin-orbit coupling effect of the low-spin iron(III) ions for 2 and for two [323 Cu(2+)] and a [Cr(CN)](3+) unit for 3. In aqueous solution, trinuclear [(323)(2)Cu(2)M(CN)(6)](+) and dinuclear [(323)CuM(CN)(6)](-) species were observed.     

Preparations, crystal structures, and magnetic properties of N,N-dipyridylaminoxyl as a new magnetic coupler and its one-dimensional cobalt(II) chains

N,N-Dipyridilaminoxyl, NOpy(2), having a stable aminoxyl, was prepared as a new magnetic coupler for heterospin systems. Solutions of NOpy(2) were mixed with those of bis{1,1,1,5,5,5, hexafluoro-4-(phenylimino)-2-pentanonate}cobalt derivatives, Co(hfpip-X)(2), at a 1:1 ratio to afford the polymeric cobalt(II) complexes, [Co(hfpip-X)(2)(NOpy(2))](n); X = H (1), F (2), F(3) (3), F(5) (4), Cl (5), Cl(3) (6), Br (7), and I (8) as single crystals. In all complexes, the local structures of the cobalt-complex units were compressed octahedra and the pyridine ligands in NOpy(2) units coordinated to the cobalt ions in trans configuration to form linear chains for 1-4 and in cis configuration to form helical chains for 5-8. In the chains, the aminoxyl in NOpy(2) ferromagnetically interacted with the cobalt ions to produce the ferromagnetic chains with J(intra)/k(B) = 9-14 K. In the magnetic susceptibility experiments of aligned sample of 6, the magnetic easy axis was determined to be the a* axis, which was the direction perpendicular to the b axis of the chain axis. The resulting chains, all except 4, interacted antiferromagnetically among each other, and especially in 1, 5, 7, and 8, the magnetic behaviors characteristic to canted two-dimensional (2D) antiferromagnets with T(c) = 5.6, 4.0, 4.0, and 6.2 K, respectively, were observed. All complexes showed slow magnetic relaxations affected by the interchain antiferromagnetic interaction. The effective activation barriers, Δ(eff)/k(B), for the reorientation of the magnetism for all complexes except 4 were estimated to be 25-39 K in the presence of a direct current (dc) field.     

Isomorphic coordination polymers of cobalt(II), zinc(II) with a flexible ligand of cis,cis,cis-1,2,3,4-cyclopentanetetracarboxylic acid and their molecular alloys: crystal structures, thermal decomposition mechanisms and magnetic properties

Three new isomorphic coordination polymers of Co(2+), Zn(2+) ions with flexible multicarboxylic acid ligand of the cis,cis,cis-1,2,3,4-cyclopentanetetracarboxylic acid (H(4)L), [Co(4)L(2)(H(2)O)(8)]·3H(2)O (1), [Zn(4)L(2)(H(2)O)(8)]·3H(2)O (2) and [Co(0.8)Zn(3.2)L(2)(H(2)O)(8)]·3H(2)O (3), have been synthesized under hydrothermal conditions and by means of controlling the pH of the reaction mixtures (with an initial pH of 6.0 for 1, 4.0 for 2, and 5.0 for 3, respectively). In the crystal of 1, two crystallographically different Co(2+) ions (Co1 and Co2) form a negatively-charged coordination polymeric chain, which contains a centrosymmetric, linear, trinuclear Co(2+) cluster (Co(3)L(2)) subunit; another crystallographically independent Co(2+) ion (Co3) coordinated to six water molecules acts as a counter ions to link the neighboring coordination polymeric chains via intermolecular H-bond interactions. The Co(2+) ions in 1 were completely and partially replaced by Zn(2+) ions to give 2 and 3, respectively. Complex 3 shows a novel molecular alloy nature, due to the random distributions of the Co(2+) and Zn(2+) ions. Three isomorphic complexes exhibit distinct thermal decomposition mechanisms. The deprotonated cis,cis,cis-1,2,3,4-cyclopentanetetracarboxylic acid ligands decompose at 420-750 °C to give the residue CoO in 1, ZnO + C in 2 and CoO + ZnO in 3. Complex 1 shows a complicated magnetic behavior with co-existence of antiferromagnetic exchange interactions between neighboring Co(2+) ions as well as strong spin-orbital coupling interactions for each Co(2+) ion; complex 3 exhibits a magnetically isolated high-spin Co(2+) ion behavior with strong spin-orbital coupling interactions.     

High-pressure synthesis and neutron diffraction investigation of the crystallographic and magnetic structure of TeNiO3 perovskite

TeNiO(3) has been prepared under moderate pressure conditions (3.5 GPa), starting from a reactive TeO(2) and Ni(OH)(2) mixture contained in a sealed platinum capsule under the reaction conditions (850 °C for 2 h). The sample has been studied by neutron powder diffraction (NPD) data and magnetization measurements. TeNiO(3) crystallizes in an orthorhombically-distorted perovskite structure (space group Pnma) with the unit cell parameters a = 5.9588(1) ?, b = 7.5028(1) ? and c = 5.2143(1) ?. The NiO(6) octahedral network is extremely tilted, shaping a trigonal-pyramidal environment for the Te, where it is effectively coordinated to three oxygen atoms at Te-O distances of 1.92 ?. Below T(N) ≈ 130 K, it experiences an antiferromagnetic ordering, as demonstrated by susceptibility and NPD measurements. Above the Néel temperature, a paramagnetic moment of 3.24(1) μ(B)/f.u. and θ(Weiss) = -199(1) K are obtained from the reciprocal susceptibility. Below T(N), the magnetic reflections observed in the neutron patterns can be indexed with a propagation vector k = 0. The magnetic structure corresponds to the magnetic mode G(y)F(z). The magnetic moments are oriented along the y-direction, with a canting along the z-axis. This ferromagnetic component explains the weak ferromagnetism observed in the magnetization isotherms; the infrequent shape of the magnetization cycles suggests a metamagnetic transition below 0.2 T. At T = 2.5 K, the ordered magnetic moment for the Ni(2+) ions is 1.88(5) μ(B) for the G(y) mode and 0.9(2) μ(B) for the F(x) mode.     

Structure and magnetic properties of oxychalcogenides A2F2Fe2OQ2 (A = Sr, Ba; Q = S, Se) with Fe2O square planar layers representing an antiferromagnetic checkerboard spin lattice

The oxychalcogenides A2F2Fe2OQ2 (A = Sr, Ba; Q = S, Se), which contain Fe2O square planar layers of the anti-CuO2 type, were predicted using a modular assembly of layered secondary building units and subsequently synthesized. The physical properties of these compounds were characterized using magnetic susceptibility, electrical resistivity, specific heat, (57)Fe Mossbauer, and powder neutron diffraction measurements and also by estimating their exchange interactions on the basis of first-principles density functional theory electronic structure calculations. These compounds are magnetic semiconductors that undergo a long-range antiferromagnetic ordering below 83.6-106.2 K, and their magnetic properties are well-described by a two-dimensional Ising model. The dominant antiferromagnetic spin exchange interaction between S = 2 Fe(2+) ions occurs through corner-sharing Fe-O-Fe bridges. Moreover, the calculated spin exchange interactions show that the A2F2Fe2OQ2 (A = Sr, Ba; Q = S, Se) compounds represent a rare example of a frustrated antiferromagnetic checkerboard lattice.     

High-nuclearity 3d-4f clusters as enhanced magnetic coolers and molecular magnets

Four 52-metal-ion 3d-4f cluster complexes featuring a common core of Ln(42)M(10) (Ln = Gd(3+), Dy(3+); M = Co(2+/3+), Ni(2+)) were obtained through self-assembly of the metal ions templated by mixed anions (ClO(4)(-) and CO(3)(2-)). Magnetic studies revealed that the Gd(42)Co(10) and Gd(42)Ni(10) clusters exhibit the largest magnetocaloric effect (MCE) among any known 3d-4f complexes. Replacement of Gd(3+) ions with anisotropic Dy(3+) ions caused significant changes in the magnetic behavior of the clusters; both Dy(42)Co(10) and Dy(42)Ni(10) displayed slow relaxation of the magnetization.     

Synthesis, crystal structure and magnetic properties of the new one-dimensional manganate Cs3Mn2O4

Cs(3)Mn(2)O(4), a new member of the small family of ternary manganese (II/III) mixed-valent compounds, has been synthesized via the azide/nitrate route and studied using powder and single crystal X-ray diffraction, magnetic susceptibility measurements and density functional theory (DFT). Its crystal structure (P2(1)/c, Z = 8, a = 1276.33(1) pm, b = 1082.31(2) pm, c = 1280.29(2) pm, β = 118.390(2)°) is based on one-dimensional MnO(2)(1.5-) chains built up from edge-sharing MnO(4) tetrahedra. The title compound is the first example of an intrinsically doped transition metalate of the series A(x)MnO(2), (A = alkali metal) where a complete 1:1 charge ordering of Mn(2+) and Mn(3+) is observed along the chains (-Mn(2+)-Mn(3+)-Mn(2+)-Mn(3+)-). From the magnetic point of view it basically consists of ferrimagnetic MnO(2) chains, where the Mn(2+) and Mn(3+) ions are strongly antiferromagnetically coupled up to high temperatures. Very interestingly, their long-range three-dimensional ordering below the Néel temperature (T(N)) ~12 K give rise to conspicuous field dependent magnetic ordering phenomena, for which we propose a consistent picture based on the change from antiferromagnetic to ferromagnetic coupling between the chains. Electronic structure calculations confirm the antiferromagnetic ordering as the ground state for Cs(3)Mn(2)O(4) and ferrimagnetic ordering as its nearly degenerate state.     

Assembly of a cubic nanocage Co8L12 and a hydrogen-bonded 3D NbO net based on the [(HCO3)2]2- synthon and water

Two new complexes [Co(H2O)6 Co8(L1)12]X6 x n H2O (X = NO3(-), n = 12 (1); X = HCO3-, n = 24, (2); HL1 = 4,6-bis(2-pyridyl)-1,3,5-triazin-2-ol) have been synthesized and characterized by single-crystal X-ray diffraction. A [Co(H2O)6](2+) ion is encapsuled in the central cavity of the cubelike nanocage [Co(H2O)6 Co8(L1)12](6+) cation, assembled by eight cobalt ions at the corners and twelve bis-bidentate ligands L1 as the edges, via the formation of 12-fold strong hydrogen bonds between the six coordinated water molecules and the oxygen atoms of twelve L1 as a guest. Complex 1 crystallizes in a centrosymmetric space group P1, while 2 is in a very high symmetric space group Im3. In 2, a planar [(HCO3)2](2-) dimer motif R2(2)(8) synthon plus six lattice water molecules constitute a planar supramolecular synthon R8(8)(20), which acts as a four connector, generating a 3D hydrogen-bonded NbO net with cubelike host cavities of approximately 20 A diameter. Interestingly, the cubelike nanocage [Co(H2O)6 Co8(L1)12](6+) cations fill in the cavities as templates. The magnetic properties of 1 have also been studied in the temperature range of 2-300 K, and its magnetic susceptibility obeys the Curie-Weiss law, showing antiferromagnetic coupling.     

Density functional theory analysis of the interplay between Jahn-Teller instability, uniaxial magnetism, spin arrangement, metal-metal interaction, and spin-orbit coupling in Ca3CoMO6 (M = Co, Rh, Ir)

In the isostructural oxides Ca(3)CoMO(6) (M = Co, Rh, Ir), the CoMO(6) chains made up of face-sharing CoO(6) trigonal prisms and MO(6) octahedra are separated by Ca atoms. We analyzed the magnetic and electronic properties of these oxides on the basis of density functional theory calculations including on-site repulsion and spin-orbit coupling, and examined the essential one-electron pictures hidden behind results of these calculations. Our analysis reveals an intimate interplay between Jahn-Teller instability, uniaxial magnetism, spin arrangement, metal-metal interaction, and spin-orbit coupling in governing the magnetic and electronic properties of these oxides. These oxides undergo a Jahn-Teller distortion, but their distortions are weak, so that their trigonal-prism Co(n+) (n = 2, 3) ions still give rise to strong easy-axis anisotropy along the chain direction. As for the d-state split pattern of these ions, the electronic and magnetic properties of Ca(3)CoMO(6) (M = Co, Rh, Ir) are consistent with d(0) < (d(2), d(-2)) < (d(1), d(-1)) but not with (d(2), d(-2)) < d(0) < (d(1), d(-1)). The trigonal-prism Co(3+) ion in Ca(3)Co(2)O(6) has the L = 2 configuration (d(0))(1)(d(2), d(-2))(3)(d(1), d(-1))(2) because of the metal-metal interaction between adjacent Co(3+) ions in each Co(2)O(6) chain, which is mediated by their z(2) orbitals, and the spin-orbit coupling of the trigonal-prism Co(3+) ion. The spins in each CoMO(6) chain of Ca(3)CoMO(6) prefer the ferromagnetic arrangement for M = Co and Rh but the antiferromagnetic arrangement for M = Ir. The octahedral M(4+) ion of Ca(3)CoMO(6) has the (1a)(1)(1e)(4) configuration for M = Rh but the (1a)(2)(1e)(3) configuration for M = Ir, which arises from the difference in the spin-orbit coupling of the M(4+) ions and the Co···M metal-metal interactions.     

Crystal refinement and magnetic structure of KNi4(PO4)3: a novel example of three interacting magnetic sub-lattices

KNi(4)(PO(4))(3) has been synthesised following a method previously reported by some of us and studied on the basis of magnetization and neutron powder diffraction (NPD) data. Magnetization measurements suggest the coexistence of ferromagnetic (FM) and antiferromagnetic (AFM) interactions: magnetization versus magnetic field curves present a remanent magnetization of around 2.15 micro(B) at T=2 K. The magnetic structure of the KNi(4)(PO(4))(3) has been determined at low temperature from the NPD data. These measurements show that there are three magnetic sub-lattices of Ni(2+) ions, which interact through common oxygen or phosphate groups, giving rise to FM and AFM couplings. The resulting interactions are FM in nature. Such a complex behaviour could provide an interesting model to analyse magnetic interactions in more condensed systems, such in mixed metal oxides.     

Single crystal EPR of Cu(II) doped [Co(tbz)2(NO3)(H2O)]NO3: probe into copper-thiabendazole interaction

EPR of Cu(II) doped, low symmetry Co(II)-thiabendazole complex [Co(tbz)2(NO3)(H2O)](NO3) is investigated at 300 K. The spin Hamiltonian parameters are found to be orthorhombic with g33=2.305, g22=2.1351, g11=2.0626 and A33=147.0 x 10(-4), A22=33.5 x 10(-4) and A11=23.1 x 10(-4) cm(-1). Computer simulation of isofrequency plots reveal that the Cu(II) ions is substitutionally incorporated in the host lattice. Angular variation of the spectra shows the presence of two magnetic sites in the lattice. The low magnitude of A33 of the complex is rationalized in terms of admixture of d(x2-y2)/d(z2) ground state and delocalization of unpaired spin density onto the ligands.     

Modeling spin interactions in a cyclic trimer and a cuboidal Co4O4 core with Co(II) in tetrahedral and octahedral environments

The X-ray crystallographic structures, the magnetic susceptibilities from 2 to 300 K, and a theoretical analysis of the magnetism for a triangular and a tetranuclear molecule consisting of linked high-spin cobalt(II) centers are described. The interpretation of the magnetic data for the triangular compound [Co(depa)Cl](3) (depa is the anion of 2,2'-(bis-4-ethylpyridyl)amine), which has tetrahedrally coordinated Co(2+) ions, entails isotropic antiferromagnetic exchange interaction and antisymmetric exchange acting within the two low-lying spin doublets. Two strong isotropic ferromagnetic interactions have been modeled in the cuboidal compound Co(4)(DPM)(4)(CH(3)O)(4)(CH(3)OH)(4) (DPM represents the anion of dipivaloylmethane), which has octahedral coordination, and the system can be approximately considered as two weakly coupled S = 3 species.     

Metamagnetism in cobalt phosphates with pillared layer structures: [Co3(pyz)(HPO4)2F2] and [Co3(4,4'-bpy)(HPO4)2F2].xH2O

Two new cobalt phosphates, [Co(3)(pyz)(HPO(4))(2)F(2)] (1) and [Co(3)(4,4'-bpy)(HPO(4))(2)F(2)].xH(2)O (x approximately 0.7) (2), have been synthesized by hydrothermal methods in the presence of aromatic amines, and characterized by single-crystal X-ray diffraction and magnetic susceptibility. Their structures consist of neutral sheets of fluorinated cobalt phosphate which are pillared through pyrazine and 4,4'-bipyridine molecules to form 3D frameworks. The structures are related to that of the mineral lazulite. Both compounds show long-range antiferromagnetic ordering below 15 K and metamagnetic behaviors. Compound 1 reveals a two-step magnetic phase transition. Crystal data for 1: monoclinic, space group C2/c (No. 15), a = 21.809(4) A, b = 7.370(1) A, c = 7.395(1) A, beta = 103.753(3) degrees, and Z = 4. Crystal data for 2 are the same as those for 1 except a = 29.940(2) A, b = 7.4421(5) A, c = 7.4170(5) A, and beta = 93.444(1) degrees.     

Surface Complexation Modeling of K(+), NO(3)(-), SO(4)(2-), Ca(2+), F(-), Co(2+), and Cr(3+) Ion Adsorption on Silica Gel

Surface complex formation of K(+), NO(3)(-), SO(4)(2-), Ca(2+), F(-), Co(2+), and Cr(3+) ions was determined on the surface of silica gel. Experimental data obtained by acid-base titration of suspensions were interpreted in terms of the triple-layer model. The value of the deprotonation constant of surface OH could be determined precisely but the protonation constant was rather uncertain. The logarithms of ion pair formation constants for K(+), NO(3)(-), Ca(2+), and SO(4)(2-) adsorbed in the beta-plane are log K(ipM,X) approximately 0, therefore these species can be considered inert ions in the investigated pH range. F(-), Co(2+), and Cr(3+) ions were found to be strongly sorbed in the o-plane. In order to provide a good fit and to obtain parameters independent of their initial values, all possible equilibrium must be accounted for in the models. Copyright 2001 Academic Press.     

Multireversible redox processes in pentanuclear bis(triple-helical) manganese complexes featuring an oxo-centered triangular {Mn(II)2Mn(III)(μ3-O)}5+ or {Mn(II)Mn(III)2(μ3-O)}6+ core wrapped by two {Mn(II)2(bpp)3}-

A new pentanuclear bis(triple-helical) manganese complex has been isolated and characterized by X-ray diffraction in two oxidation states: [{Mn(II)(μ-bpp)(3)}(2)Mn(II)(2)Mn(III)(μ-O)](3+) (1(3+)) and [{Mn(II)(μ-bpp)(3)}(2)Mn(II)Mn(III)(2)(μ-O)](4+) (1(4+)). The structure consists of a central {Mn(3)(μ(3)-O)} core of Mn(II)(2)Mn(III) (1(3+)) or Mn(II)Mn(III)(2) ions (1(4+)) which is connected to two apical Mn(II) ions through six bpp(-) ligands. Both cations have a triple-stranded helicate configuration, and a pair of enantiomers is present in each crystal. The redox properties of 1(3+) have been investigated in CH(3)CN. A series of five distinct and reversible one-electron waves is observed in the -1.0 and +1.50 V potential range, assigned to the Mn(II)(4)Mn(III)/Mn(II)(5), Mn(II)(3)Mn(III)(2)/Mn(II)(4)Mn(III), Mn(II)(2)Mn(III)(3)/Mn(II)(3)Mn(III)(2), Mn(II)Mn(III)(4)/Mn(II)(2)Mn(III)(3), and Mn(III)(5)/Mn(II)Mn(III)(4) redox couples. The two first oxidation processes leading to Mn(II)(3)Mn(III)(2) (1(4+)) and Mn(II)(2)Mn(III)(3) (1(5+)) are related to the oxidation of the Mn(II) ions of the central core and the two higher oxidation waves, close in potential, are thus assigned to the oxidation of the two apical Mn(II) ions. The 1(4+) and 1(5+) oxidized species and the reduced Mn(4)(II) (1(2+)) species are quantitatively generated by bulk electrolyses demonstrating the high stability of the pentanuclear structure in four oxidation states (1(2+) to 1(5+)). The spectroscopic characteristics (X-band electron paramagnetic resonance, EPR, and UV-visible) of these species are also described as well as the magnetic properties of 1(3+) and 1(4+) in solid state. The powder X- and Q-band EPR signature of 1(3+) corresponds to an S = 5/2 spin state characterized by a small zero-field splitting parameter (|D| = 0.071 cm(-1)) attributed to the two apical Mn(II) ions. At 40 K, the magnetic behavior is consistent for 1(3+) with two apical S = 5/2 {Mn(II)(bpp)(3)}(-) and one S = 2 noninteracting spins (11.75 cm(3) K mol(-1)), and for 1(4+) with three S = 5/2 noninteracting spins (13.125 cm(3) K mol(-1)) suggesting that the {Mn(II)(2)Mn(III)(μ(3)-O)}(5+) and {Mn(II)Mn(III)(2)(μ(3)-O)}(6+) cores behave at low temperature like S = 2 and S = 5/2 spin centers, respectively. The thermal behavior below 40 K highlights the presence of intracomplex magnetic interactions between the two apical spins and the central core, which is antiferromagnetic for 1(3+) leading to an S(T) = 3 and ferromagnetic for 1(4+) giving thus an S(T) = 15/2 ground state.     

Heterotrimetallic 4f-3d coordination polymers: synthesis, crystal structure, and magnetic properties

A series of cyano-bridged heterotrimetallic complexes [CuL](2)Ln(H(2)O)(2)M(CN)(6).7H(2)O have been synthesized by the reactions of CuL (L(2)(-) = dianion of 1,4,8,11-tetraazacyclotradecane-2,3-dione), Ln(3+) (Ln = Gd or La), and [M(CN)(6)](3)(-) (M = Co, Fe, or Cr). X-ray diffraction analysis reveals that these complexes are isostructural and have a novel chain structure. The Ln(3+) ion is eight-coordinated by six oxygen atoms of two CuL and two water molecules and two nitrogen atoms of the bridging cyano ligands of two [M(CN)(6)](3)(-), while the [M(CN)(6)](3)(-) anion connects two Ln(3+) using two trans-CN(-) ligands giving rise to a chainlike structure. In the chain, every CuL group tilts toward the CN(-) ligand of adjacent [M(CN)(6)](3)(-) with the Cu-N(cyano) contacts ranging from 2.864(6) to 2.930(6) A. Magnetic studies on the CuGdCo complex (1) indicate the presence of ferromagnetic coupling between Cu(II) and Gd(III). The CuLaCr (5) and CuLaFe (2) complexes exhibit ferromagnetic interaction between paramagnetic Cu(II) and Cr(III)/Fe(III) ions through the weak cyano bridges (Cu-N(cyano) = 2.930(6) A for 2). A global ferromagnetic interaction is operative in the CuGdFe complex (3) with the concurrence of dominant ferromagnetic Cu(II)-Gd(III) and minor antiferromagnetic Gd(III)-Fe(III) as well as the ferromagnetic Cu(II)-Fe(III) interaction. For the CuGdCr complex (4), an overall antiferromagnetic behavior was observed, which is attributed to the presence of dominant antiferromagnetic Cr(III)-Gd(III) coupling and the minor ferromagnetic Cu(II)-Gd(III) and Cu(II)-Cr(III) interaction. Moreover, a spin frustration phenomenon was found in complex 4, which results from the ferro-ferro-antiferromagnetic exchanges in the trigonal Cu-Gd-Cr units. The magnetic susceptibilities of these complexes were simulated using suitable models. The magneto-structural correlation was investigated. These complexes did not show a magnetic phase transition down to 1.8 K.     

Structural, magnetic, and gas adsorption study of a series of partially fluorinated metal-organic frameworks (HF-MOFs)

Four new partially fluorinated metal organic frameworks (HF-MOFs) have been synthesized under different solvothermal conditions (H(2)O or dimethylformamide (DMF)) from transition metal cations [Co(2+) and Mn(2+)], 3-methyl pyridine (3-mepy) and 4,4'-(hexafluoroisopropylidene) bis(benzoic acid) (C(17)H(10)F(6)O(4), H(2)hfbba), to determine the influence of reaction conditions on the formation of MOFs. This family of materials displays a striking degree of structural similarity depending on the solvent of synthesis. HF-MOFs synthesized from H(2)O [Co-HFMOF-W, Co(hfbba)(3-mepy)(H(2)O) and Mn-HFMOF-W, Mn(hfbba)(3-mepy)(H(2)O)] contain three-dimensional connectivity whereas HF-MOFs synthesized from DMF Co-HFMOF-D, [Co(2)(hfbba)(2)(3-mepy)(2)]·(DMF)(3) and Mn-HFMOF-D, [Mn(2)(hfbba)(2)(3-mepy)]·(H(2)O) are two-dimensional in nature. Co-HFMOF-W and Mn-HFMOF-W are iso-structural polymeric materials. Thermal gravimetric analysis performed on as-synthesized HF-MOFs revealed that these compounds have high thermal stability (～350 °C). The continuous decrease of the χT product with decreasing T for Co-HFMOF-D and Co-HFMOF-W respectively indicates the presence of antiferromagnetic exchange interaction between two Co(2+) (S = 3/2) metal centers within a cluster.