Antiferromagnetic Spin Coupling Research Articles

Thermal low spin–high spin equilibrium of Fe(II) in thiospinels CuFe 0.5(Sn (1− x) Ti x) 1.5S 4 (0≤ x≤1)

A series of spinel compounds with composition CuFe 0.5(Sn (1− x) Ti x ) 1.5S 4 (0≤ x≤1) is analysed by X-ray diffraction, measurements of magnetic susceptibilities and 57Fe Mössbauer spectroscopy. All samples show a temperature-dependent equilibrium between an electronic low spin 3 d( t 2 g ) 6( e g ) 0 and a high spin 3 d( t 2 g ) 4( e g ) 2 state of the Fe(II) ions. The spin crossover is of the continuous type and extends over several hundred degrees in all samples. The Sn/Ti ratio influences the thermal equilibrium between the two spin states. Substitution of Sn(IV) by the smaller Ti(IV) ions leads to a more compact crystal lattice, which, in contrast to many metal–organic Fe(II) complexes, does not stabilise the low spin state, but increases the residual high spin fraction for T→0 K. The role played by antiferromagnetic spin coupling in the stabilisation of the high spin state is discussed. The results are compared with model calculations treating the effect of magnetic interactions on spin state equilibria.

Instability of one-dimensional dangling-bond wires on H-passivated C(001), Si(001), and Ge(001) surfaces

We investigate the instability of one-dimensional dangling-bond (DB) wires fabricated on the H-terminated C(001), Si(001), and Ge(001) surfaces by using density-functional theory calculations. The three DB wires are found to show drastically different couplings between charge, spin, and lattice degrees of freedom, resulting in an insulating ground state. The C DB wire has an antiferromagnetic spin coupling between unpaired DB electrons, caused by strong electron–electron interactions, whereas the Ge DB wire has a strong charge-lattice coupling, yielding a Peierls-like lattice distortion. For the Si DB wire, the antiferromagnetic spin ordering and the Peierls instability are highly competing with each other. The physical origin of such disparate features in the three DB wires can be traced to the different degree of localization of 2 p, 3 p, and 4 p DB orbitals.

Robust magnetic polarons in type-II (Zn,Mn)Te/ZnSe magnetic quantum dots

We present a magneto-optical study of magnetic polarons in type-II (Zn,Mn) Te quantum dots. The polarons are formed due to the exchange coupling between the spins of the holes and those of the Mn ions, both of which are localized in the dots. In our photoluminescence studies, the magnetic polarons are detected at temperatures up to $\ensuremath{\sim}150\text{ }\text{K}$, with a formation energy of $\ensuremath{\sim}40\text{ }\text{meV}$. The emission from these dots exhibits an unusually small Zeeman shift with applied magnetic field ($\ensuremath{\sim}2\text{ }\text{meV}$ at 8 T) and at the same time a very large circular polarization. We attribute this apparently contradictory behavior by a low and weakly temperature-dependent magnetic susceptibility due to antiferromagnetic coupling of the Mn spins.

Europium dimer: van der Waals molecule with extremely weak antiferromagnetic spin coupling

High-level ab initio calculations reveal that the Eu(2) dimer is a van der Waals molecule with extremely weak antiferromagnetic spin coupling. The Heisenberg spin-exchange model, validated by the multireference configuration interaction method, is used to construct the full set of model interaction potentials for the states with the total spin S ranging from 0 to 7 at the coupled cluster level of theory. This model establishes the singlet (1) summation operator(+) (g) state as the ground one of the dimer with the binding energy of 710 cm(-1), the vibrational frequency of 23 cm(-1) and the effective spin-coupling constant J estimated approximately -0.3 cm(-1).

Magnetic order, Bose-Einstein condensation, and superfluidity in a bosonict−Jmodel ofCP1spinons and doped Higgs holons

We study the three-dimensional U(1) lattice gauge theory of a ${\text{CP}}^{1}$ spinon (Schwinger boson) field and a Higgs field. It is a bosonic $t\text{\ensuremath{-}}J$ model in slave-particle representation, describing the antiferromagnetic (AF) Heisenberg spin model with doped bosonic holes expressed by the Higgs field. The spinon coupling term of the action favors AF long-range order, whereas the holon hopping term in the ferromagnetic channel favors Bose-Einstein condensation (BEC) of holons. We investigate the phase structure by means of Monte Carlo simulations and study an interplay of AF order and BEC of holes. We consider the two variations in the model; (i) the three-dimensional model at finite temperatures, and (ii) the two-dimensional model at vanishing temperature. In the model (i) we find that the AF order and BEC coexist at low temperatures and certain hole concentrations. In the model (ii), by varying the hole concentration and the stiffness of AF spin coupling, we find a phase diagram similar to the model (i). Implications of the results to systems of cold atoms and the fermionic $t\text{\ensuremath{-}}J$ model of strongly correlated electrons are discussed.

Ionic fullerene complex (DMI+)3·(C60˙−)·(I−)2 with 2H-hexagonal fullerene packing and 3-D DMI+-I− network

A new ionic complex of fullerene (DMI+)3·(C60˙−)·(I−)2 (1) (DMI+ is the N,N`-dimethylimidazolium cation) was obtained as single crystals. The complex has a unique structure with a three-dimensional (3D) DMI+–I − network which arranges C60˙−radical anions. The network consists of the (DMI+)3·(I−) units and is held together by the hydrogen H(DMI+)–(I−) bonds. The C60˙−radical anions are located in the center of a trihedral prism from fullerenes with a center-to-center distance of 11.05 A, providing the 2H-hexagonal 3D packing. The relatively large distances between C60˙− prevent their dimerization but allow the manifestation of a magnetic interaction between them. The Weiss temperature of −9.6 K in the 40–300 K range indicates antiferromagnetic coupling of spins. This coupling can be accompanied by the spin frustration due to the triangular arrangement of C60˙−. The complex demonstrates an unusually broad EPR signal (21.5–23.1 mT) in the 82–297 K range attributed to C60˙− which has a g-factor value of 2.0035 at 297 K. The signal abruptly narrows below 82 K, probably due to the freezing of C60˙− rotation.

Inter-impurity and impurity–host magnetic correlations in semiconductors with low-density transition–metal impurities

Experiments on (Ga,Mn)As in the low-doping insulating phase have shown evidence for the presence of an impurity band at 110 meV above the valence band. The motivation of this paper is to investigate the role of the impurity band in determining the magnetic correlations in the low-doping regime of the dilute magnetic semiconductors. For this purpose, we present results on the Haldane–Anderson model of transition–metal impurities in a semiconductor host, which were obtained by using the Hirsch–Fye quantum Monte Carlo (QMC) algorithm. In particular, we present results on the impurity–impurity and impurity–host magnetic correlations in two- and three-dimensional semiconductors with quadratic band dispersions. In addition, we use the tight-binding approximation with experimentally determined parameters to obtain the host band structure and the impurity–host hybridization for Mn impurities in GaAs. When the chemical potential is located between the top of the valence band and the impurity bound state (IBS), the impurities exhibit ferromagnetic (FM) correlations with the longest range. We show that these FM correlations are generated by the antiferromagnetic coupling of the host electronic spins to the impurity magnetic moment. Finally, we obtain an IBS energy of 100 meV, which is consistent with the experimental value of 110 meV, by combining the QMC technique with the tight-binding approach for an Mn impurity in GaAs.

Mechanism on Two-Electron Oxidation of Ubiquinol at the Qp Site in Cytochrome bc1 Complex: B3LYP Study with Broken Symmetry

The molecular and electronic structures of the Rieske iron-sulfur [2Fe-2S] cluster with an imidazolate and imidazole were investigated by using usual unrestricted and broken symmetry B3LYP methods for the highest and lowest spin states, respectively. The electronic structures of the lowest spin states were determined by the spin contaminations and natural orbital analyses. It was shown that the spin contamination presents the number of pairs of the antiferromagnetic spin couplings. The oxidation mechanism of the ubquinol at the Q(p) site of the cytochrome bc(1) complex was also examined by the broken symmetry B3LYP methods. In the [2Fe-2S] clusters with an imidazolate, the oxidized and lowest spin state, [(Imz(-))FeS](ox)LS, was lowest in energy among four possible states, consistent with experimental observations. In the examination of the mechanism of the ubquinol oxidation, it was confirmed that the ubiquinol docks between the imidazolate of [2Fe-2S] clusters and Glu272(-) of cytochrome b by the hydrogen bonds before the oxidation proceeds, consistent with the experimental proposals. Our results support a "Glu272-first" mechanism that Glu272 serves as an acceptor of the first proton from the ubiquinol and subsequently the proton-coupled electron transfer (PCET) occurs from the ubisemiquinone anion to the Rieske iron-sulfur [2Fe-2S] cluster.

Ligand-Derived Oxidase Activity. Catalytic Aerial Oxidation of Alcohols (Including Methanol) by Cu(II)-Diradical Complexes

Seven new bis(o-iminosemiquinonato)copper(II) complexes, 1- 5, 1a, 1b, derived from differently substituted N-phenyl-2-aminophenol-based ligands, are described. Their crystal structures were determined by X-ray diffraction, and their electronic structures were established by various physical methods including electron paramagnetic resonance and variable-temperature (2-290 K) susceptibility measurements. Like complex 6, which was reported recently by us, all complexes exhibit an S t = (1)/ 2 ground state, based on the "isolated" copper(II)-spin character resulting from the dominating antiferromagnetic spin coupling between the two radicals; the ground-state electronic configuration can thus be designated as (increasing, increasing, decreasing)[R-Cu-R]. In addition, broken spin symmetry density functional solutions have been obtained. From the set of unrestricted canonical Kohn-Sham orbitals, the magnetic orbitals have been identified. The identification procedure is based on the nonvanishing overlap integrals between the space parts of orbitals occupied by electrons of opposite spin. The theoretically determined magnetic orbitals support the spin configurations suggested by the experiments. Electrochemical measurements (cyclic voltammetry and square-wave voltammetry) indicate ligand-centered redox processes. Complex 1 is found to be the best catalyst among the Cu(II) complexes for oxidation of primary alcohols with aerial oxygen as the sole oxidant to afford aldehydes under mild conditions. Thus, the function of the copper-containing enzyme Galactose Oxidase has been mimicked. Kinetic measurements in conjunction with electron paramagnetic resonance and electronic spectral studies have been used to decipher the catalytic oxidation process. A ligand-derived redox activity has been proposed as a mechanism for the aerial oxidation of primary alcohols.

Magnetic ordering and spin structure in Ca-bearing clinopyroxenes Ca M2+(Si, Ge) 2O 6, M=Fe, Ni, Co, Mn

The compounds CaFeSi 2O 6 (hedenbergite), CaNiGe 2O 6, CaCoGe 2O 6 and CaMnGe 2O 6 have been synthesized by hydrothermal or ceramic sintering techniques and were subsequently characterized by SQUID magnetometry and powder neutron diffraction in order to determine the magnetic properties and the spin structure at low temperature. All four compounds reveal the well-known clinopyroxene structure-type with monoclinic symmetry, space group C2/ c, Z=4 at all temperatures investigated. Below 35 K hedenbergite shows a ferromagnetic (FM) coupling of spins within the infinite M1 chains of edge-sharing octahedra. This FM coupling dominates an antiferromagnetic (AFM) coupling between neighbouring chains. The magnetic moments lie within the a– c plane and form an angle of 43° with the crystallographic a-axis. Magnetic ordering in CaFeSi 2O 6 causes significant discontinuities in lattice parameters, Fe–O bond lengths and interatomic Fe–Fe distances through the magnetic phase transition, which could be detected from the Rietveld refinements of powder neutron diffraction data. CaCoGe 2O 6 and CaNiGe 2O 6 show magnetic ordering below 18 K, the spin structures are similar to the one in hedenbergite with an FM coupling within and an AFM coupling of spins between the M1 chains. The moments lie within the a– c plane. The paramagnetic Curie temperature, however, decreases from CaFeSi 2O 6 (+40.2 K) to CaCoGe 2O 6 (+20.1 K) and CaNiGe 2O 6 (−13.4 K), suggesting an altered interplay between the concurring AFM and FM interaction in and between the M1 chains. CaMnGe 2O 6 finally shows an AFM ordering below 11 K. Here the magnetic moments are mainly oriented along the a-axis with a small tilt out from the a– c plane.

Antiferromagnetic spin-coupling between Mn II and amminium radical cation ligands: Models for coordination polymer magnets

One- and two-electron oxidation of the manganese(II) complex [L 2Mn(hfac) 2] (L = 4″,4‴-di- tert-butyl-2′,2″,2‴trimethoxy-{4-(4′-diphenylaminophenyl)pyridine}) were studied by ultra violet/visible/near infra red spectroscopy, cyclic voltammetry and magnetometry. A one-electron oxidation converts the triarylamine ligand to its radical cation and gives a complex in which the antiferromagnetic coupling between the spin on the ligand and that on the metal J/ k b is −1.5 K. In a dilute frozen matrix and at low temperature this behaves as an S = 2 system. A two-electron oxidation gives [L 2Mn(hfac) 2] 2 + which at low enough temperatures behaves as an S = 3/2 system but the spin-coupling between the metal and the ligand is weaker ( J/ k b = −0.3 K). The weakness of these spin-couplings mean that Mn II/amminium radical cation complexes are not promising systems on which to base coordination polymer magnets. The equivalent copper(II) complex [L 2Cu(hfac) 2] was also investigated but this decomposes when an attempt is made to oxidise the ligand to its amminium radical cation.

Fullerene complexes with divalent metal dithiocarbamates: structures, magnetic properties, and photoconductivity

A series of complexes of fullerenes C60 and C70 with metal dithiocarbamates {MII(R2dtc)2}·Cm (m = 60 or 70) and metal dithiocarbamates coordinated to nitrogen-containing ligands (L), {MII(R2dtc)2)x·L}·C60 (x = 1 or 2), where M = Cu, Zn, Cd, Hg, Mn, or Fe, R = Me, Et, Prn, Pri, or Bun, L is 1,4-diazabicyclo[2.2.2]octane (DABCO), N,N′-dimethylpiperazine, or hexamethylenetetramine, were synthesized. The shape of dithiocarbamate molecules is sterically compatible with the spherical shape of C60, resulting in an efficient interaction between their π systems. The resulting compounds are characterized by a layered or three-dimensional packing of the fullerene molecules. In the C60 complexes, iron(II) and manganese(II) dithiocarbamates exist in the high-spin states (S = 2 and 5/2). The magnetic susceptibility of {MII(Et2dtc)2}2·Cm (M = Fe or Mn, m = 60 or 70) in the temperature range of 200–300 K is described by the Curie-Weiss law with Θ = −250 and −96 K and with maxima at 110 and 46 K, respectively, which is indicative of a strong antiferromagnetic spin coupling between MII. The Weiss constants for the [{MII(Et2dtc)2}2·DABCO]·C60·(DABCO)2 complexes (M = Fe or Mn) are 1.7 and 0.3 K, respectively. The magnetic moments of the complexes containing Fe and Mn dithiocarbamates slightly increase at temperatures below 50 and 35 K, respectively, which is evidence of the ferromagnetic spin coupling between MII in {MII(Et2dtc)2}2·DABCO. Single crystals of the complexes exhibit low dark conductivity (10−10–10−11 S cm−1). The visible light irradiation of these crystals leads to an increase in the photocurrent by two–three orders of magnitude. The photogeneration of free charge carriers in the complexes occurs both due to the photoexcitation of metal dithiocarbamate (CuII(Et2dtc)2) and through the charge transfer from metal dithiocarbamate (MII(Et2dtc)2, M = Zn or Cd) to C60.

Chiral plaquette polaron theory of cuprate superconductivity

Ab initio density functional calculations on explicitly doped ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ find that doping creates localized holes in out-of-plane orbitals. A model for cuprate superconductivity is developed based on the assumption that doping leads to the formation of holes on a four-site Cu plaquette composed of the out-of-plane ${A}_{1}$ orbitals apical $\mathrm{O}\phantom{\rule{0.2em}{0ex}}{p}_{z}$, planar $\mathrm{Cu}\phantom{\rule{0.2em}{0ex}}{d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$, and planar $\mathrm{O}\phantom{\rule{0.2em}{0ex}}{p}_{\ensuremath{\sigma}}$. This is in contrast to the assumption of hole doping into planar $\mathrm{Cu}\phantom{\rule{0.2em}{0ex}}{d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ and $\mathrm{O}\phantom{\rule{0.2em}{0ex}}{p}_{\ensuremath{\sigma}}$ orbitals as in the $t\text{\ensuremath{-}}J$ model. Allowing these holes to interact with the ${d}^{9}$ spin background leads to chiral polarons with either a clockwise or anticlockwise charge current. When the polaron plaquettes percolate through the crystal at $x\ensuremath{\approx}0.05$ for ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$, a $\mathrm{Cu}\phantom{\rule{0.2em}{0ex}}{d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ and planar $\mathrm{O}\phantom{\rule{0.2em}{0ex}}{p}_{\ensuremath{\sigma}}$ band is formed. The computed percolation doping of $x\ensuremath{\approx}0.05$ equals the observed transition to the ``metallic'' and superconducting phase for ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$. Spin exchange Coulomb repulsion with chiral polarons leads to $d$-wave superconducting pairing. The equivalent of the Debye energy in phonon superconductivity is the maximum energy separation between a chiral polaron and its time-reversed partner. This energy separation is on the order of the antiferromagnetic spin coupling energy, ${J}_{dd}\ensuremath{\sim}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, suggesting a higher critical temperature. An additive skew-scattering contribution to the Hall effect is induced by chiral polarons and leads to a temperature dependent Hall effect that fits the measured values for ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$. The integrated imaginary susceptibility, observed by neutron spin scattering, satisfies $\ensuremath{\omega}∕T$ scaling due to chirality and spin-flip scattering of polarons along with a uniform distribution of polaron energy splittings. The derived functional form is compatible with experiments. The static spin structure factor for chiral spin coupling of the polarons to the undoped antiferromagnetic $\mathrm{Cu}\phantom{\rule{0.2em}{0ex}}{d}^{9}$ spins is computed for classical spins on large two-dimensional lattices and is found to be incommensurate with a separation distance from $(\ensuremath{\pi}∕a,\ensuremath{\pi}∕a)$ given by $\ensuremath{\delta}Q\ensuremath{\approx}(2\ensuremath{\pi}∕a)x$, where $x$ is the doping. When the perturbed ${x}^{2}\ensuremath{-}{y}^{2}$ band energy in mean field is included, incommensurability along the Cu-O bond direction is favored. A resistivity $\ensuremath{\sim}{T}^{\ensuremath{\mu}+1}$ arises when the polaron energy separation density is of the form $\ensuremath{\sim}{\ensuremath{\Delta}}^{\ensuremath{\mu}}$ due to Coulomb scattering of the ${x}^{2}\ensuremath{-}{y}^{2}$ band with polarons. A uniform density leads to linear resistivity. The coupling of the ${x}^{2}\ensuremath{-}{y}^{2}$ band to the undoped $\mathrm{Cu}\phantom{\rule{0.2em}{0ex}}{d}^{9}$ spins leads to the angle-resolved photoemission pseudogap and its qualitative doping and temperature dependence. The chiral plaquette polaron leads to an explanation of the evolution of the bilayer splitting in Bi-2212.

A [2 × 2] nickel(ii) grid and a copper(ii) square result from differing binding modes of a pyrazine-based diamide ligand

The potentially bis-terdentate diamide ligand N,N'-bis[2-(2-pyridyl)ethyl]pyrazine-2,3-dicarboxamide (H(2)L(Et)) was structurally characterised. Potentiometric titrations revealed rather low pK(a) values for the deprotonation of the first amide group of H(2)L(Et) (14.2) and N,N'-bis(2-pyridylmethyl)pyrazine-2,3-dicarboxamide (H(2)L(Me), 13.1). Two tetranuclear copper(ii) square complexes of H(2)L(Et) with a paddle-wheel appearance, in which each ligand strand acts as a linear N(3)-NO hybrid terdentate-bidentate chelate, have been isolated and structurally characterised. Complex [Cu(II)(4)(H(2)L(Et))(2)(HL(Et))(2)](BF(4))(6).3MeCN.0.5H(2)O (.3MeCN.0.5H(2)O), with two nondeprotonated zwitterionic ligand strands and two monodeprotonated ligand strands, is formed in the 1 : 1 reaction of H(2)L(Et) and Cu(BF(4))(2).4H(2)O. It has a polymeric chain structure of tetranuclear subunits connected by N-H[dot dot dot]N hydrogen bonds. The same reaction carried out with one equivalent of base gives the related compound [Cu(II)(4)(HL(Et))(4)](BF(4))(4) (), with all four ligand strands monodeprotonated. It consists of isolated tetranuclear units. In both .3MeCN.0.5 H(2)O and the copper(ii) ions are in five-coordinate N(4)O environments but the degree of trigonality (tau) differs [.3MeCN.0.5H(2)O 0.14 </=tau</= 0.26; tau = 0.45]. Under the same reaction conditions as for but using Ni(BF(4))(2).6H(2)O a tetranuclear [2 x 2] grid-type complex, [Ni(II)(4)(HL(Et))(4)](BF(4)).10H(2)O (.10H(2)O), is formed. The structure determination showed that the nickel(ii) ions have N(6) distorted octahedral coordination spheres and all four ligand strands are monodeprotonated and act as N(3)-N(3) bis-terdentate chelates. Magnetic susceptibility data show that the complexes .4H(2)O, and .10H(2)O exhibit very weak antiferromagnetic spin coupling. The energies and multiplicities of the spin states of .4H(2)O and were determined from the temperature dependence of the magnetic susceptibility and indicate that a singlet state is lowest and the quintet state highest. This is consistent with the X-band EPR spectra of polycrystalline powders of .4H(2)O and , measured down to 2.3 K, where the resonances observed at the lowest temperature are due to a triplet state. The g-values of the individual ions of are consistent with the expected d(x(2)-y(2)) ground state for five-coordinate copper(ii) in an approximately square pyramidal configuration.

Pyrazolate-based copper(ii) and nickel(ii) [2 × 2] grid complexes: protonation-dependent self-assembly, structures and properties

The pyrazole-based diamide ligand N,N'-bis(2-pyridylmethyl)pyrazole-3,5-dicarboxamide (H(3)L) has been structurally characterised and successfully employed in the preparation of [2 x 2] grid-type complexes. Thus, the reaction of H(3)L with Cu(ClO(4))2.6H(2)O or Ni(ClO(4))2.6H(2)O in the presence of added base (NaOH) affords the tetranuclear complexes [M(4)(HL(4))].8H(2)O (1: M = Cu, 2: M = Ni). Employment of a mixture of the two metal salts under otherwise identical reaction conditions leads to the formation of the mixed-metal species [Cu(x)Ni(4-x)(HL)(4)].8H(2)O (x<or= 4; for x = 2: 3). Complexes 1-3 have been structurally characterised and found to be isomorphous, with each ligand strand acting as a hybrid N3-NO chelator. The copper ions in 1 are in a distorted square-pyramidal N(4)O coordination environment with rather long M-O(apical) distances. The coordination sphere about the nickel ions in 2 is roughly the same, but with even longer M...O distances, and it is therefore best described as N4 square-planar with low-spin nickel(II) ions. The single crystal X-ray data obtained for the mixed-metal complex 3 gave the best results assuming a statistical distribution of copper and nickel ions. X-Band EPR spectra of 1 and 2 indicate magnetically coupled copper(II) ions and low-spin nickel(II), respectively. EPR spectra of a powdered sample of a complex with the general formulation [Cu(x)Ni(4-x)(HL)4].8H(2)O with a large excess of Ni(2+) (95%) was shown to be characteristic for individual copper(II) ions in the tetranuclear grid system. Magnetic susceptibility data of 1 indicate weak antiferromagnetic spin coupling between the copper ions (J = -8.2 +/- 0.4 cm(-1)), which is explained by the particular spatial arrangement of the magnetic orbitals.

Direct observation of the induced moment on nonmagnetic Ru: A magnetic Compton study ofCaRu0.85Fe0.15O3

The magnetic Compton profile of $\mathrm{Ca}{\mathrm{Ru}}_{0.85}{\mathrm{Fe}}_{0.15}{\mathrm{O}}_{3}$ has been measured to elucidate the spin moment structure. The obtained profile is decomposed into two partial profiles of Ru $4d$ and Fe $3d$, calculated on the basis of a Hartree-Fock molecular orbital calculation. This decomposition reveals that a spin moment of $0.38\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{B}$ per formula unit is induced on Ru and the moment aligns antiparallel to the spin moment ($\ensuremath{-}0.18\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{B}$ per formula unit) of Fe. The observed antiferromagnetic spin coupling can be explained by the superexchange interaction between Ru and Fe ions via an O ion.

Tetranuclear Copper(II) Complexes Bridged by α-d-Glucose-1-Phosphate and Incorporation of Sugar Acids through the Cu4 Core Structural Changes

Tetranuclear copper(II) complexes containing alpha-D-glucose-1-phosphate (alpha-D-Glc-1P), [Cu4(mu-OH){mu-(alpha-D-Glc-1P)}2(bpy)4(H2O)2]X3 [X = NO3 (1a), Cl (1b), Br (1c)], and [Cu4(mu-OH){mu-(alpha-D-Glc-1P)}2(phen)4(H2O)2](NO3)3 (2) were prepared by reacting the copper(II) salt with Na2[alpha-D-Glc-1P] in the presence of diimine ancillary ligands, and the structure of 2 was characterized by X-ray crystallography to comprise four {Cu(phen)}2+ fragments connected by the two sugar phosphate dianions in 1,3-O,O' and 1,1-O mu4-bridging fashion as well as a mu-hydroxo anion. The crystal structure of 2 involves two chemically independent complex cations in which the C2 enantiomeric structure for the trapezoidal tetracopper(II) framework is switched according to the orientation of the alpha-D-glucopyranosyl moieties. Temperature-dependent magnetic susceptibility data of 1a indicated that antiferromagnetic spin coupling is operative between the two metal ions joined by the hydroxo bridge (J = -52 cm(-1)) while antiferromagnetic interaction through the Cu-O-Cu sugar phosphate bridges is weak (J = -13 cm(-1)). Complex 1a readily reacted with carboxylic acids to afford the tetranuclear copper(II) complexes, [Cu4{mu-(alpha-D-Glc-1P)}2(mu-CA)2(bpy)4](NO3)2 [CA = CH3COO (3), o-C6H4(COO)(COOH) (4)]. Reactions with m-phenylenediacetic acid [m-C6H4(CH2COOH)2] also gave the discrete tetracopper(II) cationic complex [Cu4{mu-(alpha-D-Glc-1P)}2(mu-m-C6H4(CH2COO)(CH2COOH))2(bpy)4](NO3)2 (5a) as well as the cluster polymer formulated as {[Cu4{mu-(alpha-D-Glc-1P)}2(mu-m-C6H4(CH2COO)2)(bpy)4](NO3)2}n (5b). The tetracopper structure of 1a is converted into a symmetrical rectangular core in complexes 3, 4, and 5b, where the hydroxo bridge is dissociated and, instead, two carboxylate anions bridge another pair of Cu(II) ions in a 1,1-O monodentate fashion. The similar reactions were applied to incorporate sugar acids onto the tetranuclear copper(II) centers. Reactions of 1a with delta-D-gluconolactone, D-glucuronic acid, or D-glucaric acid in dimethylformamide resulted in the formation of discrete tetracopper complexes with sugar acids, [Cu4{mu-(alpha-D-Glc-1P)}2(mu-SA)2(bpy)4](NO3)2 [SA = D-gluconate (6), D-glucuronate (7), D-glucarateH (8a)]. The structures of 6 and 7 were determined by X-ray crystallography to be almost identical with that of 3 with additional chelating coordination of the C-2 hydroxyl group of D-gluconate moieties (6) or the C-5 cyclic O atom of D-glucuronate units (7). Those with D-glucaric acid and D-lactobionic acid afforded chiral one-dimensional polymers, {[Cu4{mu-(alpha-D-Glc-1P)}2(mu-D-glucarate)(bpy)4](NO3)2}n (8b) and {[Cu4{mu-(alpha-D-Glc-1P)}2(mu-D-lactobionate)(bpy)4(H2O)2](NO3)3}n (9), respectively, in which the D-Glc-1P-bridged tetracopper(II) units are connected by sugar acid moieties through the C-1 and C-6 carboxylate O atoms in 8b and the C-1 carboxylate and C-6 alkoxy O atoms of the gluconate chain in 9. When complex 7 containing d-glucuronate moieties was heated in water, the mononuclear copper(II) complex with 2-dihydroxy malonate, [Cu(mu-O2CC(OH)2CO2)(bpy)] (10), and the dicopper(II) complex with oxalate, [Cu2(mu-C2O4)(bpy)2(H2O)2](NO3)2 (11), were obtained as a result of oxidative degradation of the carbohydrates through C-C bond cleavage reactions.

2D Hydrogen-Bonded Square-Grid Coordination Networks with a Substitution-Active Metal Site

Reported here is the preparation and property of 2D coordination networks composed of rodlike ligands with ethylene glycol side chains (1). Two 2D coordination networks, [[Co(1)2(H2O)2](NO3)2.1.5H2O]n and [[Ni(1)2(H(2)O)2](NO3)2.1.5H2O]n, have been synthesized and characterized by single-crystal X-ray diffraction, TG, DSC, UV-vis spectroscopy, and magnetic measurements. The structural analyses clarified that infinite 1D hydrogen-bond arrays composed of ethylene glycol chains contribute to the stabilization of 2D coordination frameworks, keeping the environment of substitution-active metal sites unchanged. They are more stable than a similar square-grid coordination network that does not possess an ethylene glycol chain on the ligand. We also succeeded in the direct observation of a reversible apical-ligand-exchange reaction at the cobalt(II) and nickel(II) ions in a single-crystal-to-single-crystal fashion because of the considerable stability as well as moderate flexibility of the framework. The cobalt-containing coordination network crystal showed chromic behavior depending on temperatures. Crystallographic and spectroscopic studies revealed that the color change of the crystal was attributed to the ligand-exchange process between H2O and a NO3 anion on the cobalt metal. Magnetic measurements indicated weak antiferromagnetic nearest-neighbor spin coupling between cobalt(II) ions.

Synthesis, Crystal Structures, Magnetic Properties and Photoconductivity of C60 and C70 Complexes with Metal Dialkyldithiocarbamates M(R2dtc)x, where M = CuII, CuI, AgI, ZnII, CdII, HgII, MnII, NiII, and PtII; R = Me, Et, and nPr

AbstractNew complexes of fullerenes C60 and C70 with metal dialkyldithiocarbamates, [M(R2dtc)x]·[C60(70)]·[Solvent], R = Et [M = CuII (C60, 1; C70, 2), CuI (C60, 3; C70, 4), AgI (C60, 5), ZnII (C60, 6), CdII (C60, 7; C70, 8), HgII (C60, 9), MnII (C70, 10)], R = Et and Me [M = CuII (C60, 11), and ZnII (C60, 12)], and R = nPr [M = CuII (C60, 13), NiII (C60, 14), and PtII (C60, 15)] were obtained. M(R2dtc)x efficiently cocrystallized with fullerene molecules as tetrahedral monomers (6, 12), dimers (1, 7, 11), and tetramers (3, 4). Fullerene molecules form closely packed hexagonal and square layers in 1, 7, and 11, hexagonal and tetragonal 3D structures in 6 and 12, and island motifs in 3 and 4. Complexes 1–15 have a neutral ground state. However, the formation of the complexes with fullerenes changes the environment of paramagnetic CuII and MnII. The EPR spectra of 1, 2, 11, and 13 are essentially modified relative to those of pristine Cu(R2dtc)2 because of a weak coordination of CuII to fullerene and a flattening of the central (NCS2)2Cu fragments. Complex 10 shows a spectrum exhibiting features from 50 to 600 mT and manifests strong antiferromagnetic coupling of spins with a Weiss temperature of –96 K and the maximum of magnetic susceptibility at 46 K. Such magnetic behavior can be explained by the formation of [Mn(Et2dtc)2]2 dimers in 10. The illumination of the crystals of 1, 2, and 7 by white light results in up to a 103 increase in photocurrent. The photoconductivity spectra have maxima at 470, 450–650, and 660 nm for 1, 2, and 7, respectively. Photogeneration of free charge carriers is realized by photoexcitation of Cu(Et2dtc)2 in 1 and 2, and by charge transfer from Cd(Et2dtc)2 to C60 molecules in 7. The decrease of photocurrent in 1 and 7 in a weak magnetic field with B0 &lt; 0.5 T was found. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2006)

Mössbauer study of the invar Fe–Ni and Fe–Ni–C alloys in magnetic field

F.C.C. Fe-30.3%Ni and Fe-30.5%Ni-1.5%C (wt.%) alloys were studied by means of Mossbauer spectroscopy in external magnetic field B ext=2.5, 5, 7 T parallel to the gamma-beam. It is shown that distribution of effective magnetic field in the alloys is broad and that carbon expands the range of B eff. The external magnetic field increases B eff in the Fe-Ni alloy and decreases it more evidently in the Fe-Ni-C alloy. Antiferromagnetic spin coupling along the ferromagnetic component is proposed to explain data.