Isotropic Exchange Model Research Articles

Synthesis, crystal structure and magnetic properties of new trinuclear Copper(II) complexes with Biphenol−Based dinucleating ligands

Treatment of (3,3′-bis-((2,6-dimethylphenyl)-imino)methyl)-(1,1′)-biphenyl-2,2′-diol) (H2L2) and (3,3′-bis-((2-methoxyphenyl))imino)methyl)-(1,1′)-biphenyl-2,2′-diol) (H2L3) with excess [Cu2(OAc)4(H2O)2] in the presence of triethylamine afforded new trinuclear complexes [Cu(3,3′-bis-((R)-iminomethyl)-(1,1′)-biphenyl-2,2′-dioxo)]3 [R = 2,6-Me2C6H3 (3e) and R = 2- OMeC6H3(4e)] respectively. The resulting complexes were characterized by elemental analysis, magnetic susceptibility, EPR, UV–Vis, IR, CV and Single crystal X-ray diffraction analysis. The magnetic susceptibility study of 3e and 4e was performed in the 5–300 K and revealed the existence of antiferromagnetic interaction in both complexes. The experimental data could be satisfactorily reproduced using an isotropic exchange model, H = -J (S1S2 + S2S3 + S1S3), yielding as best fit parameters: J = −15 cm−1, g = 1.99 for 3e and J = −18 cm−1, g = 1.99 for 4e. Both complexes have an angular C2/c-symmetric trinuclear core and each Cu(II) ion is in similar environments coordinated to two (O, N) donor sets as revealed from single-crystal X-ray diffraction studies. The coordination geometry can best be described as distorted square planar which could be judged using the τ4 index, τ4 = 360 – (α + β)/141°, where α and β are the two largest angles subtended by the ligand donor atoms in the four-coordinate complex. The average indexes for 3e and 4e were found to be 0.42 and 0.43 respectively, signifying a considerable distortion from an idealized square planar, D4h geometry (τ4 = 0.0) or idealized tetrahedral, Td geometry (τ4 = 1.0). The average Cu⋯Cu distance between closest copper(II) ions in the complex 3e is 4.48 Å. On the other hand, the single crystal X-ray analysis showed that each of the three Cu(II) centers in complex 4e is strongly tetracoordinated to two N and two O atoms & weakly to two O atoms of the methoxy groups of the ligand, N,N′,O,O′-3, 3′-bis ((2-methoxyphenyl)-iminomethyl)-[1, 1′-biphenyl]-2, 2′-diol (H2L3). Generally, Cu−N & Cu−O bond lengths are (1.983–1.994) & (1.882–1.893) respectively, comparable to similar systems.

Ytterbium and Europium Complexes of Redox-Active Ligands: Searching for Redox Isomerism.

The reaction of (dpp-Bian)EuII(dme)2 (3) (dpp-Bian is dianion of 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene; dme is 1,2-dimethoxyethane) with 2,2'-bipyridine (bipy) in toluene proceeds with replacement of the coordinated solvent molecules with neutral bipy ligands and affords europium(II) complex (dpp-Bian)EuII(bipy)2 (9). In contrast the reaction of related ytterbium complex (dpp-Bian)YbII(dme)2 (4) with bipy in dme proceeds with the electron transfer from the metal to bipy and results in (dpp-Bian)YbIII(bipy)(bipy-̇) (10) - ytterbium(III) derivative containing both neutral and radical-anionic bipy ligands. Noteworthy, in both cases dianionic dpp-Bian ligands retain its reduction state. The ligand-centered redox-process occurs when complex 3 reacts with N,N'-bis[2,4,6-trimethylphenyl]-1,4-diaza-1,3-butadiene (mes-dad). The reaction product (dpp-Bian)EuII(mes-dad)(dme) (11) consists of two different redox-active ligands both in the radical-anionic state. The reduction of 3,6-di-tert-butyl-4-(3,6-di-tert-butyl-2-ethoxyphenoxy)-2-ethoxycyclohexa-2,5-dienone (the dimer of 2-ethoxy-3,6-di-tert-butylphenoxy radical) with (dpp-Bian)EuII(dme)2 (3) caused oxidation of the dpp-Bian ligand to radical-anion to afford (dpp-Bian)(ArO)EuII(dme) (ArO = OC6H2-3,6-tBu2-2-OEt) (12). The molecular structures of complexes 9-12 have been established by the single crystal X-ray analysis. The magnetic behavior of newly prepared compounds has been investigated by the SQUID technique in the range 2-310 K. The isotropic exchange model has been adopted to describe quantitatively the magnetic properties of the exchange-coupled europium(II) complexes (11 and 12). The best-fit isotropic exchange parameters are in good agreement with their density functional theory-computed counterparts.

A new family of multinuclear mixed-ligand copper(II) clusters: Crystal structures, magnetic properties and catecholase-like activity

In this work we report a family of mixed-ligand copper(II) clusters obtained with different nuclearities: [Cu3(μ-OH)(μ-dppi)3(μ-hfac)3] (1), [Cu4(μ-dppi)4(Hdppi)2(hfac)4] (2), [Cu5(μ-O)2(μ-dppi)(μ-Hdppi)2(μ-hfac)3(hfac)2] (3) and [Cu6(μ-O)2(μ-dppi)2(μ-Hdppi)(μ-hfac)6] (4), where hfac=hexafluoroacetylacetonate, dppi=diphenylphosphinate and Hdppi=diphenylphosphinic acid. Complexes 3 and 4 are the first examples of copper(II) clusters containing a μ-oxo oxygen atom and hfac as bridging ligands. The magnetic properties of all compounds showed predominant antiferromagnetic interactions and the magnetic coupling constants were evaluated using an isotropic exchange model based on the spin topology. A spin frustration phenomena was observed for the equilateral cyclic trinuclear complex (1), thus an antisymmetric exchange term was also considered to fit the experimental magnetic data. The obtained J values are in the range from −89cm−1 to 1.7cm−1 and were interpreted based on structural features. The electrochemical properties and catalytic activity of complexes 1 and 2 were investigated and compared with the [Cu(hfac)2] precursor. Catecholase-like activity of copper(II) complexes via oxidation of 3,5-di-tert-butylcatechol (3,5-DTBC) to 3,5-di-tert-butylquinone (3,5-DTBQ) was performed.

Theoretical Modeling of the Magnetic Behavior of Thiacalix[4]arene Tetranuclear Mn(II)2Gd(III)2 and Co(II)2Eu(III)2 Complexes.

In view of a wide perspective of 3d-4f complexes in single-molecule magnetism, here we propose an explanation of the magnetic behavior of the two thiacalix[4]arene tetranuclear heterometallic complexes Mn(II)2Gd(III)2 and Co(II)2Eu(III)2. The energy pattern of the Mn(II)2Gd(III)2 complex evaluated in the framework of the isotropic exchange model exhibits a rotational band of the low-lying spin excitations within which the Landé intervals are affected by the biquadratic spin-spin interactions. The nonmonotonic temperature dependence of the χT product observed for the Mn(II)2Gd(III)2 complex is attributed to the competitive influence of the ferromagnetic Mn-Gd and antiferromagnetic Mn-Mn exchange interactions, the latter being stronger (J(Mn, Mn) = -1.6 cm(-1), Js(Mn, Gd) = 0.8 cm(-1), g = 1.97). The model for the Co(II)2Eu(III)2 complex includes uniaxial anisotropy of the seven-coordinate Co(II) ions and an isotropic exchange interaction in the Co(II)2 pair, while the Eu(III) ions are diamagnetic in their ground states. Best-fit analysis of χT versus T showed that the anisotropic contribution (arising from a large zero-field splitting in Co(II) ions) dominates (weak-exchange limit) in the Co(II)2Eu(III)2 complex (D = 20.5 cm(-1), J = -0.4 cm(-1), gCo = 2.22). This complex is concluded to exhibit an easy plane of magnetization (arising from the Co(II) pair). It is shown that the low-lying part of the spectrum can be described by a highly anisotropic effective spin-(1)/2 Hamiltonian that is deduced for the Co(II)2 pair in the weak-exchange limit.

Polarized neutron diffraction and anisotropic exchange in pyrochlores

Rare earth frustrated pyrochlore magnets are among the materials that exhibit no long range order down to the lowest explored temperatures. To better understand the many complex phenomena observed in the pyrochlores, knowledge of the local interactions present in these materials is necessary. In the pyrochlore lattice, selection between Ising, Heisenberg, or XY types of the anisotropy cannot be based on the analysis of the macroscopic properties because of the presence of four equivalent local anisotropy (<111> -type) axes. Then only an average over the four axes can be measured by classical methods. Polarized neutron diffraction based on the `site susceptibility approach' (A Gukasov and P. J. Brown, J. Phys.: Cond. Matt., 14, 8831, 2002) allows to determine the local susceptibility tensor on the magnetic site. Its temperature dependence in the paramagnetic phase can be accounted for by the crystal field anisotropy and a molecular field tensor that encompasses exchange and dipolar interactions. It was found that for rare earth pyrochlores an isotropic exchange model is not suitable to explain their magnetic behavior and that, instead, the exchange interaction appear anisotropic to large extent (H. Cao et al., PRL 103, 056402, 2009). The experimental evaluation of the anisotropic exchange in pyrochlores can provide a clue to the richness of the phase diagram observed in rare earth frustrated magnets.

Peculiarities of crystal structures and magnetic properties of Cu(II) and Ni(II) mixed-ligand complexes on the 1,3-dithiole-2-thione-4,5-dithiolate basis

Mixed-ligand Cu(II) and Ni(II) complexes, [Cu(dmit)(bpy)] 2 ( I), [Ni(dmit)(phen) 2] ( II) and [Ni(dmit)(phen) 2]·CH 2Cl 2 ( III) (dmit=1.3-dithiole-2-thione-4.5-dithiolate, phen=1.10-phenantroline, bpy=2.2′-bipyridine) have been prepared by ligand exchange between phen or bpy and (Bu 4N) 2[M(dmit) 2] (M=Ni, Cu) and characterized by elemental analysis, IR spectroscopy, single-crystal X-ray analysis and by investigation of magnetic and resonance properties. In complex I, the monomeric units form dimers in a head-to-tail arrangement by weak coordination bonds between copper and dithiolate sulfur atoms and π–π interactions between dmit and bpy from neighboring monomers. Dimers in I are further extended into chains by weak Cu–S(thione) contacts. In crystal packing of complex II and III, there exists a weak π–π interaction between two parallel phen molecules of the adjacent complexes. As a consequence, the magnetic and resonance characteristics of copper complex may be described in approximation of exchange-coupled pairs of Cu 2+ ions with ion spin S=1/2. The nickel complexes are described by isotropic exchange model for single-site spin S=1.

An equatorial tri-iron substituted Wells–Dawson type tungstophosphate with magnetic and bifunctional electrocatalytic properties

Interaction of the hexa-lacunary polyanion precursor [α-H 2P 2W 12O 48] 12− and the Fe III in aqueous solution results in the formation of an equatorial tri-iron substituted Wells–Dawson type compound, K 4Cs 2Fe 2[P 2W 15(FeOH) 3O 59]·22H 2O ( 1). Compound 1 was characterized by IR, elemental, single-crystal X-ray diffraction, thermogravimetric, magnetic, as well as electrochemical analysis. The polyoxoanion [P 2W 15(FeOH) 3O 59] 12− can be viewed as a derivative of the parent polyoxoanion [α-P 2W 18O 62] 6− by removal of three belt W O groups and then inhabited by three Fe OH groups. The compound 1-modified carbon paste electrode ( 1-CPE) presents good electrocatalytic activity not only toward the reduction of nitrite which is attributed to the function of tungstophosphate, but also toward the oxidation of ascorbic acid which is primarily attributed to the function of Fe III. The magnetic properties of 1 have been studied by magnetic susceptibility and fitted according to an isotropic exchange model. Compound 1 exhibits strong antiferromagnetic spin exchange interactions between the Fe III centers.

Synthesis, crystal structure and magnetic properties of a trinuclear Cu(II)-pyrazolate complex containing μ 3-OH

A new pyrazolate-bridged quasi-hexacopper(II) complex [Cu3(μ3-OH)(μ-pz)3(pzH)3 (μ3-ClO4)]2(μ-ClO4)2 · (CH3OH)0.33 · (C2H5OH)0.66 · H2O (pz = pyrazolate), 1, has been isolated and characterized by single crystal X-ray diffraction, elemental analysis, IR and UV–visible spectra and magnetic susceptibilities. The complex crystallizes in the orthorhombic system, space group Pbca (, No. 61), a = 17.771(4) Å, b = 16.143(4) Å, c = 25.502(6) Å, V = 7316(3) Å3, Z = 8, F(000) = 3521, D c = 1.576 g cm−3. The structure of 1 consists of two [Cu3(μ3-OH)(μ-pz)3(pzH)3(μ3-ClO4)]2+ trimers, which are weakly bridged by two perchlorate ions via centrosymmetric operation to form a trigonal prism with the six Cu atoms at six vertexes. Magnetic measurements were performed in the temperature range 2–300 K. The experimental data above 9 K could be satisfactorily reproduced by using an isotropic exchange model, (1), with parameters of J = −241.91 cm−1, ZJ′ = −23.04 cm−1 and g = 2.07, showing strong antiferromagnetic behaviors in Cu3 unit and weak intertrimer magnetic interaction.

Synthesis, Crystal Structure, and Magnetic Studies of Oxo-Centered Trinuclear Chromium(III) Complexes: [Cr3(μ3-O)(μ2-PhCOO)6(H2O)3]NO3· 4H2O·2CH3OH, a Case of Spin-Frustrated System, and [Cr3(μ3-O)- (μ2-PhCOO)2(μ2-OCH2CH3)2(bpy)2(NCS )3], a New Type of [Cr3O] Core

The synthesis, crystal structure, and magnetic properties of two trinuclear oxo-centered carboxylate complexes are reported and discussed: [Cr3(mu3-O)(mu2-PhCOO)6(H2O)3]NO3.4H2O.2CH3OH (1) and [Cr3(mu3-O)(mu2-PhCOO)2(mu2-OCH2CH3)2(bpy)2(NCS)3] (2). For both complexes the crystal system is monoclinic, with space group C2/c for 1 and P1/n for 2. The structure of complex 1 consists of discrete trinuclear cations, associated NO3- anions, and lattice methanol and water molecules. The structure of complex 2 is built only by neutral discrete trinuclear entities. The most important feature of 2 is the unusual skeleton of the [Cr3O] core due to the lack of peripheral bridging ligands along one side of the triangular core, which is unique among the structurally characterized (mu3-oxo)trichromium(III) complexes. Magnetic measurements were performed in the 2-300 K temperature range. For complex 1, in the high-temperature region (T > 8 K), experimental data could be satisfactorily reproduced by using an isotropic exchange model, H = -2J12S1S2 - 2J13S1S3 - 2J23S2S3 (J12 = J13 = J23) with Jij = -10.1 cm(-1), g = 1.97, and TIP = 550 x 10(-6) emu mol(-1). The antisymmetric exchange interaction plays an important role in the magnetic behavior of the system, so in order to fit the experimental magnetic data at low temperature, a new magnetic model was used where this kind of interaction was also considered. The resulting fitting parameters are the following: Gzz = 0.25 cm(-1), delta = 2.5 cm(-1), and TIP = 550 x 10(-6) emu mol(-1). For complex 2, the experimental data could be satisfactorily reproduced by using an isotropic exchange model, H = -2J1(S1S2 + S1S3) - 2J2(S2S3) with J1 = -7.44 cm(-1), J2 = -51.98 cm(-1), and g = 1.99. The magnetization data allows us to deduce the ground term of S = 1/2, characteristic of equilateral triangular chromium(III) for complex 1 and S = 3/2 for complex 2, which is confirmed by EPR measurements.

Two Iron-Containing Tungstogermanates: [K(H2O)(β-Fe2GeW10O37(OH))(γ-GeW10O36)]12-and [{β-Fe2GeW10O37(OH)2}2]12-

Interaction of the dilacunary polyanion precursor [gamma-GeW(10)O(36)](8-) with Fe(3+) ions in aqueous buffer medium (pH 4.8) results in the formation of two dimeric tungstogermanates depending on the reactant ratios. When using an Fe3+ to [gamma-GeW(10)O(36)](8-) ratio of 1:1, the asymmetric anion [K(H(2)O)(beta-Fe(2)GeW(10)O(37)(OH))(gamma-GeW(10)O(36))](12-) (1) is formed, whereas [{beta-Fe(2)GeW(10)O(37)(OH)2}2]12- (2) is formed when using a ratio of 2:1. Single-crystal X-ray analyses were carried out on Cs(3)K(9)[K(H(2)O)(beta-Fe(2)GeW(10)O(37)(OH))(gamma-GeW(10)O(36))].19H(2)O (CsK-1), which crystallizes in the triclinic system, space group P1, a = 11.4547(2), b = 19.9181(5), c = 21.0781(6) A, alpha = 66.7977(12), beta = 89.1061(12), gamma = 84.4457(11) degrees, and Z = 2 and on Cs(7)K(4)Na[{beta-Fe(2)GeW(10)O(37)(OH)(2)}(2)].39H(2)O (CsKNa-2), which crystallizes in the monoclinic system, space group C2/m, a = 32.7569(13), b = 12.2631(5), c = 14.2895(5) A, beta = 104.135(2) degrees , and Z = 2. Polyanion 1 consists of (beta-Fe(2)GeW(10)O(37)) and (gamma-GeW(10)O(36)) units linked via two Fe-O-W bridges and a central potassium ion. Two equivalent FeO(6) octahedra complete the belt of the beta-Keggin unit and link to the (gamma-GeW(10)O(36)) fragment. On the other hand, 2 consists of two {beta-Fe(2)GeW(10)O(37)(OH)(2)} units with four bridging hydroxo groups linking the four Fe(3+) ions, forming an eight-membered ring. The magnetic properties of CsK-1 and CsKNa-2 have been studied by magnetic susceptibility and magnetization measurements and fitted according to an isotropic exchange model. Both polyanions 1 and 2 exhibit diamagnetic ground states with a small amount of paramagnetic impurity. Electrochemistry studies on 1 and 2 were carried out in a pH 5 acetate medium. The two polyanions have in common the simultaneous reduction of all of their Fe(3+) centers. This observation suggests the existence of identical or almost-identical influences on these centers and their equivalence, especially in the reduced state. Controlled potential coulometry results indicate the presence of two Fe(3+) centers in 1 and four in 2. The splitting of the tungsten wave of 1 compared to the single tungsten wave of 2 is attributed to a difference in acid-base properties of the polyanions. Voltammetric peak-potential shifts as a function of pH were studied in the case of 2.

Antisymmetric exchange and pseudo Jahn–Teller instability in spin-frustrated metal clusters

In this article, we analyze the interplay between the antisymmetric (AS) exchange interaction and Jahn–Teller (JT) vibronic coupling in spin-frustrated systems with triangular units. AS exchange in these systems creates a strong first order magnetic anisotropy related to the orbitally degenerate frustrated ground state that is analyzed for the unique structure of the cluster anion present in K 6 [ V 15 IV As 6 O 42 ( H 2 O ) ] · 8 H 2 O (V 15 cluster) exhibiting layers of different magnetization. Spin-frustration inherently related to the orbital degeneracy creates a structural instability that is shown to be competitive to the AS exchange. The vibronic pseudo JT coupling arising from the modulation of the exchange interaction by the double degenerate molecular vibrations is shown to reduce AS exchange giving rise to a restoration of magnetization quenched by AS exchange. This leads to an essential reduction of the magnetic anisotropy in spin-frustrated triangular clusters so that in the limit of strong JT coupling the isotropic exchange model becomes adequate.

Imidazolate bridged Cu(ii)–Cu(ii) and Cu(ii)–Zn(ii) complexes of a terpyridinophane azamacrocycle: a solution and solid state study

The dinuclear Cu2+ and Zn2+ as well as the mixed Cu2+-Zn2+ complexes of a 5,5''-pentaazaterpyridinophane ligand (L) are able to incorporate imidazolate (Im-) as a bridging ligand. The crystal structure of [Cu(2)L(Im)(Br)(H2O)](CF(3)SO(3))(2).3H2O (1) shows one copper coordinated by the three pyridine nitrogens of the terpyridine unit, one nitrogen of the imidazolate bridge (Im-) and one bromide anion occupying the axial position of a distorted square pyramid. The second copper atom is coordinated by the remaining imidazolate nitrogen, the three secondary nitrogens at the centre of the polyamine bridge and one water molecule that occupies the axial position. Magnetic measurements have been performed in the 2.0-300.0 K temperature range. Experimental data could be satisfactorily reproduced by using an isotropic exchange model H = -JS(1)S(2) with J = -52.3 cm(-1) and g = 2.09. Potentiometric studies have provided details of the speciation and stability constants for the mixed Cu2+-L-HIm, Zn2+-L-HIm (HIm = imidazole) and Cu2+-Zn2+-L-HIm systems. The apparent stability constant obtained at pH = 9 for the addition of imidazole to the dinuclear Cu2+ complexes is one of the highest so far reported (log K = 7.5). UV-Vis spectroscopy and paramagnetic NMR data show that imidazole coordinates to the Cu2+ ions as a bridging imidazolate ligand from pH 5 to 10. Electrochemical reduction of the Cu2+-Zn2+-L complex occurs in two successive one-electron per copper ion quasi-reversible steps. The formal potential of the Cu2+-Zn2+-L/Cu+-Zn2+-L couple is close to that of SOD. The IC50 values measured at pH 7.8 by means of the nitro blue tetrazolium method show significant SOD activity for the dinuclear Cu2+ complexes (IC50 = 2.5 microM) and moderate activity for the Cu2+-Zn2+ mixed systems (IC50 = 30 microM).

An antiferromagnetically coupled trinuclear Cu(II) complex containing μ( O, O′), μ( O) carboxylates and μ( O) phenoxide bridges

The trinuclear complex [L 2Cu 3(CF 3CO 2) 4] ( 1) has been synthesized and its crystal structure determined. It consists of a linear arrangement of Cu(II) centers. The central copper atom is bonded to six oxygen atoms and has a tetragonally distorted octahedral geometry, while the terminal copper atoms are bonded to three oxygen and two nitrogen atoms and show a distorted square pyramidal geometry. The complex shows di-μ( O, O′) syn– syn carboxylate bridging as well as monoatomic (μ-O) bridging, along with phenolate (μ-O) oxygen bridging. Cryomagnetic investigations in the range 2–300 K revealed an antiferromagnetic spin exchange interaction with J = −95.7 cm −1, based on the isotropic exchange model H ex = −2 J[ S 1 · S 2 + S 2 · S 3].

Trinuclear Cu(II) complexes containing peripheral ketonic oxygen bridges and a μ 3-OH core: Steric influence on their structures and existence

Two tridentate Schiff bases, HL 1(6-amino-3-methyl-1-phenyl-4-azahex-2-en-1-one), and HL 2 (6-amino-3,6-dimethyl-1-phenyl-4-azahex-2-en-1-one) on reaction with Cu(II) perchlorate in the presence of triethyl amine yielded two new trinuclear copper(II) complexes, [(CuL 1) 3(μ 3-OH)](ClO 4) 2 ( 1) and [(CuL 2) 3(μ 3-OH)](ClO 4) 2 · 0.75H 2O ( 2), whereas another tridentate ligand HL 3 (7-amino-3-methyl-1-phenyl-4-azahept-2-en-1-one) underwent hydrolysis under the same reaction conditions to result in the formation of a mononuclear complex, [Cu(bn)(pn)ClO 4] ( 3) [where bn = 1-benzoylacetonate and pn = 1,3-propanediamine]. All three complexes have been characterized by X-ray crystallography. For both 1 and 2 the cationic part is trinuclear with a [Cu 3OH] core held by three carbonyl oxygen bridges between each pair of copper(II) atoms. The structure of 3 is a monomer with a chelating 1,3-propanediamine and a benzoyl acetone moiety. Magnetic measurements of 1 and 2 have been performed in the 2–300 K temperature range. The experimental data could be satisfactorily reproduced by using an isotropic exchange model, H = − J( S 1 S 2 + S 2 S 3 + S 1 S 3), yielding as best fit parameters: J = −25.6 cm −1, g = 2.21 for 1 and J = −11.2 cm −1, g = 2.10 for 2. The EPR spectra at low temperature could be indicative of spin frustration in complex 1.

Trinuclear CuII Complexes Containing Peripheral Ketonic Oxygen Bridges and a μ3‐OH Core: Syntheses, Crystal Structures, Spectroscopic and Magnetic Properties

AbstractFour new trinuclear copper(II) complexes, [(CuL1)3(μ3‐OH)](ClO4)2·H2O (1), [(CuL2)3(μ3‐OH)](ClO4)2 (2), [(CuL3)3(μ3‐OH)](ClO4)2·H2O (3), and [(CuL4)3(μ3‐OH)](ClO4)2·H2O (4), where HL1 = 8‐amino‐4,7,7‐trimethyl‐5‐azaoct‐3‐en‐2‐one, HL2 = 7‐amino‐4‐methyl‐5‐azaoct‐3‐en‐2‐one, HL3 = 7‐(ethylamino)‐4‐methyl‐5‐azahept‐3‐en‐2‐one, and HL4 = 4‐methyl‐7‐(methylamino)‐5‐azahept‐3‐en‐2‐one, have been derived from the four tridentate Schiff bases (HL1, HL2, HL3, and HL4) and structurally characterized by X‐ray crystallography. For all compounds, the cationic part is trinuclear with a Cu3OH core held by three carbonyl oxygen bridges between each pair of copper(II) atoms. The copper atoms are five‐coordinate with a distorted square‐pyramidal geometry; the equatorial plane consists of the bridging oxygen atom of the central OH group together with three atoms (N, N, O) from one ligand whereas an oxygen atom of a second ligand occupies the axial position. Magnetic measurements have been performed in the 2–300 K temperature range. The experimental data could be satisfactorily reproduced by using an isotropic exchange model, H = –J(S1S2+S2S3+S1S3) yielding as best‐fit parameters: J = –66.7 and g = 2.19 for 1, J =–36.6 and g = 2.20 for 2, J = –24.5 and g = 2.20 for 3, andJ = –14.9 and g = 2.05 for 4. EPR spectra at low temperature show the existence of spin frustration in complexes 3 and 4, but it has not been possible to carry out calculations of the antisymmetric exchange parameter, G, from magnetic data. In frozen methanolic solution, at 4 K, hyperfine splitting in all complexes and spin frustration in complex 4 seem to be confirmed. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Cyclic trinuclear and chain of cyclic trinuclear copper(II) complexes containing a pyramidal Cu(3)O(H) core. Crystal structures and magnetic properties of [Cu(3)(mu(3)-OH)(aaat)(3)(H(2)O)(3)](NO(3))(2).H(2)O [aaat = 3-acetylamino-5-amino-1,2,4-triazolate] and ([Cu(3)(mu(3)-OH)(aat)(3)(mu(3)-SO(4))].6H(2)O)(n) [aat = 3-acetylamino-1,2,4-triazolate]: new cases of spin-frustrated systems.

New copper(II) complexes of the cyclic trinuclear type with 1,2,4-triazole ligands, [Cu(3)(mu(3)-OH)(aaat)(3)(H(2)O)(3)](NO(3))(2).H(2)O [Haaat = 3-acetylamino-5-amino-1,2,4-triazole] (1) and ([Cu(3)(mu(3)-OH)(aat)(3)(mu(3)-SO(4))].6H(2)O)(n) [Haat = 3-acetylamino-1,2,4-triazole] (2), have been prepared and characterized by X-ray crystallography and magnetic measurements. Compound 1, the first reported with the ligand (H)aaat, consists of discrete trinuclear cations, associated NO(3)(-) anions and lattice water molecules. Compound 2 consists of unusual chains of trinuclear units with a tridentate sulfato group linking the trimeric units and water molecules stabilizing the crystal lattice. In both complexes, 1 and 2, the trinuclear [Cu(3)(OH)L(3)] unit contains a pyramidal Cu(3)-mu(3)OH core, and an almost flat Cu(3)N(6) ring formed by the N,N-bridging triazolato groups. The Cu...Cu' intratrimeric distances are 3.35-3.37-3.39 A in 1 and 3.34-3.34-3.36 A in 2. The copper atoms are five-coordinated with a distorted square-pyramidal geometry. Magnetic measurements have been performed in the 1.9-300 K temperature range. In the high-temperature region (T > 90 K), experimental data could be satisfactorily reproduced by using an isotropic exchange model, H = -J(S(1)S(2) + S(2)S(3) + S(1)S(3)), with J = -194.6 cm(-1) and g = 2.08 for 1, and J = -185.1 cm(-1) and g = 2.10 for 2. The magnitude of the antiferromagnetic exchange in both complexes is discussed on the basis of their structural features by comparison with reported N,N-pheripherically bridged trinuclear systems. In order to fit the experimental magnetic data at low temperature, an antisymmetric exchange term, H(AS) = G(S(1)xS(2) + S(2)xS(3) + S(1)xS(3)), had to be introduced, with G = 27.8 (1) and 31.0 (2) cm(-1). Crystal data: C(12)H(27)Cu(3)N(17)O(14) (1) (MW = 824.13) crystallizes in the triclinic space group, P(-)1, Z = 2, with the cell dimensions a = 8.852(2) A, b = 11.491(3) A, c = 15.404(3) A, alpha = 70.43(3) degrees, beta = 75.11(2) degrees, gamma = 88.43(2) degrees, and V = 1423.8(5) A(3), D(calcd) = 1.922 g cm(-)(3); the final agreement values were R1 = 0.0822 and wR2 = 0.2300 for 4989 unique reflections. C(12)H(28)Cu(3)N(12)O(14)S (2) (MW = 787.14) crystallizes in the triclinic space group, P(-)1, Z = 2, with the cell dimensions a = 7.146(6) A, b = 14.26(1) A, c = 15.35(2) A, alpha = 109.0(9) degrees, beta = 93.6(9) degrees, gamma = 99.5(7) degrees, and V = 1448(2) A(3), D(calcd) = 1.806 g cm(-3); the final agreement values were R1 = 0.0628 and wR2 = 0.1571 for 3997 "observed" reflections.

Synthesis, structure, and magnetism of the trinuclear copper(II) complex [Cu(CuL)2][ClO4]2[H2L = 3,3′-(trimethylenedinitrilo)bis(2-butanone oxime)

The trinuclear copper(II) complex [Cu(CuL)2][ClO4]2[H2L = 3,3′-(trimethylenedinitrilo)bis-(2-butanone oxime)] has been synthesized. It crystallizes in monoclinic space group C2/c, with Z= 4, a= 14.253(1), b= 18.798(2), c= 12.675(1)A, and β= 106.92(1)°. The structure consists of trinuclear cations [Cu(CuL)2]2+ and perchlorate ions. The central copper is co-ordinated by oximate oxygens of two CuL entities and the configuration around the metal is significantly distorted from tetrahedral with a dihedral angle of 33.31 (6)°. Each terminal copper adopts an essentially planar configuration with the nitrogen atoms of L2–. The three chromophores in the cation are bent at the edges of the bridging oximate oxygens, with a dihedral angle of 55.88(9)°, to afford a butterfly shape for the cation. In the crystal the trinuclear cations interact with each other at the terminal copper, in the so-called out-of-plane mode, through oximate oxygen to form an infinite chain along the c axis. Cryomagnetic investigations in the range 80–300 K revealed a strong antiferromagnetic spin-exchange interaction within the molecule; the magnetic moment per cation is 2.00 µB at room temperature but converges to 1.80 ± 0.01 µB at 170 K. The exchange integral J, based on the isotropic exchange model ℋ=–2∑jijŜiŜj, is –290.8 cm–1.

ESR aspects of the Mn2+− Cu2+ coupled pair

Recent ESR experimental data on Mn 2+−Cu 2+ pairs, obtained by doping with manganese the dimeric Cu (pyridine N-oxide) Cl 2·.H 2O complex, are interpreted by means of a strong isotropic exchange model. The g values of the pair and the hyperfine constants at the manganese and copper nuclei, for the observed effective spin S = 2, are given by the relations g = 7 6 g Mn− 1 6 g Cu , A ́ Mn = 7 6 A Mn, A ́ Cu = − 1 6 A Cu , where g Mn, g Cu, A Mn and A Cu are the characteristic single-ion values for the quoted constants. The antiferromagnetic coupling between the Mn 2+ spin ( 5 2 ) and the Cu 2+ spin ( 1 2 ) is responsible for the observed g values, all measurably lower than the free electron value. Also to be noted are the enhancement of the hyperfine constant at the manganese nucleus, and the curious change of sign for that at the copper nucleus.

Relaxation of Local-moment Nuclei in Metals

The theory of nuclear-spin relaxation for magnetic impurities in non-magnetic metals is reviewed and compared with experiment. Both local-moment as well as localized spin-fluctuation limits of the impurity problem are discussed. The influence of the Kondo effect and of impurity-impurity interactions are briefly considered. The conditions under which observation of the local-moment NMR is practical and the information that can be obtained from such experiments are developed in some detail. Illustrative examples are drawn from recent work on Cu:55Mn and W:59Co alloys. Finally, the limitations of the isotropic conduction electron-local moment exchange model are explored.

Low-Temperature Sublattice Magnetization Of Antiferromagnetic CrCl3

The temperature dependence and the magnetic-field dependence of the Cr${\mathrm{Cl}}_{3}$ sublattice magnetization have been studied in the temperature range 0.39-4.02\ifmmode^\circ\else\textdegree\fi{}K by means of the ${\mathrm{Cr}}^{53}$ nuclear magnetic resonance. The experimental results are in quantitative agreement with predictions of the isotropic Heisenberg exchange model in the low-temperature spin-wave approximation. The antiferromagnetic interlayer-exchange interaction in this hexagonal layer-type crystal is shown to be sufficiently weak, compared to the ferromagnetic intralayer-exchange interaction, to justify its treatment as an effective anisotropy field. In addition to the exchange interactions, both dipolar and single-ion, $D{({S}^{y})}^{2}$, anisotropies are required to fit the data. The resulting two-dimensional ferromagnetic model accurately accounts for the strong temperature dependence of the sublattice magnetization and the approach of this dependence to linearity with increasing temperature. A fit of the theory to the experimental data gives an intralayer exchange constant $\frac{J}{k}=4.52\ifmmode\pm\else\textpm\fi{}0.05\ifmmode^\circ\else\textdegree\fi{}$K. The intensity enhancement of the ${\mathrm{Cr}}^{53}$ resonance in weak external magnetic fields, as well as in zero field, is discussed. The strong field-induced enhancement is explained by a domain rotation model which is based on the torque exerted by the driving field ${H}_{1}$ on the net field-induced magnetization, in the presence of a restoring force provided by a weak magnetocrystalline anisotropy ($K{sin}^{2}3\ensuremath{\theta}$) in the (001) plane. Comparison of calculated and observed intensities as a function of field strength yields $K=150\ifmmode\pm\else\textpm\fi{}50$ ergs/${\mathrm{cm}}^{3}$ and a spin flopping transition in the (001) plane at a field strength of \ensuremath{\sim} 160 Oe. The enhancement mechanism in external fields leads to selective excitation resulting in observable splittings of the nuclear resonance in polycrystalline samples when ${H}_{1}\ensuremath{\parallel}H$. The parallel magnetic susceptibility of the Cr${\mathrm{Cl}}_{3}$ spin system has been calculated from these splittings at several temperatures between 1 and 4\ifmmode^\circ\else\textdegree\fi{}K. The results (e.g., 0.28\ifmmode\pm\else\textpm\fi{}0.01 emu/mole at 4.00\ifmmode^\circ\else\textdegree\fi{}K) agree within experimental error with susceptibilities calculated from the spin-wave model using the interaction constants obtained from the temperature dependence of the sublattice magnetization.