Uniform Antiferromagnetic Chain Research Articles

Copper(II) halide complexes of aminopyridines: Synthesis, structure, and magnetic behavior of neutral compounds of 5-IAP: (5-IAP)2CuCl2·H2O, [(5-IAP)2CuBr2]2, (5-IAP)2CuBr2 and (5-IAP)3CuCl2·nH2O (5IAP = 2-amino-5-iodopyridine)

Reaction of 2-amino-5-iodopyridine (5IAP) with copper(II) bromide or chloride in alcoholic solution led to four coordination complexes: [(5-IAP)2CuCl2](H2O) (1), (5-IAP)2CuBr2 (2), [(5-IAP)2CuBr2]2 (3) and [(5-IAP)3CuCl2](H2O)n (4). The compounds were characterized by single crystal X-ray diffraction and variable temperature magnetic susceptibility measurements. 1 crystallizes as monomeric units with the 5IAP ligands in the syn-conformation and one lattice water molecule. 2 also exhibits syn-conformation 5IAP ligands, but forms a common bromide bibridged dimer structure. 3 is a conformational polymorph of 2 with anti-oriented 5IAP ligands. Long Cu⋯Br contacts link the molecules into chains. 4 crystallizes with 1.67 lattice water molecules, disordered over three positions. All four compounds exhibit strong halogen bonding. Variable field and temperature magnetic data were obtained on 1–3. 1 is paramagnetic, with no indication of magnetic exchange down to 1.8 K. 2 is well modeled as an antiferromagnetic dimer (J/kB = −22 K) with significant antiferromagnetic interactions between the dimers. 3 is well modeled as a uniform antiferromagnetic chain (J/kB = −6 K) with weak interchain interactions.

Synthesis, structure, and magnetic properties of a family of copper(II) complexes and salts of isoquinoline: (Isoquinoline)nCu(X)2 [X = Cl, Br] and (isoquinolinium)2CuX4(H2O)n [X = Cl, Br; n = 0,1

The compounds (iQuin)2CuX2 (X = Br (1), Cl (2)), (iQuinH)2CuBr4·H2O (3) and two polymorphs of (iQuinH)2CuCl4 (4, 5) (iQuin = isoquinoline) have been prepared and studied via X-ray crystallography and variable temperature magnetic susceptibility measurements. Compound 1 crystallizes in the monoclinic space group P21/n while 3, 4 and 5 all crystallize in the triclinic space group P1¯). Magnetic susceptibility measurements for 1 are best fit by an antiferromagnetic alternating chain model (J/kB = −56.6(2) K, J′/kB = 5.1(8) K) although the crystal structure suggests a uniform chain. Compound 2 is well described as an isolated uniform antiferromagnetic chain (J/kB = −25.6(1) K), while 3 agrees well with the antiferromagnetic rectangular model (J/kB = −5. 03(2) K, J′/kB = −1.0(1) K). The two polymorphs, 4 and 5, exhibit distinctly different behavior as 4 is well described by the 2D-square antiferromagnetic layer model (J/kB = −4.24(2) K), but 5 crystallizes as well isolated magnetic dimers (J/kB = −15.8(1) K). Magnetic superexchange is proposed to occur via either the bihalide or two-halide pathways. Finally, compound 6, isoquinolinium tribromide, was isolated as a biproduct of the synthesis of 3 and was characterized via X-ray diffraction. The formation of the tribromide ion in situ provides support for the mechanism of electrophilic aromatic bromination reactions which have been previously observed in syntheses similar to that of 3.

Synthesis, structure and magnetic properties of copper(II) complexes of 2-amino-5-trifluoromethylpyridine (5-TFMAP): (5-TFMAP)2CuX2 (X = Cl or Br), (5-TFMAP)3CuBr2 and [(5-TFMAP)2CuBr2](5-TFMAPH)Br

Reaction of 2-amino-5-trifluoromethylpyridine (5-TFMAP) with copper halides in water or alcohol solvents produced four coordination complexes: (5-TFMAP)2CuX2, (X=Cl (1) or Br (2)), (5-TFMAP)3CuBr2 (3) and [(5-TFMAP)2CuBr2](5-TFMAPH)Br (4). In all compounds, the 5-TFMAP ligands are coordinated to the Cu(II) ion through the pyridine nitrogen atom in a trans-geometry and a nearly square planar environment is observed in 1, 2 and 4. In 3, coordination of an additional 5-TFMAP ligand results in a distorted trigonal bipyramidal complex. All complexes are characterized by single-crystal X-ray diffraction. Compound 1 crystalizes as a chain structure via pairs of long Cl…Cu contacts between molecules and the temperature dependent magnetic susceptibility data has been fit to an alternating chain model, resulting in J=−5.97(1)K and J′=−0.95(4)K. Compound 2 exhibits weaker antiferromagnetic interactions, also via the bihalide superexchange pathway, resulting in J=−3.02(2)K from a fit to the uniform chain model. The structure of compound 3 also exhibits a uniform antiferromagnetic chain, but via the two-halide pathway (short Br…Br contacts) and susceptibility data exhibit significantly stronger interactions (J=−31.05(5)K). Compound 4 is not stable in air and loses a molecule of 5-TFMAP from the crystal lattice upon standing.

Importance of Halogen···Halogen Contacts for the Structural and Magnetic Properties of CuX2(pyrazine-N,N′-dioxide)(H2O)2 (X = Cl and Br)

The structural and magnetic properties of the newly crystallized CuX(2)(pyzO)(H(2)O)(2) (X = Cl, Br; pyzO = pyrazine-N,N'-dioxide) coordination polymers are reported. These isostructural compounds crystallize in the monoclinic space group C2/c with, at 150 K, a = 17.0515(7) Å, b = 5.5560(2) Å, c = 10.4254(5) Å, β = 115.400(2)°, and V = 892.21(7) Å(3) for X = Cl and a = 17.3457(8) Å, b = 5.6766(3) Å, c = 10.6979(5) Å, β = 115.593(2)°, and V = 950.01(8) Å(3) for X = Br. Their crystal structure is characterized by one-dimensional chains of Cu(2+) ions linked through bidentate pyzO ligands. These chains are joined together through OH···O hydrogen bonds between the water ligands and pyzO oxygen atoms and Cu-X···X-Cu contacts. Bulk magnetic susceptibility measurements at ambient pressure show a broad maximum at 7 (Cl) and 28 K (Br) that is indicative of short-range magnetic correlations. The dominant spin exchange is the Cu-X···X-Cu supersuperexchange because the magnetic orbital of the Cu(2+) ion is contained in the CuX(2)(H(2)O)(2) plane and the X···X contact distances are short. The magnetic data were fitted to a Heisenberg 1D uniform antiferromagnetic chain model with J(1D)/k(B) = -11.1(1) (Cl) and -45.9(1) K (Br). Magnetization saturates at fields of 16.1(3) (Cl) and 66.7(5) T (Br), from which J(1D) is determined to be -11.5(2) (Cl) and -46.4(5) K (Br). For the Br analog the pressure dependence of the magnetic susceptibility indicates a gradual increase in the magnitude of J(1D)/k(B) up to -51.2 K at 0.84 GPa, suggesting a shortening of the Br···Br contact distance under pressure. At higher pressure X-ray powder diffraction data indicates a structural phase transition at ∼3.5 GPa. Muon-spin relaxation measurements indicate that CuCl(2)(pyzO)(H(2)O)(2) is magnetically ordered with T(N) = 1.06(1) K, while the signature for long-range magnetic order in CuBr(2)(pyzO)(H(2)O)(2) was much less definitive down to 0.26 K. The results for the CuX(2)(pyzO)(H(2)O)(2) complexes are compared to the related CuX(2)(pyrazine) materials.

A 1D coordination polymer built on asymmetric μ1,1,3-azide bridge: from unusual topology to magnetic properties and Cu(ii)/Cu(i) redox reversibility

An azide-bridged Cu(II) one dimensional polymer was synthesized in a one pot reaction in the presence of ammonia. The refined crystal structure evidenced the unusual asymmetric μ1,1,3 coordination mode of the azide ion leading to a {Cu(μ1,1,3-N3)(μ1-N3)(NH3)0.8(H2O)1.2}n chain. The redox process of this complex was studied by cyclic voltammetry evidencing the Cu(II)/Cu(I) reversibility. Magnetic measurements were interpreted as a uniform antiferromagnetic chain (J = −15.2 cm−1) holding rather strong inter-chain exchange (zJ′ = −2.8 cm−1). Multireference difference dedicated configuration interaction (DDCI) calculations confirmed the non-negligible intensity of inter-chain interactions and evidenced a strong influence of the type of coordinated solvent, NH3 or H2O, on the nature and magnitude of the magnetic exchange.

ChemInform Abstract: Description of Anhydrous (Black) Dioptase as a S = 1/2 Uniform Antiferromagnetic Chain System.

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Description of Anhydrous (Black) Dioptase as a S = 1/2 Uniform Antiferromagnetic Chain System

AbstractThe structural and magnetic properties of anhydrous (black) dioptase, CuSiO3, were investigated by carrying out single‐crystal X‐ray diffraction, specific heat, and magnetic susceptibility measurements as well as by performing spin dimer analysis based on extended Hückel tight‐binding calculations. Our spin dimer analysis indicates that black dioptase is a S = 1/2 uniform antiferromagnetic chain system with the interchain spin exchange weaker than the intrachain exchange by three orders of magnitude. Specific heat and magnetic susceptibility measurements show that the intrachain exchange is –174(2) K, and the interchain exchange is of the order of ~2 K. They lead to long‐range magnetic ordering at about 5 K as evidenced by a broad anomaly in the specific heat.

Cooperative Jahn–Teller distortion leading to the spin-1/2 uniform antiferromagnetic chains in triclinic perovskites AgCuF 3 and NaCuF 3

Polycrystalline samples of AgCuF 3, isostructural with NaCuF 3, were synthesized by solid state reaction and characterized by powder X-ray diffraction. The magnetic properties of AgCuF 3 and NaCuF 3 were examined by measuring their magnetic susceptibilities and evaluating their spin exchange interactions. The three-dimensional CuF 3 network of corner-sharing CuF 6 octahedra present in AgCuF 3 and NaCuF 3 shows a cooperative Jahn–Teller distortion such that their magnetic susceptibilities above 50 K are well described by an S = 1/2 Heisenberg uniform antiferromagnetic chain model with average spin exchange of J/ k B ≈ −300 and −180 K, respectively. The relative strengths of these interactions are well reproduced by spin dimer analysis based on tight-binding calculations, but not by mapping analysis based on first principles density functional calculations.

On the Anisotropy of the Magnetic Properties of CsYbZnSe3

DC magnetic susceptibility measurements on CsYbZnSe 3 show a broad magnetic transition at approximately 10 K and pronounced differences between zero-field-cooled and field-cooled data that lead to experimental effective magnetic moments of 4.26(5) BM and 4.39(4) BM, respectively. Specific heat measurements confirm that there is neither long-range ordering nor a phase transition between 1.8 and 380 K. First-principles electronic structure calculations with and without inclusion of spin-orbit coupling effects show that the spins of CsYbZnSe 3 prefer to orient along [010] rather than along either [100] or [001] of this orthorhombic material and that the spin exchange between adjacent Yb3+ ions along [100] is substantially antiferromagnetic. The magnetic properties of CsYbZnSe 3 are best described by an Ising uniform antiferromagnetic chain model.

Mn(dca)2(pym)2 and Mn(dca)2(pym)(H2O) {dca = dicyanamide; pym = pyrimidine}: New coordination polymers exhibiting 1- and 2-D topologies

Two new coordination polymers comprised of Mn(2+), N(CN)(2)(-) (dicyanamide, herein denoted dca) and pyrimidine (pym) have been synthesized and structurally and magnetically characterized. Mn(dca)(2)(pym)(2), , crystallizes in the orthorhombic space group Pbcn and forms trans bi-bridged Mn-(micro(1,5)-dca)(2)-Mn ribbons that extend along the b-axis of the unit cell. Two terminally bonded pym ligands are trans-coordinated to the Mn center. Adjacent chains interdigitate in an undulating fashion presumably due to a templating effect imposed by the pym ligands where N-atoms of consecutive pym rings stack parallel to the chain axis. Mn(dca)(2)(pym)(H(2)O), , which crystallizes in the monoclinic space group P2(1)/c, has a unique interdigitated 2D network that consists of double-bridged [Mn(2)(dca)(2)(pym)(2)(H(2)O)(2)](2+)"dimers" that are connected via single-bridging dca ligands. Each MnN(5)O octahedron is comprised of a coordinated H(2)O, a monodentate pym ligand, and four micro(1,5)-bridging dca anions. The magnetic data for were fitted to a uniform antiferromagnetic chain model which yielded J/k(B) = -0.34(1) K. In contrast, is best described as an alternating chain owing to the presence of both single- and double-bridging dca anions; a least-squares fit afforded J(a) = -0.43(1) and J(b) = -0.20(2) K, respectively. A transition to long-range magnetic ordering was observed for below approximately 2.4 K in addition to a field-induced spin flop transition at 15.6 kOe (1.7 K).

Pressure Effect on Organic Radicals

The magnetic properties of galvinoxyl (4-[3,5-bis(1,1-dimethylethyl)-4-OXO-2,5-cyclohexadiene-1-ylidenemethyl]-2,6-bis (1,1-dimethyl-ethyl)-phenoxy) and F5PNN (2-pentafluorophenyl-4,4,5,5-tetramethyl-4,5-dihydro-lH-imidazol-1-oxyl 3-oxide) under pressure are presented. We have found that the structural changes of both compounds can be suppressed by pressurization. The phase transition to a diamagnetic state of galvinoxyl can be suppressed by applied pressure. It has been found that under 7 kbar, the transition is sufficiently suppressed and the ferromagnetic interactions are preserved down to low temperature. The temperature dependence of the paramagnetic susceptibility under 7 kbar obeys the ferromagnetic chain model with 2J/kB = 25 K. The room-temperature crystal structure of F5PNN involves a uniform chain, while the low-temperature magnetism can be interpreted by the alternating antiferromagnetic chain model with 2J/K B = -5.6 K and α = 0.4. We found that under 5 kbar, the low-temperature magnetism obeys the uniform antiferromagnetic chain model with 2J/K B = -5.6 K, which is consistent with the room-temperature crystal structure.

Low-temperature susceptibilities of the quasi-one-dimensional spin-½ Heisenberg antiferromagnets (6MAP)CuCl3and (3MAP)CuCl3: Spin-Peierls transition versus broken-chain effects

The magnetic susceptibilities of the two spin-\textonehalf{} Heisenberg antiferromagnetic chain systems, 6-methyl-2-aminopyridinium trichlorocuprate(II) [(6MAP)Cu${\mathrm{Cl}}_{3}$] and 3-methyl-2-aminopyridinium trichlorocuprate(II) [(3MAP)Cu${\mathrm{Cl}}_{3}$], have been reinvestigated. Their behavior cannot be described by a simple uniform Heisenberg antiferromagnetic chain model at low temperature, but rather by either a spin-Peierls model with a paramagnetic impurity contribution or a broken-chain model. The spin-Peierls model assumes a uniform antiferromagnetic chain above the transition temperature and a weakly dimerized antiferromagnetic chain below that temperature and yields uniform-chain exchange constants $\frac{J}{k}$ of -54.5 and -59 K for (6MAP)Cu${\mathrm{Cl}}_{3}$ and (3MAP)Cu${\mathrm{Cl}}_{3}$, respectively, with the alternating exchange ratio $0.98<\ensuremath{\alpha}<1$ for both. The possible spin-Peierls transition temperatures are about 2 K. It is also possible to explain the data by a broken-chain model which gives exchanges of -50.5 and -55.6 K for (6MAP)Cu${\mathrm{Cl}}_{3}$ and (3MAP)Cu${\mathrm{Cl}}_{3}$, respectively. In this model the average chain lengths of two compounds are estimated.

Specific heat and susceptibility of the 1-dimensional [formula omitted] Heisenberg antiferromagnet Cu(Pyrazine) (NO 3) 2. evidence for random exchange effects at low temperatures

Magnetic susceptibility and specific heat data are presented on the S = 1 2 antiferromagnetic Heisenberg chain Cu(pyrazine) (NO 3) 2 in the range 0.05 K < T < 18 K. The intrachain exchange is obtained as J/ k B = -5.20(5) K by a theoretical fit to data. No evidence for a transition to a 3-d ordered state could be detected down to the lowest temperatures (corresponding to k B T/| J| ≈ 0.01). The specific heat data provide a beautiful demonstration of the predicted linear chain low-temperature behaviour, c/ R = k B T/3| J|, over one decade in temperature. Superimposed upon the behaviour for the uniform antiferromagnetic chain, the data show contributions that can be interpreted in terms of a random exchange model. It is argued that these arise from random lattice imperfections (defects) which apparently break up the chains into weakly coupled finite segments with an average length of about 200 lattice sites. The segments with an odd number of spins act as a system of highly diluted, weakly interacting, spins 1 2 . The contributions due to defects may predominate the behaviour of the susceptibility and (to a lesser extent) the specific heat at very low temperatures, in case the interchain interactions are sufficiently small. In particular, the random defects will suppress the occurence of 3-d ordering between the chains. Furthermore, they lead to random exchange phenomena similar as observed in other quasi 1-d antiferromagnetic systems. It is suggested that many of the known examples of random exchange antiferromagnets can be explained in terms of the random defect model.

Instability of the uniform antiferromagnetic chain

It is shown that in the X−Y model the uniform antiferromagnetic chain spontaneously distorts into an alternating antiferromagnet. This is a Jahn-Teller effect where the distortion lifts the degeneracy between the ground state and the k = 0 magnon.