Christopher P. Landee

Synthesis, X-ray structures and magnetic properties of linear chain 4-cyanopyridine compounds: [Cu(4-CNpy)4(H2O)](ClO4)2 and M(4-CNpy)2Cl2 (M Mn, Fe, Co, Ni, Cu)

Abstract The synthesis, structures and magnetic properties of linear chain complexes of metal ions with 4-cyanopyridine are reported (4-CNpy=4-cyanopyridine): [Cu(4-CNpy) 4 (H 2 O)](ClO 4 ) 2 and M(4-CNpy) 2 Cl 2 (MMn, Fe, Co, Ni, Cu). All compounds crystallize in monoclinic systems. Crystallographic data for [Cu(4-CNpy) 4 (H 2 O)](ClO 4 ) 2 ] n ( 1 ): space group P 2 1 / c with a = 13.225(3), b = 10.567(2), c = 20.522(3) A , β = 99.71(2)°, V = 2826.7(9) A 3 , Z = 4, D calc = 1.640 Mg m −3 , μ = 3.430 mm −1 . The compound contains primer CuL 4 units with the ligands bounded to copper ions through the ring nitrogens. These units are weakly linked into chains through semi-coordinated bonds between the nitrile nitrogen and an adjacent copper ion via the axial sites. The magnetic data show only a weak antiferromagnetic interaction ( θ CW = −0.52(1) K). Crystallographic data for [Cu(4-CNpy) 2 Cl 2 ] n ( 2 ): space group P 2 1 / n with a = 3.779(2), b = 25.711(12), c = 7.104(4) A , β = 95.98(4)°, V = 686.48(4) A 3 , Z = 2, D calc = 1.658 Mg m −3 , μ = 5.575 mm −1 . Crystallographic data for [Mn(4-CNpy) 2 Cl 2 ] n ( 6 ): space group P 2 1 / c with a = 3.700(2), b = 7.198(2), c = 26.520(5) A , β = 92.13(3)°, V = 705.8(4) A 3 , Z = 2, D calc = 1.572 Mg m −3 , μ = 1.303 mm −1 . Both dichloride compounds similar structures, consisting of chains of metal ions bibridged by chlorides with the 4-CNpy ligands coordinated to the metal ions in the axial sites through the pyridine nitrogens. The difference in space groups is caused by the different modes of packing the chains into 3D lattices. Infrared spectra and powder X-ray diffraction patterns indicate that the remaining compounds form an isostructural series. The powder magnetic susceptibilities have been measured between 2 and 300 K for all compounds. Antiferromagnetic exchange (Cu, Mn) or ferromagnetic exchange (Ni, Co, Fe) exists within the chains. The Ni, Co and Fe compounds order antiferromagnetically at 7.2(2), 2.1(2) and 4.6(2) K, respectively. Magnetic field induced transitions have been observed in these three compounds below their ordering temperatures. The copper and manganese data have been fit to models for Heisenberg antiferromagnetic linear chain with S = 1 2 and S = 5 2 with exchange constants J Co / k = −13.7(2) K and J Mn / k = −0.5 7(1) K, respectively. The nickel and cobalt data have been fit to models for ferromagnetic linear chains, with the nickel compound corresponding to a S = 1 Heisenberg model with exchange constant J / lk = +4.8(2) K, and the cobalt compound to a S = 1 2 ferromagnetic Ising linear chain with exchange constant J / k = + 6.6(2) K. The iron data has not been successfully to fit to any model. It is concluded that the total magnetic interaction between the chains has been increased by approximately 50% by the substitution of 4-CNpy for pyridine.

Transition metal halide salts of 2-amino-3-methylpyridine: synthesis, crystal structures and magnetic properties of (3-MAP)2CuX4 [3-MAP=2-amino-3-methylpyridinium; X=Cl, Br]

Abstract The reaction of CuX2·nH2O with HX [X=Cl, Br] and 2-amino-3-methylpyridine in aqueous solution gave compounds of the formula (3-MAP)2(CuX4) [3-MAP=2-amino-3-methylpyridinium]. The compounds have been characterized by single-crystal X-ray diffraction. The complexes crystallize as salts with the 3-MAP cations separating layers of tetrahalometallate anions. Both compounds have two independent molecules in their asymmetric units. Weak antiferromagnetic interactions are observed, which suggest a primarily low-dimensional magnetic exchange network.

Review: A gentle introduction to magnetism: units, fields, theory, and experiment

We present an introduction to the workings, units of measure, and general properties of magnetic materials. This is intended as a “primer to interpretation of magnetic data” for those who are entering the field, or those who are encountering magnetic measurements in the literature. We expect this work will serve as an initial guide to the reader to familiarize them with the basics in the hope that those working in the field of magnetochemistry will wish to explore additional, more detailed literature as their specific investigations demand. Topics covered include: magnetic fields and units (SI and cgs), paramagnetism (magnetization and magnetic susceptibility), Curie and Curie–Weiss behavior, magnetic exchange interactions, magnetic anisotropy, dimeric systems and exchange-coupled networks (including chains, ladders, and layers), and long-range order. Graphical Abstract

Synthesis, structure, and magnetic properties of bis(3-amino-2-chloropyridinium) tetrahalocuprate(II) [halide = Cl or Br]

The reaction of CuX2 (X = Cl or Br) with 3-amino-2-chloropyridine in aqueous acids (HX; X = Cl or Br) yields bis(3-amino-2-chloropyridinium)tetrachlorocuprate(II) and bis(3-amino-2-chloropyridinium)tetrabromocuprate(II). Both compounds have been characterized by IR, powder X-ray diffraction, single-crystal X-ray diffraction and temperature dependent magnetic susceptibility. The compounds are isomorphous and exhibit weak antiferromagnetic interactions.

Strong rail spin 1/2 antiferromagnetic ladder systems: (dimethylammonium)(3,5-dimethylpyridinium)CuX4, X = Cl, Br.

The mixed cation salts, (dimethylammonium)(3,5-dimethylpyridinium)CuX4 (X = Cl, Br), henceforth (DMA)(35DMP)CuX4, are new examples of spin-ladders based on nonbonded halide...halide interactions between CuX4(2-) anions. In these structures, double rows of the CuX4(2-) anions are sheathed by the 35DMP(+) cations, while the edges are capped by the DMA(+) cations. For the Br salt, the Br...Br contacts that define the rungs of the ladder are 4.017 A in length, while those that define the rails are 3.983 A. For the Cl salt, the corresponding lengths are 3.967 and 4.045 A. The susceptibility data for the Br salt exhibits a maximum at approximately 5.5 K, and fitting the data to the spin 1/2 antiferromagnetic ladder model yields 2J(rail)/k = -7.95 K and 2J(rung)/k = -4.07 K. The exchange coupling is much weaker in the Cl salt, no maximum in chi is observed down to 1.8 K, and the corresponding exchange constants are -1.59 and -1.25 K, respectively. An analysis is made of the structural factors involved in the J(rung) pathway.

Structures and magnetic properties of transition metal complexes of 1,3,5-benzenetricarboxylic acid

Treatment of the potassium salt of 1,3,5-benzenetricarboxylic acid with transition metal ions in aqueous solution produced a series of coordination complexes. Reaction of Ni(ClO4)2·6H2O with K3[C6H3(CO2)3] gave complexes 1, Ni3[C6H3(CO2)3]2(H2O)14·4H2O. The compound crystallized as trimers with two terminal Ni(H2O)5 units bridged by two benzenetricarboxylate ligands to a central Ni(H2O)4. Magnetic susceptibility data indicate the presence of either weak antiferromagnetic interactions between metal ions, or single ion anisotropy. Reaction of Co(ClO4)2·6H2O with K3[C6H3(CO2)3] gave complexes 2, Co3[C6H3(CO2)3]2(H2O)12. The complexes exists as a pendant chain coordination polymer with both bridging and terminal Co2+ ions. Each cobalt center is also coordinated to four water molecules. A systematic linear decrease in effective moment is observed below 100 K. This decrease can be understood entirely in term of crystal field and spin-orbit coupling effects on the individual Co(II) ions and implies magnetic interaction are weak. Magnetic susceptibility data were also obtained for the Co–BTCA complexes 3, Co(BTCA)(py)2·0.67py. These data can also be described as arising solely from single-ion effects due to the non-cubic environment around the Co2+ ion with no evidence of the presence of magnetic interactions. Reaction of Mn(ClO4)2·6H2O with K3[C6H3(CO2)3] gave complexes 4, Mn3(C6H3(CO2)3)2·8H2O. Magnetic susceptibility data indicate weak antiferromagnetic interactions and suggest that the complexes have a low dimensional structures.

Quantum linear magnetic chains: structure and magnetic behavior of (2-methylpyrazine)copper(II) nitrate

Abstract The X-ray structure, susceptibility and magnetization of (2-methylpyrazine)copper(II) nitrate are reported. The compound crystallizes in the space group Pmna as a coordination polymer of Cu(II) ions bridged by 2-methylpyrazine. The resulting chains are magnetically well isolated and the susceptibility of the compound is well modeled as a one-dimensional Heisenberg antiferromagnet with 2J=9.5(1) K. High-field magnetization studies to 30 T confirm the value of the exchange constant.

Magnetic studies of the two-dimensional, S=1/2 Heisenberg antiferromagnets (5CAP)2CuCl4 and (5MAP)2CuCl4

The magnetic materials (5CAP)2CuCl4 and (5MAP)2CuCl4 form effective square planar lattices of CuCl42− anions. The Cu2+ ions have weak antiferromagnetic interactions in the plane through Cl–Cl contacts. These planes are well separated by the 5MAP and 5CAP organic groups. High temperature susceptibility measurements give J=0.57 K for (5CAP)2CuCl4 and J=0.38 K for (5MAP)2CuCl4. χ(T,H=0) for these materials has a rounded maximum at Tpeak=1.1 K for (5CAP)2CuCl4 and Tpeak=0.62 K for (5MAP)2CuCl4. A kink in χ(T,H=0) for the (5MAP)2CuCl4 suggests a phase transition to three-dimensional long range order at TC=0.44 K. Measurements of χ(T,H) show that the saturation field HC for the transition to a fieldinduced saturated state is 3.78 T for (5CAP)2CuCl4 and 2.17 T for (5MAP)2CuCl4. Thus these materials are excellent model systems in which to explore the full H–T phase diagram of the two-dimensional S=1/2 Heisenberg antiferromagnet.

New Square S=1/2 Heisenberg Antiferromagnetic Lattices: Pyridinium Tetrahalocuprates and Bispyrazinecopper(II) Tetrafluoroborate

Abstract As part of a program to create new low dimensional S=1/2 antiferromagnetic Heisenberg compounds with moderate exchange, we have prepared four new copper complexes with square magnetic lattices. The complexes L2[CuX4] have been prepared (L = 2-amino-5-methylpyridinium, X = Cl, Br; L = 2-amino-5-chloropyridinium, X = Br). The compounds are isomorphous, giving monoclinic crystals; space group C2/c. The tetrahalometallate ions lie in C-centered layers, forming a square magnetic lattice. The layers are separated by the organic cations. Preliminary analysis of the magnetic data shows that the exchange constant, J, increases in the order Br > Cl and 2-amino-5-chloropyridinium > 2-amino-5-methylpyridinium. A new member of the pyrazine bridged family M(pyrazine)2X2 has also been prepared and studied. Cu(pz)2 (BF4)2 also crystallizes in the space group C2/c. Analysis of the temperature dependence of the susceptibility yields and exchange value of −7.98 K, similar to that found for the known Cu(pz)2(ClO4)2 ...

Magnetic properties of (C6H5CH2C4H11N2Cl)2CuCl4 and (CH2OHCH2NH3)2CuCl4

The two title compounds are two‐dimensional magnetic systems with ferromagnetic interlayer exchange coupling. Magnetic susceptibility measurements ferromagnetic interlayer exchange coupling. Magnetic susceptibility measurements yield J/k=4.59 and 13.0 K for the two compounds, respectively. A smooth correlation between J and the intralayer Cu‐Cu distance is observed. The latter compound orders at TN∼8 K, while no evidence for long‐range order in the former is observed down to 1.4 K. The electron paramagnetic resonance linewidths of (C6H5CH2C4H11N2Cl)2CuCl4 are abnormally large with a dominant cos2 θ angular dependence. The assumption that the linewidths are dominated by spin anisotropies leads to an estimate of ‖dz/J‖≂0.14.