Magnetochemistry: advances in theory and experimentation

Framework-structured weak ferromagnets are new rising stars in molecule-based magnetic materials. The framework structures are powerful carriers for long-range ordering of spins. And weak ferromagnetism due to spin canting is an effective approach for magnets because of its frequent occurrence and desired spontaneous magnetization as long as the canting angle γ is large enough. In this critical review, we provide an overview of the various framework-structured weak ferromagnets based on different grades of ligands (from mono-atom to three-atom-like ligands). Particular emphasis is given to the relationships between structural features and the properties, rational employment of the ligands, and weak ferromagnetic strategies for molecule-based magnets with exciting properties and applications (273 references).

Framework-structured weak ferromagnets

Spectroscopy and coordination chemistry of cobalt in molecular sieves

The spectral and magnetic properties of simple high spin cobalt(II) complexes are reviewed with the aim of showing how it is possible to relate the experimental data to the electronic structure and the coordination geometry of the complexes. It is hoped that this may be of help to all researchers who need to characterize cobalt(II) complexes or use cobalt(II) as a spectroscopic probe. The data relative to electronic, EPR, MCD, NMR spectra as well as to magnetic susceptibility measurements are interpreted within an Angular Overlap approach. Energy level diagrams are employed to calculate the electronic spectra and then, applying the relevant perturbation μ, g, A values are calculated for different chromophores in different coordination environments. The underlying theory is sketched paying in every case much attention to show the possibility of calculating the electronic properties in low symmetry environments. An extensive compilation of available experimental data is presented. The chromophores are classified according to the coordination number and according to the donor atom set.

Spectral-structural correlations in high-spin cobalt(II) complexes

Two-dimensional transition-metal coordination polymers [MCl2(bpy)] (M = Fe, Co, Ni; Co/Ni, bpy = 4, 4‘-bipyridine) have been synthesized in superheated water solutions. The hydrothermal routes resulted in four compounds [FeCl2(bpy)](I), [CoCl2(bpy)](II), [NiCl2(bpy)](III), and [(Co/Ni)Cl2(bpy)](IV). Compounds I−IV are isostructural and belong to orthorhombic crystal system, space group Cmmm (No. 65). Crystal data for I:  a = 11.929(2) A, b = 11.447(2) A, c = 3.638(1) A, V = 496.77(18) A3, Z = 2. Crystal data for II:  a = 11.993(2) A, b = 11.374(2) A, c = 3.611(1) A, V = 492.57(18) A3, Z = 2. The structure is a noninterpenetrating two-dimensional network containing transition-metal centers octahedrally coordinated by four bridging chlorine and two bpy ligands at trans positions. The adjacent bpy ligands within a single layer are parallel to each other at a distance of 3.6 A. Spontaneous antiferromagnetic ordering was found in all compounds from the magnetic susceptibility χ(T) measurements under low fields...

Two-dimensional coordination polymers with one-dimensional magnetic chains: Hydrothermal synthesis, crystal structure, and magnetic and thermal properties of∞2 [MCl2(4,4′-bipyridine)] (M = Fe, Co, Ni, Co/Ni)

The crystal structure at room temperature and the heat-capacity and magnetic susceptibilities at low temperatures of single crystals of [${(\mathrm{C}{\mathrm{H}}_{3})}_{3}$NH]Co${\mathrm{Cl}}_{3}$ \ifmmode\cdot\else\textperiodcentered\fi{} 2${\mathrm{H}}_{2}$O are reported. The orthorhombic crystals belong to the space group $\mathrm{Pnma}$ with $a=16.671(3)$ \AA{}, $b=7.273(1)$ \AA{} $c=8.113(2)$ \AA{}, and $Z=4$. The structure consists of chains of edge-sharing trans- [Co${\mathrm{Cl}}_{4}$${(\mathrm{O}{\mathrm{H}}_{2})}_{2}$] octahedra running parallel to the $b$ axis. At 4.135 \ifmmode^\circ\else\textdegree\fi{}K, [${(\mathrm{C}{\mathrm{H}}_{3})}_{3}$NH]Co${\mathrm{Cl}}_{3}$ \ifmmode\cdot\else\textperiodcentered\fi{} 2${\mathrm{H}}_{2}$O undergoes a second-order phase transition. The heat-capacity data have been analyzed using Onsager's solution for the two-dimensional Ising model. The lattice contribution was taken into account by using the lattice of the isostructural [${(\mathrm{C}{\mathrm{H}}_{3})}_{3}$NH]Cu${\mathrm{Cl}}_{3}$ \ifmmode\cdot\else\textperiodcentered\fi{} 2${\mathrm{H}}_{2}$O and a corresponding states procedure. The three principal-axis single-crystal susceptibilities display an unusual amount of anisotropy. Two of the data sets (one parallel and one perpendicular to the chemical chain) have been fit to an Ising linear-chain model. A small molecular-field correction significantly improved one of those fits. The $a$-axis susceptibility displays an unusually sharp rise, with decreasing temperature, very close to ${T}_{N}$ along with the suggestion of a net moment persisting below the transition. Thus, a Dzyaloshinsky-Moriya antisymmetric exchange interaction was assumed to be operative, since it is symmetry alowed in this space group, and Moriya's molecular-field-susceptibility calculation was used to fit this data set. All four of the data sets are quite reasonably described, the specific heat and one of the susceptibility sets within experimental error, by the following set of parameters: rectangular lattice exchange parameters of $\frac{J}{k}=(7.7\ifmmode\pm\else\textpm\fi{}0.2)$ \ifmmode^\circ\else\textdegree\fi{}K and $\frac{{J}^{\ensuremath{'}}}{k}=(0.09\ifmmode\pm\else\textpm\fi{}0.01)$ \ifmmode^\circ\else\textdegree\fi{}K; spectroscopic splitting parameters of ${g}_{a}=2.95\ifmmode\pm\else\textpm\fi{}0.05$, ${g}_{b}=3.90\ifmmode\pm\else\textpm\fi{}0.01$, and ${g}_{c}=6.54\ifmmode\pm\else\textpm\fi{}0.01$; and a Dzyaloshinsky-Moriya antisymmetric exchange parameter of $|\frac{D}{k}|=(4.9\ifmmode\pm\else\textpm\fi{}0.2)$ \ifmmode^\circ\else\textdegree\fi{}K. Finally, a consistent spin structure is proposed in which the intrachain interaction is ferromagnetic with the compound ordering antiferromagnetically below 4.135\ifmmode^\circ\else\textdegree\fi{}K such that there is a small net moment in the $a$ direction.

An Anisotropic Low-Dimensional Ising System, [ ( C H 3 ) 3 NH]Co Cl 3 · 2 H 2 O: Its Structure and Canted Antiferromagnetic Behavior

Two anisotropic Heisenberg linear chain manganese compounds

The results of low‐temperature studies of the magnetic characteristics of Cs2CoCl4 are reported. Single‐crystal principal‐axes magnetic susceptibility measurements between 1.5 and 20 K have been fit to an exchange‐modified spin‐3/2 single‐ion model. Heat capacity measurements between 1 and 30 K on Cs2CoCl4 and between 4 and 25 K on Cs2ZnCl4 have been used with a corresponding states procedure to obtain the magnetic heat capacity of Cs2CoCl4. All the results generally indicate a large zero‐field splitting of 14 to 16 K with the possibility of a rhombic distortion of the CoCl42− ion. Both the susceptibility and heat capacity measurements indicate the presence of significant magnetic exchange. At the lower temperatures the heat capacity results appear to be describable by an XY linear chain model, in agreement with structural considerations in conjunction with the theoretically expected behavior of tetrahedral cobalt(II).

Low‐temperature magnetic characteristics of tetrahedral CoCl42−. III. Magnetic exchange in paramagnetic Cs2CoCl4

Magnetic susceptibility measurements have been made between 1.5 and 20°K on [(C2H5)4N]2CoCl4. The results show that the CoCl4 2− tetrahedra are oriented such that their unique axes lie in or near the (001) plane while maintaining the tetragonal symmetry exhibited by the crystal at room temperature. Values for the parameters D and E have been obtained, as well as the principal g values. The results indicate that the CoCl4 2− anions in this compound behave similarly to those doped into the isostructural material [(C2H5)4N]2ZnCl4.

Low‐Temperature Magnetic Characteristics of Tetrahedral CoCl4 2−. II. Nature of the Phase Transition in [(C2H5)4N]2CoCl4

Electron paramagnetic resonance measurements were made on single crystals of cobalt‐doped tetraethylammonium tetrachlorozincate (II) at 4.2°K. The results show that the CoCl4 2− tetrahedra are oriented in several magnetically nonequivalent directions in the host lattice, which is isostructural with the analogous cobalt compound, tetraethylammonium tetrachlorocobaltate (II). Each tetrahedron can be characterized by an effective spin S′= 12 and by the principal g values gx ′=5.44, gy ′=4.84, and gz ′=2.34. Although the individual CoCl4 2− anions exhibit only rhombic symmetry, these ions are so oriented at 4.2°K as to maintain the tetragonal axis exhibited by the crystal at room temperature.

G. E. Shankle

Papers

An Anisotropic Low-Dimensional Ising System, [ ( C H 3 ) 3 NH]Co Cl 3 · 2 H 2 O: Its Structure and Canted Antiferromagnetic Behavior

Two anisotropic Heisenberg linear chain manganese compounds

Low‐temperature magnetic characteristics of tetrahedral CoCl42−. III. Magnetic exchange in paramagnetic Cs2CoCl4

Low‐Temperature Magnetic Characteristics of Tetrahedral CoCl4 2−. II. Nature of the Phase Transition in [(C2H5)4N]2CoCl4

Low‐Temperature Magnetic Characteristics of Tetrahedral CoCl4 2−. I. g‐Tensor Orientation in Doped [(C2H5)4N]2ZnCl4