Organometallic- and organic-based materials exhibiting the technologically important property of bulk magnetism have been designed, prepared, and studied. These magnets are prepared via conventional organic chemistry methodologies and unlike conventional inorganic-based magnets do not require metallurgical processing. Furthermore, these magnets are frequently soluble in conventional solvents, and examples have saturation magnetizations exceeding that of iron metal on an Fe or mole basis. Also, magnets with critical temperatures exceeding room temperature, magnets with coercive fields exceeding that of Co5Sm, and thin-film magnets have been prepared. This article highlights the collective joint research executed in our, as well as Arthur J. Epstein's, laboratories.

Organometallic- and organic-based magnets: New chemistry and new materials for the new millennium

Metal-organic frameworks represent the ultimate chemical platform on which to develop a new generation of designer magnets. In contrast to the inorganic solids that have dominated permanent magnet technology for decades, metal-organic frameworks offer numerous advantages, most notably the nearly infinite chemical space through which to synthesize predesigned and tunable structures with controllable properties. Moreover, the presence of a rigid, crystalline structure based on organic linkers enables the potential for permanent porosity and postsynthetic chemical modification of the inorganic and organic components. Despite these attributes, the realization of metal-organic magnets with high ordering temperatures represents a formidable challenge, owing largely to the typically weak magnetic exchange coupling mediated through organic linkers. Nevertheless, recent years have seen a number of exciting advances involving frameworks based on a wide range of metal ions and organic linkers. This review provides a survey of structurally characterized metal-organic frameworks that have been shown to exhibit magnetic order. Section 1 outlines the need for new magnets and the potential role of metal-organic frameworks toward that end, and it briefly introduces the classes of magnets and the experimental methods used to characterize them. Section 2 describes early milestones and key advances in metal-organic magnet research that laid the foundation for structurally characterized metal-organic framework magnets. Sections 3 and 4 then outline the literature of metal-organic framework magnets based on diamagnetic and radical organic linkers, respectively. Finally, Section 5 concludes with some potential strategies for increasing the ordering temperatures of metal-organic framework magnets while maintaining structural integrity and additional function.

Metal-Organic Framework Magnets.

Organische und metallorganische molekulare magnetische Materialien: Designer‐Magnete

The employment of molecules, not atoms, as a basic building block to construct solids has lead to the development of new classes of materials exhibiting commercially useful properties.l,2 These include electrical conductivity, ferroelectricity as well as magnetic ordering. Molecules, in contrast to atoms, enables the modulation of the commercially useful properties by low-temperature organic-synthesis methodologies, that can lead to the improvement of the properties, and lead to the development of materials with a combination of properties that will expand their desirability. Herein, we focus solely upon work related to molecule-based magnets. Molecule-based magnets are defined as substances prepared from molecules (or molecular ions) that maintain aspects of the parent molecular framework, and magnetically order.

Molecule-based magnets

The synthesis and structural and magnetic properties of the pentanuclear complex [(Ni(bpm)2)3(Fe(CN)6)2]·7H2O (1) are reported. Complex 1 crystallizes in the tetragonal space group P43212 (No. 96) with the following cell parameters at 23(1) °C:  a = b = 12.888(2) A, c = 42.927(2) A, V = 7130(3) A3, and Z = 4. The structure was refined using 3197 unique reflections with I > 3σ(I) to give R = 0.047 and Rw = 0.038. The neutral [(Ni(bpm)2)3(Fe(CN)6)2] clusters contain two unconnected FeIII(CN)63- groups each bridged, via three facial CN groups, to three cis-Ni(bpm)2 moieties, thus making the cis-(CN)2NiII(bpm)2 centers six-coordinate. Metal···metal distances within the cluster are Fe···Ni = 5.042(3) and 5.058(2) A, Fe···Fe = 6.438(4) A, and Ni···Ni = 6.654(3) and 7.028(5) A. The seven water molecules form an intriguing hydrogen-bonded “cluster” which connects, via hydrogen-bonding, to the other facial CN ligands on each Fe. A wide array of magnetic susceptibility and magnetization measurements has been used t...

Structure and Magnetism of a Bimetallic Pentanuclear Cluster [(Ni(bpm)2)3(Fe(CN)6)2]·7H2O (bpm = Bis(1-pyrazolyl)methane)). The Role of the Hydrogen-Bonded 7H2O “Cluster” in Long-Range Magnetic Ordering

We report an experimental observation of spin frustration and metamagnetic behavior in a quasi‐1D ferrimagnetic chain, (MnTPP)+(TCNE)−. Metastability, hysteresis effects, and irreversibility of thermal and magnetic histories are observed in magnetization measurements. The ‘‘memory’’ phenomenon is seen in the temperature dependence of the thermoremanent and isothermal remanent magnetization. This system may be relevant to the magnetic behavior of the quantum/classical spin chains. The isothermal M(H) at T=2.25 K supports a first order transition which may be related to effects of local anisotropy.

Spin frustration and metamagnetic behavior in a molecular‐based quasi‐1D ferrimagnetic chain: (MnTPP)(TCNE)

We report field (H) and temperature (T) -dependent magnetization (M) of a member of the new class of high-${\mathit{T}}_{\mathit{c}}$ molecular-based magnets V(tetracyanoethylene${)}_{\mathit{x}}$\ensuremath{\cdot}y(solvent) with ${\mathit{T}}_{\mathit{c}}$ in an accessible range (solvent =${\mathrm{CH}}_{3}$CN). The M(H) at low T saturates slowly with increasing H. The random magnetic anisotropy model is applied to study the behavior of this disordered material, yielding values of anisotropy strengths and exchange constant. The results of an equation-of-state and scaling analysis near ${\mathit{T}}_{\mathit{c}}$ are given and compared to theoretical results.

Magnetization and static scaling of the high- T c disordered molecular-based magnet V(tetracyanoethylene ) x ⋅ y ( CH 3 CN) with x ∼1.5 and y ∼2

The effect of different spinless solvents on the magnetism of V(TCNE)x⋅y(solvent) (TCNE=tetracyanoethylene) is studied by temperature (T) dependent ac susceptibility above 2.3 K. Though V(TCNE)x⋅y(CH2Cl2) has a critical temperature Tc ∼400 K, for V(TCNE)x⋅y(C4H8O), Tc ∼205 K and the coercive field is reduced, while for V(TCNE)x⋅y(CH3CN), Tc ∼140 K or below and a spin glass phase exists below a spin freezing temperature of Tf∼7–15 K. The variations with solvent are suggested to be due to spin dilution and random anisotropy introduced by the spinless solvents.

Complex ac susceptibility studies of the disordered molecular based magnets V(TCNE)x: Role of spinless solvent

Static magnetic properties and critical behavior of V(TCNE)x·y(C4H8O), a high Tc molecular-based disordered magnet

Studies on the magnetic properties of decamethylchromocenium tetracyanoethanide, [CrCp*2][TCNE] are reported herein. Near‐zero field (H=150 mG) data indicates a ferromagnetic transition with Tc=3.65 K. The temperature dependence below Tc can be fit to a mean‐field result M=M0(1−T/Tc)1/2, and the data above Tc gives a critical exponent γ=1.2 for the dc magnetic susceptibility. Data at higher temperature indicates a strong 1‐D ferromagnetic behavior. Magnetizations at different H and T at low temperature can be scaled to a universal function with M=f(H/T3.2). The reduction in Tc in [CrCp*2][TCNE] compared with [FeCp*2][TCNE] indicates a competition between the coupling constants and the residing spins.

P. Zhou

Papers

Spin frustration and metamagnetic behavior in a molecular‐based quasi‐1D ferrimagnetic chain: (MnTPP)(TCNE)

Magnetization and static scaling of the high- T c disordered molecular-based magnet V(tetracyanoethylene ) x ⋅ y ( CH 3 CN) with x ∼1.5 and y ∼2

Complex ac susceptibility studies of the disordered molecular based magnets V(TCNE)x: Role of spinless solvent

Static magnetic properties and critical behavior of V(TCNE)x·y(C4H8O), a high Tc molecular-based disordered magnet

Ferromagnetic behavior of decamethylchromocenium tetracyanoethanide [CrCp2*][TCNE]