Abstract
The area of 3d-metal coordination clusters that behave as Single-Molecule Magnets (SMMs) is now quite mature within the interdisciplinary field of Molecular Magnetism. This area has created a renaissance in Inorganic Chemistry. From the synthetic Inorganic Chemistry viewpoint, the early years of “try and see” exercises (1993–2000) have been followed by the development of strategies and strict approaches. Our review will first summarize the early synthetic efforts and routes for the preparation of polynuclear 3d-metal SMMs, and it will be then concentrated on the description of the existing strategies. The former involve the combination of appropriate 3d-metal-containing starting materials (simple salts with inorganic anions, metal cardoxylates, and pre-formed carboxylate clusters, metal phosphonates) and one or two primary organic ligands; the importance of the end-on azido group as a ferromagnetic coupler in 3d-metal SMM chemistry will be discussed. The utility of comproportionation reactions and the reductive aggregation route for the construction of manganese SMMs will also be described. Most of the existing strategies for the synthesis of SMMs concern manganese. These involve substitution of carboxylate ligands in pre-formed SMMs by other carboxylate or non-carboxylate groups, reduction procedures for the {} SMMs, spin “tweaking,” “switching on” SMM properties upon conversion of low-spin clusters into high-spin ones, ground-state spin switching and enhancing SMM properties via targeted structural distortions, the use of radical bridging ligands and supramolecular approaches. A very useful strategy is also the “switching on” of SMM behavior through replacement of bridging hydroxide groups by end-on azido or isocyanato ligands in clusters. Selected examples will be mentioned and critically discussed. Particular emphasis will be given on the criteria for the choice of ligands.
Highlights
Today there is a multi-billion euro industry concerning magnets due to their applications in many useful and modern technologies, such as televisions, credit/debit/ATM cards, motors, computer hard drives, switches and medical equipment, among others
Magnetism has its roots in Ancient Greece; in the province of Magnesia at central Greece, the Synthesis of 3d-Metal Single-Molecule Magnets first discovery of the attraction between lodestone and iron was made
The principles of Magnetism were established through the excellent work of Ampeére, Oersted, Faraday, Maxwell and Pierre Curie, just to name few legendary scientists who contributed in this field of Physics
Summary
Today there is a multi-billion euro industry concerning magnets due to their applications in many useful and modern technologies, such as televisions, credit/debit/ATM cards, motors, computer hard drives, switches and medical equipment, among others. This organic “blend,” which contains the deprotonated primary organic ligand and the anionic auxiliary organic group, leads often to dinuclear or polynuclear 3d-metal complexes which often exhibit interesting magnetic properties This route is useful when external bases for ligand deprotonation perplex the reaction, e.g., by precipitating amorphous hydroxido- or/and oxido-containing materials. It would be naïve to believe that the above described “try and see” approach leads directly to SMMs. Considerable forethought and hindsight in the metal ions, their inorganic or auxiliary organic anions, primary organic ligands and reaction conditions (reaction ratio, solvent, “pH,” nature of external base, temperature, pressure, crystallization technique, etc.) are necessary to “switch on” or improve the SMM properties in the products. In the chemistry of 3d-metal coordination clusters, this role is often fulfilled by the solvent molecules, the inorganic anions of the 3d-metal starting materials, the auxiliary organic ligands and the “chelating” part of the bridging organic ligands, or by the purposefully addition of a chelating ligand in the reaction systems
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