Magnetic Molecular Salts as Advanced Multifunctional Materials: A “Melting Pot” Approach on the Long Way from Molecular Magnetism and Electronics Toward Molecular Spintronics and Quantum Computing

The design and the synthesis of molecule-based crystalline salts made up of simple cationic and/or anionic building blocks with multiple, occasionally stimulus-responsive, chemical (host–guest, catalytic, acid–base, or redox) and physical (optical, magnetic, or conducting) properties constitute two major goals in inorganic, organic, organometallic, and coordination chemistries. A deep knowledge of the basic features of molecular and supramolecular interactions that occur in the solid state is needed to progress along these tasks to obtain new advanced multifunctional materials. Inspired by the outstanding research of several groups on magnetic molecular salts from the mid-1970s to the present day, this review offers a personal portrayal of the history of molecular magnetism and molecular electronics and its current evolution toward molecular spintronics and quantum computing. We focus on the well-known families of molecular salts based on paramagnetic tetrathiafulvalenium/tetraselenafulvalenium or tetracyanoethenide/tetracyanoquinodimethanide organic radicals, cyclopentadienide/cyclooctatetraenide metallocenium complexes, and polyhalide/polycyanide, porphyrin/phthalocyanine, oxalate/dithiooxalate, or dithiolene/dithiolate metal complexes with first-, second-, or third-row transition metal (nd, n = 3–5) and lanthanide (4f) ions. This old but evergreen class of magnetic molecular salts provides illustrative “textbook” examples of advanced multifunctional materials such as molecular magnets and conductors, molecular magnetic conductors, molecular nanomagnets, and molecular quantum bits with potential nanotechnological applications in quantum information storage and processing.