Stabilization of intermediate spin states in mixed-valent diiron dichalcogenide complexes 
Justin T. Henthorn*, George E. Cutsail III*, Thomas Weyhermüller and Serena DeBeer

Transition metal clusters made up of iron (Fe) and sulfur (S) are essential to countless processes in both Nature and chemical industry.  The iron atoms in these clusters can be found in various oxidation-states that depend on the number of electrons each iron has.  Each of the iron’s lone electrons act as a small magnet, and all together they add up to give a total magnetic strength, something we call ‘spin.’

The spins of multiple iron sites in metal clusters interact to give a total spin we can measure in the laboratory.  Determining the “spin-state” of these metal clusters and other molecules is  very important because the “spin-state” of metal clusters controls how they will interact with other molecules and perform chemical reactions.  For iron dimers, where the two Fe atoms have different oxidation-states and thus different spins, the total spin is determined by how exactly the iron atom atoms interact with each other. All examples thus far obey rules that are quite similar to basic addition and subtraction; for iron dimers, we could only ever achieve two solutions, 2.5 + 2.0 = 4.5 or 2.5 – 2.0 = 0.5. However, quantum physics and chemistry predict that such iron dimers can have stable spin-states in between these two limits.

Corresponding authors, Justin Henthorn and George Cutsail, report in Nature Chemistry the first transition metal dimer complexes to break these rules. Demonstrating in a series of iron dimer complexes, the substitution of the bridging sulfur atoms for heavier selenium (Se) and even larger tellurium (Te) atoms yields unique physical properties.  Through a wide variety of spectroscopic techniques, magnetism measurements, and quantum chemical calculations, Henthorn and Cutsail demonstrate and explain how Se and Te substituted dimers stabilize new spin-states. The breaking of these simple rules leads to so-called intermediate spin-states and allows us to count in-between the previous limits of 0.5 and 4.5. This report is the first isolation and characterization of an intermediate spin-state in a transition metal dimer, the smallest cluster one may make.  

The understanding of physical properties such as this are key to understanding how these molecules perform various chemical reactions. Iron-sulfur clusters are abundant in all forms of life, performing crucial chemical reactions. These new complexes and their unique properties serve as excellent models to further understand related iron-sulfur cluster in Nature. Furthermore, magnetic molecules, such as this, are often explored for the potential applications in quantum computing and building the next generation of computer processors.  This demonstration of previously inaccessible spin-states in a metal cluster dimer opens the door to further development and design of new complexes. 

Original Publication: Justin T. Henthorn*, George E. Cutsail III*, Thomas Weyhermüller and Serena DeBeer. Stabilization of intermediate spin states in mixed-valent diiron dichalcogenide complexes. Nature Chemistry https://www.nature.com/articles/s41557-021-00853-5