Triplet diradical cyclobutadiene spotted for the first time

Carbon and silicon share many chemical properties, yet their low-coordinate compounds behave quite differently: For example, although C=C bonds are ubiquitous, Si=C and Si=Si bonds are rare, and the latter can be isolated only when sterically protected by large substituents.

Those bonding differences stem from parameters including differences in electronegativity and the size of bonding orbitals, according to Yitzhak Apeloig of Technion—Israel Institute of Technology.

During a lecture last week at the ACS national meeting in Washington, D.C., Apeloig described how he and his colleagues used that knowledge to observe an elusive diradical. Apeloig spoke in a symposium on theoretical modeling of chemical bonding hosted by the Division of Physical Chemistry to honor Nobel Laureate Roald Hoffmann of Cornell University.

Apeloig explained how double-bonded carbon and silicon anions and radicals have different structures and properties, and that silicon radicals are thermodynamically more stable than carbon radicals. By understanding that stability, Apeloig, working with Technion’s Boris Tumanskii, the University of Tsukuba’s Akira Sekiguchi, and others, used a combination of electron paramagnetic resonance spectroscopy (EPR) and computational work in 2015 to observe the first triplet diradical—a radical in which the two unpaired electron spins are aligned in parallel—in a Si=Si compound.

That work enabled the same team to now report the first spectroscopic observation of a triplet diradical electronic state of a heated cyclobutadiene (shown), albeit one stabilized by trimethylsilyl groups (Angew. Chem. Int. Ed. 2017, DOI: 10.1002/anie.201705228). Cyclobutadiene is a classic model compound used to study antiaromaticity, but the triplet diradical of cyclobutadiene had not been observed because of its low stability. Studying the elusive species could aid organic synthesis and help design new electronic materials, Sekiguchi told C&EN.