One-Electron Bond Caught In A Structure

Chemical bonds typically involve two atoms sharing one or more pairs of electrons. A few oddball bonds exist, such as when three atoms share a pair of electrons or two atoms share a single electron. Only a handful of compounds with a one-electron σ-bond have been reported, and none of them have been structurally characterized, until now. Marc-Etienne Moret, Limei Zhang, and Jonas C. Peters of California Institute of Technology built a cagelike copper tris(phosphine)borane complex (shown) in which the Cu(0) atom shares one of its electrons with the electron-poor boron atom (J. Am. Chem. Soc., DOI: 10.1021/ja4006578). Using electron paramagnetic resonance spectroscopy, X-ray crystallography, and other experimental and computational techniques, the researchers determined that the neutral complex is a stable radical with a one-electron Cu–B σ-bond. When they electrochemically oxidize the complex, the electron is displaced and the Cu–B bond vanishes; when they reduce the complex to add an electron, a regular two-electron Cu–B σ-bond forms. These three oxidation states in an isolable set of compounds permit an unprecedented look at the electronic structure of metal-boron σ-bonding, the Caltech team notes. “It’s a pretty cool trio of molecules to be able to compare,” Peters says.