Actinide orbitals drive C–C coupling

Researchers have found a new kind of aromatic molecule whose unusual electronic properties may open up new realms of reactivity. Jaqueline Kiplinger at Los Alamos National Laboratory and colleagues there and at the University of Vermont and the University of Minnesota Twin Cities have made the first known actinide 2-metallabiphenylene compounds through metal-mediated reductive coupling (Nature 2020, DOI: 10.1038/s41586-020-2004-7). The actinides’ 5f and 6d orbitals set off a C–C coupling reaction to form cyclobutadiene rings, sandwiched between a benzene ring and the metallacyclopentadiene; the molecule (shown) obeys Hückel’s rule. Spectroscopy and calculations suggested that in both uranium and thorium systems, the cyclobutadiene is antiaromatic, and the benzene is aromatic. However, the team found that the U 5f orbitals covalently bond with the metallabiphenylene, whereas the Th does not because it has no f electrons. “The 5f and 6d orbitals on the actinide metals are enabling chemistry that the rest of the periodic table hasn’t been able to achieve,” Kiplinger says. This highlights the differences between the transition metals and the actinides, says Trevor Hayton, an inorganic chemist at the University of California, Santa Barbara, “and suggests that many more exciting reactions are waiting to be discovered.”