Copper finally joins the metallocene club

Since its debut in the early 1950s, ferrocene has been an icon of organometallic chemistry. Its distinctive structure—a metal ion sandwiched between two cyclopentadienyl rings—paved the way for a cavalcade of other metalloceneshttps://cen.acs.org/physical-chemistry/chemical-bonding/Californium-complex-offers-bonding-insights/99/i43. But among the first-row transition metals, copper alone resisted attempts to fill that kind of sandwich, until now. Cuprocene is the first neutral, stable copper metallocene, created by a team led by William J. Evans, an organometallic chemist at the University of California, Irvine. Through oxidation and reduction reactions, the researchers also prepared its cation and anion (J. Am. Chem. Soc. 2026, DOI: 10.1021/jacs.5c19366). “I really like it,” says David Mills, an inorganic chemist at the University of Manchester who was not involved in the work. “You look at metallocene chemistry and think it’s all been done, but it hasn’t.” Previous attempts to make cuprocenes hit problems because copper tends to induce coupling reactions between the cyclopentadienyl ligands themselves. “I think that’s why these kinds of compounds weren’t observed for a long time,” says Joshua D. Queen, a postdoctoral researcher in Evans’s group who led the work. Queen and his colleagues previously made a scandium metallocene by stabilizing it with a bulky tri-tert-butylcyclopentadienyl ligand (J. Am. Chem. Soc. 2024, DOI: 10.1021/jacs.3c11922), and that success inspired Queen to try the same tactic with copper. In the new work, the team simply mixed copper(II) triflate with the bulky ligand to form the blue-green cuprocene in 65% yield, which they isolated as crystals. “If you find the right ligand, you