Heavy-Metal Ring Bags Metal Atom

Chemists have uncovered an unusual example of host-guest behavior in the form of a six-membered heterocyclic metal ring that traps and releases a neutral palladium atom. The research introduces heavy-metal heterocycles as a new class of compounds, and in reversibly trapping a neutral metal atom, the researchers add to the diversity of host-guest interactions (J. Am. Chem. Soc., DOI: 10.1021/ja205515u).

A team led by Richard D. Adams of the University of South Carolina, Columbia, and Michael B. Hall of Texas A&M University developed a process to make metalla­heterocycles by linking heavy transition metals with heavy main-group bridging ligands such as diphenyl­antimony and diphenylbismuth and explained the nature of the host-guest binding.

When the researchers took a three-membered Re₂Sb heterocycle they made and heated it with a palladium catalyst, the Re₂Sb ring dimerized to form a six-membered Re₄Sb₂ heterocyclic ring. Under certain reaction conditions, the Re₄Sb₂ ring trapped an unadorned palladium atom and held it in the center of the ring. The team further discovered that adding a phosphine ligand to a solution of the Re₄Sb₂Pd complex removes the palladium atoms; adding more palladium catalyst re­inserts some, but not all, of the palladium atoms.

“We anticipate being able to insert other metal atoms into this heterocycle and to make other heavy-atom heterocycles that exhibit similar host-guest chemistry,” Adams says.

A variety of cagelike metal clusters that take up guests, including a few that trap neutral metal atoms, have been made in the past, notes Slavi C. Sevov, a main-group cluster chemist at the University of Notre Dame. “But chemists have not ‘sold’ them as host-guest systems,” Sevov says. “It’s likely no one attempted or was able to extricate the guest atom before.” The ring structure has to be special to do that, he adds, and he would not be surprised if the reversible interaction could be done with other systems as well.