Perfluorinated ligand makes long-awaited debut in rhodium complex

The first coordination complex to contain a [C₅(CF₃)₅]⁻ ligand has been created, more than 40 years after the ligand itself was first synthesized (Angew. Chem. Int. Ed. 2022, DOI: 10.1002/anie.202211147).

The ligand is a descendant of cyclopentadienyl (Cp), found in the iconic sandwich complex, ferrocene. One common variant of this ligand is known as Cp*, in which each of the five cyclopentadienyl carbons carries a methyl group. The new ligand is the perfluorinated version of Cp*, in which every hydrogen has been replaced by fluorine.

All previous attempts to coordinate the ligand to a metal had failed because its CF₃ groups are so strongly electron withdrawing. “There’s almost no electron density in the aromatic system, so it wasn’t expected to be a coordinating ligand,” says graduate student Robin Sievers, who led the experimental work in Moritz Malischewski’s group at the Free University of Berlin.

The team overcame this hurdle by combining the ligand with an electron-rich rhodium cyclooctadiene (COD) precursor. Rhodium contributes bonding electrons back to the ligand to form the weakly bound complex Rh(COD)(C₅(CF₃)₅), a yellow solid that is stable at room temperature.

The perfluorinated Cp* is easily displaced by other ligands, so the researchers hope the complex might be exploited to make new catalysts—particularly those that could be used in conjunction with fluorinated solvents.