Cyclopentadienyl Ligands as Perfect Anion Receptors: Teamwork between π-Anion Interaction and C-H•••Anion Hydrogen Bonds

Electron-deficient arenes are now recognized as potent anion receptors, mainly through the establishment of pi-anion interactions. Cyclopentadienyl ligands (Cp's) also belong to the family of arenes that exhibit very low electron density at their centroid due to the presence of the metal oil one side. As such, they also work as anion receptors. A closer look at the reported X-ray structures in which all anion is in short contact with the Cp framework reveals that in most cases the five-membered ring is substituted, generally pentamethylated (Cp*). This is quite surprising since Cp* is electron-richer than the unsubstituted Cp ligand. This computational Study shows that Cp* functions as a ring bearing a gemstone: the anion establishes a strong pi-interaction with the centroid, but this is not sufficient to hold it straight. For that matter, each methyl group develops a C-H center dot center dot center dot anion hydrogen bond, thus working as a claw to prevent the gem from falling. Although individually small, the interaction energy provided by the five methyl groups makes the M-Cp* a perfect anion receptacle.