FLi₄X₄^- (X = Cl, Br, I): Superhalogen Anions with Planar Tetracoordinate Fluorine

The concept of superhalogen was proposed for more than 40 years, and it has never been associated with planar tetracoordinate fluorine (ptF) species. In this work, using Li as the ligands and Cl, Br, I as the auxiliary atoms, we have designed the star-like D₄ FLi₄X₄^- (X = Cl, Br, I) clusters, which contain the ptF at the centers. They are all global minima (GMs) based on unbiased searches on the potential energy surfaces. Born-Oppenheimer molecular dynamics (BOMD) simulations suggest that these ptF structures are robust against dissociation at room temperature. Chemical bonding analyses indicate that there are four lone pairs (LPs) for ptF, three LPs for each X atom, and four 3c-2e Li-X-Li σ bonds. The stabilities of these ptF clusters are dominated by multicenter ionic bonding, rather than the σ aromaticity. Interestingly, these ptF species have large first vertical detachment energies (7.37, 6.94, and 6.30 eV). According to the definition of superhalogen, they can be viewed as superhalogen anions. The current work builds an important link between superhalogen and ptF chemistry.