Weak σ-Hole Triel Bond between C5 H5 Tr (Tr=B, Al, Ga) and Haloethyne: Substituent and Cooperativity Effects.

The replacement of a CH group of benzene by a triel (Tr) atom places a positive region of electrostatic potential near the Tr atom in the plane of the aromatic ring. This σ-hole can interact with an X lone pair of XCCH (X=F, Cl, Br, and I) to form a triel bond (TrB). The interaction energy between C5 H5 Tr and FCCH lies in the range between 2.2 and 4.4 kcal/mol, in the order Tr=B<Ga<Al. This bond is strengthened by halogen substituents on the ring, particularly on the site adjacent to Tr. There is a much stronger strengthening trend as the F of the FCCH nucleophile is replaced by the heavier halogen atoms, rising up to 22 kcal/mol for ICCH. Adding a Li+ cation above the ring pulls density toward itself and thus magnifies the Tr σ-hole. The TrB to the XCCH nucleophile is thereby magnified as is the strength of the TrB. This positive cooperativity is particularly large for Tr=B.