Weak σ-Hole Triel Bond between C5H5Tr (Tr=B, Al, Ga) and Haloethyne: Substituent and Cooperativity Effects

Document Type

Article

Journal/Book Title

ChemPhysChem

Publication Date

12-14-2020

Publisher

Wiley-VCH Verlag GmbH & Co. KGaA

Award Number

NSF, Division of Chemistry (CHE) 1954310

Funder

NSF, Division of Chemistry (CHE)

Volume

22

Issue

5

First Page

481

Last Page

487

Abstract

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 C5H5Tr 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.

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