Intra- and Intermolecular Cation-π Interactions between Onium Salts and Alkynes/Acetylene: Experimental and Theoretical Insights

Cation-pi interactions between various onium salts, alkynes, and acetylene were studied, taking into account the substituents of the triple bond, the nature of the anions, and the polarity of the solvent, through a combination of MP2 calculations and experiments. In an intramolecular setting, these data (including single-crystal X-ray crystallography) concurred with the stability of folded conformers of alkynyl onium salts, even substituted with electron-withdrawing groups. To examine the contribution of these interactions on the alkyne electronic population, a thorough in silico study was carried out using natural bonding orbital analysis of the conformers. Intramolecular interactions from sulfonium salt tethered to phenylalkyne were highlighted, as illustrated above by the computed folded conformation (MP2) along with noncovalent interaction (NCI) analysis. Furthermore, investigations of intermolecular interactions, involving acetylene or phenylacetylene with various onium ions, revealed the high energy interactions of their complexes with phenyldimethylsulfonium chloride, as illustrated above with the complex PhC equivalent to CH/PhMe2SCl (MP2 calculations and NCI analysis).