Compared to the extensive studies of reactive cations paired with weakly coordinating anions (WCAs), there has been relatively little exploration into the inverse: weakly coordinating cations (WCCs) with reactive anions. In order to inform the use of WCCs in a wide variety of applications we developed a computational protocol for predicting their relative reactivities in non-polar organic media. The calculation of chloride ion dissociation enthalpies (ΔH°D) showed the high degree of coordination present in tetraalkylammoniums and the promise of phosphazeniums and tetraphenylphosphonium as WCCs. These computational results were used to inform the development of even more potent WCCs based on the tetraarylphosphonium scaffold. In addition to our computational exploration, we also developed two methods to experimentally quantify coordination and applied them to common literature cations and the tetraarylphosphonium ions. Solution state IR spectroscopy of WCC-chloride salts in CDCl3 was moderately predictive for the reactivity of various WCC-anion pairs and showed good correlation with ΔH°D. A kinetic study of the chloride and acetate salts in an SN2 reaction with octyl iodide provided a useful ranking of WCC potency in CH2Cl2 and inspired the application of WCC-chlorides as catalysts for a variety of substitution reactions in non-polar solvents. These results were used in the ongoing development of charge-enhanced Bronsted and Lewis base organocatalysts, where WCC choice has proven critical in enhancing the basicity of various anionic pyridyl borates beyond their non-charged counterparts.

Liberating the anion: Further development of weakly coordinating cations

ACS Spring 2022

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