Calcium ions, bound to weakly coordinating anions (e.g., bistriflimide or triflate ions), have gained significant utility within the last two decades as catalysts for facilitating challenging chemical reactions. This is particularly exciting because calcium, unlike the transition metals typically used as catalysts, is cheap, abundant, and non-toxic. Despite these obvious advantages, calcium catalysis is in its infancy - little is known about the mode in which calcium salts activate substrates in chemical reactions. Recent experimental work has shown that calcium bistriflimide salts activate sulfur(VI) fluorides in a sulfur(VI)-fluorine exchange (SuFEx) toward nitrogen-containing sulfur(VI) compounds of medicinal relevance. A closer look at the experimental data show that switching the anion from bistriflimide to triflate results in dramatic yield losses, indicating that the counterion plays a crucial role in facilitating the chemical process, beyond being weakly coordinating. In this work, density functional theory methods were used to compute and compare the SuFEx mechanism mediated by calcium triflate to that mediated by calcium bistriflimide. Structural, energetic, and electronic insights from computations that explain the origins of the counterion-induced reactivity differences between calcium bistriflimide and calcium triflate will be presented.

Counterions effects in calcium-mediated sulfur-fluorine exchange

ACS Spring 2022

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