Cooperative anion activation at a cobalt center through ion pairing and ligand design

Weakly coordinating anions (WCAs) are commonly used to stabilize high-valent, electrophilic transition-metal complexes, owing to their low nucleophilicity and minimal coordinating ability. Here, we present a cobalt platform supported by a tris(2-pyridyl)(N-picolyl)imidazolidine ligand (Py₄Im) that generates a directional, protic cavity for ion pairing. This environment enables a comparative study of BF₄⁻ and PF₆⁻, showing that PF₆⁻ undergoes fluoride abstraction under mild conditions, while BF₄⁻ remains unreactive unless assisted by a Lewis base. The PF₆⁻ activation yields a well-defined CoIII-F complex that acts as an efficient nucleophilic fluoride source. DFT calculations and diffusion NMR experiments highlight the critical role of anion-cation pairing in modulating reactivity, overriding intrinsic thermodynamic preferences. These findings showcase supramolecular control of WCA activation by a late transition metal and provide new design principles for reactivity modulation via counterion selection.