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<p>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 (Py<sub>4</sub>Im) that generates a directional, protic cavity for ion pairing. This environment enables a comparative study of BF<sub>4</sub><sup>–</sup> and PF<sub>6</sub>⁻, showing that PF<sub>6</sub>⁻ undergoes fluoride abstraction under mild conditions, while BF<sub>4</sub><sup>–</sup> remains unreactive unless assisted by a Lewis base. The PF<sub>6</sub>⁻ activation yields a well-defined Co<sup>III</sup>–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.</p>

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

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2026年2月7日

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  • Luis Tarifa,
  • Judit Cano-Asensio,
  • José A. López,
  • Cristina Tejel,
  • Ana M. Geer

摘要

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.