Fluoride ion complexation by a B₂/Hg heteronuclear tridentate lewis acid

The reaction of [Li(THF)₄][1,8-μ-(Mes₂B)C₁₀H₆] with HgCl₂ affords [1,1′-(Hg)-[8-(Mes₂B)C₁₀H₆]₂] (3) or [1-(ClHg)-8-(Mes₂B)C₁₀H₆] (4), depending on the stoichiometry of the reagents. These two new compounds have been characterized by ¹H, ¹³C, ¹¹B and ¹⁹⁹Hg NMR, elemental analysis and X-ray crystallography. The cyclic voltammogram of 3 in THF shows two distinct waves observed at E_1/2 −2.31 V and −2.61 V, corresponding to the sequential reductions of the two boron centers. Fluoride titration experiments monitored by electrochemistry suggest that 3 binds tightly to one fluoride anion and more loosely to a second one. Theses conclusions have been confirmed by a UV-vis titration experiment which indicates that the first fluoride binding constant (K₁) is greater than 10⁸ M⁻¹ while the second (K₂) equals 5.2 (± 0.4) × 10³ M⁻¹. The fluoride binding properties of 3 have been compared to those of [1-(Me₂B)-8-(Mes₂B)C₁₀H₆] (1) and [1-((2,6-Me₂-4-Me₂NC₆H₂)Hg)-8-(Mes₂B)C₁₀H₆] (2). Both experimental and computational results indicate that its affinity for fluoride anions is comparable to that of 2 but significantly lower than that of the diborane 1. In particular, the fluoride binding constants of 1, 2 and 3 in chloroform are respectively equal to 5.0 (± 0.2) × 10⁵ M⁻¹, 1.0 (± 0.2) × 10³ M⁻¹ and 1.7 (± 0.1) × 10³ M⁻¹. Determination of the crystal structures of the fluoride adducts [S(NMe₂)₃][1–μ₂-F] and [S(NMe₂)₃][3–μ₂-F] along with computational results indicate that the higher fluoride binding constant of 1 arises from a strong chelate effect involving two fluorophilic boron centers.