Cavitands Incorporating a Lewis Acid Dinickel Chelate Function as Receptors for Halide Anions

The halide binding properties of the cavitand [Ni₂(L^Me2H4)]²⁺ (<b>4</b>) are reported. Cavitand <b>4</b> exhibits a chelating N₃Ni­(μ-S)₂NiN₃ moiety with two square-pyramidal Ni^IIN₃S₂ units situated in an anion binding pocket of ∼4 Å diameter formed by the organic backbone of the (L^Me2H4)^2– macrocycle. The receptor reacts with fluoride, chloride (in MeCN/MeOH), and bromide (in MeCN) ions to afford an isostructural series of halogenido-bridged complexes [Ni₂(L^Me2H4)­(μ-Hal)]⁺ (Hal = F^– (<b>5</b>), Cl^– (<b>6</b>), and Br^– (<b>7</b>)) featuring a N₃Ni­(μ-S)₂(μ-Hal)­NiN₃ core structure. No reaction occurs with iodide or other polyatomic anions (ClO₄^–, NO₃^–, HCO₃^–, H₂PO₄^–, HSO₄^–, SO₄^2–). The binding events are accompanied by discrete UV–vis spectral changes, due to a switch of the coordination geometry from square-pyramidal (N₃S₂ donor set in <b>4</b>) to octahedral in the halogenido-bridged complexes (N₃S₂Hal donor environment in <b>5</b>–<b>7</b>). In MeCN/MeOH (1/1 v/v) the log <i>K</i>₁₁ values for the 1:1 complexes are 7.77(9) (F^–), 4.06(7) (Cl^–), and 2.0(1) (Br^–). X-ray crystallographic analyses for <b>4</b>(ClO₄)₂, <b>4</b>(I)₂, <b>5</b>(F), <b>6</b>(ClO₄), and <b>7</b>(Br) and computational studies reveal a significant increase of the intramolecular distance between two propylene groups at the cavity entrance upon going from F^– to I^– (for the DFT computed structure). In case of the receptor <b>4</b> and fluorido-bridged complex <b>5</b>, the corresponding distances are nearly identical. This indicates a high degree of preorganization of the [Ni₂(L^Me2H4)]²⁺ receptor and a size fit mismatch of the receptor binding cavity for anions larger than F^–