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

The halide binding properties of the cavitand [Ni-2(LMe2H4)](2+) (4) are reported. Cavitand 4 exhibits a chelating N3Ni(mu-S)2NiN3 moiety with two square-pyramidal NiIIN3S2 units situated in an anion binding pocket of similar to 4 angstrom diameter formed by the organic backbone of the (LMe2H4)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 [Ni-2(LMe2H4)(mu-Hal)](+) (Hal = F- (5), Cl- (6), and Br- (7)) featuring a N3Ni(mu-S)(2)(mu-Hal)NiN3 core structure. No reaction occurs with iodide or other polyatomic anions (ClO4(-), NO3-, HCO3-, H2PO4-, HSO4-, SO42). The binding events are accompanied by discrete UVvis spectral changes, due to a switch of the coordination geometry from square-pyramidal (N3S2 donor set in 4) to octahedral in the halogenido-bridged complexes (N3S2Hal donor environment in 5-7). In MeCN/MeOH (1/1 v/v) the log K11 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 4(ClO4)(2), 4(I)(2), 5(F), 6(ClO4), and 7(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 4 and fluorido-bridged complex 5, the corresponding distances are nearly identical. This indicates a high degree of preorganization of the [Ni-2(LMe2H4)](2+) receptor and a size fit mismatch of the receptor binding cavity for anions larger than F-.