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

The halide binding properties of
the cavitand [Ni2(LMe2H4)]2+ (4) are reported. Cavitand 4 exhibits a chelating
N3Ni­(μ-S)2NiN3 moiety with
two square-pyramidal NiIIN3S2 units
situated in an anion binding pocket of ∼4 Å 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 [Ni2(LMe2H4)­(μ-Hal)]+ (Hal = F– (5), Cl– (6), and Br– (7)) 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–, SO42–). 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 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 [Ni2(LMe2H4)]2+ receptor and a size fit mismatch
of the receptor binding cavity for anions larger than F–.