Cavitands Incorporating a Lewis Acid Dinickel Chelate
Function as Receptors for Halide Anions
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Abstract
The halide binding properties of
the cavitand [Ni<sub>2</sub>(L<sup>Me2H4</sup>)]<sup>2+</sup> (<b>4</b>) are reported. Cavitand <b>4</b> exhibits a chelating
N<sub>3</sub>Ni(μ-S)<sub>2</sub>NiN<sub>3</sub> moiety with
two square-pyramidal Ni<sup>II</sup>N<sub>3</sub>S<sub>2</sub> units
situated in an anion binding pocket of ∼4 Å diameter formed
by the organic backbone of the (L<sup>Me2H4</sup>)<sup>2–</sup> 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<sub>2</sub>(L<sup>Me2H4</sup>)(μ-Hal)]<sup>+</sup> (Hal = F<sup>–</sup> (<b>5</b>), Cl<sup>–</sup> (<b>6</b>), and Br<sup>–</sup> (<b>7</b>)) featuring
a N<sub>3</sub>Ni(μ-S)<sub>2</sub>(μ-Hal)NiN<sub>3</sub> core structure. No reaction occurs with iodide or other polyatomic
anions (ClO<sub>4</sub><sup>–</sup>, NO<sub>3</sub><sup>–</sup>, HCO<sub>3</sub><sup>–</sup>, H<sub>2</sub>PO<sub>4</sub><sup>–</sup>, HSO<sub>4</sub><sup>–</sup>, SO<sub>4</sub><sup>2–</sup>). The binding events are accompanied by discrete
UV–vis spectral changes, due to a switch of the coordination
geometry from square-pyramidal (N<sub>3</sub>S<sub>2</sub> donor set
in <b>4</b>) to octahedral in the halogenido-bridged complexes
(N<sub>3</sub>S<sub>2</sub>Hal donor environment in <b>5</b>–<b>7</b>). In MeCN/MeOH (1/1 v/v) the log <i>K</i><sub>11</sub> values for the 1:1 complexes are 7.77(9) (F<sup>–</sup>), 4.06(7) (Cl<sup>–</sup>), and 2.0(1) (Br<sup>–</sup>). X-ray crystallographic analyses for <b>4</b>(ClO<sub>4</sub>)<sub>2</sub>, <b>4</b>(I)<sub>2</sub>, <b>5</b>(F), <b>6</b>(ClO<sub>4</sub>), 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<sup>–</sup> to I<sup>–</sup> (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 [Ni<sub>2</sub>(L<sup>Me2H4</sup>)]<sup>2+</sup> receptor and a size fit mismatch
of the receptor binding cavity for anions larger than F<sup>–</sup>