Fluoride ions are added to drinking water, toothpaste and drugs used to treat osteoporosis. However, an excess of F⁻ is toxic to the body. The design of water-soluble, selective fluoride ion receptors is therefore topical. One group of receptors that has been investigated consists of the boron-containing species shown below (Mes = mesityl = 2,4,6-Me₃C₆H₂):

(a) Explain in detail how compound A binds F⁻.

(b) Suggest why the stability constant of the complex formed between B and F⁻ is greater than that formed between A and F⁻.

(c) Compounds C and D form zwitter-ionic complexes with F⁻. Explain what this means.

(d) The ¹H NMR spectrum of the complex formed between C and F⁻ shows two signals for the CH₂ protons: δ 3.82 (d, J_HH = 12.9 Hz) and 6.50 (dd, J_HH = 12.9 Hz, J_HF = 9.2 Hz). Draw the structure for a complex that is consistent with this observation, and also explains the high stability of the complex.

(e) Which of the receptors would you expect to form water-soluble complexes with F⁻?

Transcribed Image Text:

BMes3 A с BMes2 + NMe3 B BMes2 BPh₂ BMes2 + NMe3 D