Fluoride ions are added to drinidng 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): BMes3
(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 'I-! NMR spectrum of the complex formed between C and F shows two signals for the CH2 protons: 6 3.82 (d, /hh = 12.9 Hz) and 6.50 (dd, 7HH = 12.9 Hz, JHF = 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–?
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