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Keywords:

  • ammonium receptors;
  • carbon;
  • density functional calculations;
  • molecular recognition;
  • pi interactions

Abstract

A new molecular receptor (1) for ammonium recognition has been designed and constructed by using only carbon atoms. This molecular receptor can co-exist in two different isoenergetic conformations but, upon complexation, the conformers are no longer isoenergetic, and a basket-shaped conformation becomes clearly more stable. The pre-organised tetrahedral structure of this basket-shaped molecule favours the complexation of ammonium ions by N[BOND]H⋅⋅⋅π interactions with the four phenyl groups of the host. A similar behaviour is not observed in a similar, but less pre-organised, reference molecule. ESI-MS competition experiments show that 1 is able to bind NH4+ over K+ selectively. This is the first example of a neutral molecular receptor that shows a remarkable NH4+/K+ selectivity. DFT-calculations provide insight into the nature of host–guest interactions of both 1⋅NH4+ and 1⋅K+ complexes as well as in the mechanism involved in multiple cation–π interactions and the influence of these interactions on the conformational stability and the selective binding of the host.