Rational design of a nonbasic molecular receptor for selective NH4+/K+ complexation in the gas phase.

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-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 NH(4)(+) over K(+) selectively. This is the first example of a neutral molecular receptor that shows a remarkable NH(4)(+)/K(+) selectivity. DFT-calculations provide insight into the nature of host-guest interactions of both 1⋅NH(4)(+) 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.