Experimental and DFT study on the complexation of NH4+ with a hexaarylbenzene-based receptor

From extraction experiments and γ-activity measurements, the extraction constant corresponding to the equilibrium NH4+(aq)+1·Cs+(nb)⇆1·NH4+(nb)+Cs+(aq) taking part in the two-phase water-nitrobenzene system (1 = hexaarylbenzene-based receptor; aq = aqueous phase, nb = nitrobenzene phase) was evaluated as logKex(NH4+,1·Cs+)=-0.4±0.1. Further, the stability constant of the 1·NH4+ complex in nitrobenzene saturated with water was calculated for a temperature of 25 °C as logβnb(1·NH4+)=6.3±0.2. By using quantum mechanical DFT calculations, the most probable structures A and B of the 1·NH4+ complex species, which are obviously in a dynamic equilibrium, were indicated. In both of these structures of the resulting complex 1·NH4+ having C3 symmetry, the cation NH4+ synergistically interacts with the polar ethereal oxygen fence and with the central hydrophobic benzene bottom via cation-π interaction.