Interaction of protonated tyramine with a hexaarylbenzene-based receptor: Extraction and DFT study

From extraction experiments and γ-activity measurements, the extraction constant corresponding to the equilibrium HL+(aq) + 1·Cs+(nb) ⇔ 1·HL+(nb) + Cs+(aq) taking place in the two-phase water–nitrobenzene system (HL+ = protonated tyramine, 1 = hexaarylbenzene-based receptor; aq = aqueous phase, nb = nitrobenzene phase) was evaluated as log Kex (HL+, 1·Cs+) = −0.3 ± 0.1. Further, the stability constant of the 1·HL+ complex in nitrobenzene saturated with water was calculated for a temperature of 25 °C as log βnb (1·HL+) = 4.7 ± 0.2. Finally, by using quantum mechanical DFT calculations, the most probable structure of the 1·HL+ complex species was predicted. In the resulting “asymmetrical” cationic complex 1·HL+, the cation HL+ synergistically interacts with the polar ethereal oxygen fence by means of the corresponding three H-bonds and with the central hydrophobic benzene bottom of the parent receptor 1 via cation – π interaction.