Cooperative interaction of protonated hexamethylenetetramine with a hexaarylbenzene-based receptor: Experimental and theoretical 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 hexamethylenetetramine, 1 = hexaarylbenzene-based receptor; aq = aqueous phase, nb = nitrobenzene phase) was evaluated as log Kex (HL+, 1⋅Cs+) = 0.5 ± 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+) = 5.8 ± 0.2. Finally, by using quantum mechanical DFT calculations, the most probable structures A and B of the 1⋅HL+ complex species, which are obviously in a dynamic equilibrium, were indicated. In both of these structures of the resulting complex 1⋅HL+ having C3 symmetry, the cation HL+ synergistically interacts with the polar ethereal oxygen fence and with the central hydrophobic benzene bottom of the parent receptor 1 via cation–π interaction.