Voltammetric study of inclusion of p-sulfonated thiacalix[n]arenes (n = 4, 6) toward ferrocenyl guests in aqueous solutions

Inclusion behavior of negatively charged host molecules, thiacalix[4]arene-p-tetrasulfonate (TC4AS) and [6]arene-p-hexasulfonate (TC6AS), toward (ferrocenylmethyl)trimethylammonium (FcCH₂NMe₃⁺), hydroxymethylferrocene (FcCH₂OH), ferrocenecarboxylic acid (FcCOOH), and 1,1′-ferrocenedicarboxylic acid (Fc(COOH)₂) was studied in aqueous solutions (pH 7.0) with cyclic voltammetry. Upon increasing the concentration of TC4AS to 4-fold of each guest, the anodic peak current density (j_p,a) decreased, suggesting inclusion of the ferrocenyl guests in TC4AS. Also oxidation half-wave potential (E_1/2) of FcCH₂NMe₃⁺, FcCH₂OH, and FcCOOH was shifted to cathodic direction, showing preferential inclusion of the oxidative state. Inclusion of neutral guests such as FcCH₂OH and Fc⁺COO^- implies that hydrophobic interaction between TC4AS and the guests is the chief driving force for formation of host-guest assembly. The decrease of E_1/2 for each guest was in the order: FcCH₂NMe₃⁺ > FcCH₂OH > FcCOOH, suggesting that electrostatic interaction controls the preference toward oxidative form of the guest. Dicarboxylic Fc(COOH)₂ showed decrease of j_p,a but increase of E_1/2 upon inclusion, suggesting TC4AS preferred reduced form Fc(COOH)₂ to oxidized form Fc⁺(COO^-)₂. TC6AS behaved similarly to TC4AS but with larger decrease in of E_1/2 and j_p,a. The larger shift of E_1/2 for inclusion of FcCOOH, the oxidative form of which is also neutral (Fc⁺COO^-), than that attained with TC4AS endorses main role of hydrophobic interaction between TCnAS (n = 4, 6) and ferrocenyl guest molecules. Having the most preferential electrostatic interaction, kinetically stable complex was formed between TC6AS and FcCH₂NMe₃⁺.