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 (jp,a) decreased, suggesting inclusion of the ferrocenyl guests in TC4AS. Also oxidation half-wave potential (E1/2) 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 E1/2 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 jp,a but increase of E1/2 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 E1/2 and jp,a. The larger shift of E1/2 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+.