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, tiliacalix[4]arene-p-tetrasulfonate (TC4AS) and [6]arene-p-hexasulfonate (TC6AS), toward (ferrocenylmethyl)trimethylammonium (FcCH(2)NMe(3)(+)), hydroxymethylferroceiie (FcCH(2)OH), 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 FcCH(2)N Me-3(+), FcCH(2)OH, and FcCOOH was shifted to cathodic direction, showing preferential inclusion of the oxidative state. Inclusion of neutral guests such as FcCH(2)OH 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: FcCH(2)NMe(3)(+) > FcCH(2)OH > FcCOOH, suggesting that electrostatic interaction con trols 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 FcCH(2)NMe(3)(+). (C) 2007 Elsevier B.V. All rights reserved.