Bimodal Cyclodextrin Complexation of Ferrocene Derivatives Containing n-Alkyl Chains of Varying Length

The complexation of alkyldimethyl(ferrocenylmethyl)ammonium salts, where the alkyl group is methyl (FC₁⁺PF₆^-), heptyl (FC₇⁺Br^-), or hexadecyl (FC₁₆⁺Br^-), by α-, β-, and γ-cyclodextrin was studied by electrochemical methods and high-field ¹H NMR spectroscopy. The apparent diffusion coefficients of all the ferrocene derivatives were found to decrease upon addition of any cyclodextrin (CD). The oxidation potential of the ferrocene derivatives was substantially shifted to more positive values in the presence of β-CD; however, α-CD and γ-CD only caused slight shifts. FC₁⁺ and FC₇⁺ form 1:1 complexes with α-CD, β-CD, and γ-CD, respectively. FC₁₆⁺ forms a 1:2 complex with α-CD and 1:1 complexes with β-CD and γ-CD. Binding constants between FC₁⁺, FC₇⁺, and all three cyclodextrins were determined from the variation of the diffusion coefficient as a function of CD concentration. These results suggest that α-CD interacts with the aliphatic region of the derivatives, while β-CD and γ-CD interact with the ferrocene subunit. The dual mode of CD binding to these derivatives was used to build small supra-molecular aggregates in which a ferrocene derivative directs several CD hosts to bind at different molecular regions. This was exemplified by the isolation of a quaternary complex between FC₁₆⁺, α-CD, and β-CD, which exhibited the stoichiometric ratio (1:2:1) predicted in terms of the individual interactions of this ferrocene derivative with α-CD and β-CD, respectively.