Immobilization of functionalized iron(II) clathrochelates with terminal (poly)aromatic group(s) on carbonaceous materials and their detailed cyclic voltammetry study

Abstract

The detailed cyclic voltammetry (CV) study of the iron(II) clathrochelates with one, two or six terminal (poly)aromatic group(s) in a wide range of scan rate of the potential using glassy carbon (GC) and Pt working electrodes was performed. Their CVs contain the single one-electron quasireversible Fe2+/+ reduction wave in the cathodic potential range and one or two oxidation waves in the anodic range, assigned to a metal-centered Fe2+/3+ process and to oxidation of their polyaromatic terminal group(s). The potentials of both the reduction Fe2+/+ and oxidation Fe2+/3+ processes are shifted in the cathodic range as compared with their halogenoclathrochelate precursors and the values of shifts are substantially affected by a material of the working electrode; the electrochemical characteristics of the metal-centered waves suggest their diffusion-controlled character. Plots of the current peaks versus υ1/2 for the second anodic waves suggest a more complex diffusion – kinetic controlled character of these oxidations; for the dipyrenyl-terminated cage complex, the plots of the peak potentials versus lnυ allowed to obtain the corresponding average values of the apparent charge transfer coefficient α for both these electrodes, which are evidenced a very slow rate and irreversible character of this redox process. CVs of the hexafunctionalized clathrochelates contain an additional peak between the first and second (or after the second) anodic oxidation waves, an intensity of which linearly depends on a scan rate; the shapes of these CVs and the presence of an additional oxidation wave or plateau suggest their strong adsorption. The quantitative data about the physical adsorption of the iron(II) clathrochelate on activated carbon (AC) and reduced graphene oxide (RGO) were obtained. Using UV–vis spectral method; the Langmuir adsorption isotherms were used to calculate the corresponding adsorption equilibrium constants K. The observed changes of a free Gibbs energy are characteristic of the processes of physical adsorption via the stacking and Van-der-Waals interactions. Their limiting adsorption on AC decreases with increase in the number of the polyaromatic group(s) per a molecule from approximately 3·10−4 to 5.65·10−5 mol l−1, whereas in the case of RGO as an adsorbent that increases from 2.57·10−4 to 3.16·10−1 mol l−1. Values of K for AC also decrease with their number, while in the case of RGO as a substrate, its maximal value was observed for the difunctionalized iron(II) cage complex. Therefore, the physical adsorption of these clathrochelates is a structure-dependent process, which is strongly affected by the number of the functionalizing substituents at a cage framework. The value of K is determined by energy of the supramolecular binding of an adsorbate to an adsorbent. In the case of the diphenanttheryl-terminated iron(II) cage complex, its supramolecular binding with a surface of AC is stronger, thus suggesting a higher adsorption energy, while the hexafunctionalized cage complex forms the more strong supramolecular assemblies on a surface of RGO. This effect is explained by the presence of wedge-shaped pores in a macrostructure of RGO, both the size and the form of which is favorable for the physical adsorption of bulky molecules of the hexaphenanthrenyl-terminated macrobicyclic complex, while the size and the form of mesopores of AC are favorable for that of the difunctionalized iron(II) clathrochelate.