Dendritic and Ion-Pairing Effects in Oxo-anion Recognition by Giant Alkylferrocenyl Dendrimers

Abstract

Dendrimers containing 3n+2 long tethers (n = generation number from 0 to 7: G0−G7) terminated by alkylferrocenyl termini were used for the redox recognition of the oxo-anions H2PO4− and ATP2− by cyclic voltammetry (CV) in order to investigate the ion-pairing and dendritic effect in the absence of a functional group attached to the ferrocenyl termini. Such groups were previously assumed to play an essential role in the magnitude of redox recognition due to their supramolecular interaction with the anion perturbing the ferrocenyl potential. Titration in CH2Cl2 solution of these metallodendrimers with the n-Bu4N+ salt of the oxo-anion provided a new CV wave at less positive potential, while the intensity of the initial ferrocenyl CV wave decreased. Whereas ferrocene and the reference monometallic compound Fc(CH2)11OC6H4OH, 1 (Fc = ferrocenyl), undergo only reversible ion-pairing interaction with H2PO4−, the reaction of oxo-anions with the ferrocenyl dendrimers is fast and complete (1 equiv oxo-anion per ferrocenyl branch). The difference of wave potential ΔE1/2 between the initial CV wave and the new CV wave observed with the dendrimers is ΔE1/2 = 120 ± 10 mV from G0 (9 alkylferrocenyl tethers) to G3 (243 alkylferrocenyl tethers) without significant differences in this series. Strong adsorption of high-generation dendrimers G6 and G7 was favorable for the fabrication of modified Pt electrodes that recognized the oxo-anions. This study demonstrates the key role of the contact ion pairing between the ferricinium form and the oxo-anion, in synergy with the dendritic effect involving strong interaction with the oxo-anion by encapsulation even in the absence of supramolecular interaction with a functional group attached to the ferrocenyl termini.