Supramolecular Gold Nanoparticles for the Redox Recognition of Oxoanions: Syntheses, Titrations, Stereoelectronic Effects, and Selectivity

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Abstract
Stable, CH2Cl2-soluble mixed dodecanethiol/(amidoferrocenyl)alkanethiol (AFAT) gold colloids were synthesized by ligand substitution reactions from Brust's dodecanethiol gold colloids and the AFAT ligands to study the recognition and titration of oxoanions. Gold colloids were obtained with various chain lengths (C11 and C6 chains) of the AFAT ligand and different proportions of AFAT ligands in the colloids. Modification of the amidoferrocenyl structure [replacement of the free C5H5 ferrocene ring by C5Me5 (Cp*) or C5H4COCH3] has been achieved to investigate the stereoelectronic effects on the recognition. The cyclic voltammetry of these colloids in CH2Cl2 on Pt electrode shows a reversible FeII/III wave with some adsorption. With AFAT ligands, a new, less electrochemically reversible wave (with some adsorption) at a potential 220 ± 20 mV less positive than that of the initial wave appears upon titration of [n-Bu4N][H2PO4], and the initial wave completely disappears after addition of 1 equiv of anion, which allows its titration. The potential shift does not depend on the AFAT proportion nor on its chain length but is reduced with Cp* and enhanced with C5H4COCH3, showing the key role of the hydrogen bonding between the −NH− amido group and a terminal oxygen atom of the oxoanions. According to the Echegoyen−Kaifer model, the potential shift leads to the ratio K(+)/K(0) of apparent association constants. In the presence of both [n-Bu4N][HSO4] and [n-Bu4N]Cl, a shift of the initial wave (rather than its replacement) allows an easy titration, ideally with 20-Fc. Upon addition of [n-Bu4N][HSO4] alone, a weak wave shift (30 mV) of the colloids is also observed, allowing the titration of the HSO4- anion by the colloids containing a low percentage of AFAT ligand. The Echegoyen−Kaifer model provides access to the apparent association constant K(+) in this case for which the interaction between the anion and the neutral form of the host is not significant. With the C5H4COCH3 modification of the amidoferrocenyl branch, a new wave appears at a potential 70 mV more positive than the initial wave, signifying a stronger interaction with this modified ligand than with the parent AFAT ligand. These colloids favorably compare with ferrocenyl dendrimers in terms of rapid synthesis and selectivity of H2PO4- over HSO4- and with gold surfaces for the recognition of HSO4-.