A voltammetric investigation of the reactions between surface-confined amines and quinones on gold electrodes: evidence for imine formation

Abstract

Abstract A surface-confined layer containing naphthoquinone was prepared by reacting 2,3-dichloronaphthoquinone with the amino tail groups of an aminoethanethiolate-modified vapor-deposited gold surface; this surface layer was then reacted with β -ferrocenylethylamine. These modified gold electrodes were examined with cyclic staircase voltammetry in aqueous HClO 4 at the completion of each step in this sequence of reactions. The voltammetric signature for the 2e − , 2H + reduction of the quinoid redox centers disappeared after the ferrocene compound was reacted with the naphthoquinone surface layer; furthermore, the surface coverage of ferrocene was twice that expected on the basis of a 1:1 reaction between the surface-bound naphthoquinone and β -ferrocenylethylamine. These results suggest that the latter reacts with the naphthoquinone carbonyl groups to form a surface-confined di-imine, which cannot be reduced within the electrochemical potential window of aqueous HClO 4 . In similar experiments, 2-[4′-( β -ferrocenylethylaminocarbonyl)phenyl]-1,4-naphthoquinone and 2-[2′-( β -ferrocenylethylaminocarbonyl)ethyl]anthraquinone were synthesized and reacted with aminoethanethiolate-modified gold surfaces. When examined at a platinum electrode in nonaqueous solvents, both of the freely diffusing compounds exhibited two pairs of voltammetric waves characteristic of quinoid and ferrocene functionalities. However, only the ferrocene redox centers of the resulting surface-confined layers were electroactive, suggesting that the reaction of these compounds with the surface-confined aminoethanethiolate involves conversion of the quinoid carbonyls to imines.