Anodic Methods for Covalent Attachment of Ethynylferrocenes to Electrode Surfaces: Comparison of Ethynyl Activation Processes

Abstract

The electrochemical oxidation of ferrocenes having an H- or Li-terminated ethynyl group has been studied, especially as it relates to their covalent anchoring to carbon surfaces. The anodic oxidation of lithioethynylferrocene (1-Li) results in rapid loss of Li(+) and formation of the ethynyl-based radical FeCp(η(5)-C5H4)(C≡C), (1, Cp = η(5)-C5H5), which reacts with the electrode. Chemically modified electrodes (CMEs) were thereby produced containing strongly bonded, ethynyl-linked monolayers and electrochemically controlled multilayers. Strong attachments of ethynylferrocenes to gold and platinum surfaces were also possible. The lithiation/anodic oxidation process is a mirror analogue of the diazonium/cathodic reduction process for preparation of aryl-modified CMEs. A second method produced an ethynylferrocene-modified electrode by direct anodic oxidation of the H-terminated ethynylferrocene (1-H) at a considerably more positive potential. Both processes produced robust modified electrodes with well-defined ferrocene-based surface cyclic voltammetry waves that remained unchanged for as many as 10(4) scans. Ferrocene derivatives in which the ethynyl moiety was separated from the cyclopentadienyl ring by an ether group showed very similar behavior. DFT calculations were performed on the relevant redox states of 1-H, 1-Li, and 1, with emphasis on the ferrocenyl vs ethynyl character of their high valence orbitals. Whereas the HOMOs of both 1-H and 1-Li have some ethynyl character, the SOMOs of the corresponding monocations are strictly ferrocenium in makeup. Predominant ethynyl character returns to the highest valence orbitals after loss of Li(+) from [1-Li](+) or loss of H(+) from [1-H](2+). These anodic processes hold promise for the controlled chemical modification of carbon and other electrode surfaces by a variety of ethynyl or alkynyl-linked organic and metal-containing systems.

Anodic Oxidation of Ethynylferrocene Derivatives in Homogeneous Solution and Following Anodic Deposition onto Glassy Carbon Electrodes

“…The anodic deposition processes of compounds 1 , 4 , and 5 have already been partially described . The present paper offers additional examples of ethynyl‐attached ferrocene derivatives and provides new information on how the electrolyte medium may affect both the surface deposition process and the voltammetry of the surface‐bound species.…”

“…Recently it has been shown that radical‐based electrografting processes are effective in preparing chemically modified electrodes to which molecules are covalently attached through an ethynyl group . We have previously described an assortment of methods whereby metallocenes derivatized at a cyclopentadienyl ring with a terminal ethynyl group can be covalently attached to carbon surfaces .…”

“…In some cases, the so‐called “direct oxidation” method was also utilized. In view of the fact that the advantages and disadvantages of these two methods have been previously detailed, only the basic ideas behind them will be reviewed here.…”

“…These include the depositions of a nickel cyclam complex that is an effective catalyst for CO 2 reduction, using the direct oxidation method, and several calixarenes anchored through multiple ethynyl groups, using the lithio activation method . Both Sheridan et al . and Jouikov and Simonet have shown that the direct oxidation method may be successful even when the leaving group of a terminal ethynyl moiety is SiMe 3 + .…”

Effect of Electrolyte Ions on the Formation, Electroactivity, and Rectification Properties of Films Obtained by Electrografting

“…The covalent modification of carbon surfaces by electrografting began in the last decade of the past century with the oxidation of primary amines and the reduction of aryldiazonium cations, which allowed the attachment of organic moieties on carbon surfaces through N−C and C−C bonds respectively . After these pioneering studies, a wide variety of alternative electrografting methods have been developed, which involve the generation of free radicals at the electrode‐solution interface, either in reduction or oxidation processes . An advantage of these covalently modified surfaces is the fact that they have high chemical and physical stability and show similar properties to the attached molecules.…”