Electrochemistry of Cobalta Bis(dicarbollide) Ions Substituted at Carbon Atoms with Hydrophilic Alkylhydroxy and Carboxy Groups

Abstract

In this study we explore the effect on the electrochemical signals in aqueous buffers of the presence of hydrophilic alkylhydroxy and carboxy groups on the carbon atoms of cobalta bis(dicarbollide) ions. The oxygen-containing exo-skeletal substituents of cobalta bis(dicarbollide) ions belong to the perspective building blocks that are considered for bioconjugation. Carbon substitution provides wider versatility and applicability in terms of the flexibility of possible chemical pathways. However, until recently, the electrochemistry of compounds substituted only on boron atoms could be studied, due to the unavailability of carbon-substituted congeners. In the present study, electrochemistry in aqueous phosphate buffers is considered along with the dependence of electrochemical response on pH and concentration. The compounds used show electrochemical signals around −1.3 and +1.1 V of similar or slightly higher intensities than in the parent cobalta bis(dicarbollide) ion. The signals at positive electrochemical potential correspond to irreversible oxidation of the boron cage (the C2B9 building block) and at negative potential correspond to the reversible redox process of (CoIII/CoII) at the central atom. Although the first signal is typically sharp and its potential can be altered by a number of substituents, the second signal is complex and is composed of three overlapping peaks. This signal shows sigmoidal character at higher concentrations and may be used as a diagnostic tool for aggregation in solution. Surprisingly enough, the observed effects of the site of substitution (boron or carbon) and between individual groups on the electrochemical response were insignificant. Therefore, the substitutions would preserve promising properties of the parent cage for redox labelling, but would not allow for the further tuning of signal position in the electrochemical window.

Chemistry of Carbon-Substituted Derivatives of Cobalt Bis(dicarbollide)(1−) Ion and Recent Progress in Boron Substitution

“…Growing attention to the use of the cobalt bis(dicarbollide) anion in biological applications led to the first attempts of electrochemistry in the aqueous milieu [11,12,14,153]. The authors report not only the reversible redox signal of the Co(III) central atom (as was reported previously in non-aqueous media [240]) but also the irreversible signal of the core boron cage (Figure 34) [63,82,244]. The electrochemical response for the parent cobalt bis(dicarbollide) anion (using a polished glassy carbon electrode in a phosphate buffer of pH = 8) is rather complicated.…”

“…Cobalt bis(dicarbollide) can be reduced to its Co(II) dianion by sodium amalgam or metallic cesium and, subsequently, oxidized back into its Co(III) state [46]. Its ability to undergo a Co(II/III) redox process makes it suitable for use in electrochemical and electronic applications [63]. Its ability to undergo a reversible 1e − oxidation 1e − reduction cycle was first reported in 1971, suggesting an Electron transfer, Chemical reaction, and Electron transfer (ECE-type) mechanism supported by cyclic voltammetry [64].…”

“…This behavior could be explained by the presence of the three main rotamers of the parent cobalt bis(dicarbollide) anion, where the short bridging linker does not allow the transition between the different states. Study [63] compares the electrochemical behavior of B-and C-substituted derivatives, namely the alkylhydroxy and carboxy groups. Although the electronic properties of the C-and B-derivatives (with comparable ligand structure) markedly differ, this work does not find any significant difference between the electrochemical behavior of these derivatives.…”