Hydrophobic vitamin B₁₂. Part 18. Preparation of a sol–gel modified electrode trapped with a vitamin B₁₂ derivative and its photoelectrochemical reactivity

A vitamin B₁₂ derivative, heptapropyl cobyrinate perchlorate, was readily trapped onto an indium tin oxide (ITO) electrode by a sol–gel reaction. The complex was physically retained in a silica gel film which is formed on an ITO electrode. The thickness of the film could be controlled by the withdrawing speed of the dip coating process. Formation of a sol–gel film was confirmed by SEM measurements, and the total amount of the complex in the film was determined by UV–VIS absorption spectra. The complex exhibits the Co^II/Co^I redox couple at −0.42 V vs. Ag–AgCl. The amount of the electroactive complex in the sol–gel film deduced from electrochemical measurements is 3.0 × 10⁻¹¹ and 6.2 × 10⁻¹¹ mol cm⁻² for thicknesses of 170 and 330 nm, respectively. This electroactive complex shows a high reactivity towards organic halides, and the controlled-potential electrolysis of benzyl bromide using the sol–gel modified electrode at −1.20 V vs. Ag–AgCl in aqueous solution containing 0.1 M KCl afforded dehalogenated products, bibenzyl and toluene, with a total turnover number of >1000 for 1 h.