Electroreduction Ability of Organoborohydride Compounds

Abstract

Sodium borohydride (BH, NaBH4) has wide application as a reducing agent in organic and inorganic synthesis. Herein, sodium benzylborohydride (BBH, NaC7H7BH3), sodium phenylborohydride (PBH, NaC6H5BH3) and sodium 4-chlorophenylborohydride (CPBH, NaC6H4BH3Cl) are synthesized with high purity, as confirmed by NMR and FTIR. A direct relationship between the organic substituents and the reducing power of the borohydride center is investigated by electrochemical methods. Gold (Au) and platinum (Pt) disk electrodes are used to study the electrooxidation of these organoborohydride (OGB) compounds in dimethyl sulfoxide. Linear scan voltammetry and chronoamperometry measurements are carried out in 0.001–0.01 M solutions of each OGB at temperatures ranging from 25 to 65°C. The results are directly compared to those obtained with BH solutions in the same conditions. A Pt microelectrode is used to perform diffusion coefficient measurements, whose values range from 9.10 × 10−8 cm2 s−1 for CPBH to 2.87 × 10−7 cm2 s−1 for BH. The number of exchanged electrons range between 1.6 for CPBH to 6.6 to BH and charge transfer coefficients are ca. 0.9–1.0. First order reactions are evaluated for the electrooxidation of the four studied borohydride compounds and the corresponding apparent activation energies range between 15 and 33 kJ mol−1.