Sodium borohydride (NaBH₄) is a solid white powder that finds wide application as a reducing agent in organic and inorganic synthesis [1]. This compound possesses relatively mild reducing abilities, but certain applications call for reducing powers greater than NaBH₄, while others require the reducing powers to be attenuated. Organic chemists have successfully tuned the reducing power of NaBH₄by attaching organic electron donating or withdrawing groups to the boron center, which can also drive regio- and stereoselectivity.

These organoborohydrides have received much recent attention for their role in frustrated Lewis pair systems [2], as well as in hydrogen fuel carriers. Computational methods have established the tuning power of different organic substituents on the boron center, but experimentally quantifying reducing power remains difficult [3]. The present study seeks to quantify reducing power using electrochemical methods and elucidate the relationship between organic tuning group and reducing ability.

In this work, benzylborohydride (C₇H₇BH₃Na) was prepared by reduction of potassium benzyltrifluoroborate with a metal hydride. The compound structure was further characterized by multinuclear NMR and FTIR. Whereas, the electrochemical characterization was carried out using a standard three-electrode setup, by cyclic voltammetry (CV) in static conditions and by linear scan voltammetry (LSV) using a rotating ring-disk electrode (RRDE) system. Platinum and gold disk macroelectrodes and microelectrodes were used to study the electrooxidation of these compounds. The organoborohydrides were tested in DMSO + 0.03 M C₇H₇BH₃Na at temperatures ranging from 25 to 65 ºC. The results were directly compared with NaBH₄solutions in the same experimental conditions.

The main electrochemical parameters were calculated, including number of exchanged electrons, charge transfer coefficient, reaction rate constant, and diffusion coefficient for the selected range of temperatures. Moreover, the activation energy for the benzylborohydride oxidation reaction was calculated assuming Arrhenius behavior.

The formation of intermediate compounds was checked by RRDE studies, elucidating the possible steps involved in the oxidation mechanism. Several other organoborohydride compounds are planned to be studied and that will allow finding a direct relationship between organic substituents and the reducing power of the borohydride center.

1. H.O. House, Modern synthesis reactions, Benjamin, W.A., Menlo Park, CA, 1972.

2. J. Paradies, Metal-free hydrogenation of unsaturated hydrocarbons employing molecular hydrogen, Angew. Chem. Int. Ed. 53 (2014) 3552.

3. Z.M Heiden, A.P. Lathem, Establishing the hydride donor abilities of main group hydrides, Organometallics 34 (2015) 1818.