Electrocatalytic Reduction of Heteroaromatic Halides on Gold and Silver Electrodes: Synergy in Electrocatalysis

The electrocatalytic reductive cleavage of carbon–halogen C–X bonds on electrode surfaces with high halide affinity like Ag and Au is of potential great interest in analytical, synthetic and environmental contexts, resulting in many cases in impressive positive shifts of the halide reduction potentials, often with significant changes in mechanism and products, too. [1] The process has also been thoroughly studied as an excellent model for dissociative electron transfer (DET) in electrocatalytic conditions, modulated by many factors, including molecular structure (halide leaving group and aliphatic/benzylic/aromatic residue), working medium (our parallel presentation), as well as electrode surface.[1] In particular, comparing Ag and Au as catalytic electrode materials for the reductive activation of many organic halides, we have observed that Au, in spite of its higher intrinsic affinity for halide ions, generally results in much lower catalytic effects than Ag (in terms of positive peak potential shifts vs glassy carbon GC as non-catalytic reference), which we justified considering that, compared to Ag, Au should have a much more positive pzc in our working media, resulting in higher negative surface charge in the working potential range, and therefore in higher repulsive effects towards halide anions. However, the presence of S or N heteroatoms on the halide organic residue can have a remarkable impact on this situation, as we have recently observed comparing the behaviour of aromatic halides with a rich palette of heteroaromatic ones, like halo- and polyhalo- thiophenes [2], bithiophenes and benzodithiophenes, as well as halo- and polyhalo cyclic triimidazoles. The S or N atoms included in such conjugated systems not only make the aromatic ring(s) asymmetric from the perspective of the electron density, but, importantly, can themselves have specific interactions with the electrode surface, in addition to the interactions of the C–X group undergoing DET; therefore heteroatoms can act as “adsorption auxiliary groups” on the catalytic surface. Thus, as we will show, the presence of heteroatom anchoring groups can promote the electrocatalytic cleavage of C-X bonds on Au and Ag, enhancing the "intrinsic" catalytic effects of the electrode surface. However, such enhancement is much more impressive in the case of the less catalytic Au surface, which appears to recover most of its intrinsically high catalytic activity, overcoming the repulsive effect of the surface charge in the working potential range, thanks to the presence of the heteroatom "anchor(s)". Several cases are particularly impressive, with the Au performance appearing to approach or even overcome the Ag one. Interestingly, such impressive enhancement of the Au catalytic activity can be modulated by the relative position(s) of the heteroatom(s) with respect to the halide leaving group(s) in the molecule undergoing DET. For example, a higher catalytic shift is observed for the reduction of 2-Br-thiophene, having the leaving Br group adjacent to the S atom, as compared to 3-Br-thiophene; instead in the Ag case the leaving group position is less determining, possibly on account of prevailing silver-halide interactions. The support of Fondazione Cariplo/Regione Lombardia "Avviso congiunto per l’incremento dell’attrattività del sistema di ricerca lombardo e della competitività dei ricercatori candidati su strumenti ERC - edizione 2016” (Project 2016-0923) is gratefully acknowledged. [1] A. Gennaro, A.A. Isse, P.R. Mussini, Activation of the Carbon-Halogen Bond, in: O. Hammerich, B. Speiser (Eds.), Organic Electrochemistry 5th Edn., Ch. 24, CRC Press/Taylor & Francis, Boca Raton, 2016. [2] S. Arnaboldi, A. Bonetti, E. Giussani, P.R. Mussini, T. Benincori, S. Rizzo, A.A. Isse, A. Gennaro, Electrochem. Commun. 38 (2014) 100-103.