Electrochemically activated carbon–halogen bond cleavage and C–C coupling monitored by in situ shell-isolated nanoparticle-enhanced Raman spectroscopy.

The electroreductive cleavage of carbon–halogen bonds has attracted increasing attention in both electrosynthesis and pollution remediation. Herein, by employing the in situ electrochemical shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) technique, we have successfully investigated the electroreductive dehalogenation process of aryl halides with the thiol group on a smooth Au electrode in aqueous solution at different pH values. The obtained potential-dependent Raman spectra directly reveal a mixture of the reduction products 4,4′-biphenyldithiol (BPDT) and thiophenol (TP). The conversion ratios of the C–Cl and C–Br bonds at pH = 7 are 37% and 55%, respectively. Furthermore, quantitative analysis of the intensity variations of ν(C–Cl), ν(C–Br) and aromatic ν(CC) stretching modes suggests electroreductive dehalogenation via both direct electron transfer reduction and electrocatalytic hydrodehalogenation. Molecular evidence for the C–C cross coupling process through TP reaction with benzene free radical intermediates is found at negative potentials, which leads to the increasing selectivity of biphenyl products. [ABSTRACT FROM AUTHOR]