On the Mechanism of Electrocatalytic Carbon–Carbon Coupling of Conjugated Aromatic Aldehydes on Cu Cathodes

In this work, we show that Cu effectively catalyzes the electroreductive C-C coupling of <i>only</i> conjugated aromatic aldehydes (like benzaldehyde). We demonstrate that C-C coupling is not observed for aliphatic aldehydes (like pentanal), nonconjugated aromatic aldehydes (like hydrocinnamaldehyde), or conjugated nonaromatic aldehydes (like crotonaldehyde). The fact that only conjugated aromatic aldehydes undergo C-C coupling on Cu points to the importance of their planar structure and, consequently, their strong interaction with the metal surface. This direct interaction enables electron transfer that eventually leads to the Eley-Rideal (ER)-type reaction with the electrophilic carbon of the reacting physisorbed aldehyde molecule. Two additional factors are indispensable for the C-C coupling pathway: (i) the preferential first hydrogenation of the carbonyl oxygen resulting in the formation of a hydroxy intermediate (i.e., ArCHOH*), and (ii) a relatively slow second H addition resulting in a stable hydroxy intermediate on the surface. In contrast, when other metals or nonconjugated aldehydes are involved, preferential hydrogenation of the carbonyl carbon, fast second H addition, or a high intrinsic barrier for C-C bond formation inhibits the C-C coupling pathway.