All-alkynyl-protected coinage metal nanoclusters: from synthesis to electrocatalytic CO2 reduction applications

Atomically precise metal nanoclusters have been attracting considerable research interests in the last two decades, thanks to their quantum confinement effect and molecule-like properties. Such properties are significantly affected, if not dominated, by the interfacial metal-ligand bonding motifs. Recently, ligand engineering has been applied to alkynyl molecules, as they can bind to metal atoms with sigma and/or pi bonds, yielding versatile interfacial bonding motifs hence drastically different properties and functionalities can be realized, as compared to the most commonly employed thiolate ligands. This review first describes the unique advantages of alkynyl-protected metal nanoclusters, with elaboration on the comparison of the interfacial coordination mode and the optical features between thiolate/alkynyl protection. Following that, the recent progress regarding the synthetic strategy is discussed, with an emphasis on the direction reduction method and the synchronous nucleation and passivation strategy. Next, alkynyl-protected metal nanoclusters for the electrochemical CO2 reduction reaction (eCO(2)RR) are mainly discussed, with some explicit examples to elucidate the metal core effect and surface ligand effect as well as to explain the structure-performance relationship. At last, the challenges and perspectives from synthesis to the eCO(2)RR of alkynyl-protected metal nanoclusters are proposed. We envision this review to stimulate more research efforts to be dedicated to developing effective synthetic strategies and advancing the catalytic mechanistic understanding of alkynyl-protected metal nanoclusters toward the eCO(2)RR and beyond.