Alkynyl and halogen co-protected (AuAg) 44 nanoclusters: a comparative study on their optical absorbance, structure, and hydrogen evolution performance.

We report the synthesis, structure, and electrochemical hydrogen evolution reaction (HER) performance of two alkynyl and halogen coprotected AuAg alloy nanoclusters, namely Au 24 Ag 20 ( t BuPh-CC) 24 Cl 2 (NC 1 for short) and Au 22 Ag 22 ( t BuCC) 16 Br 3.28 Cl 2.72 (NC 2 for short). Single crystal X-ray structural analysis revealed that the two nanoclusters possess a rather similar core@shell@shell keplerate metal core configuration to M 12 @M 20 @M 12 with the main difference in the outermost shell (Au 12 vs. Au 10 Ag 2 ). Interestingly, such a subtle difference in the two-metal-atoms results in different optical absorbance features and drastically different HER performances. Both NCs have excellent long-term stability for the HER, but NC 1 possesses superior activity to NC 2, and density functional theory calculations disclosed that the binding energy of hydrogen to form the key *H intermediate for NC 1 is much lower and hence it adopts a more energetically feasible HER pathway.