Construction of a homologous series of metal nanoclusters and implications for structure-activity correlations

The precise structures of metal nanoclusters (NCs) can serve as an accurate model for the establishment of a structure-activity correlation, thereby attracting attention as a new type of heterogeneous catalyst. However, it remains challenging to definitively establish the relationship between the structure of local sites on NCs and catalytic activity. Herein, a homologous series of metal NCs were constructed for the structure-activity correlation. Four NCs including Au₂₄(SAdm)₁₄(OPDP)₁ (Au₂₄), Au₂₃Cd₁(SR)₁₄(OPDP)₁Cl (Au₂₃Cd₁), Au₂₂Cd₂(SAdm)₁₄(OPDP)₁Cl₂ (Au₂₂Cd₂) and Au₂₁Cd₂(SAdm)₁₃(OPDP)₁Cl₂ (Au₂₁Cd₂) (HSAdm = 1-adamantanethiol; OPDP = (oxydi-2,1-phenylene)bis(diphenylphosphine)) were synthesized. These four NCs exhibit the same common Au₂₁M₁(SR)₁₀(OPDP)₁ (M = Au/Cd) structural skeleton but two different surface motifs. These two motifs in the four NCs exhibit regular evolution mode, that is, 2[Au₁(SR)₂] for Au₂₄, [Au₁(SR)₂] + [Cd₁(SR)₃Cl] for Au₂₃Cd₁, 2[Cd₁(SR)₃Cl] for Au₂₂Cd₂, and [Cd₁(SR)₃Cl] + [CdCl] for Au₂₁Cd₂. The electrocatalytic oxygen reduction reaction (ORR) is performed with these four NCs as catalysts, revealing a catalytic activity sequence of Au₂₁Cd₂ > Au₂₂Cd₂ > Au₂₃Cd₁ > Au₂₄. Based on the differences in surface motifs, the structure-activity relationship can be rigorously correlated with certain motif types, showing an activity sequence of [CdCl] > [Cd₁(SR)₃Cl] > [Au₁(SR)₂]. Moreover, compared with the other three NCs, Au₂₁Cd₂ showed a higher electron transfer number and a smaller Tafel slope in catalyzing the ORR, indicating a different kinetic reaction process and higher reactivity of [CdCl] than [Cd₁(SR)₃Cl] and [Au₁(SR)₂], which revealed a configuration effect in determining catalytic activity of NCs.