Suppressing Isomerization of Phosphine-Protected Au₉ Cluster by Bond Stiffening Induced by a Single Pd Atom...

The fluxional nature of small gold clusters has been exemplified by reversible isomerization between [Au₉(PPh₃)₈]³⁺ with a crown motif (Au₉(C)) and that with a butterfly motif (Au₉(B)) induced by association and dissociation with compact counteranions (NO₃^-, Cl^-). However, structural isomerization was suppressed by substitution of the central Au atom of the Au₉ core in [Au₉(PPh₃)₈]³⁺ with a Pd atom: [PdAu₈(PPh₃)₈]²⁺ with a crown motif (PdAu₈(C)) did not isomerize to that with a butterfly motif (PdAu₈(B)) upon association with the counteranions. Density functional theory calculation showed that the energy difference between PdAu₈(C) and PdAu₈(B) is comparable to that between Au₉(C) and Au₉(B), indicating that the relative stabilities of the isomers are not a direct cause for the suppression of isomerization. Temperature dependence of Debye-Waller factors obtained by X-ray absorption fine-structure analysis revealed that the intracluster bonds of PdAu₈(C) were stiffer than the corresponding bonds in Au₉(C). Natural bond orbital analysis suggested that the radial Pd-Au and lateral Au-Au bonds in PdAu₈(C) are stiffened due to the increase in the ionic nature and decrease in electrostatic repulsion between the surface Au atoms, respectively. We conclude that the formation of stiffer metal-metal bonds by Pd atom doping inhibits the isomerization from PdAu₈(C) to PdAu₈(B).