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

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).