Shapeshifting: Ligation by 1,4-cyclohexadiene induces a structural change in Ag5+

Abstract

Relatively little is known about structural transformations of very small metal clusters that result from the adsorption of molecules. Here, the ligand-induced structural transformation of Ag₅⁺(g) by 1,4-cyclohexadiene, which is capable of binding metal clusters as a bidentate ligand, is investigated using equilibrium mass spectrometry experiments and theory. Based on the measured sequential ligand binding free energies of Ag_n⁺(cyclohexene)_m and Ag_n⁺(1,4-cyclohexadiene)_m (n = 3 and 5; m up to 3), it is found that Ag₅⁺(1,4-cyclohexadiene) is a particularly stable cluster relative to the other ion-molecule association complexes investigated. These results together with those from electronic structure calculations suggest that upon addition of 1,4-cyclohexadiene to Ag₅⁺, the metal cluster core undergoes a structural transformation from a “bowtie” structure(s), in which two Ag₂ units are bridged side-on by a central Ag atom, into a bidentate Ag₅⁺(1,4-cyclohexadiene) structure that resembles a “razorback” arrangement of the five Ag atoms. These results raise the prospect of using multidentate ligands to transform larger ionic silver clusters from relatively compact 3D geometries into 2D elongated “razorback” nanowires. However, results from electronic structure calculations for clusters in which the razorback nanowire structural motif is propagated to larger sizes (up to Ag₉⁺) indicate that the energy required to form such templated structures becomes increasingly unfavourable with increasing size. By calculating the vertical and adiabatic ligand binding energies, the competing effects that contribute to the energy required to form such structures, such as the metal cluster reorganization energy, can be quantified. These results indicate that the tendency for metal clusters to form compact shapes dominates other effects that contribute to the energy for forming templated nanowire structures, and this effect dramatically increases with increasing cluster size.