Ligand-free supermolecules: [Pd₂@Ge₁₈]⁴⁻ and [Pd₂@Sn₁₈]⁴⁻ as multiple-bonded Zintl-ion clusters based on Pd@Ge₉ and Pd@Sn₉ assembled units

Understanding intercluster bonding interactions is important in the rational synthesis of building blocks for molecular materials. Such characteristics have been developed for coinage metal clusters resembling single-, double-, and triple-bonded species, coined as supermolecules. Herein, we extend such an approach for understanding main-group clusters, thus evaluating [Pd₂@E₁₈]⁴⁻ clusters (E = Ge, Sn) involving the fusion of parent spherical aromatic [Pd@E₁₂]²⁻ building units. Our results indicate intercluster bonding provided by contribution from 2P and 1G shells centered at each building motif, leading to an overall bond order of 2.70 and 2.31 for [Pd₂@Ge₁₈]⁴⁻ and [Pd₂@Sn₁₈]⁴⁻, respectively. In addition, ¹¹⁹Sn-NMR patterns were evaluated to complement the experimental characterization of a single peak owing to the insolution fluxional behavior of [Pd₂@Sn₁₈]⁴⁻ as three peaks owing to the three sets of unique Sn atoms within the structure. Magnetic response properties revealed that spherical aromatic characteristics from parent [Pd@E₁₂]²⁻ building units are retained in the overall [Pd₂@E₁₈]⁴⁻ oblate cluster as two spherical aromatic units. Hence, the notion of superatomic molecules is extended to Zintl-ion clusters, favoring further rationalization for the fabrication of cluster-assembled solids.