σ-Aromaticity-Induced Stabilization of Heterometallic Supertetrahedral Clusters [Zn6 Ge16 ]4- and [Cd6 Ge16 ]4.

In this work, the largest heterometallic supertetrahedral clusters, [Zn6 Ge16 ]4- and [Cd6 Ge16 ]4- , were directly self-assembled through highly-charged [Ge4 ]4- units and transition metal cations, in which 3-center-2-electron σ bonding in Ge2 Zn or Ge2 Cd triangles plays a vital role in the stabilization of the whole structure. The cluster structures have an open framework with a large central cavity of diameter 4.6 Å for Zn and 5.0 Å for Cd, respectively. Time-dependent HRESI-MS spectra show that the larger clusters grow from smaller components with a single [Ge4 ]4- and ZnMes2 units. Calculations performed at the DFT level indicate a very large HOMO-LUMO energy gap in [M6 Ge16 ]4- (2.22 eV), suggesting high kinetic stability that may offer opportunities in materials science. These observations offer a new strategy for the assembly of heterometallic clusters with high symmetry.