Aromaticity in a Palladium-Capped Al Trigonal Prismatic Cluster.

Aromaticity, a fundamental concept for understanding molecular stability, has long been established in organic chemistry. However, isolating examples of all-metal aromaticity has remained a rare achievement, despite its initial proposal in an Al cluster two decades ago. In this study, we successfully isolated and characterized the first all-metal aluminum cluster with a trigonal prismatic geometry. X-ray crystallography reveals that each rectangular face of the Al trigonal prism is capped by a Pd atom, which is further stabilized by six chelating monoanionic ligands and two counter potassium ions. The Pd-capped Al trigonal prismatic cluster features three 3c-2e σ bonds in each of the top and bottom Al layers, and a unique 6c-2e π-π interacting bond connecting the two Al faces. Quantum chemical calculations provide strong evidence for aromaticity in the Pd-capped Al trigonal prism. This discovery not only extends the concept of aromaticity to polyhedral aluminum clusters but also opens new avenues for the synthesis and exploration of novel all-metal aromatic systems.