Electronic Delocalization and σ-Aromaticity in Heterometallic Cluster with Multiple Thorium-Palladium Bonds

Research on metal-metal bonds involving f-block actinides, such as thorium, lags far behind the well-studied metal-metal bonds of d-block transition metals. The complexes with Th-TM bonds are extremely rare; all previously identified examples have only a single Th-TM bond with the Th center at an invariably +IV oxidation state. Herein, we report a series of Th₂Pd (n = 2, 3, and 6) clusters (complexes 3, 4, and 7) with multiple Th(III)-Pd bonds. Theoretical studies reveal that the Th₂Pd unit allows electronic delocalization and σ aromaticity, leading to unexpected closed-shell singlet structures for these Th(III) species. This electronic delocalization is evident in the highest occupied molecular orbital of Th(III) complexes and facilitates a 2e reduction of alkyne by complex 7, resulting in the formation of 8. Complexes 7 and 8 are distinctive in featuring a Th₂Pd₆ core with six and eight Th-Pd bonds, respectively, making them the largest known d-f heterometallic clusters exhibiting metal-metal bonding.