Arene-Bridged Dithorium Complexes: Inverse Sandwiches Supported by a δ Bonding Interaction

A series of arene-bridged dithorium complexes was synthesized via the reduction by potassium graphite of a Th(IV) precursor in the presence of arenes. All these compounds adopt an inverse-sandwich structure, with the arene bridging two thorium centers in a μ-η⁶,η⁶-mode. Structural and spectroscopic data support the assignment of two Th(IV) ions and an arene tetraanion, which is an aromatic structure according to Hückel's rule. Arene exchange reactions revealed that the stability of the corresponding compounds follows the series naphthalene ≪ toluene < benzene ≈ biphenyl. Reactivity studies showed that they function as four-electron reductants capable to reduce anthracene, cyclooctatetraene, alkynes, and azobenzene, while a mononuclear thorium anthracene complex could reduce benzene. Density functional theory calculations unveiled that the bonding interactions consist of δ bonds between thorium 6d and 5f orbitals and arene π* orbitals, showing a significant covalent character, able to stabilize highly reduced arene ligands.