Heteroleptic actinocenes: a thorium(iv)–cyclobutadienyl–cyclooctatetraenyl–di-potassium-cyclooctatetraenyl complex

Abstract

Despite the vast array of ηⁿ-carbocyclic C_5–8 complexes reported for actinides, cyclobutadienyl (C₄) remain exceedingly rare, being restricted to six uranium examples. Here, overcoming the inherent challenges of installing highly reducing C₄-ligands onto actinides when using polar starting materials such as halides, we report that reaction of [Th(η⁸-C₈H₈)₂] with [K₂{C₄(SiMe₃)₄}] gives [{Th(η⁴-C₄[SiMe₃]₄)(μ-η⁸-C₈H₈)(μ-η²-C₈H₈)(K[C₆H₅Me]₂)}₂{K(C₆H₅Me)}{K}] (1), a new type of heteroleptic actinocene. Quantum chemical calculations suggest that the thorium ion engages in π- and δ-bonding to the η⁴-cyclobutadienyl and η⁸-cyclooctatetraenyl ligands, respectively. Furthermore, the coordination sphere of this bent thorocene analogue is supplemented by an η²-cyclooctatetraenyl interaction, which calculations suggest is composed of σ- and π-symmetry donations from in-plane in- and out-of-phase CC 2p-orbital combinations to vacant thorium 6d orbitals. The characterisation data are consistent with this being a metal–alkene-type interaction that is integral to the bent structure and stability of this complex.