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

Despite the vast array of η(n)-carbocyclic C(5–8) complexes reported for actinides, cyclobutadienyl (C(4)) remain exceedingly rare, being restricted to six uranium examples. Here, overcoming the inherent challenges of installing highly reducing C(4)-ligands onto actinides when using polar starting materials such as halides, we report that reaction of [Th(η(8)-C(8)H(8))(2)] with [K(2){C(4)(SiMe(3))(4)}] gives [{Th(η(4)-C(4)[SiMe(3)](4))(μ-η(8)-C(8)H(8))(μ-η(2)-C(8)H(8))(K[C(6)H(5)Me](2))}(2){K(C(6)H(5)Me)}{K}] (1), a new type of heteroleptic actinocene. Quantum chemical calculations suggest that the thorium ion engages in π- and δ-bonding to the η(4)-cyclobutadienyl and η(8)-cyclooctatetraenyl ligands, respectively. Furthermore, the coordination sphere of this bent thorocene analogue is supplemented by an η(2)-cyclooctatetraenyl interaction, which calculations suggest is composed of σ- and π-symmetry donations from in-plane in- and out-of-phase C[double bond, length as m-dash]C 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.