f-Element Half-Sandwich Complexes: A TetrasilylcyclobutadienylUranium(IV)-Tris(tetrahydroborate) Anion Pianostool Complex

Despite there being numerous examples of f-element compounds supported by cyclopentadienyl, arene, cycloheptatrienyl, and cyclooctatetraenyl ligands (C5-8), cyclobutadienyl (C4) complexes remain exceedingly rare. Here, we report that reaction of [Li2{C4(SiMe3)4}(THF)2] (1) with [U(BH4)3(THF)2] (2) gives the pianostool complex [U{C4(SiMe3)4}(BH4)3][Li(THF)4] (3), where use of a borohydride and preformed C4-unit circumvents difficulties in product isolation and closing a C4-ring at uranium. Complex 3 is an unprecedented example of an f-element half-sandwich cyclobutadienyl complex, and it is only the second example of an actinidecyclobutadienyl complex, the other being an inverse-sandwich. The UC distances are short (av. 2.513 Å), reflecting the formal 2- charge of the C4-unit, and the SiMe3 groups are displaced from the C4-plane, which we propose maximises U-C4 orbital overlap. DFT calculations identify two quasi-degenerate U-C4 π -bonds utilising the ψ 2 and ψ 3 molecular orbitals of the C4-unit, but the potential δ -bond using the ψ 4 orbital is vacant.