f‐Element Half‐Sandwich Complexes: A Tetrasilylcyclobutadienyl–Uranium(IV)–Tris(tetrahydroborate) Anion Pianostool Complex

Abstract

Despite there being numerous examples of f-element compounds supported by cyclopentadienyl, arene, cycloheptatrienyl, and cyclooctatetraenyl ligands (C_5-8 ), 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 actinide-cyclobutadienyl complex, the other being an inverse-sandwich. The U-C 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 ψ₂ and ψ₃ molecular orbitals of the C₄ -unit, but the potential δ-bond using the ψ₄ orbital is vacant.