f-Element Half-Sandwich Complexes: A Tetrasilylcyclobutadienyl-Uranium(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 (C-4) complexes remain exceedingly rare. Here, we report that reaction of [Li-2{C-4(SiMe3)(4)}(THF)(2)] (1) with [U(BH4)(3)(THF)(2)] (2) gives the pianostool complex [U{C-4(SiMe3)(4)}(BH4)(3)][Li(THF)(4)] (3), where use of a borohydride and preformed C-4-unit circumvents difficulties in product isolation and closing a C-4-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 angstrom), reflecting the formal 2- charge of the C-4-unit, and the SiMe3 groups are displaced from the C-4-plane, which we propose maximises U-C-4 orbital overlap. DFT calculations identify two quasi-degenerate U-C-4 pi-bonds utilising the psi(2) and psi(3) molecular orbitals of the C-4-unit, but the potential delta-bond using the psi(4) orbital is vacant.