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 (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 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.

Dominance of Cyclobutadienyl Over Cyclopentadienyl in the Crystal Field Splitting in Dysprosium Single‐Molecule Magnets

“…The silyl substituents are also prone to undergoing activation by deprotonation. Compounds 1 – 3 therefore expand the very small family of lanthanide and actinide complexes of the pristine Cb′′′′ ligand, [38–41] and represent the first lanthanide metallocene‐like sandwich complexes of such a ligand. Viewed from the perspective of improving SMM properties, the broader significance of complex 1 is that if a method of removing the equatorial borohydride ligand can be devised, the (currently) hypothetical species [(η 4 ‐Cb′′′′)Dy(η 5 ‐C 5 Me 4 t Bu)] could be synthesized.…”

Anomalous magnetism of uranium(IV)-oxo and -imido complexes reveals unusual doubly degenerate electronic ground states

“…The answer to (1) appears to be predominantly related to the presence of di-or trianionic ligands [23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39] ; indeed, we have previously found that pseudo-C 3v uranium(V) complexes with terminal nitrido-and oxo ligands tend to behave as belonging to a high-symmetry point group, unlike what their C 1 structures would dictate. 51,57 We found that some of the nitrido-complexes have electron paramagnetic resonance (EPR)-silent m J = G3/2 ground states (silent in true C 3v symmetry), with excited m J = G5/2 states observable by EPR (active even in true C 3v symmetry) lying within a few tens of cm À1 , 51 given the very sensitive nature of EPR, and that we could only observe EPR transitions in the excited state, the ground m J = G3/2 states must be very pure.…”

“…[19][20][21] Outside of ''simple'' high-symmetry complexes, sufficiently low-symmetry coordination geometries will usually exhibit a singlet (A) spin-orbit ground state 5,7,22 ; this is because uranium(IV) has two unpaired electrons and is thus a non-Kramers ion; hence, there is no requirement for any electronic degeneracies in the J = 4 spin-orbit multiplet after the action of the CF, because the CF effect for 5f-orbitals is significant. However, there are now a growing number of formally low-symmetry uranium(IV) complexes where with innocent ligands the low-temperature (<2 K) magnetic moments are quite high (R1 m B ), [23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39] suggesting that something is differentiating these complexes. Empirically, singlet ground states tend to be observed with monoanionic ligands, regardless of the (pseudo)symmetry, and higher magnetic moments have increasingly been observed at low temperatures when stronger di-or trianionic ligands are present, implying a (pseudo)doublet (E) spin-orbit ground state.…”