Arene-, Chlorido-, and Imido-Uranium Bis- and Tris(boryloxide) Complexes

We introduce the boryloxide ligand {(HCNDipp)₂BO}^- (NBO^Dipp, Dipp = 2,6-di-isopropylphenyl) to actinide chemistry. Protonolysis of [U{N(SiMe₃)₂}₃] with 3 equiv of NBO^DippH produced the uranium(III) tris(boryloxide) complex [U(NBO^Dipp)₃] (1). In contrast, treatment of UCl₄ with 3 equiv of NBO^DippK in THF at room temperature or reflux conditions produced only [U(NBO^Dipp)₂(Cl)₂(THF)₂] (2) with 1 equiv of NBO^DippK remaining unreacted. However, refluxing the mixture of 2 and unreacted NBO^DippK in toluene instead of THF afforded the target complex [U(NBO^Dipp)₃(Cl)(THF)] (3). Two-electron oxidation of 1 with AdN₃ (Ad = 1-adamantyl) afforded the uranium(V)-imido complex [U(NBO^Dipp)₃(NAd)] (4). The solid-state structure of 1 reveals a uranium-arene bonding motif, and structural, spectroscopic, and DFT calculations all suggest modest uranium-arene δ-back-bonding with approximately equal donation into the arene π₄ and π₅ δ-symmetry π* molecular orbitals. Complex 4 exhibits a short uranium(V)-imido distance, and computational modeling enabled its electronic structure to be compared to related uranium-imido and uranium-oxo complexes, revealing a substantial 5f-orbital crystal field splitting and extensive mixing of 5f |m_l,m⟩ states and m projections. Complexes 1-4 have been variously characterized by single-crystal X-ray diffraction, ¹H NMR, IR, UV/vis/NIR, and EPR spectroscopies, SQUID magnetometry, elemental analysis, and CONDON, F-shell, DFT, NLMO, and QTAIM crystal field and quantum chemical calculations.