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

We introduce the boryloxide ligand {(HCNDipp) 2 BO} - (NBO Dipp , Dipp = 2,6-di-isopropylphenyl) to actinide chemistry. Protonolysis of [U{N(SiMe 3 ) 2 } 3 ] with 3 equiv of NBO Dipp H produced the uranium(III) tris(boryloxide) complex [U(NBO Dipp ) 3 ] ( 1 ). In contrast, treatment of UCl 4 with 3 equiv of NBO Dipp K in THF at room temperature or reflux conditions produced only [U(NBO Dipp ) 2 (Cl) 2 (THF) 2 ] ( 2 ) with 1 equiv of NBO Dipp K remaining unreacted. However, refluxing the mixture of 2 and unreacted NBO Dipp K in toluene instead of THF afforded the target complex [U(NBO Dipp ) 3 (Cl)(THF)] ( 3 ). Two-electron oxidation of 1 with AdN 3 (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 π 4 and π 5 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 s ⟩ states and m j projections. Complexes 1 - 4 have been variously characterized by single-crystal X-ray diffraction, 1 H 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.