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.