Homoleptic Aryl Complexes of Uranium (IV)

An Allyl Uranium(IV) Sandwich Complex: Are ϕ Bonding Interactions Possible?

A method to explore head‐to‐head ϕ back‐bonding from uranium f‐orbitals into allyl π* orbitals has been pursued. Anionic allyl groups were coordinated to uranium with tethered anilide ligands, then the products were investigated by using NMR spectroscopy, single‐crystal XRD, and theoretical methods. The (allyl)silylanilide ligand, N‐((dimethyl)prop‐2‐enylsilyl)‐2,6‐diisopropylaniline (LH), was used as either the fully protonated, singly deprotonated, or doubly deprotonated form, thereby highlighting the stability and versatility of the silylanilide motif. A free, neutral allyl group was observed in UI₂(L1)₂ (1), which was synthesized by using the mono‐deprotonated ligand [K][N‐((dimethyl)prop‐2‐enyl)silyl)‐2,6‐diisopropylanilide] (L1). The desired homoleptic sandwich complex U[L2]₂ (2) was prepared from all three ligand precursors, but the most consistent results came from using the dipotassium salt of the doubly deprotonated ligand [K]₂[N‐((dimethyl)propenidesilyl)‐2,6‐diisopropylanilide] (L2). This allyl‐based sandwich complex was studied by using theoretical techniques with supporting experimental spectroscopy to investigate the potential for phi (ϕ) back‐bonding. The bonding between U^IV and the allyl fragments is best described as ligand‐to‐metal electron donation from a two carbon fragment‐localized electron density into empty f‐orbitals.

Synthesis and Characterization of Two Uranyl-Aryl "Ate" Complexes

Reaction of [UO₂ Cl₂ (THF)₃ ] with 3 equivalents of LiC₆ Cl₅ in Et₂ O resulted in the formation of first uranyl aryl complex [Li(Et₂ O)₂ (THF)][UO₂ (C₆ Cl₅ )₃ ] ([Li][1]) in good yields. Subsequent dissolution of [Li][1] in THF resulted in conversion into [Li(THF)₄ ][UO₂ (C₆ Cl₅ )₃ (THF)] ([Li][2]), also in good yields. DFT calculations reveal that the U-C bonds in [Li][1] and [Li][2] exhibit appreciable covalency. Additionally, the ¹³ C NMR chemical shifts for their C_ipso environments are strongly affected by spin-orbit coupling-a consequence of 5f orbital participation in the U-C bonds.

The Exceptional Diversity of Homoleptic Uranium-Methyl Complexes

Homoleptic σ-bonded uranium-alkyl complexes have been a synthetic target since the Manhattan Project. The current study describes the synthesis and characterization of several unprecedented uranium-methyl complexes. Amongst these complexes, the first example of a homoleptic uranium-alkyl dimer, [Li(THF)₄ ]₂ [U₂ (CH₃ )₁₀ ], as well as a seven-coordinate uranium-methyl monomer, {Li(OEt₂ )Li(OEt₂ )₂ UMe₇ Li}_n were both crystallographically identified. The diversity of complexes reported herein provides critical insight into the structural diversity, electronic structure and bonding in uranium-alkyl chemistry.