Reduced Uranium Complexes: Synthetic and DFT Study of the Role of π Ligation in the Stabilization of Uranium Species in a Formal Low-Valent State

Reaction of UCl₄(THF)₄ with 1,3-[2,5-(i-Pr)₂PhNC(CH₂)]₂C₆H₄Li₂ produced a complex formulated as [{1,3-[2,5-(i-Pr)₂PhNC(CH₂)]₂C₆H₄}UCl₃][Li(THF)₄] (1) that exhibits a nonagostic interaction between one of the carbon atoms of the central phenyl ring and the U metal center. This interaction leads to significant weakening of the corresponding C−H bond, thereby facilitating proton removal in consecutive transformations. Attempts to form trivalent uranium derivatives were carried out by reacting the same ligand dianion with in situ-prepared “UCl₃”. The reaction indeed afforded a trivalent species formulated as {1,3-[2,5-(i-Pr)₂PhNC(CH₂)]₂C₆H₄}U(μ-Cl)₃[Li(THF)₂]₂ (2). The configuration of the ligand system in this complex is similar to that in 1, with the same type of arrangement of the central phenyl ring. Further reduction chemistry with a variety of reagents and conditions was examined. Reaction of 1 with 1 equiv of lithium naphthalenide at 0 °C did not afford 2 but instead gave a closely related U(III) complex formulated as {1,3-[2,5-(i-Pr)₂PhNC(CH₂)]₂C₆H₄}U(THF)(μ-Cl)₂[Li(Et₂O)₂] (3). Both of the trivalent complexes 2 and 3 reacted thermally in boiling THF, undergoing oxidation of the metal center to afford a new tetravalent compound {1,3-[2,5-(i-Pr)₂PhNC(CH₂)]₂C₆H₃}U(THF)(μ-Cl)₂[Li(THF)₂] (4) in which the oxidation of the trivalent center occurred at the expense of the central phenyl ring C−H bond. Reaction of 1 with 3 equiv of lithium naphthalenide at room temperature afforded {{1,3-[2,5-(i-Pr)₂PhNC(CH₂)]₂C₆H₃}U(μ-Cl)(μ-[O(CH₂)₃CH₂])[Li(DME)]}[Li(DME)₃] (5). In this species, the tetravalent metal center forms a six-membered metallacycle ring with a moiety arising from THF ring opening. Reaction in DME afforded reductive cleavage of the solvent accompanied by reoxidation of U to the tetravalent state. Reduction of 1 in DME with 2 equiv of potassium naphthalenide at room temperature gave a mixture of two compounds having very similar structures. The two different species [{1,3-[2,5-(i-Pr)₂PhNC(CH₂)]₂C₆H₃}UCl(OCH₃)][Li(DME)₃] (6a) and [{1,3-[2,5-(i-Pr)₂PhNC(CH₂)]₂C₆H₃}UCl₂][Li(DME)₃] (6b) cocrystallized in a ratio very close to 1:1 within the same unit cell. The methoxide group was generated from cleavage of the DME solvent. We also attempted the reduction of 1 with a different reducing agent such as NaH in DME. After a slow reaction, a new species formulated as {1,3-[2,5-(i-Pr)₂PhNC(CH₂)]₂C₆H₃}U(μ-OCH₃)₃(μ,η⁶-Na)[η³-Na(DME)] (7) was isolated in significant yield. Once again, the crystal structure revealed the presence of several methoxy groups coordinated to the U center in addition to the metalation of the ligand phenyl ring. To minimize solvent cleavage, reduction of 1 was also carried out at low temperature (−35 °C) and with a larger amount (4 equiv) of lithium naphthalenide. After suitable workup, the new species {[{1,3-[2,5-(i-Pr)₂PhNC(CH₂)]₂C₆H₃}U{1,3-[2,5-(i-Pr)₂PhNC(CH₃)]₂C₆H₄}][Li(DME)(THF)]}·Et₂O (8) was isolated in significant yield. Even in this case, the uranium atom is surrounded by the expected trianionic, ring-metalated ligand. However, a second ligand unit surrounds the metal center, being bonded through a part of the π system. Reaction of 1 with excess NaH in toluene proceeded slowly at room temperature, affording a significant yield of {[{1,3-[2,5-(i-Pr)₂PhNC(CH₂)]₂C₆H₃}U{1,3-[2,5-(i-Pr)₂PhNC(CH₃)]₂C₆H₄}{Na(DME)₂}][Na(DME)₃]}·¹/₂C₇H₈ (9) after crystallization from DME/toluene. Similar to 8, the complex still contains one ring-metalated trianionic ligand and one intact ligand that has regained the H atoms and restored the two imine functions. Although according to their connectivities, complexes 8 and 9 could be assigned with the formal oxidation states +2 and +1, respectively, density functional theory calculations clearly indicated that these species contain additional spin density on the ligand system with the metal center in its more standard trivalent state.