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

Reaction of UCl<sub>4</sub>(THF)<sub>4</sub> with 1,3-[2,5-(<i>i</i>-Pr)<sub>2</sub>PhNC(CH<sub>2</sub>)]<sub>2</sub>C<sub>6</sub>H<sub>4</sub>Li<sub>2</sub> produced a complex formulated as [{1,3-[2,5-(<i>i</i>-Pr)<sub>2</sub>PhNC(CH<sub>2</sub>)]<sub>2</sub>C<sub>6</sub>H<sub>4</sub>}UCl<sub>3</sub>][Li(THF)<sub>4</sub>] (<b>1</b>) 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 “UCl<sub>3</sub>”. The reaction indeed afforded a trivalent species formulated as {1,3-[2,5-(<i>i</i>-Pr)<sub>2</sub>PhNC(CH<sub>2</sub>)]<sub>2</sub>C<sub>6</sub>H<sub>4</sub>}U(μ-Cl)<sub>3</sub>[Li(THF)<sub>2</sub>]<sub>2</sub> (<b>2</b>). The configuration of the ligand system in this complex is similar to that in <b>1</b>, 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 <b>1</b> with 1 equiv of lithium naphthalenide at 0 °C did not afford <b>2</b> but instead gave a closely related U(III) complex formulated as {1,3-[2,5-(<i>i</i>-Pr)<sub>2</sub>PhNC(CH<sub>2</sub>)]<sub>2</sub>C<sub>6</sub>H<sub>4</sub>}U(THF)(μ-Cl)<sub>2</sub>[Li(Et<sub>2</sub>O)<sub>2</sub>] (<b>3</b>). Both of the trivalent complexes <b>2</b> and <b>3</b> reacted thermally in boiling THF, undergoing oxidation of the metal center to afford a new tetravalent compound {1,3-[2,5-(<i>i</i>-Pr)<sub>2</sub>PhNC(CH<sub>2</sub>)]<sub>2</sub>C<sub>6</sub>H<sub>3</sub>}U(THF)(μ-Cl)<sub>2</sub>[Li(THF)<sub>2</sub>] (<b>4</b>) in which the oxidation of the trivalent center occurred at the expense of the central phenyl ring C−H bond. Reaction of <b>1</b> with 3 equiv of lithium naphthalenide at room temperature afforded {{1,3-[2,5-(<i>i</i>-Pr)<sub>2</sub>PhNC(CH<sub>2</sub>)]<sub>2</sub>C<sub>6</sub>H<sub>3</sub>}U(μ-Cl)(μ-[O(CH<sub>2</sub>)<sub>3</sub>CH<sub>2</sub>])[Li(DME)]}[Li(DME)<sub>3</sub>] (<b>5</b>). 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 <b>1</b> 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>i</i>-Pr)<sub>2</sub>PhNC(CH<sub>2</sub>)]<sub>2</sub>C<sub>6</sub>H<sub>3</sub>}UCl(OCH<sub>3</sub>)][Li(DME)<sub>3</sub>] (<b>6a</b>) and [{1,3-[2,5-(<i>i</i>-Pr)<sub>2</sub>PhNC(CH<sub>2</sub>)]<sub>2</sub>C<sub>6</sub>H<sub>3</sub>}UCl<sub>2</sub>][Li(DME)<sub>3</sub>] (<b>6b</b>) 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 <b>1</b> with a different reducing agent such as NaH in DME. After a slow reaction, a new species formulated as {1,3-[2,5-(<i>i</i>-Pr)<sub>2</sub>PhNC(CH<sub>2</sub>)]<sub>2</sub>C<sub>6</sub>H<sub>3</sub>}U(μ-OCH<sub>3</sub>)<sub>3</sub>(μ,η<sup>6</sup>-Na)[η<sup>3</sup>-Na(DME)] (<b>7</b>) 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 <b>1</b> 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>i</i>-Pr)<sub>2</sub>PhNC(CH<sub>2</sub>)]<sub>2</sub>C<sub>6</sub>H<sub>3</sub>}U{1,3-[2,5-(<i>i</i>-Pr)<sub>2</sub>PhNC(CH<sub>3</sub>)]<sub>2</sub>C<sub>6</sub>H<sub>4</sub>}][Li(DME)(THF)]}·Et<sub>2</sub>O (<b>8</b>) 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 <b>1</b> with excess NaH in toluene proceeded slowly at room temperature, affording a significant yield of {[{1,3-[2,5-(<i>i</i>-Pr)<sub>2</sub>PhNC(CH<sub>2</sub>)]<sub>2</sub>C<sub>6</sub>H<sub>3</sub>}U{1,3-[2,5-(<i>i</i>-Pr)<sub>2</sub>PhNC(CH<sub>3</sub>)]<sub>2</sub>C<sub>6</sub>H<sub>4</sub>}{Na(DME)<sub>2</sub>}][Na(DME)<sub>3</sub>]}·<sup>1</sup>/<sub>2</sub>C<sub>7</sub>H<sub>8</sub> (<b>9</b>) after crystallization from DME/toluene. Similar to <b>8</b>, 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 <b>8</b> and <b>9</b> could be assigned with the <i>formal</i> 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.