Identification of Oxidation State +1 in a Molecular Uranium Complex

The concept of oxidation state plays a fundamentally important role in defining the chemistry of the elements. In the f block of the periodic table, well-known oxidation states in compounds of the lanthanides include 0, +2, +3 and +4, and oxidation states for the actinides range from +7 to +2. Oxidation state +1 is conspicuous by its absence from the f-block elements. Here we show that the uranium(II) metallocene [U(η5-C5iPr5)2] and the uranium(III) metallocene [IU(η5-C5iPr5)2] can be reduced by potassium graphite in the presence of 2.2.2-cryptand to the uranium(I) metallocene [U(η5-C5iPr5)2]− (1) (C5iPr5 = pentaisopropylcyclopentadienyl) as the salt of [K(2.2.2-cryptand)]+. An X-ray crystallographic study revealed that 1 has a bent metallocene structure, and theoretical studies and magnetic measurements confirmed that the electronic ground state of uranium(I) adopts a 5f3(7s/6dz2)1(6dx2–y2/6dxy)1 configuration. The metal–ligand bonding in 1 consists of contributions from uranium 5f, 6d, and 7s orbitals, with the 6d orbitals engaging in weak but non-negligible covalent interactions. Identification of the oxidation state +1 for uranium expands the range of isolable oxidation states for the f-block elements and potentially signposts a synthetic route to this elusive species for other actinides and the lanthanides.

T he oxidation state of an element strongly influences the stability, reactivity, and physical properties of the compounds it forms. There is considerable motivation for isolating elements in new oxidation states since this can lead to new chemistry while also providing a deeper fundamental understanding of bonding and electronic structure. In the lanthanide series, the oxidation state +3 is thermodynamically the most stable species by far. Recent reports of molecular compounds containing praseodymium 1 and terbium 2−4 in the oxidation state +4 are therefore notable advances. Similarly, the synthesis and isolation of compounds containing the full series of lanthanides (except promethium) in the oxidation state +2 is a significant achievement 5−7 and builds on the earlier discovery of formally lanthanide(0) sandwich compounds. 8,9 In the actinide series, oxidation states range from as high as +7 in the neptunyl cation 10 to, most recently, +2 in molecular compounds of thorium, 11 uranium, 12−14 neptunium, 15 and plutonium. 16 We recently reported the linear uranium(II) metallocene [U(η 5 -C 5 i Pr 5 ) 2 ], in which uranium has an electron configuration consisting of a 5f 3 component and an electron residing in a 6d z 2 /7s hybrid orbital. 17 A cyclic voltammetry study of this uranocene revealed two electrochemical events, assigned to the uranium(II)/uranium(III) and uranium(III)/uranium(IV) couples. Unexpectedly, further reduction to an unstable species was tentatively attributed to a complex of uranium(I). Notably, no molecular compound of an f-block element in the oxidation state +1 has been isolated. Lanthanum monoiodide (LaI) is best described as a nominally lanthanum(I)-containing extended solid, with metallic properties arising from two electrons per La + donated to the conduction band. 18,19 A uranium(II) complex that by virtue of ligand noninnocence reacts as a uranium(I) "synthon" was also recently described, 20 and the formally uranium(I)-containing complex [UFe-(CO) 3 ] − was detected in the gas phase. 21 Beyond these examples, a theoretical study has predicted that well-defined uranium(I) compounds should be stable and therefore synthetically accessible from a suitable precursor. 22 We attempted the reduction of [U(η 5 -C 5 i Pr 5 ) 2 ] in hexane using an excess of potassium graphite (KC 8 ) and 1 equiv of 2.2.2-cryptand. Over 3 days, the initial green color faded, and a brown solid precipitated. Extraction of the solid into benzene followed by recrystallization produced brown crystals, which Xray crystallography revealed to be the uranium(I) complex [U(η 5 Figures S10 and S11). To obtain further insight into the electron configuration and bonding in 1, density functional theory (DFT) calculations were carried out. The electron configuration was determined to be 5f 3 (7s/6d z 2 ) 1 (6d x 2 −y 2 /6d xy ) 1 . Three electrons occupy orbitals with strong atomic-like 5f character. One electron occupies a quasi-σ-symmetric orbital that is an admixture of the 7s and 6d z 2 atomic orbitals. Based on decomposition of the orbital into a basis of uranium(I) orbitals, the orbital has 63% 7s character and 28% 6d character. The orbital has a toroidal shape that is typical for lanthanide(II) and uranium(II) metallocenes. 17,24,25 The one remaining electron occupies a quasi-δ-symmetric 6d x 2 −y 2 and 6d xy set of orbitals with significant delocalization into the ligands. Several calculations were carried out to see whether a low-lying lower-spin electronic configuration existed, but all lower-spin states discovered lie at higher energy than the highest-spin state.
The bonding in 1 is shown in Figure 2, with quantitative contributions of the uranium and cyclopentadienyl orbitals to the molecular orbitals (MOs) provided in Tables S2 and S3. The occupied 5f, 6d, and 7s orbitals form an energetically closely packed manifold. The three 5f orbitals occupied by three unpaired electrons all have more than 91% 5f character and show very little covalency. The 6d orbitals are weakly mixed with the nearly doubly degenerate highest-occupied MOs of the ligands. The 6d contribution in the main metal− ligand bonding orbitals varies from 0 to 14% depending on the orbital and is evidence of weak but non-negligible uranium− cyclopentadienyl covalency in 1.
The molar magnetic susceptibility (χ M ) of an unrestrained polycrystalline sample of [K(2.2.2-crypt)] [1] was measured from 2.5 to 200 K in a direct current (dc) field of 1 kOe (Figure 3, left). Above 90 K, χ M T is strongly temperaturedependent, reaching a value 3.43 cm 3 K mol −1 at 200 K, equivalent to an effective magnetic moment (μ eff ) of 5.35μ B . This magnetic moment is much larger than any reported value for a molecular uranium complex even at 300 K, 26 including those for [U(η 5 -C 5 i Pr 5 ) 2 ] 17 and [IU(η 5 -C 5 i Pr 5 ) 2 ]. 23 Between 90 and 10 K, χ M T varies only slightly in the range of 0.98−1.17 cm 3 K mol −1 or 2.80−2.90μ B before decreasing sharply to 0.43 cm 3 K mol −1 or 1.85μ B at 2.5 K. The temperature-dependence of χ M T is unusual and suggests gradual population of a thermally accessible excited electronic state at higher temperatures. At lower temperatures, the sharp drop in χ M T is consistent with the onset of single-molecule magnet (SMM) behavior. Interpretation of the susceptibility is difficult because of the large number of low-lying states in the uranium(I) ion involving strong interactions of the 7s and 6d orbitals with the ligands. 27 This leads to a densely packed manifold of thermally accessible states with largely varying magnetic properties. The low-temperature susceptibility can be interpreted in terms of a coupling model where the intershell exchange coupling is stronger than the intrashell LS coupling (where L and S are the orbital and spin angular momenta, respectively; Table S4). The increase in the susceptibility at higher temperatures most likely results from population of thermally accessible states where the spin and orbital momenta are not coupled fully antiparallel, leading to a larger total momentum.
Since complexes of uranium in low oxidation states can be multielectron donors toward small molecules, 29 (Table S1 and Figures S21−S24). This one-electron reduction process contrasts to that shown by the uranium(II) complex [U{N(SiMe 3 ) 2 } 3 ] − toward azobenzene, which initiates a four-electron reduction and cleavage of the nitrogen−nitrogen double bond in the substrate. 30 This reactivity, combined with the disproportionation of [K(2.2.2crypt)] [1] into its uranium(II) precursor and uranium metal, points to unexpected stability of [U(η 5 -C 5 i Pr 5 ) 2 ], most likely due to the stabilizing effect of the bulky ligands.
In conclusion, we have shown that a metallocene of uranium in the oxidation state +1 can be synthesized by reduction of uranium(II) and uranium(III) precursors. Reactivity studies, magnetic and spectroscopic measurements, and a DFT study are consistent with the presence of uranium(I) with a 5f 3 (7s/ 6d z 2 ) 1 (6d x 2 −y 2 /6d xy ) 1 ground-state electron configuration. The broader significance of [K(2.2.2-crypt)] [1] is that soluble molecular compounds of other actinides and some lanthanides in the oxidation state +1 should also be viable targets. The synthesis of a much larger family of compounds containing lanthanide(I) and actinide(I) centers introduces new possibilities for designing molecular magnets and luminescent materials and for developing new f-element reactivity.

■ ASSOCIATED CONTENT Data Availability Statement
Additional research data supporting this publication are available at 10.25377/sussex.21184705.
Synthesis, spectroscopic characterization, crystallographic details, magnetic property measurements, and computational details (PDF)