Uranium, the heaviest ingredient considerable in nature, can exhibit a number of oxidation states and wealthy redox chemistry¹. 4 oxidation states, uranium(III) to uranium(VI), have been nicely established. In 2013, Evans and colleagues reported the primary molecular uranium(II) advanced². Since then, the low-valent uranium chemistry has seen fast growth within the final decade³, together with the latest isolation of a uranium(I) advanced⁴. It’s extremely desired to stabilise as many oxidation states of uranium as attainable in a retained ligand framework, which is able to enable direct comparability of various oxidation states and controllable redox transformations. Properly-defined uranium complexes stabilised by chelating ligands have generated some most enjoyable chemistry in recent times, together with the isolation of a linear, O-coordinated η¹-CO₂ sure to uranium⁵ and the primary terminal uranium nitride advanced⁶, in addition to the electrocatalytic water discount to provide dihydrogen⁷. Nevertheless, no chelating ligand has been proven to help all 5 well-established oxidation states of uranium, uranium(II–VI). On this research, we goal to design a chelating ligand able to stabilising uranium(II–VI) with a preserved coordination setting.

Since established in 2017, our analysis group has centered on learning “f-block metallic–arene interactions” and using this idea in exploring new properties and reactivity of f-block metals. We reported the primary inverse-sandwich thorium arene complexes⁸ (Fig. 1a), and unveiled the distinct digital buildings of samarium and ytterbium biphenyl complexes⁹ (Fig. 1b). Impressed by uranium–arene δ interactions in inverse-sandwich uranium complexes^10,11 and Meyer’s tris(aryloxide)arene ligand^12,13 in addition to studies on π interactions between electrophilic uranium ions and impartial arenes^14,15, we anticipated that the ambiphilic nature of arenes may be utilized to stability the soundness of low and high-valent uranium ions. In 2021, our group reported two tripodal tris(amido)arene ligands that includes an anchoring arene (because the centre of the 1,3,5-triphenylbenezene spine) and N-aryl substituents, and launched them to coordination chemistry of rare-earth metals (Fig. 1c)¹⁶. Within the present research (https://www.nature.com/articles/s41467-023-40403-w)¹⁷, the N-adamantyl model of the tris(amido)arene pro-ligand, H₃[^AdTPBN₃], was ready and utilized to the coordination chemistry of uranium (Fig. 1d). The pre-organized (C₃-syn) construction and higher crystallinity of [^AdTPBN₃]^3– (in comparison with the N-aryl analogues) present ease for work-up and crystallization of uranium complexes. In the end, 5 oxidation states of uranium, i.e., uranium(II–VI), may very well be stabilised by [^AdTPBN₃]^3– by way of ambiphilic uranium–arene interactions.

As illustrated in Fig. 2a, deprotonation of H₃[^AdTPBN₃] and subsequent salt metathesis with the uranium triiodide precursor generated the uranium(III) advanced 1. Whereas discount of 1 by KC₈ produced the uranium(II) advanced 2, 1 might additionally reacted with oxygen-atom-transfer reagents, equivalent to N₂O and pyridine-N-oxide, to furnish the uranium(V) terminal oxo advanced 3. 3 may very well be additional oxidized by AgSbF₆ or diminished by KC₈ to afford the corresponding uranium(VI) and uranium(IV) terminal oxides, 5 and 4, respectively. These compounds had been obtained in good yields and exhibited glorious thermal stability beneath inert ambiance. For example, no decomposition of 2 in THF was detected even after extend heating at 50 ℃. Compounds 1–5 had been all characterised by single crystal X-ray diffraction. Notably, 3–5 characterize the primary trio of structurally authenticated uranium(IV–VI) terminal oxo complexes with the identical ancillary ligand. The superposition of molecular buildings of 1–5 revealed that the U–C_centroid distances fluctuate considerably, which peak within the uranium(IV) oxo advanced 4 and reduce upon discount or oxidation (Fig. 2b).

Electrochemical research had been carried out for the uranium(III) advanced 1 and the uranium(V) oxo advanced 3 to probe their redox properties. Two reversible one-electron redox occasions had been revealed by cyclic voltammetry for every advanced (Fig. 3a), matching nicely with the artificial findings and suggesting that the direct one-electron oxidation of 1 may be chemically possible. Reactivity research (Fig. 3b) certainly confirmed that oxidizing 1 by AgF or 1,2-C₂H₄I₂ (a handy substitute of I₂) generated uranium halides 6 or 7, respectively. Additional oxidation of the uranium(IV) iodide 7 with AgNO₂ resulted within the formation of 3. As well as, 1 was transformed to the terminal uranium(IV) oxide 4 by reacting with KNO₂ within the presence of crypt, whereas 4 may be synthesized from the response between the uranium(II) advanced 2 and oxygen-atom-transfer reagents.

The oxidation state project of uranium in these complexes was supported by NMR spectroscopy, digital absorption spectroscopy, X-ray photoelectron spectroscopy, SQUID magnetometry, and digital paramagnetic spectroscopy. The experimental characterisation supplied the premise for computational research on their digital buildings. DFT calculations supported the 5f⁴ (f²δ²) digital floor state for the uranium(II) advanced 2, and likewise revealed the U–O a number of bonding character of the trio of terminal uranium(IV–VI) oxo complexes 3–5 in addition to the potential inverse trans affect therein. To additional elucidate the uranium–arene interactions, prolonged transition state–pure orbitals for chemical valence (ETS–NOCV) calculations had been carried out on compounds 1–5 with applicable molecule fragmentation. The σ, π, and δ-type uranium–arene interactions had been labeled primarily based on the symmetry of the NOCV pairs. The outcomes present that δ backdonation from 5f orbitals (uranium) to π^* orbitals (the anchoring arene) dominate in 1 and 2, whereas π donations from π orbitals of the anchoring arene to uranium-based orbitals monotonically strengthen because the oxidation states of uranium improve (Fig. 4a). Notably, the development of complete stabilisation energies of uranium–arene interactions correlate nicely with the development of U–C_centroid distances for 1–5 (Fig. 4b), additional supporting our anticipation that the ambiphilic uranium–arene interactions play a key function in stabilising each low and high-valent uranium ions.

In conclusion, a collection of uranium(II–VI) complexes supported by a tripodal tris(amido)arene ligand had been synthesized and characterised. Managed one- or two-electron redox transformations may very well be readily achieved with these uranium complexes. Mixed experimental and computational research supported that the ambiphilic uranium–arene interactions play a pivotal function in balancing the soundness of low- (II) and high-valent (VI) uranium ions. We anticipate that this chelating ligand framework might be a promising platform to discover redox chemistry of uranium in small molecule activations. Such efforts are at the moment ongoing in our lab. Furthermore, the ligand design technique to include metallic–arene interplay could also be prolonged to different metals to stabilise uncommon oxidation states and understand difficult redox chemistry.

The funding sources, establishments, and experimental collaborators and supporters have been acknowledged within the authentic article¹⁷. Moreover, we additionally wish to thank Excessive-performance Computing Platform of Peking College, the place we carried out all calculations for this work.

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