•Uranium(IV)-imido/-oxo complexes have been made by a disproportionation method•The imido and oxo complexes show anomalously high low-temperature magnetic moments•The strong imido and oxo ligands generate pseudosymmetric electronic structures•The resulting pseudo-doublet ground states explain the anomalous magnetism Uranium (U) and its derivatives play key roles in nuclear technologies; therefore, it is of interest to study U-complexes. Variable-temperature magnetism normally enables clear assignment of U-oxidation states, as each state has a typical magnetic profile. However, a growing number of 5f2 U(IV) complexes exhibit anomalous magnetic data. We report two U(IV) complexes that also exhibit anomalous magnetic data and find that this is because of pseudo-doublet, not the usual singlet, spin-orbit ground states. This occurs because imido and oxo ligands generate pseudosymmetric electronic structures, showing that traditional crystal field arguments apply surprisingly well to uranium. Thus, existing and new U(IV)-complexes with such anomalous magnetic behavior also likely have doublet rather than singlet ground states. Studies, such as this one, will assist in studying U-electronic structure and thus compound assignments, and in principle, this approach can be applied to any axially symmetric 5f2 actinide complex. A fundamental part of characterizing any metal complex is understanding its electronic ground state, for which magnetometry provides key insight. Most uranium(IV) complexes exhibit low-temperature magnetic moments tending to zero, consistent with a non-degenerate spin-orbit ground state. However, there is a growing number of uranium(IV) complexes with low-temperature magnetic moments ≥1 μB, suggesting a degenerate ground state, but the electronic structure implications and origins have been unclear. We report uranium(IV)-oxo and -imido complexes with low-temperature magnetic moments (ca. 1.5–1.6 μB) and show that they exhibit near-doubly degenerate spin-orbit ground states. We determine that this results from the strong point-charge-like donor properties of oxo and imido anions generating pseudosymmetric electronic structures and that traditional crystal field arguments are useful for understanding electronic structure and magnetic properties of uranium(IV). This suggests that a significant number of uranium(IV) complexes might benefit from a close re-evaluation of the nature of their spin-orbit ground states. A fundamental part of characterizing any metal complex is understanding its electronic ground state, for which magnetometry provides key insight. Most uranium(IV) complexes exhibit low-temperature magnetic moments tending to zero, consistent with a non-degenerate spin-orbit ground state. However, there is a growing number of uranium(IV) complexes with low-temperature magnetic moments ≥1 μB, suggesting a degenerate ground state, but the electronic structure implications and origins have been unclear. We report uranium(IV)-oxo and -imido complexes with low-temperature magnetic moments (ca. 1.5–1.6 μB) and show that they exhibit near-doubly degenerate spin-orbit ground states. We determine that this results from the strong point-charge-like donor properties of oxo and imido anions generating pseudosymmetric electronic structures and that traditional crystal field arguments are useful for understanding electronic structure and magnetic properties of uranium(IV). This suggests that a significant number of uranium(IV) complexes might benefit from a close re-evaluation of the nature of their spin-orbit ground states. The nature of coordinated ligands and the formal oxidation state of uranium modulate the key effects of inter-electronic repulsion (IER), spin-orbit coupling (SOC), and the crystal field (CF), which together determine the electronic structure of any uranium complex.1Liddle S.T. The renaissance of non-aqueous uranium chemistry.Angew. Chem. Int. Ed. Engl. 2015; 54: 8604-8641Crossref PubMed Scopus (282) Google Scholar Some or all of these effects can be of comparable magnitudes where early actinides are concerned. Therefore, more than anywhere else in the periodic table, the electronic structure of early actinides can be intrinsically very complex and challenging to study, and yet, it is fundamentally important to understand because it dictates the nature of the electronic ground state, which in turn is intimately connected to the bonding, reactivity, and physicochemical properties of a molecule. As uranium is a central element in civil nuclear energy production,2Kaltsoyannis N. Liddle S.T. Catalyst: nuclear power in the 21st century.Chem. 2016; 1: 659-662Abstract Full Text Full Text PDF Scopus (24) Google Scholar, 3Taylor R. Reaction: a role for actinide chemists.Chem. 2016; 1: 662-663Abstract Full Text Full Text PDF Scopus (18) Google Scholar, 4Ion S. Reaction: recycling and generation IV systems.Chem. 2016; 1: 663-665Abstract Full Text Full Text PDF Scopus (9) Google Scholar resolving the long-standing challenge of nuclear waste could, in the future, utilize selective extraction methods that exploit a better understanding of covalency differences in uranium-ligand bonding, which are intrinsically connected to the underlying electronic structure. One of the most valuable and informative methods for characterizing paramagnetic open-shell uranium complexes is by variable-temperature magnetometry, as this can give direct insight into the nature of the ground state and formal oxidation state. The free uranium(IV) ion, which has a 3H4 ground state in Russell-Saunders formalism, is predicted to exhibit a magnetic moment of 3.58 μB. However, due to significant CF effects in molecular complexes, this is often around 2.0–2.5 μB at 298 K and usually decreases smoothly toward ∼0.3–0.5 μB at 2 K.5Day J.P. Venanzi L.M. Some octahedral complexes of uranium(IV).J. Chem. Soc. A. 1966; 1966: 197-200Crossref Google Scholar, 6Castro-Rodríguez I. Meyer K. Small molecule activation at uranium coordination complexes: control of reactivity via molecular architecture.Chem. Commun. (Camb). 2006; : 1353-1368Crossref PubMed Scopus (195) Google Scholar, 7Gardner B.M. King D.M. Tuna F. Wooles A.J. Chilton N.F. Liddle S.T. Assessing crystal field and magnetic interactions in diuranium-μ-chalcogenide triamidoamine complexes with UIV-E-UIV cores (E = S, Se, Te): implications for determining the presence or absence of actinide-actinide magnetic exchange.Chem. 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Chem. 2010; 49: 1595-1606Crossref PubMed Scopus (56) Google Scholar Therefore, there is increasing evidence that there is a threshold CF strength for which high-symmetry arguments, and thus, a switching of spin-orbit ground state, might be invoked. Previously, we reported that the N-heterocyclic olefin H2C=C(NMeCH)2 reacts with [UIII(N″)3] (N″ = N(SiMe3)2) to produce the mesoionic carbene complex [U(N″)3{CN(Me)C(Me)N(Me)CH}] that exhibits a UIII→C 1-electron back-bond interaction.40Seed J.A. Gregson M. Tuna F. Chilton N.F. Wooles A.J. McInnes E.J.L. Liddle S.T. Rare-earth- and uranium-mesoionic carbenes: a new class of f-block carbene complex derived from an N-heterocyclic olefin.Angew. Chem. Int. Ed. Engl. 2017; 56: 11534-11538Crossref PubMed Scopus (28) Google Scholar Seeking to widen the family of uranium mesoionic carbene complexes we targeted uranium(V) derivatives. However, we instead found that the basic reactivity properties of the N-heterocyclic olefin become a complicating factor, promoting cyclometallation and disproportionation reactions, generating rare examples of uranium(IV)-oxo and -imido complexes. We found that these complexes exhibit unusually high low-temperature magnetic moments, for complexes formally of C1 symmetry, and therefore, we investigated the electronic structure of these complexes to address the nature of the electronic ground state. This has permitted us to unambiguously verify that pseudo-C3-symmetric uranium complexes with a strong axial ligand can have paramagnetic pseudo-doublet (E) spin-orbit ground states, showing that traditional CF symmetry arguments can dictate the electronic structure and magnetic properties when strong enough point-charge-like ligands are coordinated to uranium. Treatment of pre-prepared [UV(O)(N″)3] (N″ = N(SiMe3)2, 1)26Fortier S. Brown J.L. Kaltsoyannis N. Wu G. Hayton T.W. Synthesis, molecular and electronic structure of UV(O)[N(SiMe3)2]3.Inorg. Chem. 2012; 51: 1625-1633Crossref PubMed Scopus (91) Google Scholar or in situ-prepared [UV(NSiMe3)(N″)3] (2, by oxidation of [UIII(N″)3] with N3SiMe3) with half an equivalent of the N-heterocyclic olefin H2C=C(NMeCH)2 (3) in either diethyl ether or hexane, produces, after work-up and recrystallization, brown needles of the uranium(IV)-oxo and -imido complexes [UIV(O)(N″)3][(Me)C(NMeCH)2] (4) or [UIV(NSiMe3)(N″)3][(Me)C(NMeCH)2] (5), respectively, Scheme 1. The crystalline yields of 4 and 5 are both 13%, which is low because 4 and 5 decompose in solution affording HN(SiMe3)2 and unidentified, intractable by-products and also because their formation results from disproportionation reactions in which the uranium(VI)-cyclometallate complexes concomitantly form [UVI(O)(N″)2{N(SiMe3)(SiMe2CH2)}] (6)41Fortier S. Kaltsoyannis N. Wu G. Hayton T.W. Probing the reactivity and electronic structure of a uranium(V) terminal oxo complex.J. Am. Chem. Soc. 2011; 133: 14224-14227Crossref PubMed Scopus (82) Google Scholar for 4 and [UVI(NSiMe3)(N″)2{N(SiMe3)(SiMe2CH2)}] (7) for 5, respectively, thus limiting the maximum yield in each case to 50%. When 1 and 2 are treated with one equivalent of 3, the uranium(V)-cyclometallate complexes [UV(O)(N″)2{N(SiMe3)(SiMe2CH2)}][(Me)C(NMeCH)2] (8) and [UV(NSiMe3)(N″)2{N(SiMe3)(SiMe2CH2)}][(Me)C(NMeCH)2] (9), respectively, are formed quantitatively (see supplemental information). Complexes 8 and 9 decompose when they are left in solution for prolonged periods, with complete decomposition found after 60 and 15 min, respectively. However, if 8 or 9 are treated quickly with one equivalent of 1 or 2, respectively, then 1:1 mixtures of disproportionated 4:6 or 5:7 are formed analogously to the half equivalent reactions with 3, as discussed above. The reactions between 1 or 2 with half an equivalent of 3 clearly produce 1:1 mixtures of 4:6 or 5:7, respectively, as a result of disproportionation and cyclometallation. The reactions of 1 and 2 with one equivalent of 3 provide insight into the likely mechanism of this reaction, since cyclometallated 8 or 9 are formed in this situation, but only after the addition of further 1 or 2 (which then essentially renders the 1/2:3 ratio 1:0.5) does disproportionation occur. The cyclometallation can be accounted for by the basic 3-promoting C–H activation and H-abstraction, and the fact that the extra cyclometallate donor destabilizes the uranium(V) ions in 8 and 9, as evidenced by their otherwise rapid decomposition, such that oxidation to uranium(VI) is more favorable for the cyclometallate formulation at the expense of an anionic formulation by reduction for the uranium-oxo and -imido components of 4 and 5. Certainly, the absence of D-incorporation for reactions conducted in D6-benzene are consistent with this, and [UV(O)(N″)2{N(SiMe3)(SiMe2CH2)}][MePPh3],41Fortier S. Kaltsoyannis N. Wu G. Hayton T.W. Probing the reactivity and electronic structure of a uranium(V) terminal oxo complex.J. Am. Chem. Soc. 2011; 133: 14224-14227Crossref PubMed Scopus (82) Google Scholar which is essentially 8 but with a different counter-cation, is known to be easily oxidized (E1/2 = –0.85 V versus [Cp2Fe]0/+). Once isolated, 4 and 5 are poorly soluble in aromatic solvents, and they decompose in ethers, but NMR spectroscopic data (Figures S1–S4) are consistent with their uranium(IV) formulations and show no evidence of D-incorporation from deuterated solvent (benzene). The six trimethylsilyl groups resonate as one singlet per complex in the 1H NMR spectrum, indicating a symmetric species on the NMR timescale. However, these are shifted upfield relative to 1 and 2 in agreement with the increased electron density at the uranium(IV) centers. For 5, the trimethylsilyl group of the axial imido ligand is observed in the 1H NMR spectrum at –12.55 ppm, but no 1H NMR resonance for the [M=NSiMe3] group for 2 has been reported, and therefore, no comparison can be made; however, the 29Si NMR spectra of 4 and 5 exhibit weak resonances at –37.74 and –90.74/–131.19 ppm, respectively, which is within the range of reported 29Si chemical shifts for uranium(IV) complexes.42Windorff C.J. Evans W.J. 29Si NMR spectra of silicon-containing uranium complexes.Organometallics. 2014; 33: 3786-3791Crossref Scopus (35) Google Scholar Complexes 6 and 8 were identified by comparison of NMR spectra (Figures S5–S7) of reaction mixtures compared with published data and [UV(O)(N″)2{N(SiMe3)(SiMe2CH2)}][MePPh3],41Fortier S. Kaltsoyannis N. Wu G. Hayton T.W. Probing the reactivity and electronic structure of a uranium(V) terminal oxo complex.J. Am. Chem. Soc. 2011; 133: 14224-14227Crossref PubMed Scopus (82) Google Scholar respectively. Complex 9 was identified by NMR spectroscopy with reference to 8, but 7 could not be unambiguously spectroscopically identified, most likely because the imido does not stabilize the uranium(VI) oxidation state as well as an oxo, but its fleeting existence seems all but assured, given the parallels between these oxo and imido systems with five of the six reaction partners identified. The IR (Figures S8 and S9) and UV-vis-NIR spectra (Figures S12–S15) of 4 and 5 were recorded. Unfortunately, the oxo and imido linkages of 4 and 5 were anticipated to fall in the region 800–1,000 cm−1, which has been shown to often contain absorptions from the silyl-amide complicating analysis.26Fortier S. Brown J.L. Kaltsoyannis N. Wu G. Hayton T.W. Synthesis, molecular and electronic structure of UV(O)[N(SiMe3)2]3.Inorg. Chem. 2012; 51: 1625-1633Crossref PubMed Scopus (91) Google Scholar The UV-vis-NIR spectra of 4 and 5 were dominated by strong charge transfer bands from the UV region to around 20,000 cm−1. Across the range 20,000–5,000 cm−1 the spectra were dominated by multiple but weak (ε <80 M−1 cm−1) absorptions that are characteristic of Laporte forbidden f-f transitions of uranium(IV) ions, in accordance with the pale brown color of both complexes.1Liddle S.T. The renaissance of non-aqueous uranium chemistry.Angew. Chem. Int. Ed. Engl. 2015; 54: 8604-8641Crossref PubMed Scopus (282) Google Scholar,6Castro-Rodríguez I. Meyer K. Small molecule activation at uranium coordination complexes: control of reactivity via molecular architecture.Chem. Commun. (Camb). 2006; : 1353-1368Crossref PubMed Scopus (195) Google Scholar,43Mills D.P. Moro F. McMaster J. Van Slageren J. Lewis W. Blake A.J. Liddle S.T. A delocalized arene-bridged diuranium single-molecule magnet.Nat. Chem. 2011; 3: 454-460Crossref PubMed Scopus (238) Google Scholar In order to confirm the formulations of 4 and 5, their solid-state structures were determined, Figure 1. In gross terms, each of these structures were very similar, with a separated ion pair formulation and four-coordinate uranium ions. The geometry about uranium in 4 was essentially trigonal monopyramidal, with an average O-U-Namide angle of 96.8(3)° and an average Namide-U-Namide angle of 118.6(3)°, such that the uranium ion lies only 0.279(4) Å above the plane defined by the three Namide centers. In contrast, 5 exhibited a pseudo-tetrahedral geometry about uranium, with an average Nimido-U-Namide angle of 102.39(2)° and an average Namide-U-Namide angle of 115.53(2)°. Thus, the geometries of tetravalent 4 and 5 largely mirrored those of pentavalent 1 and 2, respectively. The U-Namide distances in tetravalent 4 and 5 spanned across the ranges 2.346(7)–2.351(7) and 2.359(4)–2.368(4) Å, respectively. For comparison, the U-Namide distances in pentavalent 1 [2.235(1)–2.244(2) Å]26Fortier S. Brown J.L. Kaltsoyannis N. Wu G. Hayton T.W. Synthesis, molecular and electronic structure of UV(O)[N(SiMe3)2]3.Inorg. Chem. 2012; 51: 1625-1633Crossref PubMed Scopus (91) Google Scholar and 2 [av. 2.295(10) Å]44Zalkin A. Brennan J.G. Andersen R.A. Tris[bis(trimethylsilyl)amido](trimethylsilylimido)uranium(V).Acta Crystallogr. C Cryst. Struct. Commun. 1988; 44: 1553-1554Crossref Google Scholar are significantly shorter. For 4, the U–O distance was significantly longer than that of 1 [1.882(6) versus 1.817(1) Å, respectively] and the U-Nimido distance in 5 was significantly longer than that of 2 [1.985(4) versus 1.910(16) Å, respectively]. More widely, the U–O distance in 4 is comparable to that of [U{OK(18-crown-6)}(N″)3] [1.890(5) Å]45Smiles D.E. Wu G. Hayton T.W. Synthesis of uranium-ligand multiple bonds by cleavage of a trityl protecting group.J. Am. Chem. Soc. 2014; 136: 96-99Crossref PubMed Scopus (56) Google Scholar and the U-Nimido bond length in 5 is comparable to those of [U(NDipp)Cl2(tBu2bpy)(THF)2]46Jilek R.E. Tomson N.C. Shook R.L. Scott B.L. Boncella J.M. Preparation and reactivity of the versatile uranium(IV) imido complexes U(NAr)Cl2(R2bpy)2 (R = Me, (t)Bu) and U(NAr)Cl2(tppo)3.Inorg. Chem. 2014; 53: 9818-9826Crossref PubMed Scopus (25) Google Scholar and [K][U(=NCPh3){N(SiMe3)2}3]47Mullane K.C. Lewis A.J. Yin H. Carroll P.J. Schelter E.J. Anomalous one-electron processes in the chemistry of uranium nitrogen multiple bonds.Inorg. Chem. 2014; 53: 9129-9139Crossref PubMed Scopus (42) Google Scholar [1.981(2) Å and 1.9926(14) Å, respectively]. These structural features all support the uranium(IV) formulations of 4 and 5. Powdered samples of 4 and 5 immobilized in eicosane were studied by variable-temperature superconducting quantum interference device (SQUID) magnetometry, (Figures 2, S10, and S11). Complexes 4 and 5 exhibited magnetic moments of 2.88 and 3.01 μB at 300 K, respectively. These values are both lower than the theoretical magnetic moment of 3.58 μB for one uranium(IV) ion, which is not uncommon, but they are clearly higher than the maximal magnetic moment of 2.54 μB for one uranium(V) ion and are substantially higher than the reported magnetic moments of 1 and 2 (1.59 and 2.04 μB, respectively, at 300 K). The magnetic moments of 4 and 5 decreased slowly, reaching 2.36 and 2.30 μB, respectively, at 20 K, and then decreased more rapidly reaching 1.54 and 1.46 μB, respectively, at 2 K, Figure 2. The data for 4 and 5 do not fit the “classical” behavior of uranium(IV),1Liddle S.T. The renaissance of non-aqueous uranium chemistry.Ang