Anomalous magnetism of uranium(IV)-oxo and -imido complexes reveals unusual doubly degenerate electronic ground states

•Uranium(IV)-imido/-oxo complexes have been made by a disproportionation method•The imido and oxo show anomalously high low-temperature magnetic moments•The strong ligands generate pseudosymmetric electronic structures•The resulting pseudo-doublet ground states explain the anomalous magnetism Uranium (U) its derivatives play key roles in nuclear technologies; therefore, it is of interest to study U-complexes. Variable-temperature normally enables clear assignment U-oxidation states, as each state has typical profile. However, growing number 5f2 U(IV) exhibit data. We report two that also data find this because pseudo-doublet, not usual singlet, spin-orbit states. This occurs structures, showing traditional crystal field arguments apply surprisingly well uranium. Thus, existing new U(IV)-complexes with such behavior likely doublet rather than singlet Studies, one, will assist studying U-electronic structure thus compound assignments, principle, approach can be applied any axially symmetric actinide complex. A fundamental part characterizing metal complex understanding state, for which magnetometry provides insight. Most uranium(IV) moments tending zero, consistent non-degenerate state. there ≥1 μB, suggesting degenerate but implications origins unclear. uranium(IV)-oxo -imido (ca. 1.5–1.6 μB) they near-doubly determine results from point-charge-like donor properties anions generating structures are useful uranium(IV). suggests significant might benefit close re-evaluation nature their The coordinated formal oxidation uranium modulate effects inter-electronic repulsion (IER), coupling (SOC), (CF), together complex.1Liddle S.T. renaissance non-aqueous chemistry.Angew. Chem. Int. Ed. 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Previously, we reported N-heterocyclic olefin H2C=C(NMeCH)2 reacts [UIII(N″)3] (N″ N(SiMe3)2) produce mesoionic carbene [U(N″)3{CN(Me)C(Me)N(Me)CH}] exhibits UIII→C 1-electron back-bond interaction.40Seed Rare-earth- uranium-mesoionic carbenes: derived olefin.Angew. 11534-11538Crossref (28) Seeking widen targeted uranium(V) derivatives. instead found basic become complicating factor, promoting cyclometallation reactions, unusually moments, C1 symmetry, investigated address permitted us unambiguously verify pseudo-C3-symmetric axial dictate enough Treatment pre-prepared [UV(O)(N″)3] N(SiMe3)2, 1)26Fortier situ-prepared [UV(NSiMe3)(N″)3] (2, N3SiMe3) half equivalent either diethyl ether hexane, produces, work-up recrystallization, brown needles [UIV(O)(N″)3][(Me)C(NMeCH)2] (4) [UIV(NSiMe3)(N″)3][(Me)C(NMeCH)2] (5), respectively, Scheme 1. yields 5 both 13%, decompose solution affording HN(SiMe3)2 unidentified, intractable by-products formation reactions uranium(VI)-cyclometallate concomitantly form [UVI(O)(N″)2{N(SiMe3)(SiMe2CH2)}] (6)41Fortier Probing complex.J. 133: 14224-14227Crossref (82) [UVI(NSiMe3)(N″)2{N(SiMe3)(SiMe2CH2)}] (7) 5, limiting maximum yield case 50%. When 1 treated one 3, uranium(V)-cyclometallate [UV(O)(N″)2{N(SiMe3)(SiMe2CH2)}][(Me)C(NMeCH)2] (8) [UV(NSiMe3)(N″)2{N(SiMe3)(SiMe2CH2)}][(Me)C(NMeCH)2] (9), formed quantitatively (see supplemental information). Complexes 8 9 left prolonged periods, decomposition 60 15 min, respectively. if quickly 2, then 1:1 mixtures disproportionated 4:6 5:7 analogously discussed above. between 3 clearly 5:7, result cyclometallation. provide mechanism reaction, since cyclometallated situation, only addition further (which essentially renders 1/2:3 ratio 1:0.5) does occur. accounted 3-promoting C–H H-abstraction, fact extra cyclometallate destabilizes 9, evidenced otherwise rapid decomposition, uranium(VI) favorable formulation expense anionic reduction uranium-oxo components 5. Certainly, D-incorporation conducted D6-benzene this, [UV(O)(N″)2{N(SiMe3)(SiMe2CH2)}][MePPh3],41Fortier counter-cation, known easily oxidized (E1/2 –0.85 V versus [Cp2Fe]0/+). Once isolated, poorly soluble aromatic solvents, ethers, NMR spectroscopic (Figures S1–S4) formulations deuterated solvent (benzene). six trimethylsilyl groups resonate per 1H spectrum, indicating species timescale. shifted upfield relative agreement increased electron density centers. group spectrum –12.55 ppm, [M=NSiMe3] reported, comparison made; however, 29Si weak resonances –37.74 –90.74/–131.19 within range chemical complexes.42Windorff C.J. silicon-containing complexes.Organometallics. 33: 3786-3791Crossref (35) 6 were identified S5–S7) reaction compared published Complex was reference 8, 7 could spectroscopically identified, stabilize oxo, fleeting existence seems assured, given parallels systems five partners identified. IR S8 S9) UV-vis-NIR S12–S15) recorded. Unfortunately, linkages anticipated fall region 800–1,000 cm−1, shown contain absorptions silyl-amide analysis.26Fortier dominated charge transfer bands UV 20,000 cm−1. Across 20,000–5,000 cm−1 multiple (ε <80 M−1 cm−1) characteristic Laporte forbidden f-f transitions ions, accordance pale color complexes.1Liddle Scholar,6Castro-Rodríguez Scholar,43Mills Moro Van Slageren delocalized arene-bridged single-molecule magnet.Nat. 3: 454-460Crossref (238) In order confirm determined, Figure gross terms, similar, separated ion pair four-coordinate ions. geometry about trigonal monopyramidal, average O-U-Namide angle 96.8(3)° Namide-U-Namide 118.6(3)°, lies 0.279(4) Å above plane defined three Namide contrast, exhibited pseudo-tetrahedral uranium, Nimido-U-Namide 102.39(2)° 115.53(2)°. largely mirrored those pentavalent U-Namide distances spanned across ranges 2.346(7)–2.351(7) 2.359(4)–2.368(4) Å, comparison, [2.235(1)–2.244(2) Å]26Fortier [av. 2.295(10) Å]44Zalkin Brennan J.G. Andersen Tris[bis(trimethylsilyl)amido](trimethylsilylimido)uranium(V).Acta Crystallogr. C Cryst. 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