Steric control of redox events in organo-uranium chemistry: synthesis and characterisation of U(v) oxo and nitrido complexes

Two-Electron Redox Reactivity of Thorium Supported by Redox-Active Tripodal Frameworks

The high stability of the + IVoxidation state limits thorium redox reactivity. Here we report the synthesis and the redox reactivity of two Th(IV) complexes supported by the arene-tethered tris(siloxide) tripodal ligands [(KOSiR2 Ar)3 -arene)]. The two-electron reduction of these Th(IV) complexes generates the doubly reduced [KTh((OSi(Ot Bu)2 Ar)3 -arene)(THF)2 ] (2OtBu ) and [K(2.2.2-cryptand)][Th((OSiPh2 Ar)3 -arene)(THF)2 ](2Ph -crypt) where the formal oxidation state of Th is +II. Structural and computational studies indicate that the reduction occurred at the arene anchor of the ligand. The robust tripodal frameworks store in the arene anchor two electrons that become available at the metal center for the two-electron reduction of a broad range of substrates (N2 O, COT, CHT, Ph2 N2 , Ph3 PS and O2 ) while retaining the ligand framework. This work shows that arene-tethered tris(siloxide) tripodal ligands allow implementation of two-electron redox chemistry at the thorium center while retaining the ligand framework unchanged.

Multimetallic Uranium Nitride Cubane Clusters from Dinitrogen Cleavage

Dinitrogen cleavage provides an attractive but poorly studied route to the assembly of multimetallic nitride clusters. Here, we show that the monoelectron reduction of the dinitrogen complex [{U(OC6H2-But3-2,4,6)3}2(μ-η2:η2-N2)], 1, allows us to generate, for the first time, a uranium complex presenting a rare triply reduced N2 moiety ((μ-η2:η2-N2)•3-). Importantly, the bound dinitrogen can be further reduced, affording the U4N4 cubane cluster, 3, and the U6N6 edge-shared cubane cluster, 4, thus showing that (N2)•3- can be an intermediate in nitride formation. The tetranitride cluster showed high reactivity with electrophiles, yielding ammonia quantitatively upon acid addition and promoting CO cleavage to yield quantitative conversion of nitride into cyanide. These results show that dinitrogen reduction provides a versatile route for the assembly of large highly reactive nitride clusters, with U6N6 providing the first example of a molecular nitride of any metal formed from a complete cleavage of three N2 molecules.

Progress in the chemistry of molecular actinide-nitride compounds

The chemistry of actinide-nitrides has witnessed significant advances in the last ten years with a large focus on uranium and a few breakthroughs with thorium. Following the early discovery of the first terminal and bridging nitride complexes, various synthetic routes to uranium nitrides have since been identified, although the range of ligands capable of stabilizing uranium nitrides still remains scarce. In particular, both terminal- and bridging-nitrides possess attractive advantages for potential reactivity, especially in light of the recent development of uranium complexes for dinitrogen reduction and functionalization. The first molecular thorium bridged-nitride complexes have also been recently identified, anticipating the possibility of expanding nitride chemistry not only to low-valent thorium, but also to the transuranic elements.

Nonaqueous Electrochemistry of Uranium Complexes: A Guide to Structure-Reactivity Tuning

Uranium complexes can be stabilized in a wide range of oxidation states, ranging from U^II to U^VI and a very recent example of a U^I complex. This review provides a comprehensive summary of electrochemistry data reported on uranium complexes in nonaqueous electrolyte, to serve as a clear point of reference for newly synthesized compounds, and to evaluate how different ligand environments influence experimentally observed electrochemical redox potentials. Data for over 200 uranium compounds are reported, together with a detailed discussion of trends observed across larger series of complexes in response to ligand field variations. In analogy to the traditional Lever parameter, we utilized the data to derive a new uranium-specific set of ligand field parameters ^UE_L(L) that more accurately represent metal-ligand bonding situations than previously existing transition metal derived parameters. Exemplarily, we demonstrate ^UE_L(L) parameters to be useful for the prediction of structure-reactivity correlations in order to activate specific substrate targets.

Unprecedented pairs of uranium (iv/v) hydroxido and (iv/v/vi) oxido complexes supported by a seven-coordinate cyclen-anchored tris-aryloxide ligand

We present the synthesis and reactivity of a newly developed, cyclen-based tris-aryloxide ligand precursor, namely cyclen(Me)( t-Bu,t-BuArOH)3, and its coordination chemistry to uranium. The corresponding uranium(iii) complex [UIII((OAr t-Bu,t-Bu)3(Me)cyclen)] (1) was characterized by 1H NMR analysis, CHN elemental analysis and UV/vis/NIR electronic absorption spectroscopy. Since no single-crystals suitable for X-ray diffraction analysis could be obtained from this precursor, 1 was oxidized with methylene chloride or silver fluoride to yield [(cyclen(Me)( t-Bu,t-BuArO)3)UIV(X)] (X = Cl (2), F (3)), which were unambiguously characterized and successfully crystallized to gain insight into the molecular structure by single-crystal X-ray diffraction analysis (SC-XRD). Further, the activation of H2O and N2O by 1 is presented, resulting in the U(iv) complex [(cyclen(Me)( t-Bu,t-BuArO)3)UIV(OH)] (4) and the U(v) complex [(cyclen(Me)( t-Bu,t-BuArO)3)UV(O)] (6). Complexes 2, 3, 4, and 6 were characterized by 1H NMR analysis, CHN elemental analysis, UV/vis/NIR electronic absorption spectroscopy, IR vibrational spectroscopy, and SQUID magnetization measurements as well as cyclic voltammetry. Furthermore, chemical oxidation of 3, 4, and 6 with AgF or AgSbF6 was achieved leading to complexes [(cyclen(Me)( t-Bu,t-BuArO)3)UV(F)2] (5), [(cyclen(Me)( t-Bu,t-BuArO)3)UV(OH)][SbF6] (7), and [(cyclen(Me)( t-Bu,t-BuArO)3)UVI(O)][SbF6] (8). Finally, reduction of 7 with KC8 yielded a U(iv) complex, spectroscopically and magnetochemically identified as K[(cyclen(Me)( t-Bu,t-BuArO)3)UIV(O)].

Photochemical Synthesis of Transition Metal-Stabilized Uranium(VI) Nitride Complexes

Uranium nitrides play important roles in dinitrogen activation and functionalization and in chemistry for nuclear fuels, but the synthesis and isolation of the highly reactive uranium(VI) nitrides remains challenging. Here, we report an example of transition metal (TM) stabilized U(VI) nitride complexes, which are generated by the photolysis of azide-bridged U(IV)-TM (TM = Rh, Ir) precursors. The U(V) nitride intermediates with bridged azide ligands are isolated successfully by careful control of the irradiation time, suggesting that the photolysis of azide-bridged U(IV)-TM precursors is a stepwise process. The presence of two U(VI) nitrides stabilized by three TMs is clearly demonstrated by an X-ray crystallographic study. These TM stabilized U(V) nitride intermediates and U(VI) nitride products exhibit excellent stability both in the solid-state and in THF solution under ambient light. Density functional theory calculations show that the photolysis necessary to break the N-N bond of the azide ligands implies excitation from uranium f-orbital to the lowest unoccupied molecular orbital (LUMO), as suggested by the strong antibonding N-(N₂) character present in the latter.

Uranium-nitride chemistry: uranium-uranium electronic communication mediated by nitride bridges

Treatment of [U^IV(N₃)(Tren^TIPS)] (1, Tren^TIPS = {N(CH₂CH₂NSiPrⁱ₃)₃}^3-) with excess Li resulted in the isolation of [{U^IV(μ-NLi₂)(Tren^TIPS)}₂] (2), which exhibits a diuranium(IV) 'diamond-core' dinitride motif. Over-reduction of 1 produces [U^III(Tren^TIPS)] (3), and together with known [{U^V(μ-NLi)(Tren^TIPS)}₂] (4) an overall reduction sequence 1 → 4 → 2 → 3 is proposed. Attempts to produce an odd-electron nitride from 2 resulted in the formation of [{U^IV(Tren^TIPS)}₂(μ-NH)(μ-NLi₂)Li] (5). Use of heavier alkali metals did not result in the formation of analogues of 2, emphasising the role of the high charge-to-radius-ratio of lithium stabilising the charge build up at the nitride. Variable-temperature magnetic data for 2 and 5 reveal large low-temperature magnetic moments, suggesting doubly degenerate ground states, where the effective symmetry of the strong crystal field of the nitride dominates over the spin-orbit coupled nature of the ground multiplet of uranium(IV). Spin Hamiltonian modelling of the magnetic data for 2 and 5 suggest U⋯U anti-ferromagnetic coupling of -4.1 and -3.4 cm^-1, respectively. The nature of the U⋯U electronic communication was probed computationally, revealing a borderline case where the prospect of direct uranium-uranium bonding was raised, but in-depth computational analysis reveals that if any uranium-uranium bonding is present it is weak, and instead the nitride centres dominate the mediation of U⋯U electronic communication. This highlights the importance of obtaining high-level ab initio insight when probing potential actinide-actinide electronic communication and bonding in weakly coupled systems. The computational analysis highlights analogies between the 'diamond-core' dinitride of 2 and matrix-isolated binary U₂N₂.

Heterometallic uranium/molybdenum nitride synthesis via partial N-atom transfer

The reaction of a terminal Mo(II) nitride with a U(III) complex yields a heterodimetallic U-Mo nitride which is the first example of a transition metal-capped uranium nitride. The nitride is triply bonded to U(V) and singly bonded to Mo(0) and supports a U-Mo interaction. This compound shows reactivity toward CO oxidation.

Reactivity of Multimetallic Thorium Nitrides Generated by Reduction of Thorium Azides

Thorium nitrides are likely intermediates in the reported cleavage and functionalization of dinitrogen by molecular thorium complexes and are attractive compounds for the study of multiple bond formation in f-element chemistry, but only one example of thorium nitride isolable from solution was reported. Here, we show that stable multimetallic azide/nitride thorium complexes can be generated by reduction of thorium azide precursors─a route that has failed so far to produce Th nitrides. Once isolated, the thorium azide/nitride clusters, M₃Th═N═Th (M = K or Cs), are stable in solutions probably due to the presence of alkali ions capping the nitride, but their synthesis requires a careful control of the reaction conditions (solvent, temperature, nature of precursor, and alkali ion). The nature of the cation plays an important role in generating a nitride product and results in large structural differences with a bent Th═N═Th moiety found in the K-bound nitride as a result of a strong K-nitride interaction and a linear arrangement in the Cs-bound nitride. Reactivity studies demonstrated the ability of Th nitrides to cleave CO in ambient conditions yielding CN^-.

Nitrogen activation and cleavage by a multimetallic uranium complex

Multimetallic-multielectron cooperativity plays a key role in the metal-mediated cleavage of N₂ to nitrides (N³⁻). In particular, low-valent uranium complexes coupled with strong alkali metal reducing agents can lead to N₂ cleavage, but often, it is ambiguous how many electrons are transferred from the uranium centers to cleave N₂. Herein, we designed new dinuclear uranium nitride complexes presenting a combination of electronically diverse ancillary ligands to promote the multielectron transformation of N₂. Two heteroleptic diuranium nitride complexes, [K{U^IV(OSi(O^tBu)₃)(N(SiMe₃)₂)₂}₂(μ-N)] (1) and [Cs{U^IV(OSi(O^tBu)₃)₂(N(SiMe₃)₂)}₂(μ-N)] (3-Cs), containing different combinations of OSi(O^tBu)₃ and N(SiMe₃)₂ ancillary ligands, were synthesized. We found that both complexes could be reduced to their U(iii)/U(iv) analogues, and the complex, [K₂{U^IV/III(OSi(O^tBu)₃)₂(N(SiMe₃)₂)}₂(μ-N)] (6-K), could be further reduced to a putative U(iii)/U(iii) species that is capable of promoting the 4e⁻ reduction of N₂, yielding the N₂⁴⁻complex [K₃{U^V(OSi(O^tBu)₃)₂(N(SiMe₃)₂)}₂(μ-N)(μ-η²:η²-N₂)], 7. Parallel N₂ reduction pathways were also identified, leading to the isolation of N₂ cleavage products, [K₃{U^VI(OSi(O^tBu)₃)₂(N(SiMe₃)₂)( Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> N)}(μ-N)₂{U^V(OSi(O^tBu)₃)₂(N(SiMe₃)₂)}]₂, 8, and [K₄{(OSi(O^tBu)₃)₂U^V)(N)}(μ-NH)(μ-κ²:C,N-CH₂SiMe₂NSiMe₃)-{U^V(OSi(O^tBu)₃)₂][K(N(SiMe₃)₂]₂, 9. These complexes provide the first example of N₂ cleavage to nitride by a uranium complex in the absence of reducing alkali metals.

Combinations of ligands were used to tune U Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> NU complexes yielding a U(iii)/U(iii) nitride, which activates N₂. Parallel N₂ reduction pathways were identified, leading to the first example of N₂ cleavage by U without external alkali reducing agents.

Cation assisted binding and cleavage of dinitrogen by uranium complexes

N2 binding affinity decreases markedly in a series of isostructural U(iii)–alkali ions complexes with increasing cation size. N2 binding is undetectable in the Cs analogue, but the first example of cesium-assisted N2 cleavage to bis-nitride was observed at ambient condition.

Backbonding in Thorium(IV) and Uranium(IV) Diarsenido Complexes with tBuNC and CO

The coordination of tBuNC and CO with the diarsenido complexes (C₅ Me₅ )₂ An(η² -As₂ Mes₂ ), An=Th, U, has been investigated. For the first time, a comparison between isostructural complexes of Th^IV and U^IV has been possible with CO; density functional calculations indicated an appreciable amount of π backbonding that originates from charge transfer from an actinide-arsenic sigma bond. The calculated CO stretching frequencies in the Th^IV and U^IV diarsenido complexes are consistent with the experimental measurements, both show large shifts to lower frequency. We demonstrate that the π backbonding is crucial to explaining the red shifts of CO frequency upon An^IV complex formation. Interestingly, this interaction essentially correlates to the parallel orientation of π*(C-O) orbitals relative to the An-As bond.

Stepwise Reduction of Dinitrogen by a Uranium-Potassium Complex Yielding a U(VI)/U(IV) Tetranitride Cluster

Multimetallic cooperativity is believed to play a key role in the cleavage of dinitrogen to nitrides (N^3-), but the mechanism remains ambiguous due to the lack of isolated intermediates. Herein, we report the reduction of the complex [K₂{[U^V(OSi(O^tBu)₃)₃]₂(μ-O)(μ-η²:η²-N₂)}], B, with KC₈, yielding the tetranuclear tetranitride cluster [K₆{(OSi(O^tBu)₃)₂U^IV}₃{(OSi(O^tBu)₃)₂U^VI}(μ₄-N)₃(μ₃-N)(μ₃-O)₂], 1, a novel example of N₂ cleavage to nitride by a diuranium complex. The structure of complex 1 is remarkable, as it contains a unique uranium center bound by four nitrides and provides the second example of a trans-N═U^VI═N core analogue of UO₂²⁺. Experimental and computational studies indicate that the formation of the U(IV)/U(VI) tetrauranium cluster occurs via successive one-electron transfers from potassium to the bound N₂^4- ligand in complex B, resulting in N₂ cleavage and the formation of the putative diuranium(V) bis-nitride [K₄{[U^V(OSi(O^tBu)₃)₃]₂(μ-O)(μ-N)₂}], X. Additionally, cooperative potassium binding to the U-bound N₂^4- ligand facilitates dinitrogen cleavage during electron transfer. The nucleophilic nitrides in both complexes are easily functionalized by protons to yield ammonia in 93-97% yield and with excess ¹³CO to yield K¹³CN and KN¹³CO. The structures of two tetranuclear U(IV)/U(V) bis- and mononitride clusters isolated from the reaction with CO demonstrate that the nitride moieties are replaced by oxides without disrupting the tetranuclear structure, but ultimately leading to valence redistribution.

Amidinate Supporting Ligands Influence Molecularity in Formation of Uranium Nitrides

Uranium nitride complexes are attractive targets for chemists as molecular models for the bonding, reactivity, and magnetic properties of next-generation nuclear fuels, but these molecules are uncommon and can be difficult to isolate due to their high reactivity. Here, we describe the synthesis of three new multinuclear uranium nitride complexes, [U(BCMA)₂]₂(μ-N)(μ-κ¹:κ¹-BCMA) (7), [(U(BIMA)₂)₂(μ-N)(μ-NⁱPr)(K₂(μ-η³:η³-CH₂CHNⁱPr)]₂ (8), and [U(BIMA)₂]₂(μ-N)(μ-κ¹:κ¹-BIMA) (9) (BCMA = N,N-bis(cyclohexyl)methylamidinate, BIMA = N,N-bis(iso-propyl)methylamidinate), from U(III) and U(IV) amidinate precursors. By varying the amidinate ligand substituents and azide source, we were able to influence the composition and size of these nitride complexes. ¹⁵N isotopic labeling experiments confirmed the bridging nitride moieties in 7-9 were formed via two-electron reduction of azide. The tetra-uranium cluster 8 was isolated in 99% yield via reductive cleavage of the amidinate ligands; this unusual molecule contains nitrogen-based ligands with formal 1-, 2-, and 3- charges. Additionally, chemical oxidation of the U(IV) precursor U(N₃)(BCMA)₃ yielded the cationic U(V) species [U(N₃)(BCMA)₃][OTf]. Magnetic susceptibility measurements confirmed a U(IV) oxidation state for the uranium centers in the three nitride-bridged complexes and provided a comparison of magnetic behavior in the structurally related U(III)-U(IV)-U(V) series U(BCMA)₃, U(N₃)(BCMA)₃, and [U(N₃)(BCMA)₃][OTf]. At 240 K, the magnetic moments in this series decreased with increasing oxidation state, i.e., U(III) > U(IV) > U(V); this trend follows the decreasing number of 5f valence electrons along this series.

Synthesis, structure, and reactivity of uranium(vi) nitrides

Uranium nitride compounds are important molecular analogues of uranium nitride materials such as UN and UN2 which are effective catalysts in the Haber-Bosch synthesis of ammonia, but the synthesis of molecular nitrides remains a challenge and studies of the reactivity and of the nature of the bonding are poorly developed. Here we report the synthesis of the first nitride bridged uranium complexes containing U(vi) and provide a unique comparison of reactivity and bonding in U(vi)/U(vi), U(vi)/U(v) and U(v)/U(v) systems. Oxidation of the U(v)/U(v) bis-nitride [K2{U(OSi(O t Bu)3)3(μ-N)}2], 1, with mild oxidants yields the U(v)/U(vi) complexes [K{U(OSi(O t Bu)3)3(μ-N)}2], 2 and [K2{U(OSi(O t Bu)3)3}2(μ-N)2(μ-I)], 3 while oxidation with a stronger oxidant ("magic blue") yields the U(vi)/U(vi) complex [{U(OSi(O t Bu)3)3}2(μ-N)2(μ-thf)], 4. The three complexes show very different stability and reactivity, with N2 release observed for complex 4. Complex 2 undergoes hydrogenolysis to yield imido bridged [K2{U(OSi(O t Bu)3)3(μ-NH)}2], 6 and rare amido bridged U(iv)/U(iv) complexes [{U(OSi(O t Bu)3)3}2(μ-NH2)2(μ-thf)], 7 while no hydrogenolysis could be observed for 4. Both complexes 2 and 4 react with H+ to yield quantitatively NH4Cl, but only complex 2 reacts with CO and H2. Differences in reactivity can be related to significant differences in the U-N bonding. Computational studies show a delocalised bond across the U-N-U for 1 and 2, but an asymmetric bonding scheme is found for the U(vi)/U(vi) complex 4 which shows a U-N σ orbital well localised to U[triple bond, length as m-dash]N and π orbitals which partially delocalise to form the U-N single bond with the other uranium.

Single metal four-electron reduction by U(ii) and masked "U(ii)" compounds

The redox chemistry of uranium is dominated by single electron transfer reactions while single metal four-electron transfers remain unknown in f-element chemistry. Here we show that the oxo bridged diuranium(iii) complex [K(2.2.2-cryptand)]₂[{((Me₃Si)₂N)₃U}₂(μ-O)], 1, effects the two-electron reduction of diphenylacetylene and the four-electron reduction of azobenzene through a masked U(ii) intermediate affording a stable metallacyclopropene complex of uranium(iv), [K(2.2.2-cryptand)][U(η ²-C₂Ph₂){N(SiMe₃)₂}₃], 3, and a bis(imido)uranium(vi) complex [K(2.2.2-cryptand)][U(NPh)₂{N(SiMe₃)₂}₃], 4, respectively. The same reactivity is observed for the previously reported U(ii) complex [K(2.2.2-cryptand)][U{N(SiMe₃)₂}₃], 2. Computational studies indicate that the four-electron reduction of azobenzene occurs at a single U(ii) centre via two consecutive two-electron transfers and involves the formation of a U(iv) hydrazide intermediate. The isolation of the cis-hydrazide intermediate [K(2.2.2-cryptand)][U(N₂Ph₂){N(SiMe₃)₂}₃], 5, corroborated the mechanism proposed for the formation of the U(vi) bis(imido) complex. The reduction of azobenzene by U(ii) provided the first example of a "clear-cut" single metal four-electron transfer in f-element chemistry.

Isocyanide adducts of tri- and tetravalent uranium metallocenes supported by tetra(isopropyl)cyclopentadienyl ligands

Reaction of the uranium(iii) metallocenium salt [(CpiPr4)2U][B(C6F5)4] with tert-butyl isocyanide (tBuNC) yielded the dicationic uranium(iv) complex [(CpiPr4)2U(CNtBu)4][B(C6F5)4]2 (1), which displays a linear metallocene geometry. Use of crude mixtures of [(CpiPr4)2U][B(C6F5)4], which contain a soluble source of iodide, led instead to isolation of the monocationic uranium(iv) iodide complex [(CpiPr4)2U(I)(CNtBu)2][B(C6F5)4] (2). Adduct formation with no change in oxidation state was observed upon addition of tBuNC to the neutral uranium(iii) species (CpiPr4)2UI, resulting in isolation of (CpiPr4)2U(I)(CNtBu) (3). X-ray crystallographic and IR spectroscopic studies both showed effects ascribed to the presence of multiple strongly donating isocyanide ligands in 1.

Heteroleptic actinocenes: a thorium(iv)-cyclobutadienyl-cyclooctatetraenyl-di-potassium-cyclooctatetraenyl complex

Despite the vast array of η n -carbocyclic C5-8 complexes reported for actinides, cyclobutadienyl (C4) remain exceedingly rare, being restricted to six uranium examples. Here, overcoming the inherent challenges of installing highly reducing C4-ligands onto actinides when using polar starting materials such as halides, we report that reaction of [Th(η8-C8H8)2] with [K2{C4(SiMe3)4}] gives [{Th(η4-C4[SiMe3]4)(μ-η8-C8H8)(μ-η2-C8H8)(K[C6H5Me]2)}2{K(C6H5Me)}{K}] (1), a new type of heteroleptic actinocene. Quantum chemical calculations suggest that the thorium ion engages in π- and δ-bonding to the η4-cyclobutadienyl and η8-cyclooctatetraenyl ligands, respectively. Furthermore, the coordination sphere of this bent thorocene analogue is supplemented by an η2-cyclooctatetraenyl interaction, which calculations suggest is composed of σ- and π-symmetry donations from in-plane in- and out-of-phase C[double bond, length as m-dash]C 2p-orbital combinations to vacant thorium 6d orbitals. The characterisation data are consistent with this being a metal-alkene-type interaction that is integral to the bent structure and stability of this complex.

C-H Bond Activation by an Isolated Dinuclear U(III)/U(IV) Nitride

Synthetic studies of bimetallic uranium nitride complexes with the N(SiMe₃)₂ ligand have generated a new nitride complex of U(III), which is highly reactive toward C-H bonds and H₂. Treatment of the previously reported U(IV)/U(IV) nitride complex [Na(DME)₃][((Me₃Si)₂N)₂U(μ-N)(μ-κ²:CN̵-CH₂SiMe₂NSiMe₃)U(N(SiMe₃)₂)₂] (DME = 1,2-dimethoxyethane), 1, with 2 equiv of HNEt₃BPh₃ yielded the cationic U(IV)/U(IV) nitride complex, [{((Me₃Si)₂N)₂U(THF)}₂(μ-N)][BPh₄] (THF = tetrahydrofuran), 3, by successive protonolysis of one N(SiMe₃)₂ ligand and the uranium-methylene bond. Reduction of 3 with KC₈ afforded a rare example of a U(III) nitride, namely, the U(III)/U(IV) complex, [{((Me₃Si)₂N)₂U(THF)}₂(μ-N)], 4. Complex 4 is highly reactive and undergoes 1,2-addition of the C-H bond of the N(SiMe₃)₂ ligand across the uranium-nitride moiety to give the U(III)/U(IV) imide cyclometalate complex, [((Me₃Si)₂N)₂(THF)U(μ-NH)(μ-κ²:C,N̵-CH₂SiMe₂NSiMe₃)U(N(SiMe₃)₂))(THF)], 5. Complex 4 also reacts with toluene at -80 °C to yield an inverse sandwich imide complex arising from C-H bond activation of toluene, [{((Me₃Si)₂N)₂U(THF)}₂(μ-N)][{((Me₃Si)₂N)₃U(μ-NH)U(N(SiMe₃)₂)}₂(C₇H₈)], 6. Complex 4 effects the heterolytic cleavage of the C-H of phenylacetylene to yield the imide acetylide [{((Me₃Si)₂N)₂U(THF)}₂(μ-N)][((Me₃Si)₂N)₂U(η¹-CCPh)(μ₂-NH)(μ₂-η²:η¹-CCPh)U(N(SiMe₃)₂)₂], 7. Complex 4 also reacts with H₂ to produce an imide hydride U(III)/U(IV) complex, [{((Me₃Si)₂N)₂U(THF)}₂(μ-NH)(μ-H)], 9. These data demonstrate that nitride complexes of U(III) are accessible with amide ligands and show the high reactivity of molecular U(III) nitrides in C-H bond activation.

Intra- and intermolecular interception of a photochemically generated terminal uranium nitride

The photochemically generated synthesis of a terminal uranium nitride species is here reported and an examination of its intra- and intermolecular chemistry is presented. Treatment of the U(iii) complex L^ArUI(DME) ((L^Ar)²⁻ = 2,2′′-bis(Dippanilide)-p-terphenyl; Dipp = 2,6-diisopropylphenyl) with LiNIm^Dipp ((NIm^Dipp)⁻ = 1,3-bis(Dipp)-imidazolin-2-iminato) generates the sterically congested 3N-coordinate compound L^ArU(NIm^Dipp) (1). Complex 1 reacts with 1 equiv. of Ph₃CN₃ to give the U(iv) azide L^ArU(N₃)(NIm^Dipp) (2). Structural analysis of 2 reveals inequivalent N_α–N_β > N_β–N_γ distances indicative of an activated azide moiety predisposed to N₂ loss. Room-temperature photolysis of benzene solutions of 2 affords the U(iv) amide (N-L^Ar)U(NIm^Dipp) (3) via intramolecular N-atom insertion into the benzylic C–H bond of a pendant isopropyl group of the (L^Ar)²⁻ ligand. The formation of 3 occurs as a result of the intramolecular interception of the intermediately generated, terminal uranium nitride (L^Ar)U(N)(NIm^Dipp) (3′). Evidence for the formation of 3′ is further bolstered by its intermolecular capture, accomplished by photolyzing solutions of 2 in the presence of an isocyanide or PMe₃ to give (L^Ar)U[NCN(C₆H₃Me₂)](NIm^Dipp) (5) and (N,C-L^Ar*)U(N Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> PMe₃)(NIm^Dipp) (6), respectively. These results expand upon the limited reactivity studies of terminal uranium–nitride moieties and provide new insights into their chemical properties.

Photolysis of the U(iv) azide L^ArU(NIm^Dipp) generates a reactive uranium nitride intermediate that can be intercepted by nucleophilic substrates – the first example of intermolecular chemistry of a rare photochemically generated uranium nitride.

Lewis acid capping of a uranium(v) nitride via a uranium(iii) azide molecular square

Addition of B(C₆F₅)₃ to a tetrameric uranium(iii) azide-bridged molecular square induced N₂ loss and formation of a uranium(v) borane-capped nitride.

The synthesis and versatile reducing power of low-valent uranium complexes

This synthesis and diverse reactivity of uranium(iii) and uranium(ii) complexes is discussed.

Uranium(iv) cyclobutadienyl sandwich compounds: synthesis, structure and chemical bonding

The 1 : 1 reactions of uranium(iv) tetrakis(borohydride) with the sodium and potassium salts of the cyclobutadienyl anion [C₄(SiMe₃)₄]^2- (Cb'''') produce the half-sandwich complexes [Na(12-crown-4)₂][U(η⁴-Cb'''')(BH₄)₃] and [U(η⁴-Cb'''')(μ-BH₄)₃{K(THF)₂}]₂. In the 1 : 2 reaction of U(BH₄)₄ with Na₂Cb'''', formation of [U(η⁴-Cb'''')(η³-C₄H(SiMe₃)₃-κ-(CH₂SiMe₂)(BH₄))]^- reveals that a Cb'''' ligand undergoes an intramolecular deprotonation, resulting in an allyl/tuck-in bonding mode. A computational study reveals that the uranium-Cb'''' bonding has an appreciable covalent component with contributions from the uranium 5f and 6d orbitals.

Small molecule activation by multimetallic uranium complexes supported by siloxide ligands

The synthesis and reactivity of uranium compounds supported by the tris-tert-butoxysiloxide ligand is surveyed. The multiple binding modes of the tert-butoxysiloxide ligand have proven very well suited to stabilize highly reactive homo- and heteropolymetallic complexes of uranium that have shown an unusual high reactivity towards small molecules such as CO2, CS2, chalcogens and azides. Moreover, these ligands have allowed the isolation of dinuclear nitride and oxide bridged complexes of uranium in various oxidation states. The ability of the tris-tert-butoxysiloxide ligands to trap alkali ions in these nitride or oxide complexes leads to unprecedented ligand based and metal based reduction and functionalization of N2, CO, CO2 and H2.

Thorium-nitrogen multiple bonds provide evidence for pushing-from-below for early actinides

Although the chemistry of uranium-ligand multiple bonding is burgeoning, analogous complexes involving other actinides such as thorium remain rare and there are not yet any terminal thorium nitrides outside of cryogenic matrix isolation conditions. Here, we report evidence that reduction of a thorium-azide produces a transient Th≡N triple bond, but this activates C-H bonds to produce isolable parent imido derivatives or it can be trapped in an N-heterocycle amine. Computational studies on these thorium-nitrogen multiple bonds consistently evidences a σ > π energy ordering. This suggests pushing-from-below for thorium, where 6p-orbitals principally interact with filled f-orbitals raising the σ-bond energy. Previously this was dismissed for thorium, being the preserve of uranium-nitrides or the uranyl dication. Recognising that pushing-from-below perhaps occurs with thorium as well as uranium, and with imido ligands as well as nitrides, suggests this phenomenon may be more widespread than previously thought.

Despite the burgeoning nature of uranium–ligand multiple bonding, analogous thorium complexes remain incredibly rare. Here the authors report evidence for a transient thorium–nitride species, which, together with data on parent imido derivatives, suggests that the pushing-from-below phenomenon may be more widespread than previously thought.

Tuning the structure, reactivity and magnetic communication of nitride-bridged uranium complexes with the ancillary ligands

The reactivity of the nitride ligand is increased in complexes of uranium(iv) when bound by the OSi(O^tBu)₃ ligand as opposed to N(SiMe₃)₂, but magnetic exchange coupling is decreased.

Molecular uranium nitride complexes were prepared to relate their small molecule reactivity to the nature of the U Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> N Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> U bonding imposed by the supporting ligand. The U⁴⁺–U⁴⁺ nitride complexes, [NBu₄][{((^tBuO)₃SiO)₃U}₂(μ-N)], [NBu₄]-1, and [NBu₄][((Me₃Si)₂N)₃U}₂(μ-N)], 2, were synthesised by reacting NBu₄N₃ with the U³⁺ complexes, [U(OSi(O^tBu)₃)₂(μ-OSi(O^tBu)₃)]₂ and [U(N(SiMe₃)₂)₃], respectively. Oxidation of 2 with AgBPh₄ gave the U⁴⁺–U⁵⁺ analogue, [((Me₃Si)₂N)₃U}₂(μ-N)], 4. The previously reported methylene-bridged U⁴⁺–U⁴⁺ nitride [Na(dme)₃][((Me₃Si)₂)₂U(μ-N)(μ-κ²-C,N-CH₂SiMe₂NSiMe₃)U(N(SiMe₃)₂)₂] (dme = 1,2-dimethoxyethane), [Na(dme)₃]-3, provided a versatile precursor for the synthesis of the mixed-ligand U⁴⁺–U⁴⁺ nitride complex, [Na(dme)₃][((Me₃Si)₂N)₃U(μ-N)U(N(SiMe₃)₂)(OSi(O^tBu)₃)], 5. The reactivity of the 1–5 complexes was assessed with CO₂, CO, and H₂. Complex [NBu₄]-1 displays similar reactivity to the previously reported heterobimetallic complex, [Cs{((^tBuO)₃SiO)₃U}₂(μ-N)], [Cs]-1, whereas the amide complexes 2 and 4 are unreactive with these substrates. The mixed-ligand complexes 3 and 5 react with CO and CO₂ but not H₂. The nitride complexes [NBu₄]-1, 2, 4, and 5 along with their small molecule activation products were structurally characterized. Magnetic data measured for the all-siloxide complexes [NBu₄]-1 and [Cs]-1 show uncoupled uranium centers, while strong antiferromagnetic coupling was found in complexes containing amide ligands, namely 2 and 5 (with maxima in the χ versus T plot of 90 K and 55 K). Computational analysis indicates that the U(μ-N) bond order decreases with the introduction of oxygen-based ligands effectively increasing the nucleophilicity of the bridging nitride.

Uranocenium: Synthesis, Structure, and Chemical Bonding

Abstraction of iodide from [(η⁵ -C₅ ⁱ Pr₅ )₂ UI] (1) produced the cationic uranium(III) metallocene [(η⁵ -C₅ ⁱ Pr₅ )₂ U]⁺ (2) as a salt of [B(C₆ F₅ )₄ ]^- . The structure of 2 consists of unsymmetrically bonded cyclopentadienyl ligands and a bending angle of 167.82° at uranium. Analysis of the bonding in 2 showed that the uranium 5f orbitals are strongly split and mixed with the ligand orbitals, thus leading to non-negligible covalent contributions to the bonding. Investigation of the dynamic magnetic properties of 2 revealed that the 5f covalency leads to partially quenched anisotropy and fast magnetic relaxation in zero applied magnetic field. Application of a magnetic field leads to dominant relaxation by a Raman process.

Actinide Organometallic Complexes with π-Ligands

Recent developments and results from the organometallic chemistry of the actinides are reviewed. In the last one and a half years the structural data of about 15 organometallic complexes of transuranium actinides (Np or Pu) have been published, all involving π-ligands in the coordination sphere of the metal ion. On the basis of these data, a comparison of these molecules is presented. Depending on the steric demands of the ligands, effects like the actinide contraction seem to be stronger or weaker in the structural features. This indicates that the interplay between the actinide ion and the π-ligand is rather flexible, enabling the formation of stable bonds over a broad range of actinide ion oxidation states.

Thorium- and uranium-azide reductions: a transient dithorium-nitride versus isolable diuranium-nitrides

Molecular uranium-nitrides are now well known, but there are no isolable molecular thorium-nitrides outside of cryogenic matrix isolation experiments. We report that treatment of [M(Tren^DMBS)(I)] (M = U, 1; Th, 2; Tren^DMBS = {N(CH₂CH₂NSiMe₂Bu ^t )₃}^3-) with NaN₃ or KN₃, respectively, affords very rare examples of actinide molecular square and triangle complexes [{M(Tren^DMBS)(μ-N₃)} _n ] (M = U, n = 4, 3; Th, n = 3, 4). Chemical reduction of 3 produces [{U(Tren^DMBS)}₂(μ-N)][K(THF)₆] (5) and [{U(Tren^DMBS)}₂(μ-N)] (6), whereas photolysis produces exclusively 6. Complexes 5 and 6 can be reversibly inter-converted by oxidation and reduction, respectively, showing that these UNU cores are robust with no evidence for any C-H bond activations being observed. In contrast, reductions of 4 in arene or ethereal solvents gives [{Th(Tren^DMBS)}₂(μ-NH)] (7) or [{Th(Tren^DMBS)}{Th(N[CH₂CH₂NSiMe₂Bu ^t ]₂CH₂CH₂NSi[μ-CH₂]MeBu ^t )}(μ-NH)][K(DME)₄] (8), respectively, providing evidence unprecedented outside of matrix isolation for a transient dithorium-nitride. This suggests that thorium-nitrides are intrinsically much more reactive than uranium-nitrides, since they consistently activate C-H bonds to form rare examples of Th-N(H)-Th linkages with alkyl by-products. The conversion here of a bridging thorium(iv)-nitride to imido-alkyl combination by 1,2-addition parallels the reactivity of transient terminal uranium(iv)-nitrides, but contrasts to terminal uranium(vi)-nitrides that produce alkyl-amides by 1,1-insertion, suggesting a systematic general pattern of C-H activation chemistry for metal(iv)- vs. metal(vi)-nitrides. Surprisingly, computational studies reveal a σ > π energy ordering for all these bridging nitride bonds, a phenomenon for actinides only observed before in terminal uranium nitrides and uranyl with very short U-N or U-O distances.

Facile N-functionalization and strong magnetic communication in a diuranium(v) bis-nitride complex

Uranium nitride complexes are of high interest because of their ability to effect dinitrogen reduction and functionalization and to promote magnetic communication, but studies of their properties and reactivity remain rare. Here we have prepared in 73% yield the diuranium(v) bis-nitride complex [K₂{[U(OSi(O ^t Bu)₃)₃]₂(μ-N)₂}], 4, from the thermal decomposition of the nitride-, azide-bridged diuranium(iv) complex [K₂{[U(OSi(O ^t Bu)₃)₃]₂(μ-N)(μ-N₃)}], 3. The bis-nitride 4 reacts in ambient conditions with 1 equiv. of CS₂ and 1 equiv. of CO₂ resulting in N-C bond formation to afford the diuranium(v) complexes [K₂{[U(OSi(O ^t Bu)₃)₃]₂(μ-N)(μ-S)(μ-NCS)}], 5 and [K₂{[U(OSi(O ^t Bu)₃)₃]₂(μ-N)(μ-O)(μ-NCO)}], 6, respectively. Both nitrides in 4 react with CO resulting in oxidative addition of CO to one nitride and CO cleavage by the second nitride to afford the diuranium(iv) complex [K₂{[U(OSi(O ^t Bu)₃)₃]₂(μ-CN)(μ-O)(μ-NCO)}], 7. Complex 4 also effects the remarkable oxidative cleavage of H₂ in mild conditions to afford the bis-imido bridged diuranium(iv) complex [K₂{[U(OSi(O ^t Bu)₃)₃]₂(μ-NH)₂}], 8 that can be further protonated to afford ammonia in 73% yield. Complex 8 provides a good model for hydrogen cleavage by metal nitrides in the Haber-Bosch process. The measured magnetic data show an unusually strong antiferromagnetic coupling between uranium(v) ions in the complexes 4 and 6 with Neel temperatures of 77 K and 60 K respectively, demonstrating that nitrides are attractives linkers for promoting magnetic communication in uranium complexes.

DFT Investigations of the Magnetic Properties of Actinide Complexes

Over the past 25 years, magnetic actinide complexes have been the object of considerable attention, not only at the experimental level, but also at the theoretical one. Such systems are of great interest, owing to the well-known larger spin–orbit coupling for actinide ions, and could exhibit slow relaxation of the magnetization, arising from a large anisotropy barrier, and magnetic hysteresis of purely molecular origin below a given blocking temperature. Furthermore, more diffuse 5f orbitals than lanthanide 4f ones (more covalency) could lead to stronger magnetic super-exchange. On the other hand, the extraordinary experimental challenges of actinide complexes chemistry, because of their rarity and toxicity, afford computational chemistry a particularly valuable role. However, for such a purpose, the use of a multiconfigurational post-Hartree-Fock approach is required, but such an approach is computationally demanding for polymetallic systems—notably for actinide ones—and usually simplified models are considered instead of the actual systems. Thus, Density Functional Theory (DFT) appears as an alternative tool to compute magnetic exchange coupling and to explore the electronic structure and magnetic properties of actinide-containing molecules, especially when the considered systems are very large. In this paper, relevant achievements regarding DFT investigations of the magnetic properties of actinide complexes are surveyed, with particular emphasis on some representative examples that illustrate the subject, including actinides in Single Molecular Magnets (SMMs) and systems featuring metal-metal super-exchange coupling interactions. Examples are drawn from studies that are either entirely computational or are combined experimental/computational investigations in which the latter play a significant role.

Reactions of cerium complexes with transition metal nitrides: synthesis and structure of heterometallic cerium complexes containing bridging catecholate ligands

Heterometallic cerium complexes containing bridging catecholate ligands have been synthesized from cerium complexes with Kläui's tripodal ligand and metal catecholates.

Carbon dioxide reduction by dinuclear Yb(ii) and Sm(ii) complexes supported by siloxide ligands

Low-coordinate dinuclear lanthanide complexes supported by siloxides effect the reduction of carbon dioxide to both carbonate and oxalate, but the cooperative binding of CO2 to the two Ln(ii) cations in the dimer favours oxalate formation.

C-H Bond Addition across a Transient Uranium-Nitrido Moiety and Formation of a Parent Uranium Imido Complex

Uranium complexes in the +3 and +4 oxidation states were prepared using the anionic PN^- (PN^- = ( N-(2-(diisopropylphosphino)-4-methylphenyl)-2,4,6-trimethylanilide) ligand framework. New complexes include the halide starting materials, (PN)₂U^IIII (1) and (PN)₂U^IVCl₂ (2), which both yield (PN)₂U^IV(N₃)₂ (3) by reaction with NaN₃. Compound 3 was reduced with potassium graphite to produce a putative, transient uranium-nitrido moiety that underwent an intramolecular C-H activation to form a rare example of a parent imido complex, [K(THF)₃][(PN)U^IV(═NH)[ ⁱPr₂P(C₆H₃Me)N(C₆H₂Me₂CH₂)]] (4). Calculated reaction energy profiles strongly suggest that a C-H insertion becomes unfavorable when a reductant is present, offering a distinctively different reaction pathway than previously observed for other uranium nitride complexes.

A diuranium carbide cluster stabilized inside a C80 fullerene cage

Unsupported non-bridged uranium-carbon double bonds have long been sought after in actinide chemistry as fundamental synthetic targets in the study of actinide-ligand multiple bonding. Here we report that, utilizing Ih(7)-C80 fullerenes as nanocontainers, a diuranium carbide cluster, U=C=U, has been encapsulated and stabilized in the form of UCU@Ih(7)-C80. This endohedral fullerene was prepared utilizing the Krätschmer-Huffman arc discharge method, and was then co-crystallized with nickel(II) octaethylporphyrin (NiII-OEP) to produce UCU@Ih(7)-C80·[NiII-OEP] as single crystals. X-ray diffraction analysis reveals a cage-stabilized, carbide-bridged, bent UCU cluster with unexpectedly short uranium-carbon distances (2.03 Å) indicative of covalent U=C double-bond character. The quantum-chemical results suggest that both U atoms in the UCU unit have formal oxidation state of +5. The structural features of UCU@Ih(7)-C80 and the covalent nature of the U(f1)=C double bonds were further affirmed through various spectroscopic and theoretical analyses.

New supporting ligands in actinide chemistry: tetramethyltetraazaannulene complexes with thorium and uranium

We report the synthesis, characterization, and preliminary reactivity of new heteroleptic thorium and uranium complexes supported by the macrocyclic TMTAA ligand (TMTAA = Tetramethyl-tetra-aza-annulene). The dihalide complexes Th(TMTAA)Cl₂(THF)₂ (1), [UCl₂(TMTAA)]₂ (2) and U(TMTAA)I₂ (3) are further functionalized to the Cp* derivatives ThCp*(TMTAA)Cl (4), UCp*(TMTAA)Cl (5) and UCp*(TMTAA)I (6) (Cp* = pentamethylcyclopentadienide). Compounds 4-6 are also obtained through a one-pot reaction from standard thorium(iv) and uranium(iv) starting materials, Li2TMTAA and KCp*. Complexes 1-6 function as valuable starting materials for salt metathesis chemistry. Treatment of precursors 4 or 5 with trimethylsilylmethyllithium (LiCH₂TMS) results in the new actinide TMTAA alkyl complexes ThCp*(TMTAA)(CH₂TMS) (7) and UCp*(TMTAA)(CH₂TMTS) (8), respectively. The TMTAA-derived alkyl complexes (7 and 8) show unexpected stability and are stable for several weeks at room temperature in solution and in the solid-state. Additionally, double substitution of the halide ligands in 1-3 shows a strong dependence on the nucleophile used. While weaker nucleophiles, such as amides, and more sterically demanding nucleophiles, such as Cp (Cp = cyclopenadienide), favour the formation of bis-TMTAA "sandwich" complexes [An(TMTAA)₂] (An = Th (9) and An = U (10)), the use of oxygen-functionalized ligands like the ODipp anion (Dipp = diisopropylphenyl) results in the formation of the doubly substituted species Th(ODipp)₂TMTAA (11) and U(ODipp)₂TMTAA (12). We also describe the divergent reactivity of the TMTAA ligand towards uranium(iii). Unlike the syntheses of actinide(iv) TMTAA complexes, the synthesis of a uranium(iii) TMTAA was not successful and only uranium(iv) species could be obtained.

Metathesis of a UV imido complex: a route to a terminal UV sulfide

The metathesis reaction of a U^V imido complex supported by sterically demanding tris(tert-butoxy)siloxide ligands with CS₂ afforded a terminal U^V thiocarbonate but metathesis with H₂S afforded the first example of a terminal U^V sulfide.

Herein, we report the synthesis and characterisation of the first terminal uranium(v) sulfide and a related U^V trithiocarbonate complex supported by sterically demanding tris(tert-butoxy)siloxide ligands. The reaction of the potassium-bound U^V imido complex, [U(NAd){OSi(OtBu)₃}₄K] (4), with CS₂ led to the isolation of perthiodicarbonate [K(18c6)]₂[C₂S₆] (6), with concomitant formation of the U^IV complex, [U{OSi(OtBu)₃}₄], and S Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CNAd. In contrast, the reaction of the U^V imido complex, [K(2.2.2-cryptand)][U(NAd){OSi(OtBu)₃}₄] (5), with one or two equivalents of CS₂ afforded the trithiocarbonate complex, [K(2.2.2-cryptand)][U(CS₃){OSi(OtBu)₃}₄] (7), which was isolated in 57% yield, with concomitant elimination of the admantyl thiocyanate product, SCNAd. Complex 7 is likely formed by fast nucleophilic addition of a U^V terminal sulfide intermediate, resulting from the slow metathesis reaction of the imido complex with CS₂, to a second CS₂ molecule. The addition of a solution of H₂S in thf (1.3 eq.) to 4 afforded the first isolable U^V terminal sulfide complex, [K(2.2.2-cryptand)][US{OSi(OtBu)₃}₄] (8), in 41% yield. Based on DFT calculations, triple-bond character with a strong covalent interaction is suggested for the U–S bond in complex 7.

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

We report the synthesis and characterisation of a family of diuranium(iv)-μ-chalcogenide complexes including a detailed examination of their electronic structures and magnetic behaviours. Treatment of [U(Tren^TIPS)] [1, Tren^TIPS = N(CH₂CH₂NSiPrⁱ₃)₃] with Ph₃PS, selenium or tellurium affords the diuranium(iv)-sulfide, selenide, and telluride complexes [{U(Tren^TIPS)}₂(μ-E)] (E = S, 2; Se, 5; Te, 6). Complex 2 is also formed by treatment of [U(Tren^TIPS){OP(NMe₂)₃}] (3) with Ph₃PS, whereas treatment of 3 with elemental sulfur gives the diuranium(iv)-persulfido complex [{U(Tren^TIPS)}₂(μ-η²:η²-S₂)] (4). Complexes 2-6 have been variously characterised by single crystal X-ray diffraction, NMR, IR, and optical spectroscopies, room temperature Evans and variable temperature SQUID magnetometry, elemental analyses, and complete active space self consistent field spin orbit calculations. The combined characterisation data present a self-consistent picture of the electronic structure and magnetism of 2, 5, and 6, leading to the conclusion that single-ion crystal field effects, and not diuranium magnetic coupling, are responsible for features in their variable-temperature magnetisation data. The presence of magnetic coupling is often implied and sometimes quantified by such data, and so this study highlights the importance of evaluating other factors, such as crystal field effects, that can produce similar magnetic observables, and to thus avoid misassignments of such phenomena.