Actinide Organometallic Complexes with π-Ligands

Substituent Positioning Effects on the Magnetic Properties of Sandwich-Type Erbium(III) Complexes with Bis(trimethylsilyl)-Substituted Cyclooctatetraenyl Ligands

Lanthanide complexes with judiciously designed ligands have been extensively studied for their potential applications as single-molecule magnets. With the influence of ligands on their magnetic properties generally established, recent research has unearthed certain effects inherent to site differentiation due to the different types and varying numbers of substituents on the same ligand platform. Using two new sandwich-type Er(III) complexes with cyclooctatetraenyl (COT) ligands featuring two differently positioned trimethylsilyl (TMS) substituents, namely, [Li(DME)Er(COT1,5-TMS2)2]n (Er1) and [Na(DME)3][Er(COT1,3-TMS2)2] (Er2) [COT1,3-TMS2 and COT1,5-TMS2 donate 1,3- and 1,5-bis(trimethylsilyl)-substituted cyclooctatetraenyl ligands, respectively; DME = 1,2-dimethoxyethane], and with reference to previously reported [Li(DME)3][Er(COT1,4-TMS2)2] (A) and [K(DME)2][Er(COT1,4-TMS2)2] (B), any possible substituent position effects have been explored for the first time. The rearrangement of the TMS substituents from the starting COT1,4-TMS2 to COT1,3-TMS2 and COT1,5-TMS2, by way of formal migration of the TMS group, was thermally induced in the case of Er1, while for the formation of Er2, the use of Na+ in the placement of its Li+ and K+ congeners is essential. Both Er1 and Er2 display single-molecule magnetic behaviors with energy barriers of 170(3) and 172(6) K, respectively. Magnetic hysteresis loops, butterfly-shaped for Er1 and wide open for Er2, were observed up to 12 K for Er1 and 13 K for Er2. Studies of magnetic dynamics reveal the different pathways for relaxation of magnetization below 10 K, mainly by the Raman process for Er1 and by quantum tunneling of magnetization for Er2, leading to the order of magnitude difference in magnetic relaxation times and sharply different magnetic hysteresis loops.

Metal-carbon bonding in early lanthanide substituted cyclopentadienyl complexes probed by pulsed EPR spectroscopy

We examine lanthanide (Ln)-ligand bonding in a family of early Ln3+ complexes [Ln(Cptt)3] (1-Ln, Ln = La, Ce, Nd, Sm; Cptt = C5H3tBu2-1,3) by pulsed electron paramagnetic resonance (EPR) methods, and provide the first characterization of 1-La and 1-Nd by single crystal XRD, multinuclear NMR, IR and UV/Vis/NIR spectroscopy. We measure electron spin T1 and Tm relaxation times of 12 and 0.2 μs (1-Nd), 89 and 1 μs (1-Ce) and 150 and 1.7 μs (1-Sm), respectively, at 5 K: the T1 relaxation of 1-Nd is more than 102 times faster than its valence isoelectronic uranium analogue. 13C and 1H hyperfine sublevel correlation (HYSCORE) spectroscopy reveals that the extent of covalency is negligible in these Ln compounds, with much smaller hyperfine interactions than observed for equivalent actinide (Th and U) complexes. This is corroborated by ab initio calculations, confirming the predominant electrostatic nature of the metal-ligand bonding in these complexes.

Carbene Complexes of Plutonium: Structure, Bonding, and Divergent Reactivity to Lanthanide Analogs

Organoplutonium chemistry was established in 1965, yet structurally authenticated plutonium-carbon bonds remain rare being limited to π-bonded carbocycle and σ-bonded isonitrile and hydrocarbyl derivatives. Thus, plutonium-carbenes, including alkylidenes and N-heterocyclic carbenes (NHCs), are unknown. Here, we report the preparation and characterization of the diphosphoniomethanide-plutonium complex [Pu(BIPMTMSH)(I)(μ-I)]2 (1Pu, BIPMTMSH = (Me3SiNPPh2)2CH) and the diphosphonioalkylidene-plutonium complexes [Pu(BIPMTMS)(I)(DME)] (2Pu, BIPMTMS = (Me3SiNPPh2)2C) and [Pu(BIPMTMS)(I)(IMe4)2] (3Pu, IMe4 = C(NMeCMe)2), thus disclosing non-actinyl transneptunium multiple bonds and transneptunium NHC complexes. These Pu-C double and dative bonds, along with cerium, praseodymium, samarium, uranium, and neptunium congeners, enable lanthanide-actinide and actinide-actinide comparisons between metals with similar ionic radii and isoelectronic 4f5 vs 5f5 electron-counts within conserved ligand fields over 12 complexes. Quantum chemical calculations reveal that the orbital-energy and spatial-overlap terms increase from uranium to neptunium; however, on moving to plutonium the orbital-energy matching improves but the spatial overlap decreases. The bonding picture that emerges is more complex than the traditional picture of the bonding of lanthanides being ionic and early actinides being more covalent but becoming more ionic left to right. Multiconfigurational calculations on 2M and 3M (M = Pu, Sm) account for the considerably more complex UV/vis/NIR spectra for 5f5 2Pu and 3Pu compared to 4f5 2Sm and 3Sm. Supporting the presence of Pu═C double bonds in 2Pu and 3Pu, 2Pu exhibits metallo-Wittig bond metathesis involving the highest atomic number element to date, reacting with benzaldehyde to produce the alkene PhC(H)═C(PPh2NSiMe3)2 (4) and "PuOI". In contrast, 2Ce and 2Pr do not react with benzaldehyde to produce 4.

Synthesis and comparison of iso-structural f-block metal complexes (Ce, U, Np, Pu) featuring η6-arene interactions

Reaction of the terphenyl bis(anilide) ligand [{K(DME)₂}₂L^Ar] (L^Ar = {C₆H₄[(2,6-ⁱPr²C₆H₃)NC₆H₄]₂}^2-) with trivalent chloride "MCl₃" salts (M = Ce, U, Np) yields two distinct products; neutral L^ArM(Cl)(THF) (1^M) (M = Np, Ce), and the "-ate" complexes [K(DME)₂][(L^Ar)Np(Cl)₂] (2^Np) or ([L^ArM(Cl)₂(μ-K(X)₂)])_∞ (2^Ce, 2^U) (M = Ce, U) (X = DME or Et₂O) (2^M). Alternatively, analogous reactions with the iodide [MI₃(THF)₄] salts provide access to the neutral compounds L^ArM(I)(THF) (3^M) (M = Ce, U, Np, Pu). All complexes exhibit close arene contacts suggestive of η⁶-interactions with the central arene ring of the terphenyl backbone, with 3^M comprising the first structurally characterized Pu η⁶-arene moiety. Notably, the metal-arene bond metrics diverge from the predicted trends of metal-carbon interactions based on ionic radii, with the uranium complexes exhibiting the shortest M-C_centroid distance in all cases. Overall, the data presents a systematic study of f-element M-η⁶-arene complexes across the early actinides U, Np, Pu, and comparison to cerium congeners.

Competing Metal-Ligand Interactions in Tris(cyclopentadienyl)-cyclohexylisonitrile Complexes of Trivalent Actinides and Lanthanides

The structure and bonding properties of 16 complexes formed by trivalent f elements (M=U, Np, Pu and lanthanides except for Pm and Pr) with cyclopentadienyl (Cp) and cyclohexylisonitrile (C≡NCy) ligands, (Cp)₃M(C≡NCy), were studied by a joint experimental (XRD, NMR) and theoretical (DFT) analysis. For the large La(III) ion, the bis-adduct (Cp)₃La(C≡NCy)₂ could also be synthesized and characterized. The metal–ligand interactions, focusing on the comparison of the actinides and lanthanides as well as on the competition of the two different ligands for M, were elucidated using the Quantum Theory of Atoms in Molecules (QTAIM) and Natural Bond Orbital (NBO) models. The results point to interactions of comparable strengths with the anionic Cp and neutral C≡NCy ligands in the complexes. The structural and bonding properties of the actinide complexes reflect small but characteristic differences with respect to the lanthanide analogues. They include larger ligand-to-metal charge transfers as well as metal–ligand electron-sharing interactions. The most significant experimental marker of these covalent interactions is the C≡N stretching frequency.

Carbene Complexes of Neptunium

Since the advent of organotransuranium chemistry six decades ago, structurally verified complexes remain restricted to π-bonded carbocycle and σ-bonded hydrocarbyl derivatives. Thus, transuranium-carbon multiple or dative bonds are yet to be reported. Here, utilizing diphosphoniomethanide precursors we report the synthesis and characterization of transuranium-carbene derivatives, namely, diphosphonio-alkylidene- and N-heterocyclic carbene–neptunium(III) complexes that exhibit polarized-covalent σ²π² multiple and dative σ² single transuranium-carbon bond interactions, respectively. The reaction of [Np^IIII₃(THF)₄] with [Rb(BIPM^TMSH)] (BIPM^TMSH = {HC(PPh₂NSiMe₃)₂}^1–) affords [(BIPM^TMSH)Np^III(I)₂(THF)] (3Np) in situ, and subsequent treatment with the N-heterocyclic carbene {C(NMeCMe)₂} (I^Me4) allows isolation of [(BIPM^TMSH)Np^III(I)₂(I^Me4)] (4Np). Separate treatment of in situ prepared 3Np with benzyl potassium in 1,2-dimethoxyethane (DME) affords [(BIPM^TMS)Np^III(I)(DME)] (5Np, BIPM^TMS = {C(PPh₂NSiMe₃)₂}^2–). Analogously, addition of benzyl potassium and I^Me4 to 4Np gives [(BIPM^TMS)Np^III(I)(I^Me4)₂] (6Np). The synthesis of 3Np–6Np was facilitated by adopting a scaled-down prechoreographed approach using cerium synthetic surrogates. The thorium(III) and uranium(III) analogues of these neptunium(III) complexes are currently unavailable, meaning that the synthesis of 4Np–6Np provides an example of experimental grounding of 5f- vs 5f- and 5f- vs 4f-element bonding and reactivity comparisons being led by nonaqueous transuranium chemistry rather than thorium and uranium congeners. Computational analysis suggests that these Np^III=C bonds are more covalent than U^III=C, Ce^III=C, and Pm^III=C congeners but comparable to analogous U^IV=C bonds in terms of bond orders and total metal contributions to the M=C bonds. A preliminary assessment of Np^III=C reactivity has introduced multiple bond metathesis to transuranium chemistry, extending the range of known metallo-Wittig reactions to encompass actinide oxidation states III-VI.

Advanced x-ray spectroscopy of actinide trichlorides

The application of core-to-core (3d-to-4f) resonant inelastic x-ray scattering (RIXS) and high-energy-resolution fluorescence-detected x-ray absorption (HERFD-XAS) at actinide M_4,5 edges, as techniques with the enhanced sensitivity to changes in the chemical state, was analyzed for trivalent actinide compounds. As an example, a series of actinide chlorides AnCl₃ (An = U, Np, Pu, Am, Cm, Bk, and Cf) was used. The crystal-field multiplet formalism was applied to calculate the 3d-4f RIXS maps, and the HERFD-XAS spectra were extracted as cuts of these RIXS maps along the incident energy axis at the constant emitted energy, corresponding to the maximum of the RIXS intensity. A relation between HERFD and conventional XAS methods was also examined. Despite some differences between profiles of the An M₅ HERFD and conventional XAS spectra of trivalent actinides, the results of calculations indicate that the HERFD method can be used at the An M₅ edge for monitoring even small variations in the An chemical state. As a whole, better agreement between the HERFD and XAS spectra was found for the An M₄ edges as compared to the An M₅ edges. By using the point charge electrostatic model, the dependence of the An M_4,5 HERFD-XAS spectra on the An coordination number was studied, which indicates the significant sensitivity of the distribution of the An 5f states to the ligand structural arrangement around the An sites.

Actinide Coordination Chemistry on Surfaces: Synthesis, Manipulation, and Properties of Thorium Bis(porphyrinato) Complexes

Actinide-based metal-organic complexes and coordination architectures encompass intriguing properties and functionalities but are still largely unexplored on surfaces. We introduce the in situ synthesis of actinide tetrapyrrole complexes under ultrahigh-vacuum conditions, on both a metallic support and a 2D material. Specifically, exposure of a tetraphenylporphyrin (TPP) multilayer to an elemental beam of thorium followed by a temperature-programmed reaction and desorption of surplus molecules yields bis(porphyrinato)thorium (Th(TPP)₂) assemblies on Ag(111) and hexagonal boron nitride/Cu(111). A multimethod characterization including X-ray photoelectron spectroscopy, scanning tunneling microscopy, temperature-programmed desorption, and complementary density functional theory modeling provides insights into conformational and electronic properties. Supramolecular assemblies of Th(TPP)₂ as well as individual double-deckers are addressed with submolecular precision, e.g., demonstrating the reversible rotation of the top porphyrin in Th(TPP)₂ by molecular manipulation. Our findings thus demonstrate prospects for actinide-based functional nanoarchitectures.

f-Block Phospholyl and Arsolyl Chemistry

The f‐block chemistry of phospholyl and arsolyl ligands, heavier p‐block analogues of substituted cyclopentadienyls (Cp^R, C₅R₅) where one or more CR groups are replaced by P or As atoms, is less developed than for lighter isoelectronic C₅R₅ rings. Heterocyclopentadienyl complexes can exhibit properties that complement and contrast with Cp^R chemistry. Given that there has been renewed interest in phospholyl and arsolyl f‐block chemistry in the last two decades, coinciding with a renaissance in f‐block solution chemistry, a review of this field is timely. Here, the syntheses of all structurally characterised examples of lanthanide and actinide phospholyl and arsolyl complexes to date are covered, including benzannulated derivatives, and together with group 3 complexes for completeness. The physicochemical properties of these complexes are reviewed, with the intention of motivating further research in this field.

Dipnictogen f-Element Chemistry: A Diphosphorus Uranium Complex

The first isolation and structural characterization of an f-element dinitrogen complex was reported in 1988, but an f-element complex with the first heavier group 15 homologue diphosphorus has to date remained unknown. Here, we report the synthesis of a side-on bound diphosphorus complex of uranium(IV) using a 7λ³-(dimethylamino)phosphadibenzonorbornadiene-mediated P atom transfer approach. Experimental and computational characterization reveals that the diphosphorus ligand is activated to its dianionic (P₂)^2- form and that in-plane U-P π-bonding dominates the bonding of the U(μ-η²:η²-P₂)U unit, which is supplemented by a weak U-P interaction of δ symmetry. A preliminary reactivity study demonstrates conversion of this diphosphorus complex to unprecedented uranium cyclo-P₃ complexes, suggesting in situ generation of transient, reactive phosphido species.

A Single Small-Scale Plutonium Redox Reaction System Yields Three Crystallographically-Characterizable Organoplutonium Complexes

An approach to obtaining substantial amounts of data from a hazardous starting material that can only be obtained and handled in small quantities is demonstrated by the investigation of a single small-scale reaction of cyclooctatetraene, C₈H₈, with a solution obtained from the reduction of Cp'₃Pu (Cp' = C₅H₄SiMe₃) with potassium graphite. This one reaction coupled with oxidation of a product has provided single-crystal X-ray structural data on three organoplutonium compounds as well as information on redox chemistry thereby demonstrating an efficient route to new reactivity and structural information on this highly radioactive element. The crystal structures were obtained from the reduction of C₈H₈ by a putative Pu(II) complex, (Cp'₃Pu^II)^1-, generated in situ, to form the Pu(III) cyclooctatetraenide complex, [K(crypt)][(C₈H₈)₂Pu^III], 1-Pu, and the tetra(cyclopentadienyl) Pu(III) complex, [K(crypt)][Cp'₄Pu^III], 2-Pu. Oxidation of the sample of 1-Pu with Ag(I) afforded a third organoplutonium complex that has been structurally characterized for the first time, (C₈H₈)₂Pu^IV, 3-Pu. Complexes 1-Pu and 3-Pu contain Pu sandwiched between parallel (C₈H₈)^2- rings. The (Cp'₄Pu^III)^- anion in 2-Pu features three η⁵-Cp' rings and one η¹-Cp' ring, which is a rare example of a formal Pu-C η¹-bond. In addition, this study addresses the challenge of small-scale synthesis imparted by radiological and material availability of transuranium isotopes, in particular that of pure metal samples. A route to an anhydrous Pu(III) starting material from the more readily available Pu^IVO₂ was developed to facilitate reproducible syntheses and allow complete spectroscopic analysis of 1-Pu and 2-Pu. Pu^IVO₂ was converted to Pu^IIIBr₃(DME)₂ (DME = CH₃OCH₂CH₂OCH₃) and subsequently Pu^IIIBr₃(THF)_x, which was used to independently synthesize 1-Pu, 2-Pu, and 3-Pu.

Tris-{hydridotris(1-pyrazolyl)borato}actinide Complexes: Synthesis, Spectroscopy, Crystal Structure, Bonding Properties and Magnetic Behaviour

The isostructural compounds of the trivalent actinides uranium, neptunium, plutonium, americium, and curium with the hydridotris(1-pyrazolyl)borato (Tp) ligand An[η3 -HB(N2 C3 H3 )3 ]3 (AnTp3 ) have been obtained through several synthetic routes. Structural, spectroscopic (absorption, infrared, laser fluorescence) and magnetic characterisation of the compounds were performed in combination with crystal field, density functional theory (DFT) and relativistic multiconfigurational calculations. The covalent bonding interactions were analysed in terms of the natural bond orbital (NBO) and quantum theory of atoms in molecules (QTAIM) models.

Stable Actinide π Complexes of a Neutral 1,4-Diborabenzene

The π coordination of arene and anionic heteroarene ligands is a ubiquitous bonding motif in the organometallic chemistry of d-block and f-block elements. By contrast, related π interactions of neutral heteroarenes including neutral bora-π-aromatics are less prevalent particularly for the f-block, due to less effective metal-to-ligand backbonding. In fact, π complexes with neutral heteroarene ligands are essentially unknown for the actinides. We have now overcome these limitations by exploiting the exceptionally strong π donor capabilities of a neutral 1,4-diborabenzene. A series of remarkably robust, π-coordinated thorium(IV) and uranium(IV) half-sandwich complexes were synthesized by simply combining the bora-π-aromatic with ThCl4 (dme)2 or UCl4 , representing the first examples of actinide complexes with a neutral boracycle as sandwich-type ligand. Experimental and computational studies showed that the strong actinide-heteroarene interactions are predominately electrostatic in nature with distinct ligand-to-metal π donation and without significant π/δ backbonding contributions.

Structure and magnetism of a tetrahedral uranium(iii) β-diketiminate complex

We describe the functionalisation of the previously reported uranium(iii) β-diketiminate complex (BDI)UI₂(THF)₂ (1) with one and two equivalents of a sterically demanding 2,6-diisopropylphenolate ligand (ODipp) leading to the formation of two heteroleptic complexes: [(BDI)UI(ODipp)]₂ (2) and (BDI)U(ODipp)₂ (3). The latter is a rare example of a tetrahedral uranium(iii) complex, and it shows single-molecule magnet behaviour.

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.

Synthesis and characterisation of light lanthanide bis-phospholyl borohydride complexes

Organometallic lanthanide (Ln) chemistry is dominated by complexes that contain substituted cyclopentadienyl (CpR) ligands. Closely related phospholyls have received less attention, and although they have proven utility in stabilising low oxidation state Ln complexes the trivalent Ln chemistry of these ligands is limited in comparison. Herein, we synthesise two families of heteroleptic Ln3+ complexes, [Ln(Htp)2(μ-BH4)]2 (Htp = 2,5-di-tert-butylphospholyl; 1-Ln; Ln = La, Ce, Nd, Sm), and [[Ln(Htp)2(μ-BH4)2K(S)]n (2-Ln, Ln = La, Ce, S = 2 DME, n = 2; 3-Ce, Ln = Ce, S = Et2O and THF, n = ∞) via the reactions of parent [Ln(BH4)3(THF)3.5] with K(Htp), to investigate differences between Ln complexes with substituted phospholyl ligands and analogous CpR complexes. Complexes 1-3-Ln were characterised as appropriate by single crystal XRD, SQUID magnetometry, elemental analysis, multinuclear NMR, ATR-IR and UV-Vis-NIR spectroscopy. Ab initio calculations reveal that small changes in the Ln3+ coordination spheres of these complexes can have relatively large influences on crystal field splitting.

Polarised covalent thorium(iv)– and uranium(iv)–silicon bonds

We report thorium- and uranium–silicon bonds in structurally analogous complexes with surprisingly similar actinide–silicon bonding regimes.

f-Element Half-Sandwich Complexes: A Tetrasilylcyclobutadienyl-Uranium(IV)-Tris(tetrahydroborate) Anion Pianostool Complex

Despite there being numerous examples of f-element compounds supported by cyclopentadienyl, arene, cycloheptatrienyl, and cyclooctatetraenyl ligands (C_5-8 ), cyclobutadienyl (C₄ ) complexes remain exceedingly rare. Here, we report that reaction of [Li₂ {C₄ (SiMe₃ )₄ }(THF)₂ ] (1) with [U(BH₄ )₃ (THF)₂ ] (2) gives the pianostool complex [U{C₄ (SiMe₃ )₄ }(BH₄ )₃ ][Li(THF)₄ ] (3), where use of a borohydride and preformed C₄ -unit circumvents difficulties in product isolation and closing a C₄ -ring at uranium. Complex 3 is an unprecedented example of an f-element half-sandwich cyclobutadienyl complex, and it is only the second example of an actinide-cyclobutadienyl complex, the other being an inverse-sandwich. The U-C distances are short (av. 2.513 Å), reflecting the formal 2- charge of the C₄ -unit, and the SiMe₃ groups are displaced from the C₄ -plane, which we propose maximises U-C₄ orbital overlap. DFT calculations identify two quasi-degenerate U-C₄ π-bonds utilising the ψ₂ and ψ₃ molecular orbitals of the C₄ -unit, but the potential δ-bond using the ψ₄ orbital is vacant.

Synthesis and Utility of Neptunium(III) Hydrocarbyl Complex

To extend organoactinide chemistry beyond uranium, reported here is the first structurally characterized transuranic hydrocarbyl complex, Np[η⁴ -Me₂ NC(H)C₆ H₅ ]₃ (1), from reaction of NpCl₄ (DME)₂ with four equivalents of K[Me₂ NC(H)C₆ H₅ ]. Unlike the U^III species, the neptunium analogue can be used to access other Np^III complexes. The reaction of 1 with three equivalents of HE₂ C(2,6-Mes₂ -C₆ H₃ ) (E=O, S) yields [(2,6-Mes₂ -C₆ H₃ )CE₂ ]₃ Np(THF)₂ , maintaining the trivalent oxidation state.

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.