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Actinides
... favored the carbon−nitrogen bond adjacent to the substituent with greater steric bulk (Figure 10C).41 When using [(L)U(N(SiMe 3 ) 2 ) 3 ] 13 (L= 5,7-diisopropyl-5,7-dihydro-6Hdibenzo-[d,f ][1,3]diazepin-6-imine) in 0.1% catalyst loading, hydroboration of dicyclohexyl ketone with HBpin occurred in greater than 99% yield ( ...
Catalysis remains one of the final frontiers in molecular uranium chemistry. Depleted uranium is mildly radioactive, continuously generated in large quantities from the production and consumption of nuclear fuels and accessible through the regeneration of “uranium waste”. Organometallic complexes of uranium possess a number of properties that are appealing for applications in homogeneous catalysis. Uranium exists in a wide range of oxidation states, and its large ionic radii support chelating ligands with high coordination numbers resulting in increased complex stability. Its position within the actinide series allows it to involve its f-orbitals in partial covalent bonding; yet, the U–L bonds remain highly polarized. This causes these bonds to be reactive and, with few exceptions, relatively weak, allowing for high substrate on/off rates. Thus, it is reasonable that uranium could be considered as a source of metal catalysts. Accordingly, uranium complexes in oxidation states +4, +5, and +6 have been studied extensively as catalysts in sigma-bond metathesis reactions, with a body of literature spanning the past 40 years. High-valent species have been documented to perform a wide variety of reactions, including oligomerization, hydrogenation, and hydrosilylation. Concurrently, electron-rich uranium complexes in oxidation states +2 and +3 have been proven capable of performing reductive small molecule activation of N2, CO2, CO, and H2O. Hence, uranium’s ability to activate small molecules of biological and industrial relevance is particularly pertinent when looking toward a sustainable future, especially due to its promising ability to generate ammonia, molecular hydrogen, and liquid hydrocarbons, though the advance of catalysis in these areas is in the early stages of development. In this Perspective, we will look at the challenges associated with the advance of new uranium catalysts, the tools produced to combat these challenges, the triumphs in achieving uranium catalysis, and our future outlook on the topic.
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)].
A method to explore head‐to‐head φ backbonding from uranium f‐orbitals into allyl π* orbitals was pursued. The coordination of anionic allyl groups to uranium was achieved via tethered anilide ligands, and the products were investigated using NMR spectroscopy, single‐crystal XRD, and theoretical methods. The (allyl)silylanilide ligand, N‐((dimethyl)prop‐2‐enylsilyl)2,6‐diisopropylaniline ( LH ), was used as either the fully protonated, singly deprotonated, or doubly deprotonated form, highlighting the stability and versatility of the silylanilide motif. A free, neutral allyl group was observed in UI 2 ( L1 ) 2 ( 1 ), which was synthesized using the mono‐deprotonated ligand [K][N‐((dimethyl)prop‐2‐enyl)silyl)2,6‐diisopropylanilide] ( L1 ). The desired homoleptic sandwich complex U( L2 ) 2 ( 2 ) was prepared from all three ligand precursors, but most consistent results came from the use of the dipotassium salt of the doubly deprotonated ligand [K] 2 [N‐((dimethyl)prop‐2‐enidesilyl)‐2,6‐diisopropylanilide] ( L2 ). This allyl‐based sandwich complex was studied using theoretical techniques with supporting experimental spectroscopy to investigate the potential for phi (φ) backbonding. The bonding between the U(IV) and the allyl fragments is best described as ligand‐to‐metal electron donation from two‐carbon‐fragment localized electron density into empty f‐orbitals.
The synthesis of a tripodal, S-based ligand, namely the mesitylene-anchored, tris-thiophenolate-functionalized (mes(Me,AdArS)3)3- (1)3-, and its coordination chemistry with low-valent uranium to form [UIII((SArAd,Me)3mes)] (1-U) are reported. Single-crystal X-ray diffraction analysis reveals a C3-symmetric molecular structure. Full characterization of 1-U was performed using nuclear magnetic resonance, UV-vis-NIR electronic absorption, and electron paramagnetic resonance spectroscopies as well as SQUID magnetometry, thus confirming the U(III) oxidation state. Alternating current magnetic studies show that 1-U exhibits single-molecule magnet behavior at low temperatures in a non-zero external field. Comparison of these results to those of the previously reported mesitylene-anchored complexes, [UIII((OArAd,Me)3mes)] and [UIII((OArtBu,tBu)3mes)], indicates a drastic change in the electronic structure when moving from phenolate-based ligands to thiophenolate-based 1, which is further discussed by means of computational analysis (NBO, DFT, and QTAIM). Despite the U-O bonds being stronger, a much higher covalency was found for the U-S analogue.
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-N
i
Pr)(K2(μ-η3:η3-CH2CHN
i
Pr)]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.
We here report the synthesis and characterization of a complete series of terminal hydrochalcogenido, U–EH, and chalcogenido uranium(IV) complexes, U≡E (with E = O, S, Se, Te), supported by the (Ad,MeArOH)3tacn (1,4,7-tris(3-(1-adamantyl)-5-methyl-2-hydroxybenzyl)-1,4,7-triazacyclononane) ligand system. Reaction of H2E with the trivalent precursor [((Ad,MeArO)3tacn)U] (1) yields the corresponding uranium(IV) hydrochalcogenido complexes [((Ad,MeArO)3tacn)U(EH)] (2). Subsequent deprotonation of the terminal hydrochalcogenido species with KN(SiMe3)2, in the presence of 2.2.2-cryptand, gives access to the uranium(IV) complexes with terminal chalcogenido ligands [K(2.2.2-crypt)][((Ad,MeArO)3tacn)U≡E] (3). In order to study the influence of the varying terminal chalogenido ligands on the overall molecular and electronic structure, all complexes were studied by single-crystal X-ray diffractometry, UV/vis/NIR, electronic absorption, and IR vibrational spectroscopy as well as SQUID magnetometry and computational analyses (DFT, MO, NBO).
Extended-coordination sphere interactions between dissolved metals and other ions, including electrolyte cations, are not known to perturb the electrochemical behavior of metal cations in water. Herein, we report the stabilization of higher-oxidation-state Np dioxocations in aqueous chloride solutions by hydrophobic tetra-n-alkylammonium (TAA⁺) cations—an effect not exerted by fully hydrated Li⁺ cations under similar conditions. Experimental and molecular dynamics simulation results indicate that TAA⁺ cations not only drive enhanced coordination of anionic Cl– ligands to NpV/VI but also associate with the resulting Np complexes via non-covalent interactions, which together decrease the electrode potential of the NpVI/NpV couple by up to 220 mV (ΔΔG = −22.2 kJ mol⁻¹). Understanding the solvation-dependent interplay between electrolyte cations and metal–oxo species opens an avenue for controlling the formation and redox properties of metal complexes in solution. It also provides valuable mechanistic insights into actinide separation processes that widely use quaternary ammonium cations as extractants or in room temperature ionic liquids.
Dative bonds between p- and d-block atoms are common but species containing a double dative bond, which donates two-electron pairs to the same acceptor, are far less common. The synthesis of complexes between UCl4 and carbodiphosphoranes (CDP), which formally possess double dative bonds Cl4U⇐CDP, is reported in this paper. Single-crystal X-ray dif- fraction shows that the uranium−carbon distances are in the range of bond lengths for uranium−carbon double bonds. A bonding analysis suggests that the molecules are uranium −carbone complexes featuring divalent carbon(0) ligands rather than uranium−carbene species. The complexes represent rare examples with a double dative bond in f-block chemistry. Our study not only introduces the concept of double dative bonds between car- bones and f-block elements but also opens an avenue for the construction of other complexes with double dative bonds, thus providing new opportunities for the applications of f-block compounds.
Cooperativity between metal centres is identified as a crucial step in dinitrogen reduction both for the industrial Haber–Bosch process and for the natural fixation of nitrogen by nitrogenase enzymes, but the mechanism of N2 reduction remains poorly understood. This is in large part because multimetallic complexes that reduce and functionalize dinitrogen in the absence of strong alkali reducing agents are crucial to establish a structure–activity relationship, but remain extremely rare. Recently, we reported a multimetallic nitride-bridged diuranium(iii) complex capable of reducing and functionalizing dinitrogen. Here we show that an analogous complex assembled with an oxo instead of a nitride linker also effects the four-electron reduction of dinitrogen, but the reactivity of the resulting oxo–(N2) complex differs significantly from that of the nitride–(N2). Computational studies show a different bonding scheme for the dinitrogen where the bridging nitride does participate in the binding and consequent activation of N2, while the oxide does not. © 2018, The Author(s), under exclusive licence to Springer Nature Limited.
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-sta- bilized, 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.
Despite the fact that non-aqueous uranium chemistry is over 60 years old, most polarised-covalent uranium-element multiple bonds involve formal uranium oxidation states IV, V, and VI. The paucity of uranium(III) congeners is because, in common with metal-ligand multiple bonding generally, such linkages involve strongly donating, charge-loaded ligands that bind best to electron-poor metals and inherently promote disproportionation of uranium(III). Here, we report the synthesis of hexauranium-methanediide nanometre-scale rings. Combined experimental and computational studies suggest overall the presence of formal uranium(III) and (IV) ions, though electron delocalisation in this Kramers system cannot be definitively ruled out, and the resulting polarised-covalent U = C bonds are supported by iodide and δ-bonded arene bridges. The arenes provide reservoirs that accommodate charge, thus avoiding inter-electronic repulsion that would destabilise these low oxidation state metal-ligand multiple bonds. Using arenes as electronic buffers could constitute a general synthetic strategy by which to stabilise otherwise inherently unstable metal-ligand linkages.
Thorium sits at a unique position on the periodic table. On one hand, there is little evidence that its 5f orbitals engage in bonding as they do in other early actinides; on the other hand, its chemistry is distinct from Lewis acidic transition metals. To gain insight into the underlying electronic structure of Th and develop trends across the actinide series, it is useful to study Th(III) and Th(II) systems with valence electrons that may engage in non-electrostatic metal–ligand interactions, although only a handful of such systems are known. To expand the range of low-valent compounds and gain deeper insight into Th electronic structure, we targeted actinide bimetallic complexes containing metal–metal bonds. Herein, we report the syntheses of Th–Al bimetallics from reactions between a di-tert-butylcyclopentadienyl supported Th(IV) dihalide (Cp‡2ThCl2) and an anionic aluminum hydride salt [K(H3AlC(SiMe3)3) (1)]. Reduction of the [Th(IV)](Cl)–[Al] product resulted in a [Th(III)]–[Al] complex [Cp‡2Th(μ-H3)AlC(SiMe3)3 (4)]. The U(III) analogue [Cp‡2U(μ-H3)AlC(SiMe3)3 (5)] could be synthesized directly from a U(III) halide starting material. Electron paramagnetic resonance studies on 4 demonstrate hyperfine interactions between the unpaired electron and the Al atom indicative of spin density delocalization from the Th metal center to the Al. Density functional theory and atom in molecules calculations confirmed the presence of An→Al interactions in 4 and 5, which represents the first examples of An→M interactions where the actinide behaves as an electron donor.
A new type of double uranium oxo cation [O–U–O–U–O] ⁴⁺ is prepared by selective oxygen-atom abstraction from macrocyclic uranyl complexes using either boranes or silanes.
We report unprecedented silyl-phosphino-carbene complexes of uranium(IV), where before all covalent actinide-carbon double bonds were stabilised by phosphorus(V) substituents or restricted to matrix isolation experiments. Conversion of [U(BIPMTMS)(Cl)(μ-Cl)2Li(THF)2] (1, BIPMTMS = C(PPh2NSiMe3)2) to [U(BIPMTMS)(Cl){CH(Ph)(SiMe3)}] (2), and addition of [Li{CH(SiMe3)(PPh2)}(THF)] and Me2NCH2CH2NMe2 (TMEDA) gave [U{C(SiMe3)(PPh2)}(BIPMTMS)(μ-Cl)Li(TMEDA)(μ-TMEDA)0.5]2 (3) by α-hydrogen abstraction. Addition of 2,2,2-cryptand or two equivalents of 4-N,N-dimethylaminopyridine (DMAP) to 3 gave [U{C(SiMe3)(PPh2)}(BIPMTMS)(Cl)][Li(2,2,2-cryptand)] (4) or [U{C(SiMe3)(PPh2)}(BIPMTMS)(DMAP)2] (5). The characterisation data for 3-5 suggest that whilst there is evidence for 3-centre P-C-U π-bonding character, the U=C double bond component is dominant in each case. These U=C bonds are the closest to a 'true' uranium-alkylidene, yet outside of matrix isolation experiments.
The nature of actinide-actinide bonds has attracted considerable attention for a long time, especially since recent theoretical studies suggest that triple and up to quintuple bonds should be possible, but little is known experimentally. Actinide-actinide bonds inside fullerene cages have also been proposed, but their existence has been debated intensively by theoreticians. Despite all the theoretical arguments, critical experimental data for a dimetallic actinide endohedral fullerene have never been obtained. Herein, for the first time, we report the synthesis and isolation of a dimetallic actinide EMF, U2@C80. This compound was fully characterized by mass spectrometry, single crystal X-ray crystallography, UV-vis-NIR, cyclic voltam-metry and X-ray absorption spectroscopy (XAS). The single crystal X-ray crystallographic analysis unambiguously assigned the molecular structure to U2@Ih(7)-C80. In particular, the crystallographic data revealed that the U-U distance is within the range of 3.46-3.79 Å, which is shorter than the 3.9 Å previously predicted for an elongated weak U-U bond inside the C80 cage. The XAS results reveal that the formal charge of the U atoms trapped inside the fullerene cage is +3, which agrees with the computational and crystallographic studies that assign a hexaanionic carbon cage, (Ih-C80)6-. Theoretical studies confirm the presence of a U-U bonding interaction and suggest that the weak U-U bond in U2@Ih(7)-C80 is strengthened upon reduc-tion and weakened upon oxidation. The comprehensive characterization of U2@Ih(7)-C80 and the overall agreement be-tween the experimental data and theoretical investigations provide experimental proof and deeper understanding for acti-nide metal-metal bonding interaction inside a fullerene cage.
Gadolinium (Gd⁰) and uranium (U⁰) nanoparticles are prepared via lithium naphthalenide ([LiNaph])-driven reduction in tetrahydrofuran (THF) using GdCl3 and UCl4, respectively, as low-cost starting materials. The as-prepared Gd⁰ and U⁰ suspensions are colloidally stable and contain metal nanoparticles with diameters of 2.5 ± 0.7 nm (Gd⁰) and 2.0 ± 0.5 nm (U⁰). Whereas THF suspensions are chemically stable under inert conditions (Ar and vacuum), nanoparticulate powder samples show high reactivity in contact with, for example, oxygen, moisture, alcohols, or halogens. Such small and highly reactive Gd⁰ and U⁰ nanoparticles are first prepared via a dependable liquid-phase synthesis and stand as representatives for further nanosized lanthanides and actinides.
We report the synthesis and characterisation of the compounds [An(TrenDMBS){Pn(SiMe3)2}] and [An(TrenTIPS){Pn(SiMe3)2}] [TrenDMBS = N(CH2CH2NSiMe2But)3, An = U, Pn = P, As, Sb, Bi; An = Th, Pn = P, As; TrenTIPS = N(CH2CH2NSiPri3)3, An = U, Pn = P, As, Sb; An = Th, Pn = P, As, Sb]. The U-Sb and Th-Sb moieties are unprecedented examples of any kind of An-Sb molecular bond, and the U-Bi bond is the first electron-precise one. The Th-Bi combination was too unstable to isolate, underscoring the fragility of these linkages. However, the U-Bi complex is the heaviest electron-precise pairing of two elements involving an actinide on a macroscopic scale under ambient conditions, and this is exceeded only by An-An pairings prepared under cryogenic matrix isolation conditions. Thermolysis and photolysis experiments suggest that the U-Pn bonds degrade by hemolytic bond cleavage, whereas the more redox robust thorium compounds engage in an acid-base/dehydrocoupling route.
The reactivity of uranium compounds towards small molecules typically occurs through stoichiometric rather than catalytic processes. Examples of uranium catalysts reacting with water are particularly scarce, because stable uranyl groups form that preclude the recovery of the uranium compound. Recently, however, an arene-anchored, electron-rich uranium complex has been shown to facilitate the electrocatalytic formation of H2 from H2O. Here, we present the precise role of uranium–arene δ bonding in intermediates of the catalytic cycle, as well as details of the atypical two-electron oxidative addition of H2O to the trivalent uranium catalyst. Both aspects were explored by synthesizing mid- and high-valent uranium–oxo intermediates and by performing comparative studies with a structurally related complex that cannot engage in δ bonding. The redox activity of the arene anchor and a covalent δ-bonding interaction with the uranium ion during H2 formation were supported by density functional theory analysis. Detailed insight into this catalytic system may inspire the design of ligands for new uranium catalysts.
Reversible single-metal two-electron oxidative addition and reductive elimination are common fundamental reactions for transition metals that underpin major catalytic transformations. However, these reactions have never been observed together in the f-block because these metals exhibit irreversible one- or multi-electron oxidation or reduction reactions. Here we report that azobenzene oxidises sterically and electronically unsaturated uranium(III) complexes to afford a uranium(V)-imido complex in a reaction that satisfies all criteria of a single-metal two-electron oxidative addition. Thermolysis of this complex promotes extrusion of azobenzene, where H-/D-isotopic labelling finds no isotopomer cross-over and the non-reactivity of a nitrene-trap suggests that nitrenes are not generated and thus a reductive elimination has occurred. Though not optimally balanced in this case, this work presents evidence that classical d-block redox chemistry can be performed reversibly by f-block metals, and that uranium can thus mimic elementary transition metal reactivity, which may lead to the discovery of new f-block catalysis.
The organometallic tris-cyclopentadienide actinide(III) (AnCp₃) complexes have been reported first about 50 years ago. Up to now however only the NpCp₃ solid state structure has been studied. Here we report on the solid state structures of UCp₃ and PuCp₃ which are isostructural to their Np analogue. The structural models are supported by theoretical calculations and compared to their lanthanide analogues. The observed trends in changes of bond lengths might be indicator for an increased covalency in the bonding in the tris-cyclopentadienide actinide(iii) complexes (AnCp₃) compared to their lanthanide homologues.
The synthesis and characterization of sterically unencumbered homoleptic organouranium aryl complexes containing U‐C σ‐bonds has been of interest to the chemical community for over 70 years. Reported herein are the first structurally characterized, sterically unencumbered homoleptic uranium (IV) aryl‐ate species of the form [U(Ar)₆]²‐ (Ar = Ph, p‐tolyl, p‐ClPh). Magnetic Circular Dichroism (MCD) spectroscopy and computational studies provide insight into electronic structure and bonding interactions in the U‐C σ‐bond across this series of complexes. Overall, these studies solve a decades long challenge in synthetic uranium chemistry, enabling new insight into electronic structure and bonding in organouranium complexes.
Here we show that a scaffold combining siloxide ligands and a bridging oxide allows the synthesis and characterization of the stable dinuclear uranium(IV) hydride complex [K 2 {[U(OSi(O t Bu) 3 ) 3 ] 2 (μ-O)(μ-H) 2 }], 2, which displays high reductive reactivity. The dinuclear bis-hydride 2 effects the reductive coupling of acetonitrile by hydride transfer to yield [K 2 {[U(OSi(O t Bu) 3 ) 3 ] 2 (μ-O)(μ-κ ² -NC(CH 3 )NCH 2 CH 3 )}], 3. Under ambient conditions, the reaction of 2 with CO affords the oxomethylene ²⁻ reduction product [K 2 {[U(OSi(O t Bu) 3 ) 3 ] 2 (μ-CH 2 O)(μ-O)}], 4, that can further add H 2 to afford the methoxide hydride complex [K 2 {[U(OSi(O t Bu) 3 ) 3 ] 2 (μ-OCH 3 )(μ-O)(μ-H)}], 5, from which methanol is released in water. Complex 2 also effects the direct reduction of CO 2 to the methoxide complex 5, which is unprecedented in f element chemistry. From the reaction of 2 with excess CO 2 , crystals of the bis-formate carbonate complex [K 2 {[U(OSi(O t Bu) 3 ) 3 ] 2 (μ-CO 3 )(μ-HCOO) 2 }], 6, could also be isolated. All the reaction products were characterized by X-ray crystallography and NMR spectroscopy.
Salt metathesis between the anionic rhenium(I) compound, Na[Re(η ⁵ -Cp)(BDI)] (BDI = N,N′-bis(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate), and the uranium(III) salt, UI 3 (1,4-dioxane) 1.5 , generated the triple inverse sandwich complex, U[(μ- η ⁵ :η ⁵ -Cp)Re(BDI)] 3 , which was isolated and structurally characterized as the Lewis base adducts, (L)U[(μ- η ⁵ :η ⁵ -Cp)Re(BDI)] 3 (1·L, L = THF, 1,4-dioxane, DMAP). The assignment as one uranium(III) and three rhenium(I) centers was supported by X-ray crystallography, NMR and EPR spectroscopies, and computational studies. An unusual shortening of the rhenium-Cp bond distances in 1·L relative to Na[Re(η ⁵ -Cp)(BDI)] was observed in the solid-state and reproduced in calculated structures of 1·THF and the anionic fragment, [Re(η ⁵ -Cp)(BDI)] ⁻ . Calculations suggest that the electropositive uranium center pulls electron density away from the electron-rich rhenium centers, reducing electron-electron repulsions in the rhenium-Cp moieties and thereby strengthening those interactions, while also making uranium-Cp bonding more favorable.
The redetermination of the crystal structure of trigonal UCl6 , which is the eponym for the UCl6 structure type, showed that certain atomic coordinates had been incorrectly reported. This led to noticeably different U-Cl distances within the octahedral UCl6 molecule (2.41 and 2.51 Å). Within the revised structure model presented here, which is based on single crystal data as well as quantum chemical calculations, all U-Cl distances are essentially equal within standard uncertainty (2.431(5), 2.437(5), and 2.439(6) Å). This room temperature modification, called rt-UCl6 , crystallizes in the trigonal space group P
3
‾
m1, No. 164, hP21, with a=10.907(2), c=5.9883(12) Å, V=616.9(2) Å3 , Z=3 at T=253 K. A new low-temperature (lt) modification of UCl6 is also presented that was obtained by cooling a single crystal of rt-UCl6. The phase change occurs between 150 and 175 K. lt-UCl6 crystallizes isotypic to a low-temperature modification of SF6 in the monoclinic crystal system, space group C2/m, No. 12, mS42, with a=17.847(4), b=10.8347(18), c=6.2670(17) Å, β=96.68(2)°, V=1203.6(5) Å3 , Z=6 at 100 K. The Cl anions form a close-packed structure corresponding to the α-Sm type with uranium atoms in the octahedral voids. During the synthesis of UBr5 a new modification was obtained that crystallizes in the triclinic crystal system, space group P
1
‾
, No. 2, aP36, with a=10.4021(6), b=11.1620(6), c=12.2942(7) Å, α=68.3340(10)°, β=69.6410(10)° and γ=89.5290(10)°, V=1231.84(12) Å3 , Z=3 at T=100 K. In this structure the UBr5 units are dimerized to U2 Br10 molecules. The Br anions also form a close-packed structure of the α-Sm type with adjacent uranium atoms in the octahedral voids. Comparisons of the crystal structures of the compounds MX5 (M=Pa, U; X=Cl, Br) show that the crystal structure of monoclinic α-PaBr5 is probably not correct.
“Lanthanides find extensive applications in displays, magnets, batteries, contrast reagents, and biological probes, and actinides are central to nuclear fuel and fire alarms. … Given the very basics of lanthanide and actinide chemistry are being frequently redefined this is a fascinating and exciting area that promises many surprises in the future. …” Read more in the Guest Editorial by S. T. Liddle.
Anhydrous americium(III) starting materials were prepared from americia (AmO 2 ) using inexpensive, commercially available reagents and mild reaction conditions. [AmCl(μ-Cl) 2 (THF) 2 ] n (1-Am) and AmBr 3 (THF) 4 (2-Am) were isolated and characterized by electronic absorption spectroscopy and X-ray diffraction. These new starting materials are soluble in organic solvents, making them useful synthons for nonaqueous organometallic americium chemistry.
New examples of uranium in the formal +2 oxidation state have been isolated by reduction of Cptet3U (Cptet = C5Me4H) and U(NR2)3 (R = SiMe3) in the presence of 2.2.2-cryptand...
Reduction of IU(NHAriPr6)2 (AriPr6 = 2,6-(2,4,6-iPr3C6H2)2C6H3) results in a rare example of a U(II) complex, U(NHAriPr6)2, and the first example that is a neutral species. Here, we show spectroscopic and magnetic studies that suggest a 5f46d0 valence electronic configuration for uranium, along with characterization of related U(III) species.
Reaction of the trivalent uranium complex [((Ad,MeArO)3N)U(DME)] with one molar equivalent [Na(OCAs)(dioxane)3], in the presence of 2.2.2‐crypt, yields [Na(2.2.2‐crypt)][{((Ad,MeArO)3N)UIV(THF)}(μ‐O){((Ad,MeArO)3N)UIV(CAs)}] (1), which is the arsenic containing analogue of the previously reported, μ‐oxo‐bridged diuranium cyaphide complex. The structural characterization of complex 1 reveals the first example of a coordinated η1‐cyaarside ligand (CAs–). The terminal CAs– anionic ligand formation is promoted by the highly reducing, oxophilic uranium(III) precursor [((Ad,MeArO)3N)U(DME)] and proceeds through reductive C–O bond cleavage of the bound arsaethynolate anion, OCAs–. If two equivalents of OCAs– are allowed to react with the uranium(III) precursor, the binuclear, µ‐oxo‐bridged diuranium(IV/IV) complex [Na(2.2.2‐crypt)]2[{((Ad,MeArO)3N)UIV}2(μ‐O)(μ‐AsCAs)] (2), comprising the hitherto unknown μ:η1,η1‐coordinated (AsCAs)2– ligand, is isolated. The mechanistic pathway en route to the metallaarsaallene complex formation likely is the decarbonylation of a dimeric intermediate formed in the reaction of 1 with OCAs–. An alternative pathway to complex 2 is the conversion of complex 1 to 2 by addition of one further equivalent of OCAs–. The mechanistic proposals are corroborated by DFT calculations.
We report the preparation of four heterobimetallic uranium- and thorium-molybdenum paddlewheel complexes. The characterisation data calculations suggest the presence of Mo→An σ-interactions in all cases. These complexes represent unprecedented actinide-group...
We have identified a polydentate aminocarboxylate ligand that stabilizes uranyl(V) in water. The mono-nuclear [UO2(dpaea)]X, (dpaeaH2=Bis(pyridyl-6-methyl-2-carboxylate)-ethylamine; X= CoCp2*+ or X= K(2.2.2.cryptand) complexes have been isolated, spectroscopically and crystallographically charac-terized. These complexes disproportionate at pH 6, but are stable in water at pH 7-10 for several days.
To expand the range of synthetic options for generating complexes of the actinide metals in the +2 oxidation state, reduction of Cp″3U (Cp″ = C5H3(SiMe3)2) and the lanthanide analogs, Cp″3La and Cp″3Ce with lithium in the absence of crown ether and cryptand chelates was explored. In each case, crystallographically characterizable [Li(THF)4][Cp″3M] complexes were obtainable in yields of 70-75% for M = La and Ce and 45-50% for M = U, that is, chelating agents are not necessary to sequester the lithium countercation to form isolable crystalline M(II) products. Reductions using Cs were also explored and X-ray crystallography revealed the formation of an oligomeric structure, [Cp″U(μ-Cp")2Cs(THF)2] n, involving Cp″ ligands that bridge "(Cp″UII)1+" moieties to "[Cp″2Cs(THF)2]1-" units.
A salophen ligand derivative incorporating naphthalene (naphthylsalophen = [H2L]) and the corresponding uranyl (UO22+) complex have been synthesized and characterized both in solution and the solid-state. A hydrogen bonding uranyl...
The facile encapsulation of U(III) and La(III) by 2.2.2-cryptand (crypt) using simple starting materials is described. Addition of crypt to UI 3 and LaCl 3 forms the crystallographically-characterizable complexes, [U(crypt)I 2 ]I and [La(crypt)Cl 2 ]Cl....
PuL2 and CeL2 (L = N,N'–bis[(4,4'-diethylamino)salicylidene]-1,2-phenylenediamine) have been synthesized, and characterized by single crystal X-ray diffraction, UV/vis/NIR spectroscopy, and cyclic voltammetry. These studies reveal enhanced stabilization of Pu(IV) versus Ce(IV)...
We report a new formal oxidation state for neptunium in a crystallographically characterizable molecular complex, namely Np2+ in [K(crypt)][NpIICp″3] [crypt = 2.2.2-cryptand, Cp″ = C5H3(SiMe3)2]. Density functional theory calculations indicate that the ground state electronic configuration of the Np2+ ion in the complex is 5f46d1.
A series of metallocene thorium complexes with mono- and bis(phosphido) ligands have been investigated with varying hues: (C5Me5)2Th(Cl)[P(Mes)2] (Mes = mesityl = 2,4,6-(CH3)3C6H2; dark red-purple), (C5Me5)2Th[P(Mes)(CH3)]2 (dark red-purple), (C5Me5)2Th(CH3)[P(Mes)2] (dark red-purple), (C5Me5)2Th(CH3)[P(Mes)(SiMe3)] (orange), (C5Me5)2Th(Cl)[P(Mes)(SiMe3)] (orange), (C5Me5)2Th[P(Mes)(SiMe3)]2 (orange), and (C5Me5)2Th[PH(Mes)]2 (pale yellow). While all of these complexes bear a mesityl group on phosphorus, the electronic structure observed differs depending on the other substituent (mesityl, methyl, trimethylsilyl, or hydrogen). This sparked an investigation of the electronic structure of these complexes using 31P NMR and electronic absorption spectroscopy in concert with time-dependent density functional theory calculations.
We report the preparation of a range of alkali metal uranyl(VI) tri- bis(silyl)amide complexes [{M(THF) x}{(μ-O)U(O)(N″)3}] (1M) (N″ = {N(SiMe3)2}-, M = Li, Na, x = 2; M = K, x = 3; M = K, Rb, Cs, x = 0) containing electrostatic alkali metal uranyl-oxo interactions. Reaction of 1M with 2,2,2-cryptand or 2 equiv of the appropriate crown ether resulted in the isolation of the separated ion pair species [U(O)2(N″)3][M(2,2,2-cryptand)] (3M, M = Li-Cs) and [U(O)2(N″)3][M(crown)2] (4M, M = Li, crown = 12-crown-4 ether; M = Na-Cs, crown = 15-crown-5 ether). A combination of crystallographic studies and IR, Raman and UV-vis spectroscopies has revealed that the 1M series adopts contact ion pair motifs in the solid state where the alkali metal caps one of the uranyl-oxo groups. Upon dissolution in THF solution, this contact is lost, and instead, separated ion pair motifs are observed, which is confirmed by the isolation of [U(O)2(N″)3][M(THF) n] (2M) (M = Li, n = 4; M = Na, K, n = 6). The compounds have been characterized by single crystal X-ray diffraction, multinuclear NMR spectroscopy, IR, Raman, and UV-vis spectroscopies, and elemental analyses.
Redox stability of tetravalent Np and Pu in THF is explored, leading to facile access routes into anhydrous Np( iii ) chemistry.
New uranyl derivatives featuring the amide ligand, -N(SiHMe2)tBu, were synthesized and characterized by X-ray crystallography, multinuclear NMR spectroscopy, and absorption spectroscopies. Steric properties of these complexes were also quantified using the computational program Solid-G. The increased basicity of the free ligand -N(SiHMe2)tBu was demonstrated by direct comparison to -N(SiMe3)2, a popular supporting ligand for uranyl. Substitutional lability on a uranyl center was also demonstrated by exchange with the -N(SiMe3)2ligand. The increased basicity of this ligand and diverse characterization handles discussed here will make these compounds useful synthons for future reactivity.
The organoactinide catalyzed monohydroboration of carbodiimides is reported herein. The catalytic reactions proceed under very mild conditions in a highly atom-efficient and highly selective fashion to afford the corresponding monohydroborated N-borylformamidine products in high yields. A plausible mechanism is proposed based on stoichiometric and kinetic studies.
The bis(silyl)amide {N(SiMe3)2} (N′′) has supported spectacular actinide (An) chemistry for over 40 years, yet surprisingly there are only a handful of An complexes containing larger bis(silyl)amides, e.g. [U(N**)3] [N** = {N(SiMe2tBu)2}, 1]. Herein we report the structural characterization of the UIII complexes [U(N**)2(μ-I)]2 (2), [U(N†′)2(μ-I)]2 [N†′ = {N(SiiPr3)(SiMe3)}, 3], [U(N††)(I)2(THF)2] [N†† = {N(SiiPr3)2}, 4], [U(N††)2(I)] (5), and [U(N†′)2(I)(THF)] (6), and the AnIV complexes [An(N**){N(SiMe2tBu)(SiMetBuCH2-κ2-N,C)}(μ-Cl)]2 (7-An, An = U, Th) and [Th(N**)2{N(SiMe2tBu)(SiMetBuCH2-κ2-N,C)}] (8). Low crystalline yields were obtained in all cases, presumably due to facile cyclometallation. Although this precluded full characterization of UIII 4 and 6, and AnIV 7-8, in the case of UIII 2, 3 and 5 yields were high enough to perform NMR, EPR, NIR/UV/Vis and FTIR spectroscopy, elemental analysis and magnetic measurements.
The controlled manipulation of the axial oxo and equatorial halide ligands in the uranyl dipyrrin complex, UO2Cl(L) allows the uranyl reduction potential to be shifted by 1.53 V into the range accessible to naturally occurring reductants that are present during uranium remediation and storage processes. Abstraction of the equatorial halide ligand to form the uranyl cation causes a 780 mV positive shift in the UV/UIV reduction potential. Borane-functionalization of the axial oxo groups causes the spontaneous homolysis of the equatorial U-Cl bond and a further 750 mV shift of this potential. The combined effect of chloride loss and borane coordination to the oxo groups allows reduction of UVI to UIV by H2 or other very mild reductants such as Cp*2Fe. The reduction with H2 is accompanied by a B-C bond cleavage process in the oxo-coordinated borane.
Uranium mono(imido) species have been prepared via the oxidation of Cp*U(MesPDIMe)(THF) (1-Cp*) and [CpPU(MesPDIMe)]2 (1-CpP), where Cp* = η5-1,2,3,4,5-pentamethylcyclopentadienide, CpP = 1-(7,7-dimethylbenzyl)cyclopentadienide, MesPDIMe = 2,6-[(Mes)N═CMe]2C5H3N, and Mes = 2,4,6-trimethylphenyl, with organoazides. Treating either with N3DIPP (DIPP = 2,6-diisopropylphenyl) formed uranium(IV) mono(imido) complexes, CpPU(NDIPP)(MesPDIMe) (2-CpP) and Cp*U(NDIPP)(MesPDIMe) (2-Cp*), featuring reduced [MesPDIMe]-. The addition of electron-donating 1-azidoadamantane (N3Ad) to 1-Cp* generated a dimeric product, [Cp*U(NAd)(MesHPDIMe)]2 (3), from radical coupling at the p-pyridine position of the pyridine(diimine) ligand and H-atom abstraction, formed through a monomeric intermediate that was observed in solution but could not be isolated. To support this, Cp*U(tBu-MesPDIMe)(THF) (1-tBu), which has a tert-butyl group protecting the para position, was also treated with N3Ad, and the monomeric product, Cp*U(NAd)(tBu-MesPDIMe) (2-tBu), was isolated. All isolated complexes were analyzed spectroscopically and structurally, and the dynamic solution behavior was examined using electronic absorption spectroscopy.
Attempts to synthesize the base-free dication [(C5Me5)2Th]²⁺ by reaction of the bis(allyl) complex (C5Me5)2Th(C3H5)2 with 2 equiv of [Et3NH][BPh4] in benzene yielded a cationic phenyl complex that, in the presence of THF, crystallized from toluene as [(C5Me5)2Th(C6H5)(THF)][BPh4]. The reaction of the dimethyl complex (C5Me5)2ThMe2 with [Et3NH][BPh4] in toluene in the presence of nitriles RCN generates cations of the formula [(C5Me5)2Th(NCR)5][BPh4]2 (R = Me, Ph) in 40–55% crystalline yield. The molecular structures reveal the first examples of thorium cyclopentadienyl metallocene complexes with parallel rings.
Cleavage of dihydrogen is an important step in the industrial and enzymatic transformation of N2 into ammonia. Here we report the reversible cleavage of dihydrogen in mild conditions (room temperature and 1 atmosphere of H2) by the molecular uranium nitride complex, [Cs{U(OSi(OtBu)3)3}2(N)] 1, leading to a rare hydride-imide bridged diuranium(IV) complex, [Cs{U(OSi(OtBu)3)3}2(H)(NH)], 2 that slowly releases H2 under vacuum. This complex is highly reactive and quickly transfers hydride to acetonitrile and carbon dioxide at room temperature affording the ketimide- and formate-bridged U(IV) species [Cs{U(OSi(OtBu)3)3}2(NH)(CH3CHN)], 3 and [Cs{U(OSi(OtBu)3)3}2(HCOO)(NHCOO)], 4.
Herein we describe a convenient lab scale synthesis for pure and solvent-free binary uranium(III) halides UCl3, UBr3 and UI3. This is achieved by the reduction of the respective uranium(IV) halides with elemental silicon in borosilicate ampoules at moderate temperature. The silicon tetrahalides SiX4 formed as a side product are utilized for the removal of excess starting material via a chemical vapor transport reaction. The syntheses introduced here avoid the need for pure metallic uranium and are based on uranium(IV) halides synthesized from UO2 and the respective aluminium halides and purified by chemical vapor transport. These uranium(III) halides are obtained in single crystalline form. A similar reaction yields UF3 as a microcrystalline powder. However, no beneficial transport reaction occurs with this halide. Also, a higher temperature has to be applied and steel ampoules have to be used. The identities and purity of the products were checked by powder X-ray diffraction as well as IR spectroscopy. The synthesis of UI3 enabled its crystal structure determination on single crystals for the first time. UI3 crystallizes in the PuBr3 structure type with space group type Cmcm and a = 4.3208(9), b = 13.923(3), c = 9.923(2) Å, V = 596.9(2) Å3, and Z = 4 at T = 100 K.
Production of certified reference materials in support of domestic nuclear forensics programs require volatile precursors for introduction into electromagnetic isotopic separation instruments. β-Diketone chelates of tetravalent actinides are known for their high volatility, but previously developed synthetic approaches require starting material (NpCl4) that is prohibitively difficult and hazardous to prepare. An alternative strategy was developed here that uses controlled potential electrolysis to reduce neptunium to the tetravalent state in submolar concentrations of hydrochloric acid. Four different β-diketone ligands of varying degrees of fluorination were reacted with an aqueous solution of Np(4+). Products of this reaction were characterized via X-ray diffraction and infrared spectroscopy, and were found to be neutral 8-coordinate complexes that adopt square antiprismatic crystal geometry. Synthesis of Np β-diketonates by this approach circumvents the necessity of using NpCl4 in tetravalent Np coordination compound synthesis. The volatility of the complexes was assessed using thermogravimetric analysis, where the temperature of sublimation was determined to be in the range of 180° to 205 °C. The extent of fluorination did not appreciably alter the sublimation temperature of the complex. Thermal decomposition of these compounds was not observed during sublimation. High volatility and thermal stability of Np β-diketonates make them ideal candidates for gaseous introduction into isotopic separation instruments.
The first thorium(IV) and uranium(IV) hydrocarbyl complexes of a trans-calix[2]benzene[2]pyrrolide macrocycle can use ligand noninnocence to enable multiple C–H bond activation reactions at the metal. Both alkyl and alkynyl complexes supported by the L dianion and L–2H tetraanion are reported. The ThIV and UIV monoalkyl-ate complexes [M(L–2H)An(R)] (M = K for R = CH2Ph, M = Li for R = Me, CH2SiMe3), in which the ligand aryl groups are metalated, add C–H bonds of terminal alkynes across the metal and ligand, forming the AnIV-alkynyl complexes [(L)An(C≡CR′)2] (R′ = SiMe3, SiⁱPr3). This ligand reprotonation from (L–2H)4– to (L)2– is accompanied by a change in coordination mode of the ligand from η⁵:η¹:η⁵:η¹ to η⁵:η⁵. Alternatively, the original alkyl group can be retained if the ligand is reprotonated using [Et3NH][BPh4], affording the ThIV cations [(L)Th(R)][BPh4] (R = CH2Ph, N(SiMe3)2). Here, ligand rearrangement to the κ¹:η⁶:κ¹:η⁶ coordination mode occurs. These complexes provide rare examples of bis(arene) actinide sandwich geometry. The two η¹-alkynides in [(L)Th(C≡CSiMe3)2] rearrange upon coordination of [Ni⁰], forming [(L)Th(C≡CSiMe3)2·Ni(PR″3)] (R″ = phenyl, cyclohexyl), featuring the shortest yet reported distance between Th and Ni and giving unprecedented insight into the changes in macrocyclic ligand coordination between κ¹:η⁶:κ¹:η⁶ and η⁵:η⁵ coordination modes. A computational study of this conformational change demonstrates the η⁵:η⁵ coordination mode to be more stable in the Th/Ni bimetallics (and hypothetical Pt analogues), an observation rationalized by detailed analysis of the Kohn–Sham orbital structure of the κ¹:η⁶:κ¹:η⁶ and η⁵:η⁵ conformers. Although remarkably inert to even high pressures of CO2 at room temperature, the bis(alkynyl) complexes [(L)An(C≡CSiMe3)2] completely cleave one CO bond of CO2 when they are heated under 1 bar pressure, resulting in the formation, and elimination from the metal, of a new, CO-inserted, bicyclic, carbonylated macrocycle with complete control over the C–C and C–N bond forming reactions.
The synthesis, characterization, and electronic spectroscopy of two ML 2 sandwich complexes, where M=Ce(IV) or Th(IV) and L=napthylsalophen 2 are described. The ThL 2 complex, unlike the isovalent CeL 2 , complex possesses unusual fluorescence...
In this work we present a facile, lab scale synthesis for thorium tetrahalides ThX4 (X = Cl, Br & I). The reaction between the easily available ThO2 and AlX3 (X = Cl, Br & I) and a subsequent in situ chemical vapour transport (CVT) leads to a product of high purity which is obtained in the form of crystals or large aggregates of crystals. Their identity and purity was evidenced by X-ray powder diffraction and IR spectroscopy. The usage of ThO2 avoids, unlike earlier syntheses, the utilization of scarcely available thorium metal or of other reactants, such as CCl4, leading to impurities. Furthermore, the reaction tolerates even less pure ThO2.
Reaction of the trivalent uranium complex [((Ad,MeArO)3N)U(DME)] with [Na(OCP)(dioxane)2.5] and 2.2.2-crypt yields the μ-oxo-bridged, diuranium complex [Na(2.2.2-crypt)][{((Ad,MeArO)3N)U(DME)}(μ-O){((Ad,MeArO)3N)U(CP)}] (1). Complex 1 features an asymmetric, dinuclear UIV–O–UIV core structure with a cyaphide (CP–) anion η¹-CP bound to one of the U ions, and a κ²-O DME coordinated to the other. The CP– ligand is unprecedented in uranium chemistry and is formed through reductive C–O bond cleavage of the phosphaethynolate anion (OCP–). An analogous reaction was performed starting from the tetravalent uranium halide complex [((Ad,MeArO)3N)U(DME)(Cl)]. This salt metathesis approach with [Na(OCP)(dioxane)2.5] results in formation of the mononuclear complex [((Ad,MeArO)3N)U(DME)(OCP)] (2) with an OCP– anion bound to the uranium(IV) center via the oxygen atom in an η¹-OCP fashion.
Crystals of a hydrated Pu(III) chloride, (C 5 H 5 NBr) 2 [PuCl 3 (H 2 O) 5 ] · 2Cl · 2H 2 O, were grown via slow evaporation from acidic aqueous, high chloride media. X-ray diffraction data reveals the neutral [PuCl 3 (H 2 O) 5 ] tecton...
The bis(NHC)borate-supported thorium-bis(mesitylphosphido) complex (1) undergoes reversible intramolecular C–H bond activation enabling the catalytic hydrophosphination of unactivated internal alkynes. Catalytic and stoichiometric experiments support a mechanism involving reactive Th–NHC metallacycle intermediates (Int and 2).
The reaction of (C5 Me5 )2 Th(CH3 )2 with the phosphonium salts [CH3 PPh3 ]X (X=Cl, Br, I) was investigated. When X=Br and I, two equivalents of methane are liberated to afford (C5 Me5 )2 Th[CHPPh3 ]X, rare terminal phosphorano-stabilized carbenes with thorium. These complexes feature the shortest thorium-carbon bonds (≈2.30 Å) reported to date, and electronic structure calculations show some degree of multiple bonding. However, when X=Cl, only one equivalent of methane is lost with concomitant formation of benzene from an unstable phosphorus(V) intermediate, yielding (C5 Me5 )2 Th[κ(2) -(C,C')-(CH2 )(CH2 )PPh2 ]Cl. Density functional theory (DFT) investigations of the reaction energy profiles for [CH3 PPh3 ]X, X=Cl and I showed that in the case of iodide, thermodynamics prevents the production of benzene and favors formation of the carbene.