Covalent Uranium Carbene Chemistry

An Actinide Complex with a Nucleophilic Allenylidene Ligand

The reaction of [Cp3Th(3,3-diphenylcyclopropenyl)] (Cp = η5-C5H5) with 1 equiv of lithium diisopropylamide (LDA) results in cyclopropenyl ring opening and formation of the thorium allenylidene complex, [Li(Et2O)2][Cp3Th(CCCPh2)] ([Li(Et2O)2][1]), in good yield. Additionally, deprotonation of [Cp3Th(3,3-diphenylcyclopropenyl)] with 1 equiv of LDA, in the presence of 12-crown-4 or 2.2.2-cryptand, results in the formation of discrete cation/anion pairs, [Li(12-crown-4)(THF)][Cp3Th(CCCPh2)] ([Li(12-crown-4)(THF)][1]) and [Li(2.2.2-cryptand)][Cp3Th(CCCPh2)] ([Li(2.2.2-cryptand)][1]), respectively. Interestingly, the complex [Li(Et2O)2][1] undergoes dimerization upon standing at room temperature, resulting in the formation of [Cp2Th(μ:η1:η3-CCCPh2)]2 (2), via loss of LiCp. The reaction of [Li(Et2O)2][1] with MeI results in electrophilic attack at the Cγ carbon atom, leading to the formation of a thorium acetylide complex, [Cp3Th(C≡CC(Me)Ph2)] (3), which can be isolated in 83% yield upon workup, whereas the reaction of [Li(Et2O)2][1] with benzophenone results in the formation of 1,1,4,4-tetraphenylbutatriene (4) in 99% yield, according to integration against an internal standard. Density functional theory (DFT) calculations performed on [1]- and 2 reveal significant electron delocalization across the allenylidene ligand. Additionally, calculations of the 13C NMR chemical shifts for the Cα, Cβ, and Cγ nuclei of the allenylidene ligand were in good agreement with the experimental shifts. The calculations reveal modest deshielding induced by spin-orbital effects originating at Th due to the involvement of the 5f orbitals in the Th-C bonds. According to a DFT analysis, the cyclopropenyl ring-opening reaction proceeds via [Cp3Th(η1-3,3-Ph2-cyclo-C3)]- (IM), which features a carbanion character at Cβ.

A Metallocene Bis(phosphoranocarbene) of Uranium and a Probe of Its Reactivity with Alcohols

The addition of 2 equiv of the phosphaylide H2C═PPh3 to the dimethyl uranium metallocene Cp*2UMe2 (Cp* = η5-C5Me5) in toluene with gentle heating at 40 °C generates the phosphorano-stabilized bis(carbene) Cp*2U[C(H)PPh3]2 (1) in good yield. Characterization of 1 by X-ray crystallographic analysis reveals two short uranium-carbon bonds, ranging from 2.301(5) to 2.322(5) Å, consistent with the presence of U═C carbene-type bonds. Monitoring the reaction by NMR spectroscopy suggests that it proceeds through the intermediate formation of the methyl carbene complex Cp*2U[C(H)PPh3](Me) (1Int); however, prolonged heating of these solutions leads to the ortho-cyclometalated carbene species Cp*2U{κ2-[C(H)PPh2(C6H4)]} (2) via intramolecular C-H activation. Rapid conversion from 1 to 2 occurs within hours upon heating its toluene solutions to 100 °C. Preliminary reactivity studies of 1 show that it readily reacts with alcohols, such as HODipp (Dipp = 2,6-diisopropylphenyl) and HOC(CF3)3, to give the mixed carbene alkoxide compounds Cp*2U[C(H)PPh3](OR) (R = Dipp (4Dipp), C(CF3)3 (5CF3)). In one case, the reaction of 1 with HODipp in the presence of adventitious water led to the formation of a few crystals of the terminal U(IV) oxo complex, [Ph3PCH3][Cp*2U(O)(ODipp)] (3oxo). The isolation of 1 marks the first instance of an unchelated, heteroatom-stabilized bis(carbene) complex of uranium that also provides an entryway to the synthesis of its monocarbene derivatives through protonolysis.

Progress in Nonaqueous Molecular Uranium Chemistry: Where to Next?

There is long-standing interest in nonaqueous uranium chemistry because of fundamental questions about uranium's variable chemical bonding and the similarities of this pseudo-Group 6 element to its congener d-block elements molybdenum and tungsten. To provide historical context, with reference to a conference presentation slide presented around 1988 that advanced a defining collection of top targets, and the challenge, for synthetic actinide chemistry to realize in isolable complexes under normal experimental conditions, this Viewpoint surveys progress against those targets, including (i) CO and related π-acid ligand complexes, (ii) alkylidenes, carbynes, and carbidos, (iii) imidos and terminal nitrides, (iv) homoleptic polyalkyls, -alkoxides, and -aryloxides, (v) uranium-uranium bonds, and (vi) examples of topics that can be regarded as branching out in parallel from the leading targets. Having summarized advances from the past four decades, opportunities to build on that progress, and hence possible future directions for the field, are highlighted. The wealth and diversity of uranium chemistry that is described emphasizes the importance of ligand-metal complementarity in developing exciting new chemistry that builds our knowledge and understanding of elements in a relativistic regime.

Quantifying Actinide-Carbon Bond Covalency in a Uranyl-Aryl Complex Utilizing Solution 13C NMR Spectroscopy

Reaction of [UO2Cl2(THF)2]2 with in situ generated LiFmes (FmesH = 1,3,5-(CF3)3C6H3) in Et2O resulted in the formation of the uranyl aryl complexes [Li(THF)3][UO2(Fmes)3] ([Li(THF)3][1]) and [Li(Et2O)3(THF)][UO2(Fmes)3] ([Li(Et2O)3(THF)][1]) in good to moderate yields after crystallization from hexanes and Et2O, respectively. Both complexes were characterized by X-ray crystallography and NMR spectroscopy. DFT calculations reveal that the Cispo resonance in [1]- exhibits a deshielding of 51 ppm from spin-orbit coupling effects originating at uranium, which indicates an appreciable covalency in the U-C bonding interaction.

Arene-, Chlorido-, and Imido-Uranium Bis- and Tris(boryloxide) Complexes

We introduce the boryloxide ligand {(HCNDipp)2BO}- (NBODipp, Dipp = 2,6-di-isopropylphenyl) to actinide chemistry. Protonolysis of [U{N(SiMe3)2}3] with 3 equiv of NBODippH produced the uranium(III) tris(boryloxide) complex [U(NBODipp)3] (1). In contrast, treatment of UCl4 with 3 equiv of NBODippK in THF at room temperature or reflux conditions produced only [U(NBODipp)2(Cl)2(THF)2] (2) with 1 equiv of NBODippK remaining unreacted. However, refluxing the mixture of 2 and unreacted NBODippK in toluene instead of THF afforded the target complex [U(NBODipp)3(Cl)(THF)] (3). Two-electron oxidation of 1 with AdN3 (Ad = 1-adamantyl) afforded the uranium(V)-imido complex [U(NBODipp)3(NAd)] (4). The solid-state structure of 1 reveals a uranium-arene bonding motif, and structural, spectroscopic, and DFT calculations all suggest modest uranium-arene δ-back-bonding with approximately equal donation into the arene π4 and π5 δ-symmetry π* molecular orbitals. Complex 4 exhibits a short uranium(V)-imido distance, and computational modeling enabled its electronic structure to be compared to related uranium-imido and uranium-oxo complexes, revealing a substantial 5f-orbital crystal field splitting and extensive mixing of 5f |ml,ms⟩ states and mj projections. Complexes 1-4 have been variously characterized by single-crystal X-ray diffraction, 1H NMR, IR, UV/vis/NIR, and EPR spectroscopies, SQUID magnetometry, elemental analysis, and CONDON, F-shell, DFT, NLMO, and QTAIM crystal field and quantum chemical calculations.

Solution and solid-state characterization of rare silyluranium(III) complexes

A uranium(III) silylate complex [K(DME)4][UI2{(Si(SiMe3)2SiMe2)2O}] (1) was stabilized by the addition of 18-crown-6, forming [K(18-crown-6)][UI2{(Si(SiMe3)2SiMe2)2O}] (1-crown). Crystallization under multiple conditions resulted in three distinct molecular structures. Compound 1-crown was further characterized in the solution state via1H, 13C, and 29Si NMR spectroscopy, and electronic absorption spectroscopy.

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.

13Ccarbene nuclear magnetic resonance chemical shift analysis confirms CeIV[double bond, length as m-dash]C double bonding in cerium(iv)-diphosphonioalkylidene complexes

Diphosphonioalkylidene dianions have emerged as highly effective ligands for lanthanide and actinide ions, and the resulting formal metal-carbon double bonds have challenged and developed conventional thinking about f-element bond multiplicity and covalency. However, f-element-diphosphonioalkylidene complexes can be represented by several resonance forms that render their metal-carbon double bond status unclear. Here, we report an experimentally-validated 13C Nuclear Magnetic Resonance computational assessment of two cerium(iv)-diphosphonioalkylidene complexes, [Ce(BIPMTMS)(ODipp)2] (1, BIPMTMS = {C(PPh2NSiMe3)2}2-; Dipp = 2,6-diisopropylphenyl) and [Ce(BIPMTMS)2] (2). Decomposing the experimental alkylidene chemical shifts into their corresponding calculated shielding (σ) tensor components verifies that these complexes exhibit Ce[double bond, length as m-dash]C double bonds. Strong magnetic coupling of Ce[double bond, length as m-dash]C σ/π* and π/σ* orbitals produces strongly deshielded σ11 values, a characteristic hallmark of alkylidenes, and the largest 13C chemical shift tensor spans of any alkylidene complex to date (1, 801 ppm; 2, 810 ppm). In contrast, the phosphonium-substituent shielding contributions are much smaller than the Ce[double bond, length as m-dash]C σ- and π-bond components. This study confirms significant Ce 4f-orbital contributions to the Ce[double bond, length as m-dash]C bonding, provides further support for a previously proposed inverse-trans-influence in 2, and reveals variance in the 4f spin-orbit contributions that relate to the alkylidene hybridisation. This work thus confirms the metal-carbon double bond credentials of f-element-diphosphonioalkylidenes, providing quantified benchmarks for understanding diphosphonioalkylidene bonding generally.

Synthesis and Characterization of U≡C Triple Bonds in Fullerene Compounds

Despite decades of efforts, the actinide-carbon triple bond has remained an elusive target, defying synthesis in any isolable compound. Herein, we report the successful synthesis of uranium-carbon triple bonds in carbide-bridged bimetallic [U≡C-Ce] units encapsulated inside the fullerene cages of C72 and C78. The molecular structures of UCCe@C2n and the nature of the U≡C triple bond were characterized through X-ray crystallography and various spectroscopic analyses, revealing very short uranium-carbon bonds of 1.921(6) and 1.930(6) Å, with the metals existing in their highest oxidation states of +6 and +4 for uranium and cerium, respectively. Quantum-chemical studies further demonstrate that the C2n cages are crucial for stabilizing the [UVI≡C-CeIV] units through covalent and coordinative interactions. This work offers a new fundamental understanding of the elusive uranium-carbon triple bond and informs the design of complexes with similar bonding motifs, opening up new possibilities for creating distinctive molecular compounds and materials.

USc2C2 and USc2NC Clusters with U-C Triple Bond Character Stabilized Inside Fullerene Cages

The chemistry of f-block metal-carbon multiple bonds is underdeveloped compared to well-established carbene complexes of the d-block transition metals. Herein, we report two new actinide-rare earth mixed metal carbides and nitrogen carbide cluster fullerenes, USc2C2@D5h(6)-C80 and USc2NC@D5h(6)-C80, which contain U-C bonds with triple bond character and were successfully synthesized and characterized by mass spectrometry, UV-vis-NIR spectroscopy, Fourier transform infrared spectroscopy, single crystal X-ray diffraction, and DFT calculations. Crystallographic studies show that the two previously unreported clusters, USc2C2 and USc2NC, are stabilized in the D5h(6)-C80 carbon cage and adopt unique trifoliate configurations, in which C2/NC units are almost vertically inserted into the plane defined by the U and two Sc atoms. Combined experimental and theoretical studies further reveal the bonding structure of USc2C2 and USc2NC, which contain C═U(VI)═C and C═U(V)═N bonding motifs. The electronic structures of the two compounds are determined as U6+(Sc2)6+(C4-)2@D5h(6)-C804- and U5+(Sc2)6+(N)3-(C)4-@D5h(6)-C804-, respectively. Quantum-chemical studies confirm that the U-C bonds in both molecules show unprecedented multicenter triple-bond character. The discovery of this unique U-C multiple bond offers a deeper understanding of the fundamentals of uranium chemistry.

Bonding and Reactivity in Terminal versus Bridging Arenide Complexes of Thorium Acting as ThII Synthons

Thorium redox chemistry is extremely scarce due to the high stability of Th^IV . Here we report two unique examples of thorium arenide complexes prepared by reduction of a Th^IV -siloxide complex in presence of naphthalene, the mononuclear arenide complex [K(OSi(O^t Bu)₃ )₃ Th(η⁶ -C₁₀ H₈ )] (1) and the inverse-sandwich complex [K(OSi(O^t Bu)₃ )₃ Th]₂ (μ-η⁶ ,η⁶ -C₁₀ H₈ )] (2). The electrons stored in these complexes allow the reduction of a broad range of substrates (N₂ O, AdN₃ , CO₂ , HBBN). Higher reactivity was found for the complex 1 which reacts with the diazoolefin IDipp=CN₂ to yield the unexpected Th^IV amidoalkynyl complex 5 via a terminal N-heterocyclic vinylidene intermediate. This work showed that arenides can act as convenient redox-active ligands for implementing thorium-ligand cooperative multielectron transfer and that the reactivity can be tuned by the arenide binding mode.

Uranyl Analogue Complexes—Current Progress and Synthetic Challenges

Uranyl ions, {UO2}n+ (n = 1, 2), display trans, strongly covalent, and chemically robust U-O multiple bonds, where 6d, 5f, and 6p orbitals play important roles. The synthesis of isoelectronic analogues of uranyl has been of interest for quite some time, mainly with the purpose of unveiling covalence and 5f-orbital participation in bonding. Significant advances have occurred in the last two decades, initially marked by the synthesis of uranium(VI) bis(imido) complexes, the first analogues with a {RNUNR}2+ core, later followed by the synthesis of unique trans-{EUO}2+ (E = S, Se) complexes, and recently highlighted by the synthesis of the first complexes featuring a linear {NUN} moiety. This review covers the synthesis, structure, bonding, and reactivity of uranium complexes containing a linear {EUE}n+ core (n = 0, 1, 2), isoelectronic to uranyl ions, {OUO}n+ (n = 1, 2), incorporating σ- and π-donating ligands that can engage in uranium–ligand multiple bonding, where oxygen may be replaced by heavier chalcogenido, imido, nitride, and carbene ligands, or by a transition metal. It focuses on synthetic methods of well-defined molecular uranium species in the condensed phase but also references gas-phase and low-temperature-matrix experiments, as well as computational studies that may lead to valuable insights.

Mesoionic Carbene Complexes of Uranium(IV) and Thorium(IV)

We report the synthesis and characterization of uranium(IV) and thorium(IV) mesoionic carbene complexes [An{N(SiMe3)2}2(CH2SiMe2NSiMe3){MIC}] (An = U, 4U and Th, 4Th; MIC = {CN(Me)C(Me)N(Me)CH}), which represent rare examples of actinide mesoionic carbene linkages and the first example of a thorium mesoionic carbene complex. Complexes 4U and 4Th were prepared via a C-H activation intramolecular cyclometallation reaction of actinide halides, with concomitant formal 1,4-proton migration of an N-heterocyclic olefin (NHO). Quantum chemical calculations suggest that the An-carbene bond comprises only a σ-component, in contrast to the uranium(III) analogue [U{N(SiMe3)2}3(MIC)] (1) where computational studies suggested that the 5f3 uranium(III) ion engages in a weak one-electron π-backbond to the MIC. This highlights the varying nature of actinide-MIC bonding as a function of actinide oxidation state. In solution, 4Th exists in equilibrium with the Th(IV) metallacycle [Th{N(SiMe3)2}2(CH2SiMe2NSiMe3)] (6Th) and free NHO (3). The thermodynamic parameters of this equilibrium were probed using variable-temperature NMR spectroscopy yielding an entropically favored but enthalpically endothermic process with an overall reaction free energy of ΔG 298.15K = 0.89 kcal mol-1. Energy decomposition analysis (EDA-NOCV) of the actinide-carbon bonds in 4U and 4Th reveals that the former is enthalpically stronger and more covalent than the latter, which accounts for the respective stabilities of these two complexes.

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.

Ring-opening of a Thorium Cyclopropenyl Complex Generates a Transient Thorium-bound Carbene

The reaction of [Cp3ThCl] with in situ generated lithium-3,3-diphenylcyclopropene results in the formation of [Cp3Th(3,3-diphenylcyclopropenyl)] (1), in good yields. Thermolysis of 1 results in isomerization to the ring-opened product, [Cp3Th(3-phenyl-1H-inden-1-yl)]...

Synthesis and electronic structure analysis of the actinide allenylidenes, [{(NR2)3}An(CCCPh2)]- (An = U, Th; R = SiMe3)

The reaction of [AnCl(NR₂)₃] (An = U, Th, R = SiMe₃) with in situ generated lithium-3,3-diphenylcyclopropene results in the formation of [{(NR₂)₃}An(CH[double bond, length as m-dash]C[double bond, length as m-dash]CPh₂)] (An = U, 1; Th, 2) in good yields after work-up. Deprotonation of 1 or 2 with LDA/2.2.2-cryptand results in formation of the anionic allenylidenes, [Li(2.2.2-cryptand)][{(NR₂)₃}An(CCCPh₂)] (An = U, 3; Th, 4). The calculated ¹³C NMR chemical shifts of the C_α, C_β, and C_γ nuclei in 2 and 4 nicely reproduce the experimentally assigned order, and exhibit a characteristic spin-orbit induced downfield shift at C_α due to involvement of the 5f orbitals in the Th-C bonds. Additionally, the bonding analyses for 3 and 4 show a delocalized multi-center character of the ligand π orbitals involving the actinide. While a single-triple-single-bond resonance structure (e.g., An-C[triple bond, length as m-dash]C-CPh₂) predominates, the An[double bond, length as m-dash]C[double bond, length as m-dash]C[double bond, length as m-dash]CPh₂ resonance form contributes, as well, more so for 3 than for 4.

Isolation of a Uranium(III)-Carbon Multiple Bond Complex

Low-valent uranium-element multiple bond complexes remain scarce, though there is burgeoning interest regarding to their bonding and reactivity. Herein, isolation of a uranium(III)-carbon double bond complex [(Cp*)2 U(CDP)](BPh4 ) (1) comprising a tridentate carbodiphosphorane (CDP) was reported for the first time. Oxidation of 1 afforded the corresponding U(IV) complex [(Cp*)2 U(CDP)](BPh4 )2 (2). The distance between U and C in 2 is 2.481 Å, indicating the existence of a typical U=C double bond, which is further confirmed by quantum chemical calculations. Bonding analysis suggested that the CDP also serves as both σ- and π-donor in complex 1, though a longer U-C bond (2.666(3) Å) is observed. It implies that 1 is the first isolable mononuclear uranium(III) carbene complex. Moreover, these results suggest that CDPs are promising ligands to establish other low-valent f-block metal-carbon multiple bond complexes.

Fragmentation, catenation, and direct functionalisation of white phosphorus by a uranium(IV)-silyl-phosphino-carbene complex

Room temperature reaction of the uranium(iv)-carbene [U{C(SiMe3)(PPh2)}(BIPMTMS)(μ-Cl)Li(TMEDA)(μ-TMEDA)0.5]2 (1, BIPMTMS = C(PPh2NSiMe3)2) with white phosphorus (P4) produces the organo-P5 compound [P5{C(SiMe3)(PPh2)}2][Li(TMEDA)2] (2) and the uranium(iv)-methanediide [U{BIPMTMS}{Cl}{μ-Cl}2{Li(TMEDA)}] (3). This is an unprecedented example of cooperative metal-carbene P4 activation/insertion into a metal-carbon double bond and also an actinide complex reacting with P4 to directly form an organophosphorus species. Conducting the reaction at low temperature permits the isolation of the diuranium(iv) complex [{U(BIPMTMS)([μ-η2:η2-P2]C[SiMe3][PPh2])}2] (4), which then converts to 2 and 3. Thus, surprisingly, in contrast to all other actinide P4 reactivity, although this reaction produces catenation overall it proceeds via P4 cleavage to functionalised P2 units. Hence, this work establishes a proof of concept synthetic cycle for direct fragmentation, catenation, and functionalisation of P4.

f-Element silicon and heavy tetrel chemistry

The last three decades have seen a significant increase in the number of reports of f-element carbon chemistry, whilst the f-element chemistry of silicon, germanium, tin, and lead remain underdeveloped in comparison. Here, in this perspective we review complexes that contain chemical bonds between f-elements and silicon or the heavier tetrels since the birth of this field in 1985 to present day, with the intention of inspiring researchers to contribute to its development and explore the opportunities that it presents. For the purposes of this perspective, f-elements include lanthanides, actinides and group 3 metals. We focus on complexes that have been structurally authenticated by single-crystal X-ray diffraction, and horizon-scan for future opportunities and targets in the area.

cis- and trans-Ligand Effects on the Inverse trans-Influence in [UVI(O)(L)Cl4]0/- (L = Unidentate Ligand) Complexes

A comprehensive exploration of the inverse trans-influence (ITI) phenomenon in a series of cis-[U^VI(O)(L)Cl₄]^0/- and trans-[U^VI(O)(L)Cl₄]^0/- complexes involving a wide variety of neutral and anionic unidentate ligands L, using relativistic density functional theory, threw light on the still-intriguing physics of ITI, elucidated its origin, and deployed the ligands L in cis- and trans-ITI sequences (ladders). ITI is produced for the complete set of L in both series of [U(O)(L)Cl₄]^0/- complexes, but this is not reflected in the thermodynamic stability of the [U(O)(L)Cl₄]^0/- isomers. In effect the hard and strong σ-donor anionic ligands stabilize the trans isomers, but the opposite is true for the soft σ-donor/π-donor neutral and anionic ligands that stabilize the cis isomers. According to the ITI%(U-L) metrics the hard strong σ-donor anionic ligands exert stronger ITI than the soft σ-donor/π-donor neutral ones, while according to the ITI%(U-O) metrics ITI is produced only for the more stable trans-[U(O)(L)Cl₄]^0/- isomers involving the anionic ligands. In contrast the neutral ligands in the more stable cis-[U(O)(L)Cl₄]^0/- isomers produce the normal cis influence (CI). Furthermore, the more electronegative ligands produce stronger ITI. ITI%(U-O) cis- and trans-philicity ladders are also built for both series of complexes employing the isotropic σ^iso(SO) ¹⁷O NMR shielding constants as a sensitive metric of the ITI phenomenon. The NMR ITI%(U-O) metrics are consistent with the ITI%(U-O) ones illustrating that the isotropic ¹⁷O NMR shifts are sensitive metrics of the covalency of the multiple U-O bonding mode and, hence, of the ITI phenomenon. Interestingly the 2σ BD(U-O) natural bond orbitals play a key role in tuning the bond length and covalency of the U-O bond through the 2σ(U≡O) → 2σ*(U≡O) hyperconjugative interactions. The assessment of the magnitude of the ITI in the [U^VI(O)(L)Cl₄]^0/- complexes and the recognition of the factors affecting ITI dispose a guide to experimentalists working in the area of uranium chemistry to develop strategies for stabilizing uranium-ligand linkages.

Nature of the Arsonium-Ylide Ph3 As=CH2 and a Uranium(IV) Arsonium-Carbene Complex

Treatment of [Ph₃ EMe][I] with [Na{N(SiMe₃ )₂ }] affords the ylides [Ph₃ E=CH₂ ] (E=As, 1As; P, 1P). For 1As this overcomes prior difficulties in the synthesis of this classical arsonium-ylide that have historically impeded its wider study. The structure of 1As has now been determined, 45 years after it was first convincingly isolated, and compared to 1P, confirming the long-proposed hypothesis of increasing pyramidalisation of the ylide-carbon, highlighting the increasing dominance of E⁺ -C^- dipolar resonance form (sp³ -C) over the E=C ene π-bonded form (sp² -C), as group 15 is descended. The uranium(IV)-cyclometallate complex [U{N(CH₂ CH₂ NSiPrⁱ ₃ )₂ (CH₂ CH₂ SiPrⁱ ₂ CH(Me)CH₂ )}] reacts with 1As and 1P by α-proton abstraction to give [U(Tren^TIPS )(CHEPh₃ )] (Tren^TIPS =N(CH₂ CH₂ NSiPrⁱ ₃ )₃ ; E=As, 2As; P, 2P), where 2As is an unprecedented structurally characterised arsonium-carbene complex. The short U-C distances and obtuse U-C-E angles suggest significant U=C double bond character. A shorter U-C distance is found for 2As than 2P, consistent with increased uranium- and reduced pnictonium-stabilisation of the carbene as group 15 is descended, which is supported by quantum chemical calculations.

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.

A complete series of uranium(iv) complexes with terminal hydrochalcogenido (EH) and chalcogenido (E) ligands E = O, S, Se, Te

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).

Double dative bond between divalent carbon(0) and uranium

Dative bonds between p- and d-block atoms are common but species containing a double dative bond, which donate 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 diffraction 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 carbones 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.

Decomposition of d- and f-Shell Contributions to Uranium Bonding from the Quantum Theory of Atoms in Molecules: Application to Uranium and Uranyl Halides

The electronic structures of a series of uranium hexahalide and uranyl tetrahalide complexes were simulated at the density functional theoretical (DFT) level. The resulting electronic structures were analyzed using a novel application of the Quantum Theory of Atoms in Molecules (QTAIM) by exploiting the high symmetry of the complexes to determine 5f- and 6d-shell contributions to bonding via symmetry arguments. This analysis revealed fluoride ligation to result in strong bonds with a significant covalent character while ligation by chloride and bromide species resulted in more ionic interactions with little differentiation between the ligands. Fluoride ligands were also found to be most capable of perturbing an existing electronic structure. 5f contributions to overlap-driven covalency were found to be larger than 6d contributions for all interactions in all complexes studied while degeneracy-driven covalent contributions showed significantly greater variation. σ-contributions to degeneracy-driven covalency were found to be consistently larger than those of individual π-components while the total π-contribution was, in some cases, larger. Strong correlations were found between overlap-driven covalent bond contributions, U–O vibrational frequencies, and energetic stability, which indicates that overlap-driven covalency leads to bond stabilization in these complexes and that uranyl vibrational frequencies can be used to quantitatively probe equatorial bond covalency. For uranium hexahalides, degeneracy-driven covalency was found to anti-correlate with bond stability.

Uranium(III)-carbon multiple bonding supported by arene δ-bonding in mixed-valence hexauranium nanometre-scale rings

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.

A Very Short Uranium(IV)-Rhodium(I) Bond with Net Double-Dative Bonding Character

Reaction of [U{C(SiMe3 )(PPh2 )}(BIPM)(μ-Cl)Li(TMEDA)(μ-TMEDA)0.5 ]2 (BIPM=C(PPh2 NSiMe3 )2 ; TMEDA=Me2 NCH2 CH2 NMe2 ) with [Rh(μ-Cl)(COD)]2 (COD=cyclooctadiene) affords the heterotrimetallic UIV -RhI2 complex [U(Cl)2 {C(PPh2 NSiMe3 )(PPh[C6 H4 ]NSiMe3 )}{Rh(COD)}{Rh(CH(SiMe3 )(PPh2 )}]. This complex has a very short uranium-rhodium distance, the shortest uranium-rhodium bond on record and the shortest actinide-transition metal bond in terms of formal shortness ratio. Quantum-chemical calculations reveal a remarkable RhI→→ UIV net double dative bond interaction, involving RhI 4dz2 - and 4dxy/xz -type donation into vacant UIV 5f orbitals, resulting in a Wiberg/Nalewajski-Mrozek U-Rh bond order of 1.30/1.44, respectively. Despite being, formally, purely dative, the uranium-rhodium bonding interaction is the most substantial actinide-metal multiple bond yet prepared under conventional experimental conditions, as confirmed by structural, magnetic, and computational analyses.

Silyl-Phosphino-Carbene Complexes of Uranium(IV)

Unprecedented silyl-phosphino-carbene complexes of uranium(IV) are presented, 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)2 Li(THF)2 ] (1, BIPMTMS =C(PPh2 NSiMe3 )2 ) into [U(BIPMTMS )(Cl){CH(Ph)(SiMe3 )}] (2), and addition of [Li{CH(SiMe3 )(PPh2 )}(THF)]/Me2 NCH2 CH2 NMe2 (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 inverse-trans-influence in tetravalent lanthanide and actinide bis(carbene) complexes

Across the periodic table the trans-influence operates, whereby tightly bonded ligands selectively lengthen mutually trans metal-ligand bonds. Conversely, in high oxidation state actinide complexes the inverse-trans-influence operates, where normally cis strongly donating ligands instead reside trans and actually reinforce each other. However, because the inverse-trans-influence is restricted to high-valent actinyls and a few uranium(V/VI) complexes, it has had limited scope in an area with few unifying rules. Here we report tetravalent cerium, uranium and thorium bis(carbene) complexes with trans C=M=C cores where experimental and theoretical data suggest the presence of an inverse-trans-influence. Studies of hypothetical praseodymium(IV) and terbium(IV) analogues suggest the inverse-trans-influence may extend to these ions but it also diminishes significantly as the 4f orbitals are populated. This work suggests that the inverse-trans-influence may occur beyond high oxidation state 5f metals and hence could encompass mid-range oxidation state actinides and lanthanides. Thus, the inverse-trans-influence might be a more general f-block principle.