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

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.

Exploring Spin-Orbit Effects in a [Cu6Tl]+ Nanocluster Featuring an Uncommon Tl-H Interaction

Reaction of [CuH(PPh3)]6 with 1 equiv. of Tl(OTf) results in formation of [Cu6TlH6(PPh3)6][OTf] ([1]OTf]), which can be isolated in good yields. Variable-temperature 1H NMR spectroscopy, in combination with density functional theory (DFT) calculations, confirms the presence of a rare Tl-H orbital interaction. According to DFT, the 1H chemical shift of the Tl-adjacent hydride ligands of [1]+ includes 7.7 ppm of deshielding due to spin-orbit effects from the heavy Tl atom. This study provides valuable new insights into a rare class of metal hydrides, given that [1][OTf] is only the third isolable species reported to contain a Tl-H interaction.

N-Heterocyclic Carbene to Actinide d-Based π-bonding Correlates with Observed Metal-Carbene Bond Length Shortening Versus Lanthanide Congeners

Comparison of bonding and electronic structural features between trivalent lanthanide (Ln) and actinide (An) complexes across homologous series' of molecules can provide insights into subtle and overt periodic trends. Of keen interest and debate is the extent to which the valence f- and d-orbitals of trivalent Ln/An ions engage in covalent interactions with different ligand donor functionalities and, crucially, how bonding differences change as both the Ln and An series are traversed. Synthesis and characterization (SC-XRD, NMR, UV-vis-NIR, and computational modeling) of the homologous lanthanide and actinide N-heterocyclic carbene (NHC) complexes [M(C5Me5)2(X)(IMe4)] {X = I, M = La, Ce, Pr, Nd, U, Np, Pu; X = Cl, M = Nd; X = I/Cl, M = Nd, Am; and IMe4 = [C(NMeCMe)2]} reveals consistently shorter An-C vs Ln-C distances that do not substantially converge upon reaching Am3+/Nd3+ comparison. Specifically, the difference of 0.064(6) Å observed in the La/U pair is comparable to the 0.062(4) Å difference observed in the Nd/Am pair. Computational analyses suggest that the cause of this unusual observation is rooted in the presence of π-bonding with the valence d-orbital manifold in actinide complexes that is not present in the lanthanide congeners. This is in contrast to other documented cases of shorter An-ligand vs Ln-ligand distances, which are often attributed to increased 5f vs 4f radial diffusivity leading to differences in 4f and 5f orbital bonding involvement. Moreover, in these traditional observations, as the 5f series is traversed, the 5f manifold contracts such that by americium structural studies often find no statistically significant Am3+vs Nd3+ metal-ligand bond length differences.

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.

Exact two-component theory becoming an efficient tool for NMR shieldings and shifts with spin-orbit coupling

We present a gauge-origin invariant exact two-component (X2C) approach within a modern density functional framework, supporting meta-generalized gradient approximations such as TPSS and range-separated hybrid functionals such as CAM-B3LYP. The complete exchange-correlation kernel is applied, including the direct contribution of the field-dependent basis functions and the reorthonormalization contribution from the perturbed overlap matrix. Additionally, the finite nucleus model is available for the electron-nucleus potential and the vector potential throughout. Efficiency is ensured by the diagonal local approximation to the unitary decoupling transformation in X2C as well as the (multipole-accelerated) resolution of the identity approximation for the Coulomb term (MARI-J, RI-J) and the seminumerical exchange approximation. Errors introduced by these approximations are assessed and found to be clearly negligible. The applicability of our implementation to large-scale calculations is demonstrated for a tin pincer-type system as well as low-valent tin and lead complexes. Here, the calculation of the Sn nuclear magnetic resonance shifts for the pincer-type ligand with about 2400 basis functions requires less than 1 h for hybrid density functionals. Further, the impact of spin-orbit coupling on the nucleus-independent chemical shifts and the corresponding ring currents of all-metal aromatic systems is studied.

31P Nuclear Magnetic Resonance Spectroscopy as a Probe of Thorium-Phosphorus Bond Covalency: Correlating Phosphorus Chemical Shift to Metal-Phosphorus Bond Order

We report the use of solution and solid-state 31P Nuclear Magnetic Resonance (NMR) spectroscopy combined with Density Functional Theory calculations to benchmark the covalency of actinide-phosphorus bonds, thus introducing 31P NMR spectroscopy to the investigation of molecular f-element chemical bond covalency. The 31P NMR data for [Th(PH2)(TrenTIPS)] (1, TrenTIPS = {N(CH2CH2NSiPri3)3}3-), [Th(PH)(TrenTIPS)][Na(12C4)2] (2, 12C4 = 12-crown-4 ether), [{Th(TrenTIPS)}2(μ-PH)] (3), and [{Th(TrenTIPS)}2(μ-P)][Na(12C4)2] (4) demonstrate a chemical shift anisotropy (CSA) ordering of (μ-P)3- > (═PH)2- > (μ-PH)2- > (-PH2)1- and for 4 the largest CSA for any bridging phosphido unit. The B3LYP functional with 50% Hartree-Fock mixing produced spin-orbit δiso values that closely match the experimental data, providing experimentally benchmarked quantification of the nature and extent of covalency in the Th-P linkages in 1-4 via Natural Bond Orbital and Natural Localized Molecular Orbital analyses. Shielding analysis revealed that the 31P δiso values are essentially only due to the nature of the Th-P bonds in 1-4, with largely invariant diamagnetic but variable paramagnetic and spin-orbit shieldings that reflect the Th-P bond multiplicities and s-orbital mediated transmission of spin-orbit effects from Th to P. This study has permitted correlation of Th-P δiso values to Mayer bond orders, revealing qualitative correlations generally, but which should be examined with respect to specific ancillary ligand families rather than generally to be quantitative, reflecting that 31P δiso values are a very sensitive reporter due to phosphorus being a soft donor that responds to the rest of the ligand field much more than stronger, harder donors like nitrogen.

Dimerization and ring-opening in bis(diisopropylamino)cyclopropenylidene (BAC) mediated by [U(NR2)3(CCPh)] (R = SiMe3)

Addition of 2 equiv. of bis(diisopropylamino)cyclopropenylidene (BAC) to [U(NR2)3(CCPh)] (1, R = SiMe3), in Et2O, results in formation of [cyclo-N(iPr)C(Me)2CH(NiPr2)C{CHC3(NiPr2)2}][U(NR2)2(N(SiMe3)SiMe2CH2)(CCPh)] (2) in moderate isolated yield. Complex 2 is the result of coupling and protonation of two BAC molecules, where complex 1 contributes the required proton. It was characterized by NMR spectroscopy and X-ray crystallography and represents a new mode of reactivity of the cyclopropenylidene fragment.

Evaluating f-Orbital Participation in the UV═E Multiple Bonds of [U(E)(NR2)3] (E = O, NSiMe3, NAd; R = SiMe3)

The reaction of 1 equiv of 1-azidoadamantane with [U^III(NR₂)₃] (R = SiMe₃) in Et₂O results in the formation of [U^V(NR₂)₃(NAd)] (1, Ad = 1-adamantyl) in good yields. The electronic structure of 1, as well as those of the related U(V) complexes, [U^V(NR₂)₃(NSiMe₃)] (2) and [U^V(NR₂)₃(O)] (3), were analyzed with EPR spectroscopy, SQUID magnetometry, NIR-visible spectroscopy, and crystal field modeling. This analysis revealed that, within this series of complexes, the steric bulk of the E^2- (E═O, NR) ligand is the most important factor in determining the electronic structure. In particular, the increasing steric bulk of this ligand, on moving from O^2- to [NAd]^2-, results in increasing U═E distances and E-U-N_amide angles. These changes have two principal effects on the resulting electronic structure: (1) the increasing U═E distances decreases the energy of the f_σ orbital, which is primarily σ* with respect to the U═E bond, and (2) the increasing E-U-N_amide angles increases the energy of f_δ, due to increasing antibonding interactions with the amide ligands. As a result of the latter change, the electronic ground state for complexes 1 and 2 is primarily f_φ in character, whereas the ground state for complex 3 is primarily f_δ.

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.

ThC2@C82 versus Th@C84: unexpected formation of triangular thorium carbide cluster inside fullerenes

Synthesis of the first thorium-containing clusterfullerenes, ThC₂@C _s (6)-C₈₂ and ThC₂@C₂(5)-C₈₂, is reported. These two novel actinide fullerene compounds were characterized by mass spectrometry, single-crystal X-ray diffraction crystallography, UV-vis-NIR spectroscopy, and theoretical calculations. Crystallographic studies reveal that the encapsulated ThC₂ clusters in both C _s (6)-C₈₂ and C₂(5)-C₈₂ feature a novel bonding structure with one thorium metal center connected by a C[triple bond, length as m-dash]C unit, forming an isosceles triangular configuration, which has not been hitherto observed for endohedral fullerenes or for solid phase thorium carbides. Electronic structure calculations assign a formal electronic structure of [Th⁴⁺(C₂)^2-]²⁺@[C₈₂]^2-, with pronounced donation bonding from (C₂)^2- to Th⁴⁺, secondary backbonding from the fullerene to thorium and Th-C double bond character in both compounds. This work presents a new family of endohedral fullerenes, MC₂@C_2n-2, being unexpected isomers of MC_2n , and provides broader understanding of thorium bonding.

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)]...