13C NMR Shifts as an Indicator of U-C Bond Covalency in Uranium(VI) Acetylide Complexes: An Experimental and Computational Study

Determination of Uranium Central-Field Covalency with 3d4f Resonant Inelastic X-ray Scattering

Understanding the nature of metal-ligand bonding is a major challenge in actinide chemistry. We present a new experimental strategy for addressing this challenge using actinide 3d4f resonant inelastic X-ray scattering (RIXS). Through a systematic study of uranium(IV) halide complexes, [UX6]2-, where X = F, Cl, or Br, we identify RIXS spectral satellites with relative energies and intensities that relate to the extent of uranium-ligand bond covalency. By analyzing the spectra in combination with ligand field density functional theory we find that the sensitivity of the satellites to the nature of metal-ligand bonding is due to the reduction of 5f interelectron repulsion and 4f-5f spin-exchange, caused by metal-ligand orbital mixing and the degree of 5f radial expansion, known as central-field covalency. Thus, this study furthers electronic structure quantification that can be obtained from 3d4f RIXS, demonstrating it as a technique for estimating actinide-ligand covalency.

Tris-Silanide f-Block Complexes: Insights into Paramagnetic Influence on NMR Chemical Shifts

The paramagnetism of f-block ions has been exploited in chiral shift reagents and magnetic resonance imaging, but these applications tend to focus on 1H NMR shifts as paramagnetic broadening makes less sensitive nuclei more difficult to study. Here we report a solution and solid-state (ss) 29Si NMR study of an isostructural series of locally D 3h -symmetric early f-block metal(III) tris-hypersilanide complexes, [M{Si(SiMe3)3}3(THF)2] (1-M; M = La, Ce, Pr, Nd, U); 1-M were also characterized by single crystal and powder X-ray diffraction, EPR, ATR-IR, and UV-vis-NIR spectroscopies, SQUID magnetometry, and elemental analysis. Only one SiMe3 signal was observed in the 29Si ssNMR spectra of 1-M, while two SiMe3 signals were seen in solution 29Si NMR spectra of 1-La and 1-Ce. This is attributed to dynamic averaging of the SiMe3 groups in 1-M in the solid state due to free rotation of the M-Si bonds and dissociation of THF from 1-M in solution to give the locally C 3v -symmetric complexes [M{Si(SiMe3)3}3(THF) n ] (n = 0 or 1), which show restricted rotation of M-Si bonds on the NMR time scale. Density functional theory and complete active space self-consistent field spin-orbit calculations were performed on 1-M and desolvated solution species to model paramagnetic NMR shifts. We find excellent agreement of experimental 29Si NMR data for diamagnetic 1-La, suggesting n = 1 in solution and reasonable agreement of calculated paramagnetic shifts of SiMe3 groups for 1-M (M = Pr and Nd); the NMR shifts for metal-bound 29Si nuclei could only be reproduced for diamagnetic 1-La, showing the current limitations of pNMR calculations for larger nuclei.

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.

Tetravalent Cerium Alkyl and Benzyl Complexes

High-valent cerium complexes of alkyl and benzyl ligands are unprecedented due to the incompatibility of the typically highly oxidizing Ce4+ ion and the reducing alkyl or benzyl ligand. Herein we report the synthesis and isolation of the first tetravalent cerium alkyl and benzyl complexes supported by the tri-tert-butyl imidophosphorane ligand, [NP(tBu)3]1-. The Ce4+ monoiodide complex, [Ce4+I(NP(tert-butyl)3)3] (1-CeI), serves as a precursor to the alkyl and benzyl complexes, [Ce4+(Npt)(NP(tert-butyl)3)3] (2-CeNpt) (Npt = neopentyl, CH2C(CH3)3) and [Ce4+(Bn)(NP(tert-butyl)3)3] (2-CeBn) (Bn = benzyl, CH2Ph). The bonding and structure of these complexes are characterized by single-crystal XRD, NMR and UV-vis-NIR spectroscopy, cyclic voltammetry, and DFT studies.

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.

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.

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.

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

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

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.

Synthesis, Covalency Sequence, and Crystal Features of Pentagonal Uranyl Acylpyrazolone Complexes along with DFT Calculation and Hirshfeld Analysis

Three uranyl acylpyrazolone complexes [UO₂(PCBPMP)₂(CH₃CH₂OH)] (complex I), [UO₂(PCBMCPMP)₂(CH₃CH₂OH)] (complex II), and [UO₂(PCBPTMP)₂(CH₃CH₂OH)] (complex III) were synthesized from σ-donating acypyrazolone ligands to analyze their sequence of covalent characteristics, reactivity, and redox properties (PCBPMP: p-chlorobenzoyl 1-phenyl 3-methyl 5-pyrazolone; PCBMCPMP: p-chlorobenzoyl 1-(m-chlorophenyl) 3-methyl 5-pyrazolone; PCBPTMP: p-chlorobenzoyl 1-(p-tolyl) 3-methyl 5-pyrazolone). An examination of the structure, pentagonal bipyramidal geometry, and composition of these complexes was conducted mainly through their single-crystal X-ray diffraction (XRD) data, ¹H nuclear magnetic resonance (NMR) δ-values, plots of thermogravimetric-differential thermal analysis (TG-DTA), significant Fourier transform infrared (FTIR) vibrations, gravimetric estimation, and molar conductivity values. The covalency order was found to be complex II > III > I, which mainly depends on values of stretching frequencies, average bond lengths of axial uranyl bonds, values of average bond lengths on the pentagonal equatorial plane, solvent coordination on the fifth site of a pentagonal plane, and the type of aryl group on the nitrogen of the pyrazolone ring. This was confirmed by FTIR spectroscopy and single-crystal spectral characterization. To verify experimental results by comparison with theoretical results, density functional theory (DFT) calculations were carried out, which further gives evidence for the covalency order through theoretical frequencies and the gap of highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energies. Theoretical bond properties were also examined by the identification of global index parameters. Intermolecular noncovalent surface interactions were studied by the Hirshfeld surface analysis. The irreversible redox behavior of uranyl species was identified through electrochemical cyclic voltammetry-differential pulse voltammetry (CV-DPV) plot analysis.

Covalency in AnCl2 (An = Th-No)

A potential connection has previously been proposed between the emergence of unexpected covalent behaviour in various transcurium complexes and the increasing stability of the +2 oxidation state in the later members of the actinide series. We recently used computational methods to study AnCl₃, finding evidence for energy degeneracy driven covalency in the later actinides, and here present a comparative study of AnCl₂. The An-Cl bond lengths of the latter divide into two data sets; Th-Np, Cm, Bk and Pu, Am, Cf-No. On average the An-Cl bond length decreases for both sets but, with significant increases between Np and Pu, and between Bk and Cf, unlike the former group (Pu, Am, Cf-No)Cl₂ have significantly larger lengths than the corresponding trichlorides. Using a range of Natural Bond Orbital (NBO), Natural Resonance Theory (NRT) and Quantum Theory of Atoms In Molecules (QTAIM) metrics, the covalency of the dichloride bonds is analysed. We find that the first group of dichlorides are similar to their trichloride counterparts and possess significantly more covalent bonds than (Pu, Am, Cf-No)Cl₂. We believe this change in covalent behaviour across the series for the dichlorides is due to a decreased involvement of the 6d orbital in the later elements (as a result of the f-d excitation energy exceeding the d-stabilisation energy of the actinide ions in question). Moreover, we find that unlike the trichlorides, where the QTAIM delocalisation index indicates that covalency plateaus/moderately increases, An-Cl covalency decreases across the second half of the series for AnCl₂. We attribute this difference in behaviour to a lack of significant energy degeneracy driven covalency for the dichlorides, with the energy difference between the dichlorides' β 5f and 3p Natural Atomic Orbitals being larger than for the trichlorides. Hence we find it is not the presence of a stable +2 oxidation state, but instead the extent of energy matching between the actinide 5f orbitals and the ligand 3p, that drives covalency in the transcurium chlorides.

Electrochemical Studies of the Cycloaddition Activity of Bismuth(III) Acetylides Towards Organic Azides Under Copper(I)-Catalyzed Conditions

Time-dependent monitoring of the reactive intermediates provides valuable information about the mechanism of a synthetic transformation. However, the process frequently involves intermediates with short lifetimes that significantly challenge the accessibility of the desired kinetic data. We report in situ cyclic voltammetry (CV) and nuclear magnetic resonance (NMR) spectroscopy studies of the cycloaddition reaction of organobismuth(III) compounds with organic azides under the copper(I)-catalyzed conditions. A series of bismuth(III) acetylides carrying diphenyl sulfone scaffolds have been synthesized to study the underlying electronic and steric effects of the tethered moieties capable of transannular oxygen O···Bi interactions and para-functionality of the parent phenylacetylene backbones. While belonging to the family of copper-catalyzed azide-alkyne cycloaddition reactions, the reaction yielding 5-bismuth(III)-triazolide is the sole example of a complex catalytic transformation that features activity of bismuth(III) acetylides towards organic azides under copper(I)-catalyzed conditions. Stepwise continuous monitoring of the copper(I)/copper(0) redox activity of the copper(I) catalyst by cyclic voltammetry provided novel insights into the complex catalytic cycle of the bismuth(III)-triazolide formation. From CV-derived kinetic data, reaction rate parameters of the bismuth(III) acetylides coordination to the copper(I) catalyst (K_A) and equilibrium concentration of the copper species [cat]_eq. are compared with the overall 5-bismuth(III)-triazolide formation rate constant k_obs obtained by ¹H-NMR kinetic analysis.

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)]...

Experimental and Computational Study of a Tetraazamacrocycle Bis(aryloxide) Uranyl Complex and of the Analogues {E═U═NR}2+ (E = O and NR)

The reaction of [U(κ⁶-{(^t-Bu2ArO)₂Me₂-cyclam})I][I] (H₂{(^t-Bu2ArO)₂Me₂-cyclam} = 1,8-bis(2-hydroxy-3,5-di-tert-butyl)-4,11-dimethyl-1,4,8,11-tetraazacyclotetradecane) with 2 equiv of NaNO₂ in acetonitrile results in the isolation of the uranyl complex [UO₂{(^t-Bu2ArO)₂Me₂-cyclam}] (3) in 31% yield, which was fully characterized, including by single-crystal X-ray diffraction. Density functional theory (DFT) computations were performed to evaluate and compare the level of covalency within the U═E bonds in 3 and in the analogous trans-bis(imido) [U(κ⁴-{(^t-Bu2ArO)₂Me₂-cyclam})(NPh)₂] (1) and trans-oxido-imido [U(κ⁴-{(^t-Bu2ArO)₂Me₂-cyclam})(O)(NPh)] (2) complexes. Natural bond orbital (NBO) analysis allowed us to determine the mixing covalency parameter λ, showing that in 2, where both U-O_oxido and U-N_imido bonds are present, the U-N_imido bond registers more covalency with regard to 1, and the opposite is seen for U-O_oxido with respect to 3. However, the covalency driven by orbital overlap in the U-N_imido bond is slightly higher in 1 than in 2. The ¹⁵N-labeled complexes [U(κ⁴-{(^t-Bu2ArO)₂Me₂-cyclam})(¹⁵NPh)₂] (1-¹⁵N) and [U(κ⁴-{(^t-Bu2ArO)₂Me₂-cyclam})(O)(¹⁵NPh)] (2-¹⁵N) were prepared and analyzed by solution ¹⁵N NMR spectroscopy. The calculated and experimental ¹⁵N chemical shifts are in good agreement, displaying the same trend of δ_N (1-¹⁵N) > δ_N (2-¹⁵N) and reveal that the ¹⁵N chemical shift may serve as a probe for the covalency of the U═NR bond.

Covalency of Trivalent Actinide Ions with Different Donor Ligands: Do Density Functional and Multiconfigurational Wavefunction Calculations Corroborate the Observed "Breaks"?

A comprehensive ab initio study of periodic actinide-ligand bonding trends for trivalent actinides is performed. Relativistic density functional theory (DFT) and complete active-space (CAS) self-consistent field wavefunction calculations are used to dissect the chemical bonding in the [AnCl₆]^3-, [An(CN)₆]^3-, [An(NCS)₆]^3-, [An(S₂PMe₂)₃], [An(DPA)₃]^3-, and [An(HOPO)]^- series of actinide (An = U-Es) complexes. Except for some differences for the early actinide complexes with DPA, bond orders and excess 5f-shell populations from donation bonding show qualitatively similar trends in 5f ⁿ active-space CAS vs DFT calculations. The influence of spin-orbit coupling on donation bonding is small for the tested systems. Along the actinide series, chemically soft vs chemically harder ligands exhibit clear differences in bonding trends. There are pronounced changes in the 5f populations when moving from Pu to Am or Cm, which correlate with previously noted "breaks" in chemical trends. Bonding involving 5f becomes very weak beyond Cm/Bk. We propose that Cm(III) is a borderline case among the trivalent actinides that can be meaningfully considered to be involved in ground-state 5f covalent bonding.

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.

Synthesis of Parent Acetylide and Dicarbide Complexes of Thorium and Uranium and an Examination of Their Electronic Structures

The reaction of [AnCl(NR₂)₃] (An = U or Th; R = SiMe₃) with NaCCH and tetramethylethylenediamine (TMEDA) results in the formation of [An(C≡CH)(NR₂)₃] (1, An = U; 2, An = Th), which can be isolated in good yields after workup. Similarly, the reaction of 3 equiv of NaCCH and TMEDA with [AnCl(NR₂)₃] results in the formation of [Na(TMEDA)][An(C≡CH)₂(NR₂)₃] (4, An = U; 5, An = Th), which can be isolated in fair yields after workup. The reaction of 1 with 2 equiv of KC₈ and 1 equiv of 2.2.2-cryptand in tetrahydrofuran results in formation of the uranium(III) acetylide complex [K(2.2.2-cryptand)][U(C≡CH)(NR₂)₃] (3). Thermolysis of 1 or 2 results in formation of the bimetallic dicarbide complexes [{An(NR₂)₃}₂(μ,η¹:η¹-C₂)] (6, An = U; 7, An = Th), whereas the reaction of 1 with [Th{N(R)(SiMe₂CH₂)}(NR₂)₂] results in the formation of [U(NR₂)₃(μ,η¹:η¹-C₂)Th(NR₂)₃] (8). The ¹³C NMR chemical shifts of the α-acetylide carbon atoms in 2, 5, and 7 exhibit a characteristic spin-orbit-induced downfield shift, due to participation of the 5f orbitals in the Th-C bonds. Magnetism measurements demonstrate that 6 displays weak ferromagnetic coupling between the uranium(IV) centers (J = 1.78 cm^-1).

Structural, Spectroscopic, and Computational Analysis of Heterometallic Thorium Phosphinidiide Complexes

To synthesize complexes with thorium-phosphorus multiple-bond character, reactions of (C₅Me₅)₂Th[P(H)Mes]₂ with monovalent alkali-metal bases, MN(SiMe₃)₂, as well as CuMes, have been investigated. The results with MN(SiMe₃)₂ are phosphinidiide complexes of the form {(C₅Me₅)₂Th[μ₂-P(Mes)][μ₂-P(H)Mes]M(L)_n}₂ (M = Na, n = 0; M = K, L = THF, n = 1; M = Rb, L = THF, n = 1; M = Cs, L = Et₂O, n = 1). With CuMes, the product is a Th₂Cu₃P₅ heterometallic structure, {(C₅Me₅)₂Th[(μ₂-P(H)Mes)P(Mes)]Cu}₂Cu[μ₂-P(H)Mes]. All complexes have been characterized using heteronuclear NMR and IR spectroscopy, density functional theory calculations, and their solid-state structure identified by X-ray crystallography. We also report the structure of {(C₅Me₅)₂Th[(μ₂-As(H)Mes)As(Mes)]Cu}₂Cu[μ₂-As(H)Mes] obtained from (C₅Me₅)₂Th[As(H)Mes]₂ with CuMes.

Exceptional uranium(VI)-nitride triple bond covalency from 15N nuclear magnetic resonance spectroscopy and quantum chemical analysis

Determining the nature and extent of covalency of early actinide chemical bonding is a fundamentally important challenge. Recently, X-ray absorption, electron paramagnetic, and nuclear magnetic resonance spectroscopic studies have probed actinide-ligand covalency, largely confirming the paradigm of early actinide bonding varying from ionic to polarised-covalent, with this range sitting on the continuum between ionic lanthanide and more covalent d transition metal analogues. Here, we report measurement of the covalency of a terminal uranium(VI)-nitride by 15N nuclear magnetic resonance spectroscopy, and find an exceptional nitride chemical shift and chemical shift anisotropy. This redefines the 15N nuclear magnetic resonance spectroscopy parameter space, and experimentally confirms a prior computational prediction that the uranium(VI)-nitride triple bond is not only highly covalent, but, more so than d transition metal analogues. These results enable construction of general, predictive metal-ligand 15N chemical shift-bond order correlations, and reframe our understanding of actinide chemical bonding to guide future studies.

Homoleptic Perchlorophenyl "Ate" Complexes of Thorium(IV) and Uranium(IV)

The reaction of AnCl₄(DME)_n (An = Th, n = 2; U, n = 0) with 5 equiv of LiC₆Cl₅ in Et₂O resulted in the formation of homoleptic actinide-aryl "ate" complexes [Li(DME)₂(Et₂O)]₂[Li(DME)₂][Th(C₆Cl₅)₅]₃ ([Li][1]) and [Li(Et₂O)₄][U(C₆Cl₅)₅] ([Li][2]). Similarly, the reaction of AnCl₄(DME)_n (An = Th, n = 2; U, n = 0) with 3 equiv of LiC₆Cl₅ in Et₂O resulted in the formation of heteroleptic actinide-aryl "ate" complexes [Li(DME)₂(Et₂O)][Li(Et₂O)₂][ThCl₃(C₆Cl₅)₃] ([Li][3]) and [Li(Et₂O)₃][UCl₂(C₆Cl₅)₃] ([Li][4]). Density functional calculations show that the An-C_ipso σ-bonds are considerably more covalent for the uranium complexes vs the thorium analogues, in line with past results. Additionally, good agreement between experiment and calculations is obtained for the ¹³C_ipso NMR chemical shifts in [Li][1] and [Li][3]. The calculations demonstrate a deshielding by ca. 29 ppm from spin-orbit coupling effects originating at Th, which is a direct consequence of 5f orbital participation in the Th-C bonds.

29Si NMR Spectroscopy as a Probe of s- and f-Block Metal(II)-Silanide Bond Covalency

We report the use of ²⁹Si NMR spectroscopy and DFT calculations combined to benchmark the covalency in the chemical bonding of s- and f-block metal-silicon bonds. The complexes [M(Si^tBu₃)₂(THF)₂(THF)_x] (1-M: M = Mg, Ca, Yb, x = 0; M = Sm, Eu, x = 1) and [M(Si^tBu₂Me)₂(THF)₂(THF)_x] (2-M: M = Mg, x = 0; M = Ca, Sm, Eu, Yb, x = 1) have been synthesized and characterized. DFT calculations and ²⁹Si NMR spectroscopic analyses of 1-M and 2-M (M = Mg, Ca, Yb, No, the last in silico due to experimental unavailability) together with known {Si(SiMe₃)₃}^--, {Si(SiMe₂H)₃}^--, and {SiPh₃}^--substituted analogues provide 20 representative examples spanning five silanide ligands and four divalent metals, revealing that the metal-bound ²⁹Si NMR isotropic chemical shifts, δ_Si, span a wide (∼225 ppm) range when the metal is kept constant, and direct, linear correlations are found between δ_Si and computed delocalization indices and quantum chemical topology interatomic exchange-correlation energies that are measures of bond covalency. The calculations reveal dominant s- and d-orbital character in the bonding of these silanide complexes, with no significant f-orbital contributions. The δ_Si is determined, relatively, by paramagnetic shielding for a given metal when the silanide is varied but by the spin-orbit shielding term when the metal is varied for a given ligand. The calculations suggest a covalency ordering of No(II) > Yb(II) > Ca(II) ≈ Mg(II), challenging the traditional view of late actinide chemical bonding being equivalent to that of the late lanthanides.

Synthesis and Characterization of Two Uranyl-Aryl "Ate" Complexes

Reaction of [UO₂ Cl₂ (THF)₃ ] with 3 equivalents of LiC₆ Cl₅ in Et₂ O resulted in the formation of first uranyl aryl complex [Li(Et₂ O)₂ (THF)][UO₂ (C₆ Cl₅ )₃ ] ([Li][1]) in good yields. Subsequent dissolution of [Li][1] in THF resulted in conversion into [Li(THF)₄ ][UO₂ (C₆ Cl₅ )₃ (THF)] ([Li][2]), also in good yields. DFT calculations reveal that the U-C bonds in [Li][1] and [Li][2] exhibit appreciable covalency. Additionally, the ¹³ C NMR chemical shifts for their C_ipso environments are strongly affected by spin-orbit coupling-a consequence of 5f orbital participation in the U-C bonds.

Isolation and characterization of a covalent CeIV-Aryl complex with an anomalous 13C chemical shift

The synthesis of bona fide organometallic CeIV complexes is a formidable challenge given the typically oxidizing properties of the CeIV cation and reducing tendencies of carbanions. Herein, we report a pair of compounds comprising a CeIV - Caryl bond [Li(THF)4][CeIV(κ2-ortho-oxa)(MBP)2] (3-THF) and [Li(DME)3][CeIV(κ2-ortho-oxa)(MBP)2] (3-DME), ortho-oxa = dihydro-dimethyl-2-[4-(trifluoromethyl)phenyl]-oxazolide, MBP2- = 2,2'-methylenebis(6-tert-butyl-4-methylphenolate), which exhibit CeIV - Caryl bond lengths of 2.571(7) - 2.5806(19) Å and strongly-deshielded, CeIV - Cipso 13C{1H} NMR resonances at 255.6 ppm. Computational analyses reveal the Ce contribution to the CeIV - Caryl bond of 3-THF is ~12%, indicating appreciable metal-ligand covalency. Computations also reproduce the characteristic 13C{1H} resonance, and show a strong influence from spin-orbit coupling (SOC) effects on the chemical shift. The results demonstrate that SOC-driven deshielding is present for CeIV - Cipso 13C{1H} resonances and not just for diamagnetic actinide compounds.

Inverse halogen dependence in anion 13C NMR

Halogens cause pronounced and systematic effects on the 13C NMR chemical shift (δ13C) of an adjacent carbon nucleus, usually leading to a decrease in the values across the halogen series. Although this normal halogen dependence (NHD) is known in organic and inorganic compounds containing the carbon atom in its neutral and cationic forms, information about carbanions is scarce. To understand how δ13C changes in molecules with different charges, the shielding mechanisms of CHX3, CX3+, and CX3- (X = Cl, Br, or I) systems are investigated via density functional theory calculations and further analyzed by decomposition into contributions of natural localized molecular orbitals. An inverse halogen dependence (IHD) is determined for the anion series as a result of the negative spin-orbit contribution instead of scalar paramagnetic effects. The presence of a carbon nonbonding orbital in anions allows magnetic couplings that generate a deshielding effect on the nucleus and contradicts the classical association between δ13C and atomic charge.

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

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

Use of 15N NMR spectroscopy to probe covalency in a thorium nitride

The first isolable molecular thorium nitride, [(NR₂)₃Th(μ-N)Th(NR₂)₃]^–, was synthesized by reaction of [Th{N(R)(SiMe₂)CH₂}(NR₂)₂] with NaNH₂ and characterized by X-ray crystallography, ¹⁵N NMR spectroscopy, and DFT calculations.

Reaction of the thorium metallacycle, [Th{N(R)(SiMe₂)CH₂}(NR₂)₂] (R = SiMe₃) with 1 equiv. of NaNH₂ in THF, in the presence of 18-crown-6, results in formation of the bridged thorium nitride complex, [Na(18-crown-6)(Et₂O)][(R₂N)₃Th(μ-N)(Th(NR₂)₃] ([Na][1]), which can be isolated in 66% yield after work-up. Complex [Na][1] is the first isolable molecular thorium nitride complex. Mechanistic studies suggest that the first step of the reaction is deprotonation of [Th{N(R)(SiMe₂)CH₂}(NR₂)₂] by NaNH₂, which results in formation of the thorium bis(metallacycle) complex, [Na(THF)_x][Th{N(R)(SiMe₂CH₂)}₂(NR₂)], and NH₃. NH₃ then reacts with unreacted [Th{N(R)(SiMe₂)CH₂}(NR₂)₂], forming [Th(NR₂)₃(NH₂)] (2), which protonates [Na(THF)_x][Th{N(R)(SiMe₂CH₂)}₂(NR₂)] to give [Na][1]. Consistent with hypothesis, addition of excess NH₃ to a THF solution of [Th{N(R)(SiMe₂)CH₂}(NR₂)₂] results in formation of [Th(NR₂)₃(NH₂)] (2), which can be isolated in 51% yield after work-up. Furthermore, reaction of [K(DME)][Th{N(R)(SiMe₂CH₂)}₂(NR₂)] with 2, in THF-d₈, results in clean formation of [K][1], according to ¹H NMR spectroscopy. The electronic structures of [1]^– and 2 were investigated by ¹⁵N NMR spectroscopy and DFT calculations. This analysis reveals that the Th–N_nitride bond in [1]^– features more covalency and a greater degree of bond multiplicity than the Th–NH₂ bond in 2. Similarly, our analysis indicates a greater degree of covalency in [1]^–vs. comparable thorium imido and oxo complexes.