Synthesis of a uranium(VI)-carbene: reductive formation of uranyl(V)-methanides, oxidative preparation of a [R2C═U═O]2+ analogue of the [O═U═O]2+ uranyl ion (R = Ph2PNSiMe3), and comparison of the nature of U(IV)═C, U(V)═C, and U(VI)═C double bonds

Understanding the origins of Oyl–U–Oylbending in the uranyl (UO22+) ion

Although rare, O_yl–U–O_ylbending in the uranyl (UO₂²⁺) ion can be effected by either steric perturbation or electronic perturbation.

Rare-Earth- and Uranium-Mesoionic Carbenes: A New Class of f-Block Carbene Complex Derived from an N-Heterocyclic Olefin

Neutral mesoionic carbenes (MICs) have emerged as an important class of carbene, however they are found in the free form or ligated to only a few d-block ions. Unprecedented f-block MIC complexes [M(N'')3 {CN(Me)C(Me)N(Me)CH}] (M=U, Y, La, Nd; N''=N(SiMe3 )2 ) are reported. These complexes were prepared by a formal 1,4-proton migration reaction when the metal triamides [M(N'')3 ] were treated with the N-heterocyclic olefin H2 C=C(NMeCH)2 , which constitutes a new, general way to prepare MIC complexes. Quantum chemical calculations on the 5f3 uranium(III) complex suggest the presence of a U=C donor-acceptor bond, composed of a MIC→U σ-component and a U(5f)→MIC(2p) π-back-bond, but for the d0 f0 Y and La and 4f3 Nd congeners only MIC→M σ-bonding is found. Considering the generally negligible π-acidity of MICs, this is surprising and highlights that greater consideration should possibly be given to recognizing MICs as potential π-acid ligands when coordinated to strongly reducing metals.

Crystalline Diuranium Phosphinidiide and μ-Phosphido Complexes with Symmetric and Asymmetric UPU Cores

Reaction of [U(TrenTIPS )(PH2 )] (1, TrenTIPS =N(CH2 CH2 NSiPri3 )3 ) with C6 H5 CH2 K and [U(TrenTIPS )(THF)][BPh4 ] (2) afforded a rare diuranium parent phosphinidiide complex [{U(TrenTIPS )}2 (μ-PH)] (3). Treatment of 3 with C6 H5 CH2 K and two equivalents of benzo-15-crown-5 ether (B15C5) gave the diuranium μ-phosphido complex [{U(TrenTIPS )}2 (μ-P)][K(B15C5)2 ] (4). Alternatively, reaction of [U(TrenTIPS )(PH)][Na(12C4)2 ] (5, 12C4=12-crown-4 ether) with [U{N(CH2 CH2 NSiMe2 But )2 CH2 CH2 NSi(Me)(CH2 )(But )}] (6) produced the diuranium μ-phosphido complex [{U(TrenTIPS )}(μ-P){U(TrenDMBS )}][Na(12C4)2 ] [7, TrenDMBS =N(CH2 CH2 NSiMe2 But )3 ]. Compounds 4 and 7 are unprecedented examples of uranium phosphido complexes outside of matrix isolation studies, and they rapidly decompose in solution underscoring the paucity of uranium phosphido complexes. Interestingly, 4 and 7 feature symmetric and asymmetric UPU cores, respectively, reflecting their differing steric profiles.

Terminal Parent Phosphanide and Phosphinidene Complexes of Zirconium(IV)

The reaction of [Zr(TrenDMBS )(Cl)] [Zr1; TrenDMBS =N(CH2 CH2 NSiMe2 But )3 ] with NaPH2 gave the terminal parent phosphanide complex [Zr(TrenDMBS )(PH2 )] [Zr2; Zr-P=2.690(2) Å]. Treatment of Zr2 with one equivalent of KCH2 C6 H5 and two equivalents of benzo-15-crown-5 ether (B15C5) afforded an unprecedented example (outside of matrix isolation) of a structurally authenticated transition-metal terminal parent phosphinidene complex [Zr(TrenDMBS )(PH)][K(B15C5)2 ] [Zr3; Zr=P=2.472(2) Å]. DFT calculations reveal a polarized-covalent Zr=P double bond, with a Mayer bond order of 1.48, and together with IR spectroscopic data also suggest an agostic-type Zr⋅⋅⋅HP interaction [∡ZrPH =66.7°] which is unexpectedly similar to that found in cryogenic, spectroscopically observed phosphinidene species. Surprisingly, computational data suggest that the Zr=P linkage is similarly polarized, and thus as covalent, as essentially isostructural U=P and Th=P analogues.

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.

Terminal Uranium(V/VI) Nitride Activation of Carbon Dioxide and Carbon Disulfide: Factors Governing Diverse and Well-Defined Cleavage and Redox Reactions

The reactivity of terminal uranium(V/VI) nitrides with CE₂ (E=O, S) is presented. Well-defined C=E cleavage followed by zero-, one-, and two-electron redox events is observed. The uranium(V) nitride [U(Tren^TIPS )(N)][K(B15C5)₂ ] (1, Tren^TIPS =N(CH₂ CH₂ NSiiPr₃ )₃ ; B15C5=benzo-15-crown-5) reacts with CO₂ to give [U(Tren^TIPS )(O)(NCO)][K(B15C5)₂ ] (3), whereas the uranium(VI) nitride [U(Tren^TIPS )(N)] (2) reacts with CO₂ to give isolable [U(Tren^TIPS )(O)(NCO)] (4); complex 4 rapidly decomposes to known [U(Tren^TIPS )(O)] (5) with concomitant formation of N₂ and CO proposed, with the latter trapped as a vanadocene adduct. In contrast, 1 reacts with CS₂ to give [U(Tren^TIPS )(κ² -CS₃ )][K(B15C5)₂ ] (6), 2, and [K(B15C5)₂ ][NCS] (7), whereas 2 reacts with CS₂ to give [U(Tren^TIPS )(NCS)] (8) and "S", with the latter trapped as Ph₃ PS. Calculated reaction profiles reveal outer-sphere reactivity for uranium(V) but inner-sphere mechanisms for uranium(VI); despite the wide divergence of products the initial activation of CE₂ follows mechanistically related pathways, providing insight into the factors of uranium oxidation state, chalcogen, and NCE groups that govern the subsequent divergent redox reactions that include common one-electron reactions and a less-common two-electron redox event. Caution, we suggest, is warranted when utilising CS₂ as a reactivity surrogate for CO₂ .

Thorium-phosphorus triamidoamine complexes containing Th-P single- and multiple-bond interactions

Despite the burgeoning field of uranium-ligand multiple bonds, analogous complexes involving other actinides remain scarce. For thorium, under ambient conditions only a few multiple bonds to carbon, nitrogen, oxygen, sulfur, selenium and tellurium are reported, and no multiple bonds to phosphorus are known, reflecting a general paucity of synthetic methodologies and also problems associated with stabilising these linkages at the large thorium ion. Here we report structurally authenticated examples of a parent thorium(IV)-phosphanide (Th-PH₂), a terminal thorium(IV)-phosphinidene (Th=PH), a parent dithorium(IV)-phosphinidiide (Th-P(H)-Th) and a discrete actinide-phosphido complex under ambient conditions (Th=P=Th). Although thorium is traditionally considered to have dominant 6d-orbital contributions to its bonding, contrasting to majority 5f-orbital character for uranium, computational analyses suggests that the bonding of thorium can be more nuanced, in terms of 5f- versus 6d-orbital composition and also significant involvement of the 7s-orbital and how this affects the balance of 5f- versus 6d-orbital bonding character.

Emergence of comparable covalency in isostructural cerium(iv)- and uranium(iv)-carbon multiple bonds

We report comparable levels of covalency in cerium- and uranium-carbon multiple bonds in the iso-structural carbene complexes [M(BIPMTMS)(ODipp)2] [M = Ce (1), U (2), Th (3); BIPMTMS = C(PPh2NSiMe3)2; Dipp = C6H3-2,6-iPr2] whereas for M = Th the M[double bond, length as m-dash]C bond interaction is much more ionic. On the basis of single crystal X-ray diffraction, NMR, IR, EPR, and XANES spectroscopies, and SQUID magnetometry complexes 1-3 are confirmed formally as bona fide metal(iv) complexes. In order to avoid the deficiencies of orbital-based theoretical analysis approaches we probed the bonding of 1-3 via analysis of RASSCF- and CASSCF-derived densities that explicitly treats the orbital energy near-degeneracy and overlap contributions to covalency. For these complexes similar levels of covalency are found for cerium(iv) and uranium(iv), whereas thorium(iv) is found to be more ionic, and this trend is independently found in all computational methods employed. The computationally determined trends in covalency of these systems of Ce ∼ U > Th are also reproduced in experimental exchange reactions of 1-3 with MCl4 salts where 1 and 2 do not exchange with ThCl4, but 3 does exchange with MCl4 (M = Ce, U) and 1 and 2 react with UCl4 and CeCl4, respectively, to establish equilibria. This study therefore provides complementary theoretical and experimental evidence that contrasts to the accepted description that generally lanthanide-ligand bonding in non-zero oxidation state complexes is overwhelmingly ionic but that of uranium is more covalent.

Giant spin–orbit effects on 1H and 13C NMR shifts for uranium(vi) complexes revisited: role of the exchange–correlation response kernel, bonding analyses, and new predictions

Visiting the previously predicted giant spin–orbit-induced ¹H and ¹³C shifts in U(vi) complexes with improved methodology retains the reported unusual shift ranges, provides better understanding of the observations and gives improved confidence in the predictions.

A monometallic lanthanide bis(methanediide) single molecule magnet with a large energy barrier and complex spin relaxation behaviour

We report a dysprosium(iii) bis(methanediide) single molecule magnet (SMM) where stabilisation of the highly magnetic states and suppression of mixing of opposite magnetic projections is imposed by a linear arrangement of negatively-charged donor atoms supported by weak neutral donors. Treatment of [Ln(BIPMTMS)(BIPMTMSH)] [Ln = Dy, 1Dy; Y, 1Y; BIPMTMS = {C(PPh2NSiMe3)2}2-; BIPMTMSH = {HC(PPh2NSiMe3)2}-] with benzyl potassium/18-crown-6 ether (18C6) in THF afforded [Ln(BIPMTMS)2][K(18C6)(THF)2] [Ln = Dy, 2Dy; Y, 2Y]. AC magnetic measurements of 2Dy in zero DC field show temperature- and frequency-dependent SMM behaviour. Orbach relaxation dominates at high temperature, but at lower temperatures a second-order Raman process dominates. Complex 2Dy exhibits two thermally activated energy barriers (U eff) of 721 and 813 K, the largest U eff values for any monometallic dysprosium(iii) complex. Dilution experiments confirm the molecular origin of this phenomenon. Complex 2Dy has rich magnetic dynamics; field-cooled (FC)/zero-field cooled (ZFC) susceptibility measurements show a clear divergence at 16 K, meaning the magnetic observables are out-of-equilibrium below this temperature, however the maximum in ZFC, which conventionally defines the blocking temperature, T B, is found at 10 K. Magnetic hysteresis is also observed in 10% 2Dy@2Y at these temperatures. Ab initio calculations suggest the lowest three Kramers doublets of the ground 6H15/2 multiplet of 2Dy are essentially pure, well-isolated |±15/2, |±13/2 and |±11/2 states quantised along the C[double bond, length as m-dash]Dy[double bond, length as m-dash]C axis. Thermal relaxation occurs via the 4th and 5th doublets, verified experimentally for the first time, and calculated U eff values of 742 and 810 K compare very well to experimental magnetism and luminescence data. This work validates a design strategy towards realising high-temperature SMMs and produces unusual spin relaxation behaviour where the magnetic observables are out-of-equilibrium some 6 K above the formal blocking temperature.

Charge control of the inverse trans-influence

The synthesis and characterization of uranium(VI) mono(imido) complexes, by the oxidation of corresponding uranium(V) species, are presented. These experimental results, paired with DFT analyses, allow for the comparison of the electronic structure of uranium(VI) mono(oxo) and mono(imido) ligands within a conserved ligand framework and demonstrate that the magnitude of the ground state stabilization derived from the inverse trans-influence (ITI) is governed by the relative charge localization on the multiply bonded atom or group.

The Renaissance of Non-Aqueous Uranium Chemistry

Prior to the year 2000, non-aqueous uranium chemistry mainly involved metallocene and classical alkyl, amide, or alkoxide compounds as well as established carbene, imido, and oxo derivatives. Since then, there has been a resurgence of the area, and dramatic developments of supporting ligands and multiply bonded ligand types, small-molecule activation, and magnetism have been reported. This Review 1) introduces the reader to some of the specialist theories of the area, 2) covers all-important starting materials, 3) surveys contemporary ligand classes installed at uranium, including alkyl, aryl, arene, carbene, amide, imide, nitride, alkoxide, aryloxide, and oxo compounds, 4) describes advances in the area of single-molecule magnetism, and 5) summarizes the coordination and activation of small molecules, including carbon monoxide, carbon dioxide, nitric oxide, dinitrogen, white phosphorus, and alkanes.

Triamidoamine uranium(IV)-arsenic complexes containing one-, two- and threefold U-As bonding interactions

To further our fundamental understanding of the nature and extent of covalency in uranium-ligand bonding, and the benefits that this may have for the design of new ligands for nuclear waste separation, there is burgeoning interest in the nature of uranium complexes with soft- and multiple-bond-donor ligands. Despite this, there have so far been no examples of structurally authenticated molecular uranium-arsenic bonds under ambient conditions. Here, we report molecular uranium(IV)-arsenic complexes featuring formal single, double and triple U-As bonding interactions. Compound formulations are supported by a range of characterization techniques, and theoretical calculations suggest the presence of polarized covalent one-, two- and threefold bonding interactions between uranium and arsenic in parent arsenide [U-AsH2], terminal arsinidene [U=AsH] and arsenido [U≡AsK2] complexes, respectively. These studies inform our understanding of the bonding of actinides with soft donor ligands and may be of use in future ligand design in this area.

Uranium triamidoamine chemistry

Triamidoamine (Tren) complexes of the p- and d-block elements have been well-studied, and they display a diverse array of chemistry of academic, industrial and biological significance. Such in-depth investigations are not as widespread for Tren complexes of uranium, despite the general drive to better understand the chemical behaviour of uranium by virtue of its fundamental position within the nuclear sector. However, the chemistry of Tren-uranium complexes is characterised by the ability to stabilise otherwise reactive, multiply bonded main group donor atom ligands, construct uranium-metal bonds, promote small molecule activation, and support single molecule magnetism, all of which exploit the steric, electronic, thermodynamic and kinetic features of the Tren ligand system. This Feature Article presents a current account of the chemistry of Tren-uranium complexes.

Reactivity of the uranium(IV) carbene complex [U(BIPM(TMS))(Cl)(μ-Cl)₂Li(THF)₂] (BIPM(TMS) = {C(PPh₂NSiMe₃)₂}) towards carbonyl and heteroallene substrates: metallo-Wittig, adduct formation, C-F bond activation, and [2 + 2]-cycloaddition reactions

The reactivity of the uranium(IV) carbene complex [U(BIPM(TMS))(Cl)(μ-Cl)2Li(THF)2] (1, BIPM(TMS) = {C(PPh2NSiMe3)2}) towards carbonyl and heteroallene substrates is reported. Reaction of 1 with benzophenone proceeds to give the metallo-Wittig terminal alkene product Ph2C=C(PPh2NSiMe3)2 (2); the likely "UOCl2" byproduct could not be isolated. Addition of the bulky ketone PhCOBu(t) to 1 resulted in loss of LiCl, coordination of the ketone, and dimerisation to give [U(BIPM(TMS))(Cl)(μ-Cl){OC(Ph)(Bu(t))}]2 (3). The reaction of 1 with coumarin resulted in ring opening of the cyclic ester and a metallo-Wittig-type reaction to afford [U{BIPM(TMS)[C(O)(CHCHC6H4O-2)]-κ(3)-N,O,O'}(Cl)2(THF)] (4) where the enolate product remains coordinated to uranium. The reaction of PhCOF with 1 resulted in C-F bond activation and oxidation resulting in isolation of [U(O)2(Cl)2(μ-Cl)2{(μ-LiDME)OC(Ph)=C(PPh2NSiMe3)(PPh2NHSiMe3)}2] (5) along with [U(Cl)2(F)2(py)4] (6). The reactions of 1 with tert-butylisocyanate or dicyclohexylcarbodiimide resulted in the isolation of the [2 + 2]-cycloaddition products [U{BIPM(TMS)[C(NBu(t)){OLi(THF)2(μ-Cl)Li(THF)3}]-κ(4)-C,N,N',N''}(Cl)3] (7) and [U{BIPM(TMS)[C(NCy)2]-κ(4)-C,N,N',N''}(Cl)(μ-Cl)2Li(THF)2] (8). Complexes 2-8 have been variously characterised by single crystal X-ray diffraction, multi-nuclear NMR and FTIR spectroscopies, Evans method solution magnetic moments, variable temperature SQUID magnetometry, and elemental analyses.

Thorium-ligand multiple bonds via reductive deprotection of a trityl group

Reaction of [Th(I)(NR₂)₃] (R = SiMe₃) (2) with KECPh₃ (E = O, S) affords the thorium chalcogenates, [Th(ECPh₃)(NR₂)₃] (3, E = O; 4, E = S), in moderate yields. Reductive deprotection of the trityl group from 3 and 4 by reaction with KC₈, in the presence of 18-crown-6, affords the thorium oxo complex, [K(18-crown-6)][Th(O)(NR₂)₃] (6), and the thorium sulphide complex, [K(18-crown-6)][Th(S)(NR₂)₃] (7), respectively. The natural bond orbital and quantum theory of atoms-in-molecules approaches are employed to explore the metal-ligand bonding in 6 and 7 and their uranium analogues, and in particular the relative roles of the actinide 5f and 6d orbitals.

U(III)-CN versus U(IV)-NC coordination in tris(silylamide) complexes

Treatment of the metallacycle [UN*2(N,C)] [N* = N(SiMe3)2; N,C = CH2SiMe2N(SiMe3)] with [HNEt3][BPh4], [HNEt3]Cl, and [pyH][OTf] (OTf = OSO2CF3) gave the cationic compound [UN*3][BPh4] (1) and the neutral complexes [UN*3X] [X = Cl (3), OTf (4)], respectively. The dinuclear complex [{UN*(μ-N,C)(μ-OTf)}2] (5) and its tetrahydrofuran (THF) adduct [{UN*(N,C)(THF)(μ-OTf)}2] (6) were obtained by thermal decomposition of 4. The successive addition of NEt4CN or KCN to 1 led to the formation of the cyanido-bridged dinuclear compound [(UN*3)2(μ-CN)][BPh4] (7) and the mononuclear mono- and bis(cyanide) complexes [UN*3(CN)] (2) and [M][UN*3(CN)2] [M = NEt4 (8), K(THF)4 (9)], while crystals of [K(18-crown-6)][UN*3(CN)2] (10) were obtained by the oxidation of [K(18-crown-6)][UN*3(CN)] with pyridine N-oxide. The THF adduct of 1, [UN*3(THF)][BPh4], and complexes 2-7, 9 and 10 were characterized by their X-ray crystal structure. In contrast to their U(III) analogues [NMe4][UN*3(CN)] and [K(18-crown-6)]2[UN*3(CN)2] in which the CN anions are coordinated to the metal center via the C atom, complexes 2 and 9 exhibit the isocyanide U-NC coordination mode of the cyanide ligand. This U(III)/U(IV) differentiation has been analyzed using density functional theory calculations. The observed preferential coordinations are well explained considering the electronic structures of the different species and metal-ligand bonding energies. A comparison of the different quantum descriptors, i.e., bond orders, NPA/QTAIM data, and energy decomposition analysis, has allowed highlighting of the subtle balance between covalent, ionic, and steric factors that govern the U-CN/NC bonding.

Achievements in uranium alkyl chemistry: celebrating sixty years of synthetic pursuits

This Perspective highlights the synthesis, characterization, and reactivity of significant organouranium complexes with σ-bonded alkyl ligands.

Si–H activation by means of metal ligand cooperation in a methandiide derived carbene complex

Si–H bond activation of a series of silanes by means of metal ligand cooperation is reported.

Instability of metal 1,3-benzodi(thiophosphinoyl)methandiide complexes: formation of hafnium, tin and zirconium complexes of 1,3-benzodi(thiophosphinoyl)thioketone dianionic ligand [1,3-C6H4(PhPS)2CS]2−

The reaction illustrates that the metal centre and ligand substituents are crucial for the stabilization of a C_methandiideHf bond.

Two-electron reductive carbonylation of terminal uranium(V) and uranium(VI) nitrides to cyanate by carbon monoxide

Two-electron reductive carbonylation of the uranium(VI) nitride [U(Tren(TIPS))(N)] (2, Tren(TIPS)=N(CH2CH2NSiiPr3)3) with CO gave the uranium(IV) cyanate [U(Tren(TIPS))(NCO)] (3). KC8 reduction of 3 resulted in cyanate dissociation to give [U(Tren(TIPS))] (4) and KNCO, or cyanate retention in [U(Tren(TIPS))(NCO)][K(B15C5)2] (5, B15C5=benzo-15-crown-5 ether) with B15C5. Complexes 5 and 4 and KNCO were also prepared from CO and the uranium(V) nitride [{U(Tren(TIPS))(N)K}2] (6), with or without B15C5, respectively. Complex 5 can be prepared directly from CO and [U(Tren(TIPS))(N)][K(B15C5)2] (7). Notably, 7 reacts with CO much faster than 2. This unprecedented f-block reactivity was modeled theoretically, revealing nucleophilic attack of the π* orbital of CO by the nitride with activation energy barriers of 24.7 and 11.3 kcal mol(-1) for uranium(VI) and uranium(V), respectively. A remarkably simple two-step, two-electron cycle for the conversion of azide to nitride to cyanate using 4, NaN3 and CO is presented.