Highlights in Uranium Coordination Chemistry

Laser-Induced Thermal Decomposition of Uranium Coordination Compounds with Non-oxidic Ligands to Produce Nitride and Carbide Materials

The production of ceramics from uranium coordination compounds can be achieved through thermal processing if an excess amount of the desired atoms (i.e., C or N), or reactive gaseous products (e.g., methane or nitrogen oxide) is made available to the reactive uranium metal core via decomposition/fragmentation of the surrounding ligand groups. Here, computational thermodynamic approaches were utilized to identify the temperatures necessary to produce uranium metal from some starting compounds─UI4(TMEDA)2, UCl4(TMEDA)2, UCl3(pyridine)x, and UI3(pyridine)4. Experimentally, precursors were irradiated by a laser under various gaseous environments (argon, nitrogen, and methane) creating extreme reaction conditions (i.e., fast heating, high temperature profile >2000 °C, and rapid cooling). Despite the fast dynamics associated with laser irradiation, the central uranium atom reacted with the thermal decomposition products of the ligands yielding uranium ceramics. Residual gas analysis identified vaporized products from the laser irradiation, and the final ceramic products were characterized by powder X-ray diffraction. The composition of the uranium precursor as well as the gaseous environment had a direct impact on the production of the final phases.

Energy-structure-property relationships in uranium metal-organic frameworks

Located at the foot of the periodic table, uranium is a relatively underexplored element possessing rich chemistry. In addition to its high relevance to nuclear power, uranium shows promise for small molecule activation and photocatalysis, among many other powerful functions. Researchers have used metal-organic frameworks (MOFs) to harness uranium's properties, and in their quest to do so, have discovered remarkable structures and unique properties unobserved in traditional transition metal MOFs. More recently, (e.g. the last 8-10 years), theoretical calculations of framework energetics have supplemented structure-property studies in uranium MOFs (U-MOFs). In this Perspective, we summarize how these budding energy-structure-property relationships in U-MOFs enable a deeper understanding of chemical phenomena, enlarge chemical space, and elevate the field to targeted, rather than exploratory, discovery. Importantly, this Perspective encourages interdisciplinary connections between experimentalists and theorists by demonstrating how these collaborations have elevated the entire U-MOF field.

Functionalized Tris(anilido)triazacyclononanes as Hexadentate Ligands for the Encapsulation of U(III), U(IV) and La(III) Cations

Tripodal multidentate ligands have become increasingly popular in f-element chemistry for stabilizing unusual bonding motifs and supporting small molecule activation processes. The steric and electronic effects of ligand donor atom substituents have proved crucial in both of these applications. In this study we functionalized the previously reported tris-anilide ligand {tacn(SiMe2NPh)3} (tacn = 1,3,7-triazacyclononane) to incorporate substituted aromatic rings, with the aim of modifying f-element complex solubility and ligand steric effects. We report the synthesis of two proligands, {tacn(SiMe2NHAr)3} (Ar = C6H3Me2-3,5 or C6H4Me-4), and their respective group 1 transfer agents—{tacn(SiMe2NKAr)3}, M(III) complexes [M{tacn(SiMe2NAr)3}] for M = La and U, and U(IV) complexes [M{tacn(SiMe2NAr)3}(Cl)]. These compounds were characterized by multinuclear NMR and FTIR spectroscopy and elemental analysis. The paramagnetic uranium complexes were also characterized by solid state magnetic measurements and UV/Vis/NIR spectroscopy. U(III) complexes were additionally studied by EPR spectroscopy. The solid state structures of all f-block complexes were authenticated by single-crystal X-ray diffraction (XRD), together with a minor byproduct [U{tacn(SiMe2NC6H4Me-4)3}(I)]. Comparisons of the characterization data of our f-element complexes with similar literature examples containing the {tacn(SiMe2NPh)3} ligand set showed minor changes in physicochemical properties resulting from the different aromatic ring substitution patterns we investigated.

Structure and magnetism of a tetrahedral uranium(iii) β-diketiminate complex

We describe the functionalisation of the previously reported uranium(iii) β-diketiminate complex (BDI)UI₂(THF)₂ (1) with one and two equivalents of a sterically demanding 2,6-diisopropylphenolate ligand (ODipp) leading to the formation of two heteroleptic complexes: [(BDI)UI(ODipp)]₂ (2) and (BDI)U(ODipp)₂ (3). The latter is a rare example of a tetrahedral uranium(iii) complex, and it shows single-molecule magnet behaviour.

The synthesis and versatile reducing power of low-valent uranium complexes

This synthesis and diverse reactivity of uranium(iii) and uranium(ii) complexes is discussed.

New supporting ligands in actinide chemistry: tetramethyltetraazaannulene complexes with thorium and uranium

We report the synthesis, characterization, and preliminary reactivity of new heteroleptic thorium and uranium complexes supported by the macrocyclic TMTAA ligand (TMTAA = Tetramethyl-tetra-aza-annulene). The dihalide complexes Th(TMTAA)Cl₂(THF)₂ (1), [UCl₂(TMTAA)]₂ (2) and U(TMTAA)I₂ (3) are further functionalized to the Cp* derivatives ThCp*(TMTAA)Cl (4), UCp*(TMTAA)Cl (5) and UCp*(TMTAA)I (6) (Cp* = pentamethylcyclopentadienide). Compounds 4-6 are also obtained through a one-pot reaction from standard thorium(iv) and uranium(iv) starting materials, Li2TMTAA and KCp*. Complexes 1-6 function as valuable starting materials for salt metathesis chemistry. Treatment of precursors 4 or 5 with trimethylsilylmethyllithium (LiCH₂TMS) results in the new actinide TMTAA alkyl complexes ThCp*(TMTAA)(CH₂TMS) (7) and UCp*(TMTAA)(CH₂TMTS) (8), respectively. The TMTAA-derived alkyl complexes (7 and 8) show unexpected stability and are stable for several weeks at room temperature in solution and in the solid-state. Additionally, double substitution of the halide ligands in 1-3 shows a strong dependence on the nucleophile used. While weaker nucleophiles, such as amides, and more sterically demanding nucleophiles, such as Cp (Cp = cyclopenadienide), favour the formation of bis-TMTAA "sandwich" complexes [An(TMTAA)₂] (An = Th (9) and An = U (10)), the use of oxygen-functionalized ligands like the ODipp anion (Dipp = diisopropylphenyl) results in the formation of the doubly substituted species Th(ODipp)₂TMTAA (11) and U(ODipp)₂TMTAA (12). We also describe the divergent reactivity of the TMTAA ligand towards uranium(iii). Unlike the syntheses of actinide(iv) TMTAA complexes, the synthesis of a uranium(iii) TMTAA was not successful and only uranium(iv) species could be obtained.

A hydrolytically stable uranyl organic framework for highly sensitive and selective detection of Fe3+ in aqueous media

A depleted uranium-based metal organic framework is synthesized and it exhibits highly sensitive and selective detection towards Fe³⁺ ions in aqueous media with an extremely low detection limit of 6.3 ppb.

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.

Characterizing pressure-induced uranium C-H agostic bonds

The diuranium(III) compound [UN''2]2(μ-η(6):η(6)-C6H6) (N''=N(SiMe3)2) has been studied using variable, high-pressure single-crystal X-ray crystallography, and density functional theory . In this compound, the low-coordinate metal cations are coupled through π- and δ-symmetric arene overlap and show close metal-CH contacts with the flexible methyl CH groups of the sterically encumbered amido ligands. The metal-metal separation decreases with increasing pressure, but the most significant structural changes are to the close contacts between ligand CH bonds and the U centers. Although the interatomic distances are suggestive of agostic-type interactions between the U and ligand peripheral CH groups, QTAIM (quantum theory of atoms-in-molecules) computational analysis suggests that there is no such interaction at ambient pressure. However, QTAIM and NBO analyses indicate that the interaction becomes agostic at 3.2 GPa.

Singlet ground state actinide chemistry with geminals

We present the first application of the variationally orbital optimized antisymmetric product of 1-reference orbital geminals (vOO-AP1roG) method to singlet-state actinide chemistry.

Structural/electronic properties and reaction energies of a series of mono- and bis-uranyl dihalides equatorially coordinated by N/O ligands

Monometallic (UO2)(X)2(L)3 (L = pyridine (py), X = F (1), Cl (2), Br (3) and I (4); L = tetrahydrofuran (thf), X = Cl (5); L = pyrrole (pl), X = Cl (6)) as well as bimetallic [(UO2)(μ2-X)(X)(L)2]2 (L = py, X = F (7), Cl (8), Br (9) and I (10); L = thf, X = Cl (11); L = pl, X = Cl (12); μ 2 = doubly bridged) were examined using relativistic density functional theory. With changing from F, Cl, Br to I irregardless of in mono- or bis-uranyl complexes, bond lengths of U = O were calculated to be decreasing, resulting from strengthening of axial U = O bonds while weakening equatorial X → U coordination. This is further evidenced by calculated bond orders of U = O and stretching vibrational frequencies. A similar situation was is found in 2, 5 and 6 as well as in 8, 11 and 12, where N/O ligands are varied but the chlorine atoms are retained. The present study reveals that all these complexes have U(f)-character low-lying unoccupied orbitals, and their π*(U = O) antibonds are located on higher-energy orbitals. Complex 1 was calculated to show σ(U = O) bonding character for HOMO, and pyridine-character for other occupied orbitals; the fluorine ligand occurs in a relatively low-energy region. In contrast, the π(p) characters of heavier halogen atoms significantly contribute to most frontier molecular orbitals of 2, 3 and 4. Unlike this electronic feature of 2, complexes 5 and 6 exhibit mainly thf and pyrrole characters, respectively, for their high-lying occupied orbitals. Electronic structures of bisuranyl complexes 7-12, albeit a little more complicated, are revealed to be similar to those of the corresponding monouranyl complexes. Finally, energies of formation reactions of the above complexes were calculated and compared with available experimental results.

Toward equatorial planarity about uranyl: synthesis and structure of tridentate nitrogen-donor {UO2}2+ complexes

The reaction of UO2Cl2·3THF with the tridentate nitrogen donor ligand 2,6-bis(2-benzimidazolyl)pyridine (H2BBP) in pyridine leads to the formation of three different complexes: [(UO2)(H2BBP)Cl2] (1), [(UO)2(HBBP)(Py)Cl] (2), and [(UO2)(BBP)(Py)2] (3) after successive deprotonation of H2BBP with a strong base. Crystallographic determination of 1-3 reveals that increased charge through ligand deprotonation and displacement of chloride leads to equatorial planarity about uranyl as well as a more compact overall coordination geometry. Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectra of 1-3 at the U-4d edges have been recorded using a soft X-ray Scanning Transmission X-ray Microscope (STXM) and reveal the uranium 4d5/2 and 4d3/2 transitions at energies associated with uranium in the hexavalent oxidation state. First-principles Density Functional Theory (DFT) electronic structure calculations for the complexes have been performed to determine and validate the coordination characteristics, which correspond well to the experimental results.

Application of Calixarenes as Macrocyclic Ligands for Uranium(VI): A Review

Calixarenes represent a well-known family of macrocyclic molecules with broad range of potential applications in chemical, analytical, and engineering materials fields. This paper covers the use of calixarenes as complexing agents for uranium(VI). The high effectiveness of calix[6]arenes in comparison to other calixarenes in uranium(VI) separation process is also presented. Processes such as liquid-liquid extraction (LLE), liquid membrane (LM) separation, and ion exchange are considered as potential fields for application of calixarenes as useful agents for bindingUO22+for effective separation from aqueous solutions containing other metal components.

Steric effects on uranyl complexation: synthetic, structural, and theoretical studies of carbamoyl pyrazole compounds of the uranyl(VI) ion

New bifunctional pyrazole based ligands of the type [C(3)HR(2)N(2)CONR'] (where R = H or CH(3); R' = CH(3), C(2)H(5), or (i)C(3)H(7)) were prepared and characterized. The coordination chemistry of these ligands with uranyl nitrate and uranyl bis(dibenzoyl methanate) was studied with infrared (IR), (1)H NMR, electrospray-mass spectrometry (ES-MS), elemental analysis, and single crystal X-ray diffraction methods. The structure of compound [UO(2)(NO(3))(2)(C(3)H(3)N(2)CON{C(2)H(5)}(2))] (2) shows that the uranium(VI) ion is surrounded by one nitrogen atom and seven oxygen atoms in a hexagonal bipyramidal geometry with the ligand acting as a bidentate chelating ligand and bonds through both the carbamoyl oxygen and pyrazolyl nitrogen atoms. In the structure of [UO(2)(NO(3))(2)(H(2)O)(2)(C(5)H(7)N(2)CON {C(2)H(5)}(2))(2)], (5) the pyrazole ligand acts as a second sphere ligand and hydrogen bonds to the water molecules through carbamoyl oxygen and pyrazolyl nitrogen atoms. The structure of [UO(2)(DBM)(2)C(3)H(3)N(2)CON{C(2)H(5)}(2)] (8) (where DBM = C(6)H(5)COCHCOC(6)H(5)) shows that the pyrazole ligand acts as a monodentate ligand and bonds through the carbamoyl oxygen to the uranyl group. The ES-MS spectra of 2 and 8 show that the ligand is similarly bonded to the metal ion in solution. Ab initio quantum chemical studies show that the steric effect plays the key role in complexation behavior.

Cation-cation complexes of pentavalent uranyl: from disproportionation intermediates to stable clusters

Three new cation-cation complexes of pentavalent uranyl, stable with respect to the disproportionation reaction, have been prepared from the reaction of the precursor [(UO(2)py(5))(KI(2)py(2))](n) (1) with the Schiff base ligands salen(2-), acacen(2-), and salophen(2-) (H(2)salen = N,N'-ethylene-bis(salicylideneimine), H(2)acacen = N,N'-ethylenebis(acetylacetoneimine), H(2)salophen = N,N'-phenylene-bis(salicylideneimine)). The preparation of stable complexes requires a careful choice of counter ions and reaction conditions. Notably the reaction of 1 with salophen(2-) in pyridine leads to immediate disproportionation, but in the presence of [18]crown-6 ([18]C-6) a stable complex forms. The solid-state structure of the four tetranuclear complexes, {[UO(2)(acacen)](4)[μ(8)-](2)[K([18]C-6)(py)](2)} (3) and {[UO(2)(acacen)](4)[μ(8)-]}⋅2 [K([222])(py)] (4), {[UO(2)(salophen)](4)[μ(8)-K](2)[μ(5)-KI](2)[(K([18]C-6)]}⋅2 [K([18]C-6)(thf)(2)]⋅2 I (5), and {[UO(2)(salen)(4)][μ(8)-Rb](2)[Rb([18]C-6)](2)} (9) ([222] = [222]cryptand, py = pyridine), presenting a T-shaped cation-cation interaction has been determined by X-ray crystallographic studies. NMR spectroscopic and UV/Vis studies show that the tetranuclear structure is maintained in pyridine solution for the salen and acacen complexes. Stable mononuclear complexes of pentavalent uranyl are also obtained by reduction of the hexavalent uranyl Schiff base complexes with cobaltocene in pyridine in the absence of coordinating cations. The reactivity of the complex [U(V)O(2)(salen)(py)][Cp*(2)Co] with different alkali ions demonstrates the crucial effect of coordinating cations on the stability of cation-cation complexes. The nature of the cation plays a key role in the preparation of stable cation-cation complexes. Stable tetranuclear complexes form in the presence of K(+) and Rb(+), whereas Li(+) leads to disproportionation. A new uranyl-oxo cluster was isolated from this reaction. The reaction of [U(V)O(2)(salen)(py)][Cp*(2)Co] (Cp* = pentamethylcyclopentadienyl) with its U(VI) analogue yields the oxo-functionalized dimer [UO(2)(salen)(py)](2)[Cp*(2)Co] (8). The reaction of the {[UO(2)(salen)(4)][μ(8)-K](2)[K([18]C-6)](2)} tetramer with protons leads to disproportionation to U(IV) and U(VI) species and H(2)O confirming the crucial role of the proton in the U(V) disproportionation.

New insights into the acid mediated disproportionation of pentavalent uranyl

The reaction of benzoic acid with the uranyl(V) complex [(UO(2)Py(5))(KI(2)Py(2))] in pyridine leads to immediate disproportionation with formation of a hexanuclear U(IV) benzoate cluster, a bis-benzoate complex of uranyl(VI) and water.