Oxidative Dissolution of Lanthanide Metals Ce and Ho in Molten GaCl3.

Gallium trichloride (GaCl3) was used as a solvent for the oxidative dissolution of the lanthanide (Ln) metals cerium (Ce) and holmium (Ho). Reactions were performed at temperatures above 100 °C in sealed vessels to maintain the liquid phase for GaCl3 during the oxidizing reactions. The best results were obtained from reactions using 8 equiv of GaCl3 to metal where the inorganic complexes [Ga][Ln(GaCl4)4] [Ln = Ce (1), Ho (2)] could be isolated. Recrystallization of 1 and 2 employing fluorobenzene (C6H5F) produced [Ga(η6-C6H5F)2][Ln(GaCl4)4] [Ln = Ce (3), Ho (4)] where reversible η6 coordination of C6H5F to [Ga]+ was observed. All complexes were characterized through elemental analysis (F and Cl), IR and UV-vis-near-IR spectroscopies, and both solution and solid-state NMR techniques.

Chlorination of Pu and U Metal Using GaCl3.

The oxidative chlorination of the plutonium metal was achieved through a reaction with gallium(III) chloride (GaCl3). In DME (DME = 1,2-dimethoxyethane) as the solvent, substoichiometric (2.8 equiv) amounts of GaCl3 were added, which consumed roughly 60% of the plutonium metal over the course of 10 days. The salt species [PuCl2(dme)3][GaCl4] was isolated as pale-purple crystals, and both solid-state and solution UV-vis-NIR spectroscopies were consistent with the formation of a trivalent plutonium complex. The analogous reaction was performed with uranium metal, generating a dicationic trivalent uranium complex crystallized as the [UCl(dme)3][GaCl4]2 salt. The extraction of [UCl(dme)3][GaCl4]2 in DME at 70 °C followed by crystallization produced [{U(dme)3}2(µ-Cl3)][GaCl4]3, a product arising from the loss of GaCl3. This method of halogenation worked on a small scale for plutonium and uranium, providing a route to cationic Pu3+ and dicationic U3+ complexes using GaCl3 in DME.

Isolation and characterization of a californium metallocene.

Californium (Cf) is currently the heaviest element accessible above microgram quantities. Cf isotopes impose severe experimental challenges due to their scarcity and radiological hazards. Consequently, chemical secrets ranging from the accessibility of 5f/6d valence orbitals to engage in bonding, the role of spin-orbit coupling in electronic structure, and reactivity patterns compared to other f elements, remain locked. Organometallic molecules were foundational in elucidating periodicity and bonding trends across the periodic table1-3, with a twenty-first-century renaissance of organometallic thorium (Th) through plutonium (Pu) chemistry4-12, and to a smaller extent americium (Am)13, transforming chemical understanding. Yet, analogous curium (Cm) to Cf chemistry has lain dormant since the 1970s. Here, we revive air-/moisture-sensitive Cf chemistry through the synthesis and characterization of [Cf(C5Me4H)2Cl2K(OEt2)]n from two milligrams of 249Cf. This bent metallocene motif, not previously structurally authenticated beyond uranium (U)14,15, contains the first crystallographically characterized Cf-C bond. Analysis suggests the Cf-C bond is largely ionic with a small covalent contribution. Lowered Cf 5f orbital energy versus dysprosium (Dy) 4f in the colourless, isoelectronic and isostructural [Dy(C5Me4H)2Cl2K(OEt2)]n results in an orange Cf compound, contrasting with the light-green colour typically associated with Cf compounds16-22.

Using molten salts to probe outer-coordination sphere effects on lanthanide(III)/(II) electron-transfer reactions.

Controlling structure and reactivity by manipulating the outer-coordination sphere around a given reagent represents a longstanding challenge in chemistry. Despite advances toward solving this problem, it remains difficult to experimentally interrogate and characterize outer-coordination sphere impact. This work describes an alternative approach that quantifies outer-coordination sphere effects. It shows how molten salt metal chlorides (MCln; M = K, Na, n = 1; M = Ca, n = 2) provided excellent platforms for experimentally characterizing the influence of the outer-coordination sphere cations (Mn+) on redox reactions accessible to lanthanide ions; Ln3+ + e1- → Ln2+ (Ln = Eu, Yb, Sm; e1- = electron). As a representative example, X-ray absorption spectroscopy and cyclic voltammetry results showed that Eu2+ instantaneously formed when Eu3+ dissolved in molten chloride salts that had strongly polarizing cations (like Ca2+ from CaCl2) via the Eu3+ + Cl1- → Eu2+ + ½Cl2 reaction. Conversely, molten salts with less polarizing outer-sphere M1+ cations (e.g., K1+ in KCl) stabilized Ln3+. For instance, the Eu3+/Eu2+ reduction potential was >0.5 V more positive in CaCl2 than in KCl. In accordance with first-principle molecular dynamics (FPMD) simulations, we postulated that hard Mn+ cations (high polarization power) inductively removed electron density from Lnn+ across Ln-Cl⋯Mn+ networks and stabilized electron-rich and low oxidation state Ln2+ ions. Conversely, less polarizing Mn+ cations (like K1+) left electron density on Lnn+ and stabilized electron-deficient and high-oxidation state Ln3+ ions.

Origin of Bond Elongation in a Uranium(IV) cis-Bis(imido) Complex.

The activation of U-N multiple bonds in an imido analogue of the uranyl ion is accomplished by using a system that is very electron-rich with sterically encumbering ligands. Treating the uranium(VI) trans-bis(imido) UI2(NDIPP)2(THF)3 (DIPP = 2,6-diisopropylphenyl and THF = tetrahydrofuran) with tert-butyl(dimethylsilyl)amide (NTSA) results in a reduction and rearrangement to form the uranium(IV) cis-bis(imido) [U(NDIPP)2(NTSA)2]K2 (1). Compound 1 features long U-N bonds, pointing toward substantial activation of the N═U═N unit, as determined by X-ray crystallography and 1H NMR, IR, and electronic absorption spectroscopies. Computational analyses show that uranium(IV)-imido bonds in 1 are significantly weakened multiple bonds due to polarization toward antibonding and nonbonding orbitals. Such geometric control has important effects on the electronic structures of these species, which could be useful in the recycling of spent nuclear fuels.

Insight into geometric preferences in uranium(VI) mixed tris(imido) systems.

Uranium tris(imido) species have been synthesized using different imido groups in the axial and equatorial positions by treating [(MesPDIMe)U(THF)]2 (1-THF), which is a uranium(iv) dimer that is supported by MesPDIMe tetraanions, with mixed organoazide solutions. While the origin of the geometric preference isn't clear, both steric and electronic factors are likely at play.

A Solid-State Support for Separating Astatine-211 from Bismuth.

Increasing access to the short-lived α-emitting radionuclide astatine-211 (211At) has the potential to advance targeted α-therapeutic treatment of disease and to solve challenges facing the medical community. For example, there are numerous technical needs associated with advancing the use of 211At in targeted α-therapy, e.g., improving 211At chelates, developing more effective 211At targeting, and characterizing in vivo 211At behavior. There is an insufficient understanding of astatine chemistry to support these efforts. The chemistry of astatine is one of the least developed of all elements on the periodic table, owing to its limited supply and short half-life. Increasing access to 211At could help address these issues and advance understanding of 211At chemistry in general. We contribute here an extraction chromatographic processing method that simplifies 211At production in terms of purification. It utilizes the commercially available Pre-Filter resin to rapidly (<1.5 h) isolate 211At from irradiated bismuth targets (Bi decontamination factors ≥876 000), in reasonable yield (68-55%) and in a form that is compatible for subsequent in vivo study. We are excited about the potential of this procedure to address 211At supply and processing/purification problems.

Manganese-Mediated Formic Acid Dehydrogenation.

A robust and rapid manganese formic acid (FA) dehydrogenation catalyst is reported. The manganese is supported by the recently developed, hybrid backbone chelate ligand tBu PNNOP (tBu PNNOP=2,6-(di-tert-butylphosphinito)(di-tert-butylphosphinamine)pyridine) (1) and the catalyst is readily prepared with MnBrCO5 to form [(tBu PNNOP)Mn(CO)2 ][Br] (2). Dehydrohalogenation of 2 generated the neutral five coordinate complex (tBu PNNOP)Mn(CO)2 (3). Dehydrogenation of FA by 2 and 3 was found to be highly efficient, exhibiting turnover frequencies (TOFs) exceeding 8500 h-1 , rivaling many noble metal systems. The parent chelate, tBu PONOP (tBu PONOP=2,6-bis(di-tert-butylphosphinito)pyridine) or tBu PNNNP (tBu PNNNP=2,6-bis (di-tert-butylphosphinamine)pyridine), coordination complexes of Mn were synthesized, respectively affording [(tBu PONOP)Mn(CO)2 ][Br] (4) and [(tBu PNNNP)Mn(CO)2 ][Br] (5). FA dehydrogenation with the hybrid-ligand supported 2 exhibits superior catalysis to 4 and 5.

Oxidation of uranium(iv) mixed imido-amido complexes with PhEEPh and to generate uranium(vi) bis(imido) dichalcogenolates, U(NR)2(EPh)2(L)2.

This work provides new routes for the conversion of U(iv) into U(vi) bis(imido) complexes and offers new information on the manner in which the U(vi) compounds form. Many compounds from the series described by the general formula U(NR)2(EPh)2(L)2 (R = 2,6-diisopropylphenyl, tert-butyl; E = S, Se, Te; L = py, EPh) were synthesized via oxidation of an in situ generated U(iv) amido-imido species with Ph2E2. This synthetic sequence provides a general route into bis(imido) U(vi) chalcogenolate complexes, circumventing the need to perform problematic salt metathesis reactions on U(vi) iodides. Investigation into the speciation of the U(iv) complexes that form prior to oxidation found a significant dependence on the identity of the ancillary ligands, with tBu2bpy forming the isolable imido-(bis)amido complex, U(NDipp)(NHDipp)2(tBu2bpy)2. Together, these data are consistent with the view that the bis(imido) U(vi) motif - much like the uranyl ion, UO22+- is a thermodynamic sink into which simple ligand frameworks are unable to prevent uranium from falling when in the presence of a suitable retinue of imido proligands.

Non-aqueous neptunium and plutonium redox behaviour in THF - access to a rare Np(iii) synthetic precursor.

Solvent exchange of NpCl4(DME)2 with THF proceeds simply to yield NpCl4(THF)3, whereas PuCl4(DME)2 is unstable in THF, partially decomposing to the mixed valent [PuIIICl2(THF)5][PuIVCl5(THF)] salt. Reduction of NpCl4(THF)3 with CsC8 ultimately afforded NpCl3(py)4, the only example of a structurally characterized solvated Np(iii) halide. The method demonstrates a route to a well-defined Np(iii) starting material without the need to employ scarcely available Np metal.

Elucidating bonding preferences in tetrakis(imido)uranate(VI) dianions.

Actinyl species, [AnO2]2+, are well-known derivatives of the f-block because of their natural occurrence and essential roles in the nuclear fuel cycle. Along with their nitrogen analogues, [An(NR)2]2+, actinyls are characterized by their two strong trans-An-element multiple bonds, a consequence of the inverse trans influence. We report that these robust bonds can be weakened significantly by increasing the number of multiple bonds to uranium, as demonstrated by a family of uranium(VI) dianions bearing four U-N multiple bonds, [M]2[U(NR)4] (M = Li, Na, K, Rb, Cs). Their geometry is dictated by cation coordination and sterics rather than by electronic factors. Multiple bond weakening by the addition of strong π donors has the potential for applications in the processing of high-valent actinyls, commonly found in environmental pollutants and spent nuclear fuels.

Reactivity of Silanes with (tBu PONOP)Ruthenium Dichloride: Facile Synthesis of Chloro-Silyl Ruthenium Compounds and Formic Acid Decomposition.

The coordination of tBu PONOP (tBu PONOP=2,6-bis(ditert-butylphosphinito)pyridine) to different ruthenium starting materials, to generate (tBu PONOP)RuCl2 , was investigated. The resultant (tBu PONOP)RuCl2 reactivity with three different silanes was then investigated and contrasted dramatically with the reactivity of (iPr PONOP)RuCl2 (DMSO) (iPr PONOP=2,6-bis(diisopropylphosphinito)pyridine) with the same silanes. The 16-electron species (tBu PONOP)Ru(H)Cl was produced from the reaction of triethylsilane with (tBu PONOP)RuCl2 . Reactions of (tBu PONOP)RuCl2 with both phenylsilane or diphenylsilane afforded the 16-electron hydrido-silyl species (tBu PONOP)Ru(H)(PhSiCl2 ) and (tBu PONOP)Ru(H)(Ph2 SiCl), respectively. Reactions of all three of these complexes with silver triflate afforded the simple salt metathesis products of (tBu PONOP)Ru(H)(OTf), (tBu PONOP)Ru(H)(PhSiCl(OTf)), and (tBu PONOP)Ru(H)(Ph2 Si(OTf)). Formic acid dehydrogenation was performed in the presence of triethylamine (TEA), and each species proved competent for gas-pressure generation of CO2 and H2 . The hydride species (tBu PONOP)Ru(H)Cl, (tBu PONOP)Ru(H)(OTf), and (tBu PONOP)Ru(H)(PhSiCl2 ) exhibited faster catalytic activity than the other compounds tested.

Investigation of Uranium Tris(imido) Complexes: Synthesis, Characterization, and Reduction Chemistry of [U(NDIPP)3(thf)3].

Addition of KC8 to trivalent [UI3(thf)4] in the presence of three equivalents of 2,6-diisopropylphenylazide (N3DIPP) results in the formation of the hexavalent uranium tris(imido) complex [U(NDIPP)3(thf)3] (1) through a facile, single-step synthesis. The X-ray crystal structure shows an octahedral complex that adopts a facial orientation of the imido substituents. This structural trend is maintained during the single-electron reduction of 1 to form dimeric [U(NDIPP)3{K(Et2O)}]2 (2). Variable-temperature/field magnetization studies of 2 show two independent U(V) 5f (1) centers, with no antiferromagnetic coupling present. Characterization of these complexes was accomplished using single-crystal X-ray diffraction, variable-temperature (1)H NMR spectroscopy, as well as IR and UV/Vis absorption spectroscopic studies.

Investigation of the electronic ground states for a reduced pyridine(diimine) uranium series: evidence for a ligand tetraanion stabilized by a uranium dimer.

The electronic structures of a series of highly reduced uranium complexes bearing the redox-active pyridine(diimine) ligand, (Mes)PDI(Me) ((Mes)PDI(Me) = 2,6-(2,4,6-Me3-C6H2-N═CMe)2C5H3N) have been investigated. The complexes, ((Mes)PDI(Me))UI3(THF) (1), ((Mes)PDI(Me))UI2(THF)2 (2), [((Mes)PDI(Me))UI]2 (3), and [((Mes)PDI(Me))U(THF)]2 (4), were examined using electronic and X-ray absorption spectroscopies, magnetometry, and computational analyses. Taken together, these studies suggest that all members of the series contain uranium(IV) centers with 5f (2) configurations and reduced ligand frameworks, specifically [(Mes)PDI(Me)](•/-), [(Mes)PDI(Me)](2-), [(Mes)PDI(Me)](3-) and [(Mes)PDI(Me)](4-), respectively. In the cases of 2, 3, and 4 no unpaired spin density was found on the ligands, indicating a singlet diradical ligand in monomeric 2 and ligand electron spin-pairing through dimerization in 3 and 4. Interaction energies, representing enthalpies of dimerization, of -116.0 and -144.4 kcal mol(-1) were calculated using DFT for the monomers of 3 and 4, respectively, showing there is a large stabilization gained by dimerization through uranium-arene bonds. Highlighted in these studies is compound 4, bearing a previously unobserved pyridine(diimine) tetraanion, that was uniquely stabilized by backbonding between uranium cations and the η(5)-pyridyl ring.

Harnessing redox activity for the formation of uranium tris(imido) compounds.

Classically, late transition-metal organometallic compounds promote multielectron processes solely through the change in oxidation state of the metal centre. In contrast, uranium typically undergoes single-electron chemistry. However, using redox-active ligands can engage multielectron reactivity at this metal in analogy to transition metals. Here we show that a redox-flexible pyridine(diimine) ligand can stabilize a series of highly reduced uranium coordination complexes by storing one, two or three electrons in the ligand. These species reduce organoazides easily to form uranium-nitrogen multiple bonds with the release of dinitrogen. The extent of ligand reduction dictates the formation of uranium mono-, bis- and tris(imido) products. Spectroscopic and structural characterization of these compounds supports the idea that electrons are stored in the ligand framework and used in subsequent reactivity. Computational analyses of the uranium imido products probed their molecular and electronic structures, which facilitated a comparison between the bonding in the tris(imido) structure and its tris(oxo) analogue.

Isolation of a uranium(III) benzophenone ketyl radical that displays redox-active ligand behaviour.

The first uranium(III) charge separated ketyl radical complex, Tp*2U(OC·Ph2), has been isolated and characterized by infrared, (1)H NMR, and electronic absorption spectroscopies, along with X-ray crystallography. Tp*2U(OC·Ph2) is a potent two-electron reductant towards N3Mes (Mes = 2,4,6-trimethylphenyl) and (2,2,6,6-tetramethyl-piperidin-1-yl)oxyl (TEMPO), with reducing equivalents derived from the metal centre and the redox-active benzophenone.

Multielectron C-O bond activation mediated by a family of reduced uranium complexes.

A family of cyclopentadienyl uranium complexes supported by the redox-active pyridine(diimine) ligand, (Mes)PDI(Me) ((Mes)PDI(Me) = 2,6-((Mes)N═CMe)2-C5H3N, Mes = 2,4,6-trimethylphenyl), has been synthesized. Using either Cp* or Cp(P) (Cp* = 1,2,3,4,5-pentamethylcyclopentadienide, Cp(P) = 1-(7,7-dimethylbenzyl)cyclopentadienide), uranium complexes of the type Cp(X)UI2((Mes)PDI(Me)) (1-Cp(X); X = * or P), Cp(X)UI((Mes)PDI(Me)) (2-Cp(X)), and Cp(X)U((Mes)PDI(Me))(THF)n (3-Cp(X); *, n = 1; P, n = 0) were isolated and characterized. The series was generated via ligand centered reduction events; thus the extent of (Mes)PDI(Me) reduction varies in each case, but the uranium(IV) oxidation state is maintained. Treating 2-Cp(X), which has a doubly reduced (Mes)PDI(Me), with furfural results in radical coupling between the substrate and (Mes)PDI(Me), leading to C-C bond formation to form Cp(X)UI((Mes)PDI(Me)-CHOC4H3O) (4-Cp(X)). Exposure of 3-Cp* and 3-Cp(P), which contain a triply reduced (Mes)PDI(Me) ligand, to benzaldehyde and benzophenone, respectively, results in the corresponding pinacolate complexes Cp*U(O2C2Ph2H2)((Mes)PDI(Me)) (5-Cp*) and Cp(P)U(O2C2Ph4)((Mes)PDI(Me)) (5-Cp(P)). The reducing equivalents required for this coupling are derived solely from the redox-active ligand, rather than the uranium center. Complexes 1-5 have been characterized by (1)H NMR and electronic absorption spectroscopies, and SQUID magnetometry was employed to confirm the mono(anionic) [(Mes)PDI(Me)](-) ligand in 1-Cp(P) and 5-Cp(P). Structural parameters of complexes 1-Cp(P), 2-Cp(X), 4-Cp*, and 5-Cp(X) have been elucidated by X-ray crystallography.