Uranium Tetrakis-Aryloxide Derivatives Supported by Tetraazacyclododecane: Synthesis of Air-Stable, Coordinatively-Unsaturated U(IV) and U(V) Complexes

We present the synthesis, characterization, and one-electron oxidation of two uranium­(IV) complexes, coordinated to the cyclen-anchored tetrakis­(aryloxide) ligands tetrakis-hydroxybenzyl-1,4,7,10 tetraazacyclododecane, (^R,RArOH)₄cyclen; R = ^<i>t</i>Bu, Me. The new uranium­(IV) and (V) complexes exhibit an eight-coordinate, tetragonal ligand environment, effecting exceptional stability of the coordinatively unsaturated uranium compounds. Cyclic voltammetry studies reveal redox events ranging from tri- to hexavalent species, covering an electrochemical window of ∼4 V

Fungicide Field Concentrations Exceed FOCUS Surface Water Predictions: Urgent Need of Model Improvement

FOCUS models are used in European regulatory risk assessment to predict the frequency and magnitude of individual pesticide surface water concentrations. A recent study showed that these models are not protective in the prediction of insecticide concentrations in surface waters and sediments. Since fungicides differ with regard to their physicochemical properties, application patterns, and entry routes, we compared a larger data set of 417 measured field concentrations (MFC) of agricultural fungicides in surface waters and sediments from 56 studies to the respective predicted environmental concentrations (PEC) calculated with FOCUS step 1–4. Although the fraction of the underestimation of fungicide MFC values was generally lower than that obtained for insecticides, 12% of step 3 and 23% of step 4 PECs were exceeded by surface water MFCs. Taking only the 90th percentile concentration of every substance and only E.U. studies into account (E.U. studies: <i>n</i> = 327; 90th percentile + E.U. studies: <i>n</i> = 136), a maximum of 25% of the step 3 and 43% of the step 4 PECs were exceeded by surface water MFCs, which is an even worse outcome than that obtained for insecticides. Our results demonstrate that FOCUS predictions are neither protective nor appropriate for predicting fungicide concentrations in the field in the context of European pesticide risk assessment

Molecular and Electronic Structures of Eight-Coordinate Uranium Bipyridine Complexes: A Rare Example of a Bipy^2– Ligand Coordinated to a U⁴⁺ Ion

Reaction of trivalent [((^{Ad,<i>t</i>Bu}ArO)₃tacn)­U] (<b>1</b>) with 2,2′-bipyridine (bipy) yields [((^{Ad,<i>t</i>Bu}ArO)₃tacn)­U­(bipy)] (<b>2</b>) and subsequent reduction of <b>2</b> with KC₈ in the presence of Kryptofix222 furnishes [K­(2.2.2-crypt)]­[((^{Ad,<i>t</i>Bu}ArO)₃tacn)­U­(bipy)] (<b>3</b>). Alternatively, complex <b>3</b> can be synthesized from <b>1</b> by addition of [K­(bipy)] in the presence of the cryptand. New complexes <b>2</b> and <b>3</b> are characterized by a variety of spectroscopic, electrochemical, and magnetochemical methods, single-crystal X-ray diffraction, computational methods, and CHN elemental analysis. Structural analyses reveal a bipyridine radical (bipy^•–) ligand in <b>2</b> and a dianionic (bipy^2–) species in <b>3</b>. Complex <b>3</b> represents a rare example of an isolated and unambiguously characterized bipy^2– ligand coordinated to a uranium ion. The electronic structure assignments are supported by UV/vis/NIR and EPR spectroscopy, as well as SQUID magnetometry. The results of CASSCF calculations indicate multiconfigurational ground states for complexes <b>2</b> and <b>3</b>. The electronic ground state for <b>2</b> consists of an open-shell doublet U⁴⁺(bipy^•–) state (91%) and a closed-shell doublet U⁵⁺(bipy^2–) state (9%). The almost degenerate multiconfigurational ground state for <b>3</b> was found to be composed of an open-shell singlet and pure triplet state 0.06 eV higher in energy, both resulting from the U⁴⁺(5f²) (bipy^2–) configuration

Molecular and Electronic Structures of Eight-Coordinate Uranium Bipyridine Complexes: A Rare Example of a Bipy^2– Ligand Coordinated to a U⁴⁺ Ion

Reaction of trivalent [((^{Ad,<i>t</i>Bu}ArO)₃tacn)­U] (<b>1</b>) with 2,2′-bipyridine (bipy) yields [((^{Ad,<i>t</i>Bu}ArO)₃tacn)­U­(bipy)] (<b>2</b>) and subsequent reduction of <b>2</b> with KC₈ in the presence of Kryptofix222 furnishes [K­(2.2.2-crypt)]­[((^{Ad,<i>t</i>Bu}ArO)₃tacn)­U­(bipy)] (<b>3</b>). Alternatively, complex <b>3</b> can be synthesized from <b>1</b> by addition of [K­(bipy)] in the presence of the cryptand. New complexes <b>2</b> and <b>3</b> are characterized by a variety of spectroscopic, electrochemical, and magnetochemical methods, single-crystal X-ray diffraction, computational methods, and CHN elemental analysis. Structural analyses reveal a bipyridine radical (bipy^•–) ligand in <b>2</b> and a dianionic (bipy^2–) species in <b>3</b>. Complex <b>3</b> represents a rare example of an isolated and unambiguously characterized bipy^2– ligand coordinated to a uranium ion. The electronic structure assignments are supported by UV/vis/NIR and EPR spectroscopy, as well as SQUID magnetometry. The results of CASSCF calculations indicate multiconfigurational ground states for complexes <b>2</b> and <b>3</b>. The electronic ground state for <b>2</b> consists of an open-shell doublet U⁴⁺(bipy^•–) state (91%) and a closed-shell doublet U⁵⁺(bipy^2–) state (9%). The almost degenerate multiconfigurational ground state for <b>3</b> was found to be composed of an open-shell singlet and pure triplet state 0.06 eV higher in energy, both resulting from the U⁴⁺(5f²) (bipy^2–) configuration

A Series of Uranium (IV, V, VI) Tritylimido Complexes, Their Molecular and Electronic Structures and Reactivity with CO₂

A series of uranium tritylimido complexes with structural continuity across complexes in different oxidation states, namely U^IV, U^V, and U^VI, is reported. This series was successfully synthesized by employing the trivalent uranium precursor, [((^nP,MeArO)₃tacn)­U^III] (<b>1</b>) (where (^nP,MeArO)₃tacn^3– = trianion of 1,4,7-tris­(2-hydroxy-5-methyl-3-neopentylbenzyl)-1,4,7-triazacyclononane), with the organic azides Me₃SiN₃, Me₃SnN₃, and Ph₃CN₃ (tritylazide). While the reaction with Me₃SiN₃ yields an inseparable mixture of both the azido and imido uranium complexes, applying the heavier Sn homologue yields the bis-μ-azido complex [{((^nP,MeArO)₃tacn)­U^IV}₂(μ-N₃)₂] (<b>2</b>) exclusively. In contrast to this one-electron redox chemistry, the reaction of precursor <b>1</b> with tritylazide solely leads to the two-electron oxidized U^V imido [((^nP,MeArO)₃tacn)­U^V(N-CPh₃)] (<b>3</b>). Oxidation and reduction of <b>3</b> yield the corresponding U^VI and U^IV complexes [((^nP,MeArO)₃tacn)­U^VI(N-CPh₃)]­[B­(C₆F₅)₄] (<b>4</b>) and K­[((^nP,MeArO)₃tacn)­U^IV(N-CPh₃)] (<b>5</b>), respectively. In addition, the U^V imido <b>3</b> engages in a H atom abstraction reaction with toluene to yield the closely related amido complex [((^nP,MeArO)₃tacn)­U^IV(N­(H)-CPh₃)] (<b>6</b>). Complex <b>6</b> and the three tritylimido complexes <b>3</b>, <b>4</b>, and <b>5</b>, with oxidation states ranging from +IV to +VI and homologous core structures, were investigated by X-ray diffraction analyses and magnetochemical and spectroscopic studies as well as density functional theory (DFT) computational analysis. The series of structurally very similar imido complexes provides a unique opportunity to study electronic properties and to probe the uranium imido reactivity solely as a function of electron count of the metal–imido entity. Evidence for the U–N bond covalency and f-orbital participation in complexes <b>3</b>–<b>6</b> was drawn from the in-depth and comparative DFT study. The reactivity of the imido and amido complexes with CO₂ was probed, and conclusions about the influence of the formal oxidation state are reported

Reactivity of U–E–U (E = S, Se) Toward CO₂, CS₂, and COS: New Mixed-Carbonate Complexes of the Types U–CO₂E–U (E = S, Se), U–CS₂E–U (E = O, Se), and U–COSSe–U

We recently reported the formation of a bridging carbonate complex [{((^AdArO)₃N)­U}₂(μ–η¹:κ²-CO₃)] via reductive cleavage of CO₂, yielding a μ-oxo bridged complex, followed by the insertion of another molecule of CO₂. In a similar strategy, we were able to isolate and characterize a series of mixed carbonate complexes U–CO₂E–U, U–CS₂E–U, and even U–OC­(S)­Se–U, by reacting bridged chalcogenide complexes [{((^AdArO)₃N)­U}₂(μ-E)] (E = S, Se) with CO₂, CS₂, and COS. These chalcogenido mixed-carbonate complexes represent the first of their kind

Molecular and Electronic Structure of Dinuclear Uranium Bis-μ-Oxo Complexes with Diamond Core Structural Motifs

In a multiple-bond metathesis reaction, the triazacyclononane (tacn)-anchored methyl- and neopentyl (nP)-substituted tris­(aryloxide) U^III complex [((^nP,MeArO)₃tacn)­U^III] (<b>1</b>) reacts with mesityl azide and CO₂ to form mesityl isocyanate and the dinuclear bis­(μ-oxo)-bridged U^V/U^V complex [{((^nP,MeArO)₃tacn)­U^V}₂(μ-O)₂] (<b>3</b>). This reaction proceeds via the mononuclear U^V imido intermediate [((^nP,MeArO)₃tacn)­U^V(NMes)] (<b>2</b>), which has been synthesized and fully characterized independently. The dimeric U^V oxo species shows rich redox behavior: complex <b>3</b> can be reduced by one and two electrons, respectively, yielding the mixed-valent U^IV/U^V bis­(μ-oxo) complex [K­(crypt)]­[{((^nP,MeArO)₃tacn)­U^IV/V}₂(μ-O)₂] (<b>7</b>) and the U^IV/U^IV bis­(μ-oxo) complex K₂[{((^nP,MeArO)₃tacn)­U^IV}₂(μ-O)₂] (<b>6</b>). In addition, complex <b>3</b> can be oxidized to provide the mononuclear uranium­(VI) oxo complexes [((^nP,MeArO)₃tacn)­U^VI(O)_eq(OTf)_ax] (<b>8</b>) and [((^nP,MeArO)₃tacn)­U^VI(O)_eq]­SbF₆ (<b>9</b>). The unique series of bis­(μ-oxo) complexes also shows notable magnetic behavior, which was investigated in detail by UV/vis/NIR and EPR spectroscopy as well as SQUID magnetization studies. In order to understand possible magnetic exchange phenomena, the mononuclear terminal oxo complexes [((^nP,MeArO)₃tacn)­U^V(O)­(O-pyridine)] (<b>4</b>) and [((^nP,MeArO)₃tacn)­U^V(O)­(O-NMe₃)] (<b>5</b>) were synthesized and fully characterized. The magnetic study revealed an unusually strong antiferromagnetic exchange coupling between the two U^V ions in <b>3</b>. Examination of the ¹⁸O-labeled bis­(μ-oxo)-bridged dinuclear complexes <b>3</b>, <b>6</b>, and <b>7</b> allowed for the first time the unambiguous assignment of the vibrational signature of the [U­(μ-O)₂U] diamond core structural motif

Reactivity of U–E–U (E = S, Se) Toward CO₂, CS₂, and COS: New Mixed-Carbonate Complexes of the Types U–CO₂E–U (E = S, Se), U–CS₂E–U (E = O, Se), and U–COSSe–U

We recently reported the formation of a bridging carbonate complex [{((^AdArO)₃N)­U}₂(μ–η¹:κ²-CO₃)] via reductive cleavage of CO₂, yielding a μ-oxo bridged complex, followed by the insertion of another molecule of CO₂. In a similar strategy, we were able to isolate and characterize a series of mixed carbonate complexes U–CO₂E–U, U–CS₂E–U, and even U–OC­(S)­Se–U, by reacting bridged chalcogenide complexes [{((^AdArO)₃N)­U}₂(μ-E)] (E = S, Se) with CO₂, CS₂, and COS. These chalcogenido mixed-carbonate complexes represent the first of their kind

Functionalization of Complexed N₂O in Bis(pentamethylcyclopentadienyl) Systems of Zirconium and Titanium

Methyl triflate reacts with the metastable azoxymetallacyclopentene complex Cp*₂Zr­(N­(O)­NCPhCPh), generated <i>in situ</i> from nitrous oxide insertion into the Zr–C bond of Cp*₂Zr­(η²-PhCCPh) at −78 °C, to afford the salt [Cp*₂Zr­(N­(O)­N­(Me)­CPhCPh)]­[O₃SCF₃] (<b>1</b>) in 48% isolated yield. A single-crystal X-ray structure of <b>1</b> features a planar azoxymetallacycle with methyl alkylation taking place only at the β-nitrogen position of the former Zr­(N­(O)­NCPhCPh) scaffold. In addition to <b>1</b>, the methoxy-triflato complex Cp*₂Zr­(OMe)­(O₃SCF₃) (<b>2</b>) was also isolated from the reaction mixture in 26% yield and fully characterized, including its independent synthesis from the alkylation of Cp*₂ZrO­(NC₅H₅) with MeO₃SCF₃. Complex <b>2</b> could also be observed, spectroscopically, from the thermolysis of <b>1</b> (80 °C, 2 days). In contrast to Cp*₂Zr­(N­(O)­NPhCCPh), the more stable titanium N₂O-inserted analogue, Cp*₂Ti­(N­(O)­NCPhCPh), reacts with MeO₃SCF₃ to afford a 1:1 mixture of regioisomeric salts, [Cp*₂Ti­(N­(O)­N­(Me)­CPhCPh)]­[O₃SCF₃] (<b>3</b>) and [Cp*₂Ti­(N­(OMe)­NCPhCPh)]­[O₃SCF₃] (<b>4</b>), in a combined 65% isolated yield. Single-crystal X-ray diffraction studies of a cocrystal of <b>3</b> and <b>4</b> show a 1:1 mixture of azoxymetallacyle salts resulting from methyl alkylation at both the β-nitrogen and the β-oxygen of the former Ti­(N­(O)­NCPhCPh ring. As opposed to alkylation reactions, the one-electron reduction of Cp*₂Ti­(N­(O)­NCPhCPh) with KC₈, followed by encapsulation with the cryptand 2,2,2-Kryptofix, resulted in the isolation of the discrete radical anion [K­(2,2,2-Kryptofix)]­[Cp*₂Ti­(N­(O)­NCPhCPh)] (<b>5</b>) in 68% yield. Complex <b>5</b> was studied by single-crystal X-ray diffraction, and its solution X-band EPR spectrum suggested a nonbonding σ-type wedge hybrid orbital on titanium, d­(<i>z</i>²)/d­(<i>x</i>²–<i>y</i>²), houses the unpaired electron, without perturbing the azoxymetallacycle core in Cp*₂Ti­(N­(O)­NCPhCPh). Theoretical studies of Ti and the Zr analogue are also presented and discussed

Reactivity of U–E–U (E = S, Se) Toward CO₂, CS₂, and COS: New Mixed-Carbonate Complexes of the Types U–CO₂E–U (E = S, Se), U–CS₂E–U (E = O, Se), and U–COSSe–U

We recently reported the formation of a bridging carbonate complex [{((^AdArO)₃N)­U}₂(μ–η¹:κ²-CO₃)] via reductive cleavage of CO₂, yielding a μ-oxo bridged complex, followed by the insertion of another molecule of CO₂. In a similar strategy, we were able to isolate and characterize a series of mixed carbonate complexes U–CO₂E–U, U–CS₂E–U, and even U–OC­(S)­Se–U, by reacting bridged chalcogenide complexes [{((^AdArO)₃N)­U}₂(μ-E)] (E = S, Se) with CO₂, CS₂, and COS. These chalcogenido mixed-carbonate complexes represent the first of their kind