Tuning Uranium–Nitrogen Multiple Bond Formation with Ancillary Siloxide Ligands

The new homoleptic <i>ate</i> U­(III) siloxide [K­(18c6)]­[U­(OSi­(O^<i>t</i>Bu)₃)₄] <b>2</b> was prepared in 69% yield by reduction of [U­(OSi­(O^<i>t</i>Bu)₃)₄] <b>3</b> with KC₈. The reaction of the neutral U­(III) siloxide complex [U­(OSi­(O^<i>t</i>Bu)₃)₂(μ-OSi­(O^<i>t</i>Bu)₃)]₂ <b>1</b> with adamantyl azide leads to the isolation of the dinuclear U­(VI) imido complex [U₂(NAd)₄(OSi­(O^<i>t</i>Bu)₃)₄] <b>4</b>. The X-ray crystal structure shows the presence of a “cation–cation interaction” between the two [U­(NAd)₂]²⁺ groups. In contrast the reactions of <b>2</b> with the trimethylsilyl and adamantyl azides afford the U­(V) imido complexes [K­(18c6)]­[U­(NSiMe₃)­(OSi­(O^<i>t</i>Bu)₃)₄] <b>5-TMS</b> and [K­(18c6)]­[U­(NAd)­(OSi­(O^<i>t</i>Bu)₃)₄] <b>5-Ad</b> pure in 48% and 66% yield, respectively. The reaction of <b>2</b> with CsN₃ in THF at −40 °C yields a mixture of products from which the azido U­(IV) complex [K­(18c6)]­[U­(N₃)­(OSi­(O^<i>t</i>Bu)₃)₄] <b>7</b> and the μ-nitrido diuranium­(V) complex [KU­(μ-N)­(OSi­(O^<i>t</i>Bu)₃)]₂ <b>8</b> were isolated. The crystal structure of <b>8</b> shows the presence of a rare U₂N₂ core with two nitrido atoms bridging two uranium centers in a diamond-shaped geometry. In contrast, the reaction of <b>1</b> with CsN₃ affords the diuranium­(IV) complex Cs­{(μ-N)­[U­(OSi­(O^<i>t</i>Bu)₃)₃]₂} <b>9</b> presenting a nitrido ligand bridging two uranium and one cesium cations. These results show the importance of the coordination environment in the outcome of the reaction of U­(III) with azides

Tuning Uranium–Nitrogen Multiple Bond Formation with Ancillary Siloxide Ligands

The new homoleptic <i>ate</i> U­(III) siloxide [K­(18c6)]­[U­(OSi­(O^<i>t</i>Bu)₃)₄] <b>2</b> was prepared in 69% yield by reduction of [U­(OSi­(O^<i>t</i>Bu)₃)₄] <b>3</b> with KC₈. The reaction of the neutral U­(III) siloxide complex [U­(OSi­(O^<i>t</i>Bu)₃)₂(μ-OSi­(O^<i>t</i>Bu)₃)]₂ <b>1</b> with adamantyl azide leads to the isolation of the dinuclear U­(VI) imido complex [U₂(NAd)₄(OSi­(O^<i>t</i>Bu)₃)₄] <b>4</b>. The X-ray crystal structure shows the presence of a “cation–cation interaction” between the two [U­(NAd)₂]²⁺ groups. In contrast the reactions of <b>2</b> with the trimethylsilyl and adamantyl azides afford the U­(V) imido complexes [K­(18c6)]­[U­(NSiMe₃)­(OSi­(O^<i>t</i>Bu)₃)₄] <b>5-TMS</b> and [K­(18c6)]­[U­(NAd)­(OSi­(O^<i>t</i>Bu)₃)₄] <b>5-Ad</b> pure in 48% and 66% yield, respectively. The reaction of <b>2</b> with CsN₃ in THF at −40 °C yields a mixture of products from which the azido U­(IV) complex [K­(18c6)]­[U­(N₃)­(OSi­(O^<i>t</i>Bu)₃)₄] <b>7</b> and the μ-nitrido diuranium­(V) complex [KU­(μ-N)­(OSi­(O^<i>t</i>Bu)₃)]₂ <b>8</b> were isolated. The crystal structure of <b>8</b> shows the presence of a rare U₂N₂ core with two nitrido atoms bridging two uranium centers in a diamond-shaped geometry. In contrast, the reaction of <b>1</b> with CsN₃ affords the diuranium­(IV) complex Cs­{(μ-N)­[U­(OSi­(O^<i>t</i>Bu)₃)₃]₂} <b>9</b> presenting a nitrido ligand bridging two uranium and one cesium cations. These results show the importance of the coordination environment in the outcome of the reaction of U­(III) with azides

Tuning Uranium–Nitrogen Multiple Bond Formation with Ancillary Siloxide Ligands

The new homoleptic <i>ate</i> U­(III) siloxide [K­(18c6)]­[U­(OSi­(O^<i>t</i>Bu)₃)₄] <b>2</b> was prepared in 69% yield by reduction of [U­(OSi­(O^<i>t</i>Bu)₃)₄] <b>3</b> with KC₈. The reaction of the neutral U­(III) siloxide complex [U­(OSi­(O^<i>t</i>Bu)₃)₂(μ-OSi­(O^<i>t</i>Bu)₃)]₂ <b>1</b> with adamantyl azide leads to the isolation of the dinuclear U­(VI) imido complex [U₂(NAd)₄(OSi­(O^<i>t</i>Bu)₃)₄] <b>4</b>. The X-ray crystal structure shows the presence of a “cation–cation interaction” between the two [U­(NAd)₂]²⁺ groups. In contrast the reactions of <b>2</b> with the trimethylsilyl and adamantyl azides afford the U­(V) imido complexes [K­(18c6)]­[U­(NSiMe₃)­(OSi­(O^<i>t</i>Bu)₃)₄] <b>5-TMS</b> and [K­(18c6)]­[U­(NAd)­(OSi­(O^<i>t</i>Bu)₃)₄] <b>5-Ad</b> pure in 48% and 66% yield, respectively. The reaction of <b>2</b> with CsN₃ in THF at −40 °C yields a mixture of products from which the azido U­(IV) complex [K­(18c6)]­[U­(N₃)­(OSi­(O^<i>t</i>Bu)₃)₄] <b>7</b> and the μ-nitrido diuranium­(V) complex [KU­(μ-N)­(OSi­(O^<i>t</i>Bu)₃)]₂ <b>8</b> were isolated. The crystal structure of <b>8</b> shows the presence of a rare U₂N₂ core with two nitrido atoms bridging two uranium centers in a diamond-shaped geometry. In contrast, the reaction of <b>1</b> with CsN₃ affords the diuranium­(IV) complex Cs­{(μ-N)­[U­(OSi­(O^<i>t</i>Bu)₃)₃]₂} <b>9</b> presenting a nitrido ligand bridging two uranium and one cesium cations. These results show the importance of the coordination environment in the outcome of the reaction of U­(III) with azides

Synthesis of Electron-Rich Uranium(IV) Complexes Supported by Tridentate Schiff Base Ligands and Their Multi-Electron Redox Chemistry

The synthesis, structure, and reactivity of a new complex of U­(IV) with the tridentate Schiff base ligand Menaphtquinolen are reported. The reduction of the bis-ligand complexes [UX₂(^Menaphtquinolen)₂] (X = Cl, (<b>1-Cl</b>) ; I (<b>1-I</b>)) with potassium metal affords the U­(IV) complex of the new tetranionic hexadentate ligand μ-bis-^Menaphtquinolen formed through the intramolecular reductive coupling of the imino groups of each ^Menaphtquinolen unit. The solid state structure of the [U­(μ-bis-^Menaphtquinolen)]₂ dimer <b>2</b> isolated from toluene confirms the presence of a U­(IV) complex of the reduced ligand. Reactivity studies with molecular oxygen and 9,10-phenanthrenequinone show that complex <b>2</b> can act as a multielectron reducing agent releasing two electrons through the cleavage of the C–C bond to restore the original imino function of the ligand. In the resulting U­(IV) and U­(VI) complexes [U­(9,10-phenanthrenediol)­(^Menaphtquinolen)₂], <b>3</b>, and [UO₂(^Menaphtquinolen)₂], <b>4</b>, the restored tridentate Schiff base allows for the coordination of the reduced substrate to the metal. Electrochemical studies of complex <b>2</b> show the presence of irreversible ligand centered reduction processes and of a reversible U­(IV)/U­(III) couple

Synthesis of Electron-Rich Uranium(IV) Complexes Supported by Tridentate Schiff Base Ligands and Their Multi-Electron Redox Chemistry

The synthesis, structure, and reactivity of a new complex of U­(IV) with the tridentate Schiff base ligand Menaphtquinolen are reported. The reduction of the bis-ligand complexes [UX₂(^Menaphtquinolen)₂] (X = Cl, (<b>1-Cl</b>) ; I (<b>1-I</b>)) with potassium metal affords the U­(IV) complex of the new tetranionic hexadentate ligand μ-bis-^Menaphtquinolen formed through the intramolecular reductive coupling of the imino groups of each ^Menaphtquinolen unit. The solid state structure of the [U­(μ-bis-^Menaphtquinolen)]₂ dimer <b>2</b> isolated from toluene confirms the presence of a U­(IV) complex of the reduced ligand. Reactivity studies with molecular oxygen and 9,10-phenanthrenequinone show that complex <b>2</b> can act as a multielectron reducing agent releasing two electrons through the cleavage of the C–C bond to restore the original imino function of the ligand. In the resulting U­(IV) and U­(VI) complexes [U­(9,10-phenanthrenediol)­(^Menaphtquinolen)₂], <b>3</b>, and [UO₂(^Menaphtquinolen)₂], <b>4</b>, the restored tridentate Schiff base allows for the coordination of the reduced substrate to the metal. Electrochemical studies of complex <b>2</b> show the presence of irreversible ligand centered reduction processes and of a reversible U­(IV)/U­(III) couple

Lanthanide Complexes Based on β‑Diketonates and a Tetradentate Chromophore Highly Luminescent as Powders and in Polymers

A new type of octacoordinated ternary β-diketonates complexes of terbium and europium has been prepared using the anionic tetradentate terpyridine-carboxylate ligand (<b>L</b>) as a sensitizer of lanthanide luminescence in combination with two β-diketonates ligands 2-thenoyltrifluoroacetyl-acetonate (tta^–) for Eu³⁺ and trifluoroacetylacetonate (tfac^–) for Tb³⁺. The solid state structures of the two complexes [Tb­(<b>L</b>)­(tfac)₂] (<b>1</b>) and [Eu­(<b>L</b>)­(tta)₂] (<b>2</b>) have been determined by X-ray crystallography. Photophysical and ¹H NMR indicate a high stability of these complexes with respect to ligand dissociation in solution. The use of the anionic tetradentate ligand in combination with two β-diketonates ligands leads to the extension of the absorption window toward the visible region (390 nm) and to high luminescence quantum yield for the europium complex in the solid state (Φ = 66(6)%). Furthermore, these complexes have been incorporated in polymer matrixes leading to highly luminescent flexible layers

Lanthanide Complexes Based on β‑Diketonates and a Tetradentate Chromophore Highly Luminescent as Powders and in Polymers

A new type of octacoordinated ternary β-diketonates complexes of terbium and europium has been prepared using the anionic tetradentate terpyridine-carboxylate ligand (<b>L</b>) as a sensitizer of lanthanide luminescence in combination with two β-diketonates ligands 2-thenoyltrifluoroacetyl-acetonate (tta^–) for Eu³⁺ and trifluoroacetylacetonate (tfac^–) for Tb³⁺. The solid state structures of the two complexes [Tb­(<b>L</b>)­(tfac)₂] (<b>1</b>) and [Eu­(<b>L</b>)­(tta)₂] (<b>2</b>) have been determined by X-ray crystallography. Photophysical and ¹H NMR indicate a high stability of these complexes with respect to ligand dissociation in solution. The use of the anionic tetradentate ligand in combination with two β-diketonates ligands leads to the extension of the absorption window toward the visible region (390 nm) and to high luminescence quantum yield for the europium complex in the solid state (Φ = 66(6)%). Furthermore, these complexes have been incorporated in polymer matrixes leading to highly luminescent flexible layers

Multimetallic Cooperativity in Uranium-Mediated CO₂ Activation

The metal-mediated redox transformation of CO₂ in mild conditions is an area of great current interest. The role of cooperativity between a reduced metal center and a Lewis acid center in small-molecule activation is increasingly recognized, but has not so far been investigated for f-elements. Here we show that the presence of potassium at a U, K site supported by sterically demanding tris­(<i>tert</i>-butoxy)­siloxide ligands induces a large cooperative effect in the reduction of CO₂. Specifically, the ion pair complex [K­(18c6)]­[U­(OSi­(O^tBu)₃)₄], <b>1</b>, promotes the selective reductive disproportionation of CO₂ to yield CO and the mononuclear uranium­(IV) carbonate complex [U­(OSi­(O^tBu)₃)₄(μ-κ²:κ¹-CO₃)­K₂(18c6)], <b>4</b>. In contrast, the heterobimetallic complex [U­(OSi­(O^tBu)₃)₄K], <b>2</b>, promotes the potassium-assisted two-electron reductive cleavage of CO₂, yielding CO and the U­(V) terminal oxo complex [UO­(OSi­(O^tBu)₃)₄K], <b>3</b>, thus providing a remarkable example of two-electron transfer in U­(III) chemistry. DFT studies support the presence of a cooperative effect of the two metal centers in the transformation of CO₂

Metal-Controlled Diastereoselective Self-Assembly and Circularly Polarized Luminescence of a Chiral Heptanuclear Europium Wheel

The chiral dissymmetric tetradentate ligand (<i>S</i>)-6′-(4-phenyloxazolin-2-yl)-2,2′-bipyridine-6-carboxylate (S-Phbipox) leads to the diastereoselective assembly of a homochiral Eu³⁺ triangle and a highly emissive (quantum yield = 27%) heptanuclear wheel that is the largest example of a chiral luminescent complex of Eu³⁺ reported to date. The nuclearity of the assembly is controlled by the solvent and the Eu³⁺ cation. All of the compounds show large circularly polarized luminescence with an activity that varies with the nature of the assembly (highest for the homochiral trimer)

Metal-Controlled Diastereoselective Self-Assembly and Circularly Polarized Luminescence of a Chiral Heptanuclear Europium Wheel

The chiral dissymmetric tetradentate ligand (<i>S</i>)-6′-(4-phenyloxazolin-2-yl)-2,2′-bipyridine-6-carboxylate (S-Phbipox) leads to the diastereoselective assembly of a homochiral Eu³⁺ triangle and a highly emissive (quantum yield = 27%) heptanuclear wheel that is the largest example of a chiral luminescent complex of Eu³⁺ reported to date. The nuclearity of the assembly is controlled by the solvent and the Eu³⁺ cation. All of the compounds show large circularly polarized luminescence with an activity that varies with the nature of the assembly (highest for the homochiral trimer)