16 research outputs found
Tuning UraniumāNitrogen Multiple Bond Formation with Ancillary Siloxide Ligands
The
new homoleptic <i>ate</i> UĀ(III) siloxide [KĀ(18c6)]Ā[UĀ(OSiĀ(O<sup><i>t</i></sup>Bu)<sub>3</sub>)<sub>4</sub>] <b>2</b> was prepared in 69% yield by reduction of [UĀ(OSiĀ(O<sup><i>t</i></sup>Bu)<sub>3</sub>)<sub>4</sub>] <b>3</b> with KC<sub>8</sub>. The reaction of the neutral UĀ(III) siloxide complex [UĀ(OSiĀ(O<sup><i>t</i></sup>Bu)<sub>3</sub>)<sub>2</sub>(Ī¼-OSiĀ(O<sup><i>t</i></sup>Bu)<sub>3</sub>)]<sub>2</sub> <b>1</b> with adamantyl azide leads to the isolation of the dinuclear
UĀ(VI) imido complex [U<sub>2</sub>(NAd)<sub>4</sub>(OSiĀ(O<sup><i>t</i></sup>Bu)<sub>3</sub>)<sub>4</sub>] <b>4</b>. The
X-ray crystal structure shows the presence of a ācationācation
interactionā between the two [UĀ(NAd)<sub>2</sub>]<sup>2+</sup> groups. In contrast the reactions of <b>2</b> with the trimethylsilyl
and adamantyl azides afford the UĀ(V) imido complexes [KĀ(18c6)]Ā[UĀ(NSiMe<sub>3</sub>)Ā(OSiĀ(O<sup><i>t</i></sup>Bu)<sub>3</sub>)<sub>4</sub>] <b>5-TMS</b> and [KĀ(18c6)]Ā[UĀ(NAd)Ā(OSiĀ(O<sup><i>t</i></sup>Bu)<sub>3</sub>)<sub>4</sub>] <b>5-Ad</b> pure in 48%
and 66% yield, respectively. The reaction of <b>2</b> with CsN<sub>3</sub> in THF at ā40
Ā°C yields a mixture of products from which the azido
UĀ(IV) complex [KĀ(18c6)]Ā[UĀ(N<sub>3</sub>)Ā(OSiĀ(O<sup><i>t</i></sup>Bu)<sub>3</sub>)<sub>4</sub>] <b>7</b> and the Ī¼-nitrido
diuraniumĀ(V) complex [KUĀ(Ī¼-N)Ā(OSiĀ(O<sup><i>t</i></sup>Bu)<sub>3</sub>)]<sub>2</sub> <b>8</b> were isolated. The crystal
structure
of <b>8</b> shows the presence of a rare U<sub>2</sub>N<sub>2</sub> core with two nitrido atoms bridging two uranium centers
in a diamond-shaped geometry. In contrast, the reaction of <b>1</b> with CsN<sub>3</sub> affords the diuraniumĀ(IV) complex CsĀ{(Ī¼-N)Ā[UĀ(OSiĀ(O<sup><i>t</i></sup>Bu)<sub>3</sub>)<sub>3</sub>]<sub>2</sub>} <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 UĀ(III) with azides
Tuning UraniumāNitrogen Multiple Bond Formation with Ancillary Siloxide Ligands
The
new homoleptic <i>ate</i> UĀ(III) siloxide [KĀ(18c6)]Ā[UĀ(OSiĀ(O<sup><i>t</i></sup>Bu)<sub>3</sub>)<sub>4</sub>] <b>2</b> was prepared in 69% yield by reduction of [UĀ(OSiĀ(O<sup><i>t</i></sup>Bu)<sub>3</sub>)<sub>4</sub>] <b>3</b> with KC<sub>8</sub>. The reaction of the neutral UĀ(III) siloxide complex [UĀ(OSiĀ(O<sup><i>t</i></sup>Bu)<sub>3</sub>)<sub>2</sub>(Ī¼-OSiĀ(O<sup><i>t</i></sup>Bu)<sub>3</sub>)]<sub>2</sub> <b>1</b> with adamantyl azide leads to the isolation of the dinuclear
UĀ(VI) imido complex [U<sub>2</sub>(NAd)<sub>4</sub>(OSiĀ(O<sup><i>t</i></sup>Bu)<sub>3</sub>)<sub>4</sub>] <b>4</b>. The
X-ray crystal structure shows the presence of a ācationācation
interactionā between the two [UĀ(NAd)<sub>2</sub>]<sup>2+</sup> groups. In contrast the reactions of <b>2</b> with the trimethylsilyl
and adamantyl azides afford the UĀ(V) imido complexes [KĀ(18c6)]Ā[UĀ(NSiMe<sub>3</sub>)Ā(OSiĀ(O<sup><i>t</i></sup>Bu)<sub>3</sub>)<sub>4</sub>] <b>5-TMS</b> and [KĀ(18c6)]Ā[UĀ(NAd)Ā(OSiĀ(O<sup><i>t</i></sup>Bu)<sub>3</sub>)<sub>4</sub>] <b>5-Ad</b> pure in 48%
and 66% yield, respectively. The reaction of <b>2</b> with CsN<sub>3</sub> in THF at ā40
Ā°C yields a mixture of products from which the azido
UĀ(IV) complex [KĀ(18c6)]Ā[UĀ(N<sub>3</sub>)Ā(OSiĀ(O<sup><i>t</i></sup>Bu)<sub>3</sub>)<sub>4</sub>] <b>7</b> and the Ī¼-nitrido
diuraniumĀ(V) complex [KUĀ(Ī¼-N)Ā(OSiĀ(O<sup><i>t</i></sup>Bu)<sub>3</sub>)]<sub>2</sub> <b>8</b> were isolated. The crystal
structure
of <b>8</b> shows the presence of a rare U<sub>2</sub>N<sub>2</sub> core with two nitrido atoms bridging two uranium centers
in a diamond-shaped geometry. In contrast, the reaction of <b>1</b> with CsN<sub>3</sub> affords the diuraniumĀ(IV) complex CsĀ{(Ī¼-N)Ā[UĀ(OSiĀ(O<sup><i>t</i></sup>Bu)<sub>3</sub>)<sub>3</sub>]<sub>2</sub>} <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 UĀ(III) with azides
Tuning UraniumāNitrogen Multiple Bond Formation with Ancillary Siloxide Ligands
The
new homoleptic <i>ate</i> UĀ(III) siloxide [KĀ(18c6)]Ā[UĀ(OSiĀ(O<sup><i>t</i></sup>Bu)<sub>3</sub>)<sub>4</sub>] <b>2</b> was prepared in 69% yield by reduction of [UĀ(OSiĀ(O<sup><i>t</i></sup>Bu)<sub>3</sub>)<sub>4</sub>] <b>3</b> with KC<sub>8</sub>. The reaction of the neutral UĀ(III) siloxide complex [UĀ(OSiĀ(O<sup><i>t</i></sup>Bu)<sub>3</sub>)<sub>2</sub>(Ī¼-OSiĀ(O<sup><i>t</i></sup>Bu)<sub>3</sub>)]<sub>2</sub> <b>1</b> with adamantyl azide leads to the isolation of the dinuclear
UĀ(VI) imido complex [U<sub>2</sub>(NAd)<sub>4</sub>(OSiĀ(O<sup><i>t</i></sup>Bu)<sub>3</sub>)<sub>4</sub>] <b>4</b>. The
X-ray crystal structure shows the presence of a ācationācation
interactionā between the two [UĀ(NAd)<sub>2</sub>]<sup>2+</sup> groups. In contrast the reactions of <b>2</b> with the trimethylsilyl
and adamantyl azides afford the UĀ(V) imido complexes [KĀ(18c6)]Ā[UĀ(NSiMe<sub>3</sub>)Ā(OSiĀ(O<sup><i>t</i></sup>Bu)<sub>3</sub>)<sub>4</sub>] <b>5-TMS</b> and [KĀ(18c6)]Ā[UĀ(NAd)Ā(OSiĀ(O<sup><i>t</i></sup>Bu)<sub>3</sub>)<sub>4</sub>] <b>5-Ad</b> pure in 48%
and 66% yield, respectively. The reaction of <b>2</b> with CsN<sub>3</sub> in THF at ā40
Ā°C yields a mixture of products from which the azido
UĀ(IV) complex [KĀ(18c6)]Ā[UĀ(N<sub>3</sub>)Ā(OSiĀ(O<sup><i>t</i></sup>Bu)<sub>3</sub>)<sub>4</sub>] <b>7</b> and the Ī¼-nitrido
diuraniumĀ(V) complex [KUĀ(Ī¼-N)Ā(OSiĀ(O<sup><i>t</i></sup>Bu)<sub>3</sub>)]<sub>2</sub> <b>8</b> were isolated. The crystal
structure
of <b>8</b> shows the presence of a rare U<sub>2</sub>N<sub>2</sub> core with two nitrido atoms bridging two uranium centers
in a diamond-shaped geometry. In contrast, the reaction of <b>1</b> with CsN<sub>3</sub> affords the diuraniumĀ(IV) complex CsĀ{(Ī¼-N)Ā[UĀ(OSiĀ(O<sup><i>t</i></sup>Bu)<sub>3</sub>)<sub>3</sub>]<sub>2</sub>} <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 UĀ(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 UĀ(IV) with
the tridentate Schiff base ligand Menaphtquinolen are reported. The
reduction of the bis-ligand complexes [UX<sub>2</sub>(<sup>Me</sup>naphtquinolen)<sub>2</sub>] (X = Cl, (<b>1-Cl</b>) ; I (<b>1-I</b>)) with potassium metal affords the UĀ(IV) complex of the
new tetranionic hexadentate ligand Ī¼-bis-<sup>Me</sup>naphtquinolen
formed through the intramolecular reductive coupling of the imino
groups of each <sup>Me</sup>naphtquinolen unit. The solid state structure
of the [UĀ(Ī¼-bis-<sup>Me</sup>naphtquinolen)]<sub>2</sub> dimer <b>2</b> isolated from toluene confirms the presence of a UĀ(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 CāC bond to restore the original imino
function of the ligand. In the resulting UĀ(IV) and UĀ(VI) complexes
[UĀ(9,10-phenanthrenediol)Ā(<sup>Me</sup>naphtquinolen)<sub>2</sub>], <b>3</b>, and [UO<sub>2</sub>(<sup>Me</sup>naphtquinolen)<sub>2</sub>], <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 UĀ(IV)/UĀ(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 UĀ(IV) with
the tridentate Schiff base ligand Menaphtquinolen are reported. The
reduction of the bis-ligand complexes [UX<sub>2</sub>(<sup>Me</sup>naphtquinolen)<sub>2</sub>] (X = Cl, (<b>1-Cl</b>) ; I (<b>1-I</b>)) with potassium metal affords the UĀ(IV) complex of the
new tetranionic hexadentate ligand Ī¼-bis-<sup>Me</sup>naphtquinolen
formed through the intramolecular reductive coupling of the imino
groups of each <sup>Me</sup>naphtquinolen unit. The solid state structure
of the [UĀ(Ī¼-bis-<sup>Me</sup>naphtquinolen)]<sub>2</sub> dimer <b>2</b> isolated from toluene confirms the presence of a UĀ(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 CāC bond to restore the original imino
function of the ligand. In the resulting UĀ(IV) and UĀ(VI) complexes
[UĀ(9,10-phenanthrenediol)Ā(<sup>Me</sup>naphtquinolen)<sub>2</sub>], <b>3</b>, and [UO<sub>2</sub>(<sup>Me</sup>naphtquinolen)<sub>2</sub>], <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 UĀ(IV)/UĀ(III)
couple
Lanthanide Complexes Based on Ī²āDiketonates and a Tetradentate Chromophore Highly Luminescent as Powders and in Polymers
A new type of octacoordinated ternary
Ī²-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 Ī²-diketonates ligands 2-thenoyltrifluoroacetyl-acetonate
(tta<sup>ā</sup>) for Eu<sup>3+</sup> and trifluoroacetylacetonate
(tfac<sup>ā</sup>) for Tb<sup>3+</sup>. The solid state structures
of the two complexes [TbĀ(<b>L</b>)Ā(tfac)<sub>2</sub>] (<b>1</b>) and [EuĀ(<b>L</b>)Ā(tta)<sub>2</sub>] (<b>2</b>) have been determined by X-ray crystallography. Photophysical and <sup>1</sup>H 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 Ī²-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 (Ī¦ = 66(6)%). Furthermore, these complexes
have been incorporated in polymer matrixes leading to highly luminescent
flexible layers
Lanthanide Complexes Based on Ī²āDiketonates and a Tetradentate Chromophore Highly Luminescent as Powders and in Polymers
A new type of octacoordinated ternary
Ī²-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 Ī²-diketonates ligands 2-thenoyltrifluoroacetyl-acetonate
(tta<sup>ā</sup>) for Eu<sup>3+</sup> and trifluoroacetylacetonate
(tfac<sup>ā</sup>) for Tb<sup>3+</sup>. The solid state structures
of the two complexes [TbĀ(<b>L</b>)Ā(tfac)<sub>2</sub>] (<b>1</b>) and [EuĀ(<b>L</b>)Ā(tta)<sub>2</sub>] (<b>2</b>) have been determined by X-ray crystallography. Photophysical and <sup>1</sup>H 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 Ī²-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 (Ī¦ = 66(6)%). Furthermore, these complexes
have been incorporated in polymer matrixes leading to highly luminescent
flexible layers
Multimetallic Cooperativity in Uranium-Mediated CO<sub>2</sub> Activation
The metal-mediated
redox transformation of CO<sub>2</sub> 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 trisĀ(<i>tert</i>-butoxy)Āsiloxide ligands induces a large cooperative effect in the
reduction of CO<sub>2</sub>. Specifically, the ion pair complex [KĀ(18c6)]Ā[UĀ(OSiĀ(O<sup>t</sup>Bu)<sub>3</sub>)<sub>4</sub>], <b>1</b>, promotes the
selective reductive disproportionation of CO<sub>2</sub> to yield
CO and the mononuclear uraniumĀ(IV) carbonate complex [UĀ(OSiĀ(O<sup>t</sup>Bu)<sub>3</sub>)<sub>4</sub>(Ī¼-Īŗ<sup>2</sup>:Īŗ<sup>1</sup>-CO<sub>3</sub>)ĀK<sub>2</sub>(18c6)], <b>4</b>. In contrast,
the heterobimetallic complex [UĀ(OSiĀ(O<sup>t</sup>Bu)<sub>3</sub>)<sub>4</sub>K], <b>2</b>, promotes the potassium-assisted two-electron
reductive cleavage of CO<sub>2</sub>, yielding CO and the UĀ(V) terminal
oxo complex [UOĀ(OSiĀ(O<sup>t</sup>Bu)<sub>3</sub>)<sub>4</sub>K], <b>3</b>, thus providing a remarkable example of two-electron transfer
in UĀ(III) chemistry. DFT studies support the presence of a cooperative
effect of the two metal centers in the transformation of CO<sub>2</sub>
Biologically Relevant Heterodinuclear IronāManganese Complexes
The heterodinuclear complexes [Fe<sup>III</sup>Mn<sup>II</sup>(L-Bn)Ā(Ī¼-OAc)<sub>2</sub>]Ā(ClO<sub>4</sub>)<sub>2</sub> (<b>1</b>) and [Fe<sup>II</sup>Mn<sup>II</sup>(L-Bn)Ā(Ī¼-OAc)<sub>2</sub>]Ā(ClO<sub>4</sub>) (<b>2</b>) with the unsymmetrical
dinucleating ligand HL-Bn
{[2-bisĀ[(2-pyridylmethyl)Āaminomethyl]]-6-[benzyl-2-(pyridylmethyl)Āaminomethyl]-4-methylphenol}
were synthesized and characterized as biologically relevant models
of the new Fe/Mn class of nonheme enzymes. Crystallographic studies
have been completed on compound <b>1</b> and reveal an Fe<sup>III</sup>Mn<sup>II</sup>Ī¼-phenoxobisĀ(Ī¼-carboxylato) core.
A single location of the Fe<sup>III</sup> ion in <b>1</b> and
of the Fe<sup>II</sup> ion in <b>2</b> was demonstrated by MoĢssbauer
and <sup>1</sup>H NMR spectroscopies, respectively. An investigation
of the temperature dependence of the magnetic susceptibility of <b>1</b> revealed a moderate antiferromagnetic interaction (<i>J</i> = 20 cm<sup>ā1</sup>) between the high-spin Fe<sup>III</sup> and Mn<sup>II</sup> ions in <b>1</b>, which was confirmed
by MoĢssbauer and electron paramagnetic resonance (EPR) studies.
The electrochemical properties of complex <b>1</b> are described.
A quasireversible electron transfer at ā40 mV versus Ag/AgCl
corresponding to the Fe<sup>III</sup>Mn<sup>II</sup>/Fe<sup>II</sup>Mn<sup>II</sup> couple appears in the cyclic voltammogram. Thorough
investigations of the MoĢssbauer and EPR signatures of complex <b>2</b> were performed. The analysis allowed evidencing of a weak
antiferromagnetic interaction (<i>J</i> = 5.72 cm<sup>ā1</sup>) within the Fe<sup>II</sup>Mn<sup>II</sup> pair consistent with
that deduced from magnetic susceptibility measurements (<i>J</i> = 6.8 cm<sup>ā1</sup>). Owing to the similar value of the
Fe<sup>II</sup> zero-field splitting (<i>D</i><sub>Fe</sub> = 3.55 cm<sup>ā1</sup>), the usual treatment within the strong
exchange limit was precluded and a full analysis of the electronic
structure of the ground state of complex <b>2</b> was developed.
This situation is reminiscent of that found in many diiron and ironāmanganese
enzyme active sites
Biologically Relevant Heterodinuclear IronāManganese Complexes
The heterodinuclear complexes [Fe<sup>III</sup>Mn<sup>II</sup>(L-Bn)Ā(Ī¼-OAc)<sub>2</sub>]Ā(ClO<sub>4</sub>)<sub>2</sub> (<b>1</b>) and [Fe<sup>II</sup>Mn<sup>II</sup>(L-Bn)Ā(Ī¼-OAc)<sub>2</sub>]Ā(ClO<sub>4</sub>) (<b>2</b>) with the unsymmetrical
dinucleating ligand HL-Bn
{[2-bisĀ[(2-pyridylmethyl)Āaminomethyl]]-6-[benzyl-2-(pyridylmethyl)Āaminomethyl]-4-methylphenol}
were synthesized and characterized as biologically relevant models
of the new Fe/Mn class of nonheme enzymes. Crystallographic studies
have been completed on compound <b>1</b> and reveal an Fe<sup>III</sup>Mn<sup>II</sup>Ī¼-phenoxobisĀ(Ī¼-carboxylato) core.
A single location of the Fe<sup>III</sup> ion in <b>1</b> and
of the Fe<sup>II</sup> ion in <b>2</b> was demonstrated by MoĢssbauer
and <sup>1</sup>H NMR spectroscopies, respectively. An investigation
of the temperature dependence of the magnetic susceptibility of <b>1</b> revealed a moderate antiferromagnetic interaction (<i>J</i> = 20 cm<sup>ā1</sup>) between the high-spin Fe<sup>III</sup> and Mn<sup>II</sup> ions in <b>1</b>, which was confirmed
by MoĢssbauer and electron paramagnetic resonance (EPR) studies.
The electrochemical properties of complex <b>1</b> are described.
A quasireversible electron transfer at ā40 mV versus Ag/AgCl
corresponding to the Fe<sup>III</sup>Mn<sup>II</sup>/Fe<sup>II</sup>Mn<sup>II</sup> couple appears in the cyclic voltammogram. Thorough
investigations of the MoĢssbauer and EPR signatures of complex <b>2</b> were performed. The analysis allowed evidencing of a weak
antiferromagnetic interaction (<i>J</i> = 5.72 cm<sup>ā1</sup>) within the Fe<sup>II</sup>Mn<sup>II</sup> pair consistent with
that deduced from magnetic susceptibility measurements (<i>J</i> = 6.8 cm<sup>ā1</sup>). Owing to the similar value of the
Fe<sup>II</sup> zero-field splitting (<i>D</i><sub>Fe</sub> = 3.55 cm<sup>ā1</sup>), the usual treatment within the strong
exchange limit was precluded and a full analysis of the electronic
structure of the ground state of complex <b>2</b> was developed.
This situation is reminiscent of that found in many diiron and ironāmanganese
enzyme active sites