6 research outputs found
η<sup>5</sup>âη<sup>1</sup> Switch in Divalent Phosphaytterbocene Complexes with Neutral Iminophosphoranyl Pincer Ligands: Solid-State Structures and Solution NMR <sup>1</sup><i>J</i><sub>YbâP</sub> Coupling Constants
This
paper reports the synthesis of a series of complexes based
on the bisÂ(pentamethylcyclopentadienyl)ÂytterbiumÂ(II) (<b>1</b>; Cp*<sub>2</sub>Yb) and bisÂ(tetramethylphospholyl)ÂytterbiumÂ(II)
(<b>2</b>; Tmp<sub>2</sub>Yb) fragments bearing an additional
neutral bisÂ(methyliminophosphoranyl)Âpyridine ligand (<b>L</b>) on which the steric demand is modulated at the phosphorus position
(triethyl, <b>L</b><sup><b>Et</b></sup>; triphenyl, <b>L</b><sup><b>Ph</b></sup>; tricyclohexyl, <b>L</b><sup><b>Cy</b></sup>) to yield the original complexes Cp*<sub>2</sub>Yb<b>L</b><sup><b>Et</b></sup> (<b>1-L</b><sup><b>Et</b></sup>), Cp*<sub>2</sub>Yb<b>L</b><sup><b>Ph</b></sup> (<b>1-L</b><sup><b>Ph</b></sup>), Tmp<sub>2</sub>Yb<b>L</b><sup><b>Et</b></sup> (<b>2-L</b><sup><b>Et</b></sup>), Tmp<sub>2</sub>Yb<b>L</b><sup><b>Ph</b></sup> (<b>2-L</b><sup><b>Ph</b></sup>), and Tmp<sub>2</sub>Yb<b>L</b><sup><b>Cy</b></sup> (<b>2-L</b><sup><b>Cy</b></sup>), while no reaction occurs between <b>1</b> and <b>L</b><sup><b>Cy</b></sup>. The crystal
structures of these sterically crowded complexes are reported as well
as room-temperature NMR data for all the complexes. The solid-state
coordination mode of <b>L</b><sup><b>R</b></sup> differs
depending on the nature of the fragments <b>1</b> and <b>2</b> and on the steric bulk of <b>L</b><sup><b>R</b></sup>. The crystal structure of the divalent Tmp<sub>2</sub>YbÂ(py)<sub>2</sub> (<b>3</b>) is also reported for structural and spectroscopic
comparisons. Interestingly, in both <b>2-L</b><sup><b>Et</b></sup> and <b>2-L</b><sup><b>Cy</b></sup>, one of the
two Tmp ligands coordinates in an η<sup>1</sup> rather than
in an η<sup>5</sup> fashion, a relevant coordination mode for
the study of sterically induced reductions. The behavior of those
complexes in solution varies with the sterics and electronics of the
ligands, as demonstrated by variable-temperature NMR experiments.
In solution, the <sup>1</sup><i>J</i><sub>YbâP</sub> coupling is used to track the coordination mode of the Tmp ligand
and a large difference in the <sup>1</sup><i>J</i><sub>YbâP</sub> coupling constant allows the distinction between an η<sup>5</sup> coordination mode and a dynamic η<sup>5</sup>âη<sup>1</sup> switch
Functional Synthetic Model for the Lanthanide-Dependent Quinoid Alcohol Dehydrogenase Active Site
The oxidation of
methanol by dehydrogenase enzymes is an essential
part of the bacterial methane metabolism cycle. The recent discovery
of a lanthanide (Ln) cation in the active site of the XoxF dehydrogenase
represents the only example of a rare-earth element in a physiological
role. Herein, we report the first synthetic, functional model of Ln-dependent
dehydrogenase and its stoichiometric and catalytic dehydrogenation
of a benzyl alcohol. Density functional theory calculations implicate
a hydride transfer mechanism for these reactions
Nickel Complexes Featuring IminophosphoraneâPhenoxide Ligands for Catalytic Ethylene Dimerization
A series of bidentate ligands associating
an iminophosphorane and
a phenoxide were synthesized and coordinated to nickelÂ(II), leading
initially to bimetallic KNi adducts. Replacement of the potassium
by another metal allowed the isolation and characterization of bimetallic
LiNi and AlNi complexes, while addition of one equivalent of triphenylphosphine
gave access to monometallic complexes. The same type of complex was
obtained with the coordination of a tridentate ligand incorporating
a supplementary amine donor. These paramagnetic complexes were characterized
by elemental analysis, and some of them by X-ray diffraction, evidencing
a tetrahedral nickel center. They were shown to efficiently catalyze
the oligomerization of ethylene in the presence of Et<sub>2</sub>AlCl
(Al/Ni = 22.5) with TOF up to 72 â000 molÂ(C<sub>2</sub>H<sub>4</sub>)/molÂ(Ni)/h, giving selectively butene (more than 97%) with
at best 93% of 1-C<sub>4</sub>
Nickel Complexes Featuring IminophosphoraneâPhenoxide Ligands for Catalytic Ethylene Dimerization
A series of bidentate ligands associating
an iminophosphorane and
a phenoxide were synthesized and coordinated to nickelÂ(II), leading
initially to bimetallic KNi adducts. Replacement of the potassium
by another metal allowed the isolation and characterization of bimetallic
LiNi and AlNi complexes, while addition of one equivalent of triphenylphosphine
gave access to monometallic complexes. The same type of complex was
obtained with the coordination of a tridentate ligand incorporating
a supplementary amine donor. These paramagnetic complexes were characterized
by elemental analysis, and some of them by X-ray diffraction, evidencing
a tetrahedral nickel center. They were shown to efficiently catalyze
the oligomerization of ethylene in the presence of Et<sub>2</sub>AlCl
(Al/Ni = 22.5) with TOF up to 72 â000 molÂ(C<sub>2</sub>H<sub>4</sub>)/molÂ(Ni)/h, giving selectively butene (more than 97%) with
at best 93% of 1-C<sub>4</sub>
Coordination Chemistry of a Strongly-Donating Hydroxylamine with Early Actinides: An Investigation of Redox Properties and Electronic Structure
Separations of f-block
elements are a critical aspect of nuclear waste processing. Redox-based
separations offer promise, but challenges remain in stabilizing and
differentiating actinides in high oxidation states. The investigation
of new ligand types that provide thermodynamic stabilization to high-valent
actinides is essential for expanding their fundamental chemistry and
to elaborate new separation techniques and storage methods. We report
herein the preparation and characterization of Th and U complexes
of the pyridyl-hydroxylamine ligand, <i>N</i>-<i>tert</i>-butyl-<i>N</i>-(pyridin-2-yl)Âhydroxylamine (pyNO<sup>â</sup>). Electrochemical studies performed on the homoleptic complexes
[MÂ(pyNO)<sub>4</sub>] (M = Th, U) revealed significant stabilization
of the U complex upon one-electron oxidation. The salt [UÂ(pyNO)<sub>4</sub>]<sup>+</sup> was isolated by chemical oxidation of [UÂ(pyNO)<sub>4</sub>]; spectroscopic and computational data support assignment
as a U<sup>V</sup> cation
Understanding and Controlling the Emission Brightness and Color of Molecular Cerium Luminophores
Molecular
cerium complexes are a new class of tunable and energy-efficient
visible- and UV-luminophores. Understanding and controlling the emission
brightness and color are important for tailoring them for new and
specialized applications. Herein, we describe the experimental and
computational analyses for series of <i>tris</i>(guanidinate)
(<b>1</b>â<b>8</b>, CeÂ{(R<sub>2</sub>N)ÂCÂ(N<sup><i>i</i></sup>Pr)<sub>2</sub>}<sub>3</sub>, R = alkyl,
silyl, or phenyl groups), guanidinate-amide [<b>GA</b>, <b>A</b> = NÂ(SiMe<sub>3</sub>)<sub>2</sub>, <b>G</b> = (Me<sub>3</sub>Si)<sub>2</sub>NCÂ(N<sup><i>i</i></sup>Pr)<sub>2</sub>], and guanidinate-aryloxide (<b>GOAr</b>, <b>OAr</b> = 2,6-di-<i>tert</i>-butylphenoxide) ceriumÂ(III) complexes
to understand and develop predictive capabilities for their optical
properties. Structural studies performed on complexes <b>1</b>â<b>8</b> revealed marked differences in the steric
encumbrance around the cerium center induced by various guanidinate
ligand backbone substituents, a property that was correlated to photoluminescent
quantum yield. Computational studies revealed that consecutive replacements
of the amide and aryloxide ligands by guanidinate ligand led to less
nonradiative relaxation of bright excited states and smaller Stokes
shifts. The results establish a comprehensive structureâluminescence
model for molecular ceriumÂ(III) luminophores in terms of both quantum
yields and colors. The results provide a clear basis for the design
of tunable, molecular, cerium-based, luminescent materials