η⁵–η¹ Switch in Divalent Phosphaytterbocene Complexes with Neutral Iminophosphoranyl Pincer Ligands: Solid-State Structures and Solution NMR ¹<i>J</i>_Yb–P Coupling Constants

This paper reports the synthesis of a series of complexes based on the bis­(pentamethylcyclopentadienyl)­ytterbium­(II) (<b>1</b>; Cp*₂Yb) and bis­(tetramethylphospholyl)­ytterbium­(II) (<b>2</b>; Tmp₂Yb) fragments bearing an additional neutral bis­(methyliminophosphoranyl)­pyridine ligand (<b>L</b>) on which the steric demand is modulated at the phosphorus position (triethyl, <b>L</b>^<b>Et</b>; triphenyl, <b>L</b>^<b>Ph</b>; tricyclohexyl, <b>L</b>^<b>Cy</b>) to yield the original complexes Cp*₂Yb<b>L</b>^<b>Et</b> (<b>1-L</b>^<b>Et</b>), Cp*₂Yb<b>L</b>^<b>Ph</b> (<b>1-L</b>^<b>Ph</b>), Tmp₂Yb<b>L</b>^<b>Et</b> (<b>2-L</b>^<b>Et</b>), Tmp₂Yb<b>L</b>^<b>Ph</b> (<b>2-L</b>^<b>Ph</b>), and Tmp₂Yb<b>L</b>^<b>Cy</b> (<b>2-L</b>^<b>Cy</b>), while no reaction occurs between <b>1</b> and <b>L</b>^<b>Cy</b>. 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>^<b>R</b> differs depending on the nature of the fragments <b>1</b> and <b>2</b> and on the steric bulk of <b>L</b>^<b>R</b>. The crystal structure of the divalent Tmp₂Yb­(py)₂ (<b>3</b>) is also reported for structural and spectroscopic comparisons. Interestingly, in both <b>2-L</b>^<b>Et</b> and <b>2-L</b>^<b>Cy</b>, one of the two Tmp ligands coordinates in an η¹ rather than in an η⁵ 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 ¹<i>J</i>_Yb–P coupling is used to track the coordination mode of the Tmp ligand and a large difference in the ¹<i>J</i>_Yb–P coupling constant allows the distinction between an η⁵ coordination mode and a dynamic η⁵–η¹ 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 Iminophosphorane–Phenoxide Ligands for Catalytic Ethylene Dimerization

A series of bidentate ligands associating an iminophosphorane and a phenoxide were synthesized and coordinated to nickel­(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 Et₂AlCl (Al/Ni = 22.5) with TOF up to 72  000 mol­(C₂H₄)/mol­(Ni)/h, giving selectively butene (more than 97%) with at best 93% of 1-C₄

Nickel Complexes Featuring Iminophosphorane–Phenoxide Ligands for Catalytic Ethylene Dimerization

A series of bidentate ligands associating an iminophosphorane and a phenoxide were synthesized and coordinated to nickel­(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 Et₂AlCl (Al/Ni = 22.5) with TOF up to 72  000 mol­(C₂H₄)/mol­(Ni)/h, giving selectively butene (more than 97%) with at best 93% of 1-C₄

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)­hydroxylamine (pyNO^–). Electrochemical studies performed on the homoleptic complexes [M­(pyNO)₄] (M = Th, U) revealed significant stabilization of the U complex upon one-electron oxidation. The salt [U­(pyNO)₄]⁺ was isolated by chemical oxidation of [U­(pyNO)₄]; spectroscopic and computational data support assignment as a U^V 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>–<b>8</b>, Ce­{(R₂N)­C­(N^<i>i</i>Pr)₂}₃, R = alkyl, silyl, or phenyl groups), guanidinate-amide [<b>GA</b>, <b>A</b> = N­(SiMe₃)₂, <b>G</b> = (Me₃Si)₂NC­(N^<i>i</i>Pr)₂], and guanidinate-aryloxide (<b>GOAr</b>, <b>OAr</b> = 2,6-di-<i>tert</i>-butylphenoxide) cerium­(III) complexes to understand and develop predictive capabilities for their optical properties. Structural studies performed on complexes <b>1</b>–<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 structure–luminescence model for molecular cerium­(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