Effects of the Grafting of Lanthanum Complexes on a Silica Surface on the Reactivity: Influence on Ethylene, Propylene, and 1,3-Butadiene Homopolymerization

In this contribution, we report full details of the ethylene, 1,3-butadiene, and propylene homopolymerization processes mediated by alkylated bis­(trimethyl)­silylamide lanthanide-grafted complexes using a density functional theory (DFT) study of the initiation and first propagation steps. These systems allows us (i) to examine the role of the grafting mode on the kinetics and thermodynamics of the three processes considered, (ii) to confirm the catalytic behavior of these grafted complexes in ethylene polymerization, (iii) to rationalize the experimental preference for 1,4-cis polymerization of 1,3-butadiene, and (iv) to provide unprecedented information on the catalytic activity of the lanthanide-grafted complex as a propylene hompolymerization catalyst

On the Interaction of Phosphines with High Surface Area Mesoporous Silica

To increase the efficiency and selectivity of homogeneous catalysts, particularly useful in the synthesis of fine chemicals and drugs, fine-tuning of the steric and electronic properties of the complexes can be achieved by modification of the ligands in the coordination sphere of the metal center. Considerable efforts have been devoted in order to immobilize such well-defined catalysts on solid substrates, e.g., silica, to facilitate catalysts’ recovery and to reduce contamination of desired products by metallic impurities. However, the presence of the silica surface can play a very important role in tuning the electronic properties of the metal, its steric environment, or in participating in the reactivity of the complex. In this context, several moieties have been used to anchor metallic catalysts on surfaces, but one of the most interesting is phosphine. Herein, we report on the addition of PPh₂Cl to mesoporous silica, which leads to the grafting and the oxidation of the phosphine species, even in the absence of oxygen, and that the nature of the surface plays an important role in secondary interactions, e.g., hydrogen bonding, and modifies the spectroscopic properties of the functional groups on the surface. In particular, the chemical shift of the phosphorus resonance in the ³¹P NMR spectra is altered by hydrogen bonding between available silanol or water molecules present on the silica surface and the phosphorus oxide. The DFT models developed for this process are in direct accordance with the experimental results and demonstrate firmly that the oxidation of the phosphine after grafting of ClPR₂ is highly favored thermodynamically and occurs with the formation of Si–Cl bonds on the surface. Passivation of the surface with hexamethyldisilazane limits the extent of the H-bonding between the surface and the oxide and also leads to some substitution reaction between bound phosphorus species and the trimethylsilyl (TMS) moieties. These findings offer new knowledge critical to fully ascertain the environment and the stability of immobilized phosphine-containing catalytic systems and thus further broaden the range of their reactivity

Yttrium Dihydride Cation [YH₂(THF)₂]⁺_<i>n</i>: Aggregate Formation and Reaction with (NNNN)-Type Macrocycles

Monocationic bis­(hydrocarbyl)­yttrium complexes [YR₂(THF)₂]­[A] (R = CH₂SiMe₃, CH₂C₆H₄-o-NMe₂; A = weakly coordinating anion) underwent hydrogenolysis using dihydrogen or phenylsilane to give a mixture of polynuclear solvent-stabilized dihydride cations [YH₂(THF)₂]_<i>n</i>[A]_<i>n</i>. The mixture composition as well as the nuclearity <i>n</i> depended on the starting material, solvent, and reaction conditions. NMR spectroscopic data in solution and X-ray diffraction data suggested that the main product was tetranuclear, although conclusive structural data were not obtained. DFT calculations supported a <i>closo</i>-type tetrahedral [YH₂(THF)₂]₄⁴⁺ core. The hydridic character of these cations was revealed by their reaction with benzophenone to give the bis­(diphenylmethoxy) cation [Y­(OCHPh₂)₂(THF)₄]­[AlR₄]. The reaction of this cluster with the (NNNN)-type macrocycle Me₄TACD under dihydrogen gave the dinuclear tetrahydride dication with quadruply bridging hydride ligands, [Y₂(μ-H)₄(Me₄TACD)₂]­[A]₂, analogous to the previously characterized lutetium derivative. NH-acidic (Me₃TACD)H gave the dinuclear dihydride dication [Y₂(μ-H)₂(Me₃TACD)₂(THF)₂]­[A]₂

Yttrium Dihydride Cation [YH₂(THF)₂]⁺_<i>n</i>: Aggregate Formation and Reaction with (NNNN)-Type Macrocycles

Monocationic bis­(hydrocarbyl)­yttrium complexes [YR₂(THF)₂]­[A] (R = CH₂SiMe₃, CH₂C₆H₄-o-NMe₂; A = weakly coordinating anion) underwent hydrogenolysis using dihydrogen or phenylsilane to give a mixture of polynuclear solvent-stabilized dihydride cations [YH₂(THF)₂]_<i>n</i>[A]_<i>n</i>. The mixture composition as well as the nuclearity <i>n</i> depended on the starting material, solvent, and reaction conditions. NMR spectroscopic data in solution and X-ray diffraction data suggested that the main product was tetranuclear, although conclusive structural data were not obtained. DFT calculations supported a <i>closo</i>-type tetrahedral [YH₂(THF)₂]₄⁴⁺ core. The hydridic character of these cations was revealed by their reaction with benzophenone to give the bis­(diphenylmethoxy) cation [Y­(OCHPh₂)₂(THF)₄]­[AlR₄]. The reaction of this cluster with the (NNNN)-type macrocycle Me₄TACD under dihydrogen gave the dinuclear tetrahydride dication with quadruply bridging hydride ligands, [Y₂(μ-H)₄(Me₄TACD)₂]­[A]₂, analogous to the previously characterized lutetium derivative. NH-acidic (Me₃TACD)H gave the dinuclear dihydride dication [Y₂(μ-H)₂(Me₃TACD)₂(THF)₂]­[A]₂

Yttrium Dihydride Cation [YH₂(THF)₂]⁺_<i>n</i>: Aggregate Formation and Reaction with (NNNN)-Type Macrocycles

Monocationic bis­(hydrocarbyl)­yttrium complexes [YR₂(THF)₂]­[A] (R = CH₂SiMe₃, CH₂C₆H₄-o-NMe₂; A = weakly coordinating anion) underwent hydrogenolysis using dihydrogen or phenylsilane to give a mixture of polynuclear solvent-stabilized dihydride cations [YH₂(THF)₂]_<i>n</i>[A]_<i>n</i>. The mixture composition as well as the nuclearity <i>n</i> depended on the starting material, solvent, and reaction conditions. NMR spectroscopic data in solution and X-ray diffraction data suggested that the main product was tetranuclear, although conclusive structural data were not obtained. DFT calculations supported a <i>closo</i>-type tetrahedral [YH₂(THF)₂]₄⁴⁺ core. The hydridic character of these cations was revealed by their reaction with benzophenone to give the bis­(diphenylmethoxy) cation [Y­(OCHPh₂)₂(THF)₄]­[AlR₄]. The reaction of this cluster with the (NNNN)-type macrocycle Me₄TACD under dihydrogen gave the dinuclear tetrahydride dication with quadruply bridging hydride ligands, [Y₂(μ-H)₄(Me₄TACD)₂]­[A]₂, analogous to the previously characterized lutetium derivative. NH-acidic (Me₃TACD)H gave the dinuclear dihydride dication [Y₂(μ-H)₂(Me₃TACD)₂(THF)₂]­[A]₂

Yttrium Dihydride Cation [YH₂(THF)₂]⁺_<i>n</i>: Aggregate Formation and Reaction with (NNNN)-Type Macrocycles

Monocationic bis­(hydrocarbyl)­yttrium complexes [YR₂(THF)₂]­[A] (R = CH₂SiMe₃, CH₂C₆H₄-o-NMe₂; A = weakly coordinating anion) underwent hydrogenolysis using dihydrogen or phenylsilane to give a mixture of polynuclear solvent-stabilized dihydride cations [YH₂(THF)₂]_<i>n</i>[A]_<i>n</i>. The mixture composition as well as the nuclearity <i>n</i> depended on the starting material, solvent, and reaction conditions. NMR spectroscopic data in solution and X-ray diffraction data suggested that the main product was tetranuclear, although conclusive structural data were not obtained. DFT calculations supported a <i>closo</i>-type tetrahedral [YH₂(THF)₂]₄⁴⁺ core. The hydridic character of these cations was revealed by their reaction with benzophenone to give the bis­(diphenylmethoxy) cation [Y­(OCHPh₂)₂(THF)₄]­[AlR₄]. The reaction of this cluster with the (NNNN)-type macrocycle Me₄TACD under dihydrogen gave the dinuclear tetrahydride dication with quadruply bridging hydride ligands, [Y₂(μ-H)₄(Me₄TACD)₂]­[A]₂, analogous to the previously characterized lutetium derivative. NH-acidic (Me₃TACD)H gave the dinuclear dihydride dication [Y₂(μ-H)₂(Me₃TACD)₂(THF)₂]­[A]₂

Yttrium Dihydride Cation [YH₂(THF)₂]⁺_<i>n</i>: Aggregate Formation and Reaction with (NNNN)-Type Macrocycles

Monocationic bis­(hydrocarbyl)­yttrium complexes [YR₂(THF)₂]­[A] (R = CH₂SiMe₃, CH₂C₆H₄-o-NMe₂; A = weakly coordinating anion) underwent hydrogenolysis using dihydrogen or phenylsilane to give a mixture of polynuclear solvent-stabilized dihydride cations [YH₂(THF)₂]_<i>n</i>[A]_<i>n</i>. The mixture composition as well as the nuclearity <i>n</i> depended on the starting material, solvent, and reaction conditions. NMR spectroscopic data in solution and X-ray diffraction data suggested that the main product was tetranuclear, although conclusive structural data were not obtained. DFT calculations supported a <i>closo</i>-type tetrahedral [YH₂(THF)₂]₄⁴⁺ core. The hydridic character of these cations was revealed by their reaction with benzophenone to give the bis­(diphenylmethoxy) cation [Y­(OCHPh₂)₂(THF)₄]­[AlR₄]. The reaction of this cluster with the (NNNN)-type macrocycle Me₄TACD under dihydrogen gave the dinuclear tetrahydride dication with quadruply bridging hydride ligands, [Y₂(μ-H)₄(Me₄TACD)₂]­[A]₂, analogous to the previously characterized lutetium derivative. NH-acidic (Me₃TACD)H gave the dinuclear dihydride dication [Y₂(μ-H)₂(Me₃TACD)₂(THF)₂]­[A]₂

To Bend or Not To Bend: Experimental and Computational Studies of Structural Preference in Ln(Tp^iPr₂)₂ (Ln = Sm, Tm)

The synthesis and characterization of Ln­(Tp^iPr2)₂ (Ln = Sm, <b>3Sm</b>; Tm, <b>3Tm</b>) are reported. While the simple ¹H NMR spectra of the compounds indicate a symmetrical solution structure, with equivalent pyrazolyl groups, the solid-state structure revealed an unexpected, “bent sandwich-like” geometry. By contrast, the structure of the less sterically congested Tm­(Tp^Me2,4Et)₂ (<b>4</b>) adopts the expected symmetrical structure with a linear B–Tm–B arrangement. Computational studies to investigate the origin of the unexpected bent structure of the former compounds indicate that steric repulsion between the isopropyl groups forces the Tp ligands apart and permits the development of unusual interligand C–H···N hydrogen-bonding interactions that help stabilize the structure. These results find support in the similar geometry of the Tm­(III) analogue [Tm­(Tp^iPr2)₂]­I, <b>3Tm</b>^<b>+</b>, and confirm that the low symmetry is not the result of a metal–ligand interaction. The relevance of these results to the general question of the coordination geometry of MX₂ and M­(C₅R₅)₂ (M = heavy alkaline earth and Ln­(II), X = halide, and C₅R₅ = bulky persubstituted cyclopentadienyl) complexes and the importance of secondary H-bonding and nonbonding interactions on the structure are highlighted

¹⁷O NMR Gives Unprecedented Insights into the Structure of Supported Catalysts and Their Interaction with the Silica Carrier

Flame silica was surface-labeled with ¹⁷O, through isotopic enrichment of both siloxanes and silanols. After heat treatment at 200 and 700 °C under vacuum, the resulting partially dehydroxylated silica materials were investigated by high-field solid-state ¹H and ¹⁷O NMR. More specifically, MQ MAS and HMQC sequences were used to probe the ¹⁷O local environment. In a further step, these ¹⁷O-tagged supports were used for the preparation of supported catalysts by reaction with perhydrocarbyl transition metal derivatives (zirconium tetraalkyl, tantalum trisalkyl–alkylidene, and tungsten trisalkyl–alkylidyne complexes). Detailed ¹⁷O 1D and 2D MQ and HMQC MAS NMR studies demonstrate that signals in the Si–<b>O</b>H, Si–<b>O</b>–Si, and Si–<b>O</b>–metal regions are highly sensitive to local structural modifications, thanks to ¹⁷O wide chemical shift and quadrupolar constant ranges. Experimental results were supported by DFT calculations. From the selective surface labeling, unprecedented information on interactions between supported catalysts and their inorganic carrier has been extracted

Well-Defined Supported Mononuclear Tungsten Oxo Species as Olefin Metathesis Pre-Catalysts

[WOCl₄] was grafted on silica dehydroxylated at 200 °C, and the structure of the surface species was elucidated by a combination of spectroscopic and theoretical methods, demonstrating the formation of [(SiO)₂WOCl₂] (<b>1a</b>) as the major species accompanied by minor monopodal species [(SiO)­WOCl₃] (<b>1b</b>). Most noteworthy, EXAFS and ¹⁷O NMR combined to DFT calculations helped elucidate the structure of the surface species. Alkylation was performed using SnMe₄, affording methyl species that were also precisely characterized. The alkylated species achieved excellent performances in isobutene metathesis to 2,3-dimethylbutene