1,721,106 research outputs found
Homoleptic Lanthanide Amide Catalysts for Organic Synthesis: Experiment and Theory
No full textHomoleptic lanthanide trisamides provide a simple and highly accessible bridge between organolanthanide chemistry and the synthetic methodology community, and although they have proven to be competent catalysts in a wide variety of unique transformations, their reactivity, scope, and mechanism remain understudied relative to lanthanide metallocenes and related organometallic catalysts. In this Perspective, we provide a critical review of the recent advances in homoleptic lanthanide-mediated catalysis as they apply to atom-efficient and environmentally benign organic syntheses. Instead of an exhaustive review, we focus on selected examples of the unexpected reactivity displayed by readily available Ln[N(SiMe3)2]3 complexes. By focusing on both the experimental as well as theoretical and mechanistic aspects of this work, we seek to highlight the value, versatility, and underlying concepts governing the unusual catalytic properties of homoleptic lanthanide amides and offer a general outlook on the prospects for this field
La[N(SiMe3)(2)](3)-Catalyzed Ester Reductions with Pinacolborane: Scope and Mechanism of Ester Cleavage
No full textTris[N,N-bis(trimethylsilyl)amido]lanthanum (La-NTMS) is an efficient, highly active, and selective homogeneous catalyst for ester reduction with pinacolborane (HBpin). Alkyl and aryl esters are cleaved to the corresponding alkoxy- and aryloxy-boronic esters which can then be straightforwardly hydrolyzed to alcohols. Ester reduction is achieved with 1 mol % catalyst loading at 25-60 degrees C, and most substrates are quantitatively reduced in 1 h. Nitro, halide, and amino functional groups are well tolerated, and ester reduction is completely chemoselective over potentially competing intra- or intermolecular alkene or alkyne hydroboration. Kinetic studies, isotopic labeling, and density functional theory calculations with energetic span analysis argue that ester reduction proceeds through a rate determining hydride-transfer step that is ligand-centered (hydride is transferred directly from bound HBpin to bound ester) and not through a metal hydride-based intermediate that is often observed in organolanthanide catalysis. The active catalyst is proposed to be a La-hemiacetal, [(Me3Si)(2)N](2)La-OCHR(OR)[HBpin], generated in situ from La-NTMS via hydroboronolysis of a single La-N(SiMe3)(2) bond. These results add to the growing compendium of selective oxygenate transformations that La-NTMS is competent to catalyze, further underscoring the value and versatility of homoleptic lanthanide complexes in homogeneous catalytic organic synthesis
Mechanism of Organoscandium-Catalyzed Ethylene Copolymerization with Amino-Olefins: A Quantum Chemical Analysis
No full textThe direct, efficient copolymerization of ethylene with polar monomers represents a "holy grail" for the synthesis of polar polyethylenes; however, developing effective catalysts for such copolymerizations remains a largely unsolved challenge. Very recently, organoscandium catalysts were shown to be very active for ethylene + polar monomer [H2C=CH(CH2)(n)CH(2)FG, FG = polar functional group] copolymerization. Interestingly, comonomer enchainment selectivity decreases with increasing linker length (n), while overall polymerization activity is largely unaffected, and the intriguing mechanistic origins are not yet understood. In this study, density functional theory (DFT) methods are employed to investigate the mechanism of organoscandium-catalyzed ethylene + amino olefin (AO) copolymerization, using (C5Me4SiMe3)Sc(CH2CH2CH3)B+(C6F5)(4)(-) (Sc-1) as the model active species and N-(1-butenyl)"Pr-2 and N-(1-octenyl)"Pr-2 as model comonomers. Among conceivable scenarios in monomer coordination, activation, and insertion, it is found that copolymerization activity is largely governed by intermolecular amino olefin N-coordination. Amino olefin n-dependent enchainment patterns arise from chain-length regulation of the energy barrier for an amino olefin chelating "self-assisted" enchainment pathway. Short-chain N-(1-butenyl)"Pr-2 enchains via a self-assisted insertion pathway (6.0 kcal/mol energy barrier), while long-chain N-(1-octenyl)"Pr-2 enchains via unassisted 1,2-insertion with exogenous amine coordination (7.2 kcal/mol energy barrier). These findings explain the experimental results, showcase the characteristic reactivity of Sc catalysts in polar monomer copolymerization, and highlight the potential and challenges in developing catalysts for polar monomer copolymerization
Silver(I) Bis(pyrazolyl)methane Complexes and Their Implementation as Precursors for Metallic Silver Deposition
No full textThe following AgIbis(pyrazolyl)methane complexes with BF4–/NO3–as counteranions were synthesized and characterized: [Ag(LpzH)]2(NO3)2(1), [Ag(LpzMe)]2(NO3)2(2), [Ag(LpzH)]n(BF4)n(3), and [Ag(LpzMe)]n(BF4)n(4) [LpzH= bis(pyrazolyl)methane; LpzMe= bis(3,5-dimethylpyrazolyl)methane]. These complexes were prepared to identify optimum precursors for the thermolytic deposition of metallic silver. The crystal structures of 1 and 2 show that the complexes are dinuclear and that the NO3–anions interact with the metals. In contrast, 3 is polymeric and the BF4–does not interact with the metal. When crystallizing 1–4 in non-anhydrous solvents, the presence of adventitious water further reacts with 3 and 4 (but not with 1 and 2) to yield dinuclear complexes [Ag(LpzH)(H2O)2]2(BF4)2(3a) and [Ag(LpzMe)(H2O)2]2(BF4)2(4a). All of the dinuclear species 1, 2, 3a and 4a exhibit an argentophilic interaction with Ag···Ag distances in the range 3.18–2.99 Å. Thermogravimetric analysis (TGA) shows that 1 and 2 have lower decomposition temperatures (231 and 255 °C, respectively) than 3 and 4 (298 and 331 °C, respectively). 2 was further investigated as a precursor for metallic silver deposition by spin-coating solutions (10–3m, THF/DMSO, 4:1), followed by annealing at 310 °C on 52100 steel substrates. According to energy-dispersive X-ray spectroscopy (EDS) and scanning electron microscopy (SEM) the metal deposition proceeds primarily via an island growth (Volmer–Weber) mechanism
Pyridylamido Bi-Hafnium Olefin Polymerization Catalysis: Conformationally Supported Hf···Hf Enchainment Cooperativity
No full textHomobimetallic Hf(IV) complexes, L2-Hf2Me5
(3) and L2-Hf2Me4 (4) (L2 = N,N′-{[naphthalene-1,4-
diylbis(pyr idine-6,2-diyl)]bis[(2-isopropylphenyl)-
methylene)]bis(2,6-diisopropylaniline}), were synthesized by
reaction of the free ligand L2 with the appropriate Hf precursor
and were characterized in solution (NMR) and in the solid
state (X-ray diffraction). In 3, L2 acts as a dianionic tridentate
ligand for one Hf metal center and as a monoanionic bidentate
ligand for the other, whereas in 4, both Hf units are
tricoordinated to opposite sides of L2. In the solid state, the
Hf···Hf distance is significantly different in 3 vs 4 (6.16 vs 8.06
Å, respectively), but in solution, the structural dynamics of the two linked metallic units in bis-activated complex 3 accesses
conformers with far closer Hf···Hf distances (∼3.2 Å). Once activated with Ph3C+B(C6F5)4
− (B1) or PhNMe2H+B(C6F5)4
−
(NB), 3 exhibits pronounced bimetallic cooperative effects in ethylene homopolymerization and ethylene +1-octene
copolymerization vs the monometallic analogue L1-HfMe2 (1, L1 = 2,6-diisopropyl-N-{(2-isopropylphenyl)[6-(naphthalen-1-
yl)pyridin-2-yl]methyl}aniline) and bimetallic 4, producing polyethylene with 5.7 times higher Mw and poly(ethylene-co-1-
octene) with 2.4 times higher Mw and 1.9 times greater 1-octene enchainment densities than 1. The activation chemistry of 3 and
4 with 1 or 2 equiv of B1 and NB is characterized in detail by NMR spectroscopy. In sharp contrast to 1, which undergoes Hf−
Cnaph protonolysis followed by naphthyl remetalation with NB as the cocatalyst, activation of 3 with B1 or NB proceeds by
consecutive −CH3 protonolysis/abstractions at each Hf center, explaining the higher polymerization activity of 3/NB versus 1/
NB. All product polymers have narrow (2−3) PDIs, and this is explained by NMR evidence for very fast exchange of alkyl
moieties between the two active Hf metal centers. Key experimental findings are supported by DFT analysis
Cationic Pyridylamido Adsorbate on Bronsted Acidic Sulfated Zirconia: A Molecular Supported Organohafnium Catalyst for Olefin Homo- and Co-Polymerization
No full textHere we report the combined application of highresolution solid-state 13C−CPMAS-NMR and FT-IR spectroscopy, elemental analysis, kinetic poisoning/active site counting, variable dielectric constant medium, and DFT computation to characterize the surface chemistry of a pyridylamido hafnium complex (Cat1, L1-HfMe2, L1 = 2,6-diisopropyl-N-{(2-isopropylphenyl)[6-(naphthalen-1-yl)pyridin-2-yl]methyl}aniline) adsorbed on Brønsted acidic sulfated zirconia (ZrS). The spectroscopic and DFT results indicate protonolytic formation of organohafnium cations having a largely electrostatic pyridylamido-Hf-CH3 +···ZrS− interaction with elongated Hf···OZrS distances of ∼2.14 Å. High-molecular-weight polyethylenes and ethylene/1-octene copolymers are obtained with this supported catalyst without an activator/cocatalyst. The DFT calculations reveal that the first ethylene insertion into the Hf-methyl bond has a lower barrier than the corresponding insertion into the Hf-aryl bond of this single-site heterogeneous catalyst
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
/Sulfated Alumina‐Catalyzed Olefin Polymerization Tacticity via Catalyst⋅⋅⋅Surface Ion‐Pairing
No full textSurface-bound organometallic molecules have recently enabled the development of single-site heterogeneous catalysts, advancing the atomic scale understanding and diversity of heterogeneous catalysis. Here we report that supporting Cp*ZrMe3 (Cat1) on acidic sulfated-alumina (AlS) affords the surface catalyst Cat1/AlS, which was characterized by multi-dimensional solid-state NMR spectroscopies, and is active in ethylene homo- and copolymerizations, as well as propylene and 1-hexene homopolymerizations. In contrast to propylene (or 1-hexene) polymerization by homogeneous Cp*ZrMe2+ B(C6F5)4− which yields atactic polyolefins, Cat1/AlS promotes remarkable isotacticity with mmmm >95 %. Complementary DFT analysis argues that the restrictive local Cat1/AlS C1-symmetry favors activation and enchainment at the propylene re enantioface, promoting isotactic polymerization via a “back-skip-like” mechanis
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