1,724,872 research outputs found
Reactions of Mo(NAr)(CH-t-Bu)(CH<sub>2</sub>-t-Bu)<sub>2</sub> with Alcohols To Give Metathesis Catalysts of the Type Mo(NAr)(CH-t-Bu)(CH<sub>2</sub>-t-Bu)(OR)
No full textThe reaction between Mo(NAr)(CH-t-Bu)(CH2-t-Bu)2 and 1 equiv of an alcohol yields either
Mo(NAr)(CH-t-Bu)(CH2-t-Bu)(OR) complexes or Mo(NAr)(CH2-t-Bu)3(OR) complexes, which evolve neopentane to yield the former. Preliminary experiments
have shown that Mo(NAr)(CH-t-Bu)(CH2-t-Bu)2(OR)
complexes are active for metathesis conversion of diallyl
ether to dihydrofuran and ethylene
Synthesis, Properties, and Coordination Chemistry of t-Bu-Xantphos Ligands
No full textThis thesis provides an account of research into a group of diphosphine ligands with a rigid xanthene backbone and tert -butyl substituents on the phosphorus atoms. The three ligands have different groups in the bridgehead position of the backbone (CMe₂, SiMe₂, or S) which change the natural (calculated) bite-angle of the ligand. The coordination chemistry of these t -Bu-xantphos ligands with late-transition metals has been investigated with a focus on metal complexes that may form in catalytic reactions. The three t -Bu-xantphos ligands were synthesised by lithiation of the backbone using sec -butyllithium/TMEDA and treatment with PtBu₂Cl. The natural biteangles of the Ph-xantphos (111.89–114.18°) and t -Bu-xantphos (126.80–127.56°) ligands were calculated using DFT. The bite-angle of the t -Bu-xantphos ligands is larger due to the increased steric bulk of the tert -butyl substituents. The electronic properties of the t -Bu-xantphos ligandswere also investigated by synthesis of their phosphine selenides. The values of ¹J PSe (689.1–698.5Hz) indicate that the t -Bu-xantphos ligands have a higher basicity than Ph-xantphos between PPh₂Me and PMe₃. The silver complexes, [Ag(t -Bu-xantphos)Cl] and [Ag(t -Bu-xantphos)]BF₄ were synthesised with the t -Bu-xantphos ligands. In contrast to systems with phenyl phosphines, all species were monomeric. [Rh(t -Bu-xantphos)Cl] complexes were synthesised, which reacted with H₂, forming [Rh(t -Bu-xantphos-ĸP,O,P ’)Cl(H)₂] complexes, and with CO, forming [Rh(t -Bu-xantphos)(CO)₂Cl] complexes. The [Rh(t -Bu-xantphos)Cl] species are air-sensitive readily forming [Rh(t -Bu-xantphos)Cl(ƞ²-O₂)] complexes. The crystal structure of [Rh(t -Bu-xantphos)Cl(ƞ²-O₂)], contained 15% of the dioxygen sites replaced with an oxo ligand. This is the first crystallographic evidence of a rhodium(III) oxo complex, and only the third rhodium oxo species reported. The coordination chemistry of the ligands with platinum(0) and palladium(0) showed some differences. [Pt(t -Bu-xantphos)(C₂H₄)] complexes were synthesised for all three ligands. However, reaction with [Pt(nb)₃] produced a mixture of [Pt(t -Bu-xantphos)] and [Pt(t -Bu-xantphos)(nb)] for t -Bu-sixantphos and t -Buthixantphos. Although few examples of isolable [Pt(PP)] complexes with diphosphines have been reported [Pt(t -Bu-thixantphos)] was isolated by removal of the norbornene. t -Bu-Xantphos formed small amounts of [Pt(t -Bu-xantphos)] initially, which progressed to [Pt(t -Bu-xantphos)H]X. The analogous reactions with [Pd(nb)₃] gave [Pd(t -Bu-xantphos)] and [Pd(t -Bu-xantphos)(nb)] complexes in all cases. [Pt(t -Bu-thixantphos)(C₂H₄)] and [M(t -Bu-thixantphos)] (M = Pd, Pt) react with oxygen forming [Pt(t -Bu-thixantphos)(ƞ²-O₂)], which reacts with CO to give [Pt(t -Bu-thixantphos-H-ĸ-C,P,P ’)OH] through a series of intermediates. [M(t -Bu-xantphos)Cl₂] (M = Pd, Pt) complexes were synthesised, showing exclusive trans coordination of the diphosphine ligands. The X-ray crystal structure of [Pt(t -Bu-thixantphos)Cl₂] has a bite-angle of 151.722(15)°. This is the first [PtCl₂(PP)] complex with a bite-angle between 114 and 171°. In polar solvents a chloride ligand dissociates from the [Pt(t -Bu-xantphos)Cl₂] complexes producing [Pt(t -Bu-xantphos-ĸP,O,P ’)Cl]⁺. The analogous [Pd(t -Bu-xantphos-ĸP,O,P ’)Cl]⁺ complexes were formed by reaction of the dichlorides complexes with NH₄PF₆. The [Pt(t -Bu-xantphos-ĸP,O,P ’)Me]⁺ pincer complexes were the only product from reaction with [Pt(C₆H₁₀)ClMe], with the stronger trans influence of the methyl ligand promoting loss of the chloride. The formation of the pincer complexes was further explored using DFT. The values of J PtC for the methyl carbons in the [Pt(t -Bu-xantphos-ĸP,O,P ’)Me]⁺ complexes, and J RhH for the hydride trans to the oxygen atom in the [Rh(t -Buxantphos-ĸP,O,P ’)Cl(H)₂] complexes were largest for t -Bu-sixantphos, then t -Buthixantphos, then t -Bu-xantphos. The trans influence of the t -Bu-xantphos oxygen donor follows the trend t -Bu-sixantphos < t -Bu-thixantphos < t -Bu-xantphos.</p
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
A homologous series of homoleptic zinc bis(1,4-di-tert-butyl-1,4-diaza-1,3-butadiene) complexes: Kx[Zn(t-BuNCHCHN-t-Bu)2], Zn(t-BuNCHCHN-t-Bu)2, and [Zn(t-BuNCHCHN-t-Bu)2](OTf)x)(X=1,2)
No full textA homologous series of mono- and dicationic, neutral, and mono- and dianionic zinc diazabutadiene complexes, Kx[Zn(t-BuNCHCHN-t-Bu)2], Zn(t-BuNCHCHN-t-Bu)2, and [Zn(t-BuNCHCHN-t-Bu)2](OTf)x (x = 1, 2), have been prepared and isolated in pure form. The crystal structures of the mono- and dicationic as well as of the monoanionic complexes are reported. In this series, the formal charge on the t-BuNCHCHN-t-Bu ligands ranges from -2 to +2, and the way in which the molecular geometry of the ligands varies with the charge is discussed. [Zn(t-BuNCHCHN-t-Bu)2](OTf)2 reacts with methanol to give 1,3-di-tert-butylimidazolium triflate. Crystal data: dicationic 2 ([Zn(t-BuNCHCHN-t-Bu)2](OTf)2, C22H40F6S2N4O6Zn), monoclinic, space group C2/c, with a = 18.015(6) Å, b = 9.257(6) Å, c = 20.012(5) Å, = 109.63(3), and Z = 4; monocationic 3·thf ([Zn(t-BuNCHCHN-t-Bu)2]OTf·thf, C25H48F3N4O3SZn), orthorhombic, space group P212121, with a = 10.3077(6) Å, b = 17.1974(6) Å, c = 17.8241(13) Å, and Z = 4; monoanionic 5·thf (K(thf)3[Zn(t-BuNCHCHN-t-Bu)2], C36H72KN4O3Zn), triclinic, space group P, with a = 10.8702(10) Å, b = 11.5175(9) Å, c = 18.2815(13) Å, = 73.795(6), = 74.227(6), = 75.736(7), and Z = 2; 7 (1,3-di-tert-butylimidazolium triflate, C12H21F3N2O3S), orthorhombic, space group Pbca, with a = 14.4086(8) Å, b = 12.0293(8) Å, c = 18.6985(12) Å, and Z = 8
CO-induced reductive elimination of P(t-Bu)2H from the platinum(II) dinuclear derivative Pt2[mu-P(t-Bu)2]2(H)2[P(t-Bu)2H]2, affording mononuclear platinum(0) or triangulo triplatinum(I,I,II) derivatives
No full textCarbon monoxide (1 atm) quantitatively converts the Pt(II) dinuclear derivative Pt-2[mu-P(t-Bu)(2)](2)(H)(2)[P(t-Bu)(2)H](2) (1) into the new (Pt2PtII)-Pt-I triangulo cluster Pt-3[mu-P(t-Bu)(2)](3)(H)(CO)(2) (2). The 44e(-) species 2 was characterized by IR and multinuclear NMR spectroscopy and by a single-crystal X-ray diffraction study. Complex 2 is formed through the slow CO-induced reductive elimination of P(t-Bu)(2)H from 1, affording the intermediate mononuclear Pt(0) carbonyl derivative Pt[P(t-Bu)(2)H](2)(CO)(2) (4), which equilibrates with the carbonyl-bridged triangulo derivative Pt-3[P(t-Bu)(2)H](3)(mu-CO)(3) (5); these are the main products under high pressures of CO. Two alternative mechanisms were examined for the subsequent formation of complex 2, the first being the rapid condensation of complex 4 with unreacted 1 while the second assumes 5 as the direct precursor of 2. Although the first mechanism cannot be conclusively rejected, the second seems the most appropriate, since under high pressures of CO/H-2 complex 2 was shown to equilibrate with 5. In the presence of an excess of other phosphines, the carbonylation of 1 yields quantitatively the mononuclear monocarbonyl derivatives Pt(PR(3))(3)(CO) (R(3) = Ph(3), Et(3), t-Bu(2)H)
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
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
- …
