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Coordinatively unsaturated alkyne complexes of tungsten: synthesis and dynamic nuclear magnetic resonance studies of some but-2-yne derivatives
No full textReaction of [{W(μ-Br)Br(CO)(MeC≡CMe)2}2] (I) with one molar equiv of Na(S2CNMe2), Na(S2PMe2), or Tl(acac) (acac = acetylacetonate) gives mononuclear derivs. [WBr(L2)(CO)(MeC≡CMe)2] (L2 = S2CNMe2, S2PMe2, acac). The 1H and 13C NMR spectra of the acac complex are temp. independent whereas dynamic 1H NMR studies of the S2CNMe2 and S2PMe2 derivs. have been interpreted in terms of a mol. rearrangement involving dechelation of the bidentate ligand which exchanges the alkyne environments and the ends of the dithiolate ligand. The reaction of I with two molar equiv of Na(S2PMe2) gives the bis-alkyne complex [W(S2PMe2)2(MeC≡CMe)2] (II) whereas Na[S2P(OMe)2] and Tl(acac) give mono-alkyne derivs. [W(L2)2(CO)(MeC≡CMe)] [III; L2 = S2P(OMe)2 or acac]. 1H NMR studies of II suggest fluxional behavior involving dechelation of S2PMe2 which exchanges both the S2PMe2 methyls and the alkyne methyls without necessarily requiring alkyne propeller rotation. In contrast, fluxional behavior in III [L2 = S2P(OMe)2] clearly involves alkyne propeller rotation in addn. to two other processes, one of which exchanges the Me groups of one S2P(OMe)2 group. However, the only dynamic process obsd. with III (L2 = acac) appears to involve alkyne propeller rotation
Coordinatively unsaturated alkyne complexes: synthesis of mono and bisalkyne complexes of tungsten(II)
No full text[WBr2(CO)4]n reacts with alkynes to give complexes [WBr2CO(RC≡CR)2]2 (I, R = R' = Me, Et, Ph; R = Me, R' = Ph), which react with nucleophiles L [L = CNCMe3, PPh3, or P(OMe)3] to give monoalkyne derivs. [WBr2(CO)(RC≡CR')L2]. An intermediate bis-alkyne adduct [WBr2CO(MeC≡CMe)2(CNCMe3)] was isolated in the reaction of [WBr2(CO)(MeC≡CMe)2]2 with CNCMe3 illustrating that cleavage of the dimer I is the first stage in these reactions
Synthesis and nuclear magnetic resonance studies of coordinatively unsaturated alkyne complexes of tungsten(II)
No full text[WBr2(CO)4]2 reacts with RC≡CR1 (R = R1 = Me, Et, Ph; R = Me, R1 = Ph) to give [WBr2(CO)(RC≡CR1)2]2. These react with nucleophiles (L) to give WBr2(CO)L2(RC≡CR1) [L = CNCMe3, P(OMe)3, PPh3; R = R1 = Me, Ph; R = Me, R1 = Ph], which exist as cis and/or trans isomers and which exhibit alkyne propeller rotation, according to 1H and 31P NMR studies. The isolation of WBr2(CO)(CNCMe3)(MeC≡CMe)2 indicates that the reactions proceed via monomeric bis(alkyne) intermediates
Coordinatively unsaturated diene complexes of tungsten(II) and their reactions with nucleophiles to give six- and seven-coordinate derivatives
No full text[WBr2(CO)4]2 reacts with dienes L1 [cycloocta-1,5-diene (L2), cyclooctatetraene (L3), norbornadiene (L4)] to give coordinatively unsatd. WBr2(CO)2L1 (I). I react with Lewis bases L [CNCMe3, PMe2Ph, P(OMe)3] in 1:1 molar ratio to give coordinatively unsatd. WBr2(CO)LL1 via intermediate adducts WBr2(CO)2LL1, isolated for L1 = L4. With a 3:1 molar ratio of ligand to metal, displacement of L2 or L3 occurs to give WBr2(CO)2L3 (II), whereas a 4:1 molar ratio (L = CNCMe3) gives WBr2(CO)L4. In contrast, I (L1 = L4) forms WBr2(CO)L2L4 [L = P(OMe)3, PMe2Ph; L2 = 2,2'-bipyridyl]. 1H and 31P NMR studies of II [L = P(OMe)3, PMe2Ph] reveal the presence of 2 isomers which exhibit dynamic stereochem. at ambient temp.; the static form of 1 isomer was detected at lower temps
Synthesis of coordinatively unsaturated diolefin complexes of tungsten(II)
No full text[WBr2(CO)4] reacts with cycloocta-1,5-diene, cyclooctatetraene, and norbornadiene (LL) to give complexes [WBr2(CO)LL]2 (I) which react with Lewis bases (L') [L' = CNCMe3, PMe2Ph, (POMe)3] to give coordinatively unsatd. diene complexes [WBr2CO(LL)L] (II) and coordinatively satd. derivs. [WBr2(CO)(LL)L'2] (LL = norbornadiene) and [WBr2(CO)2L'3]. Two isomeric forms of II exist which have been sepd. in the case of [WBr2(CO)(NBD)(PMe2Ph)] (NBD = norbornadiene)
Alkene metathesis catalyzed by [WBr2(CO)2(diene)]/AlCl2Et
No full text[WBr2(CO)2(diene)]/AlCl2Et mixts. promote metathesis of cis-2-pentene to give 2-butene and 3-hexene. For diene = norbornadiene, the reaction is catalytic, and is accompanied by cis to trans isomerization of the 2-pentene, whereas for diene = 1,5-cyclooctadiene or cyclooctatetraene, the metathesis is essentially stoichiometric. The same complexes catalyze the metathesis of trans-2-pentene to give 2-butene and 3-hexene, but at approx. twice the rate obsd. for the cis-alkene. [WBr2(CO)2(diene)]/AlCl2Et mixts. also catalyze the metathesis of 1,7-octadiene to give cyclohexene and ethene. The intermediacy of metal carbene derivs. in such reactions is supported by the reactions of [WBr2(CO)2(norbornadiene)] with alkylating agents SiMe4 and MeMgBr which give methane, whereas [WBr2(CO)2(norbornadiene)] and AlCl2Et in the absence of alkenes gave virtually stoichiometric amts. of ethane
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
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
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