1,720,968 research outputs found
Synthesis and reactivity of cycloplatinated rollover complexes, an experimental and theoretical approach
Full text linkCyclometalation reactions constitute one of the evergreen fields in synthetic chemistry because of the variety of applications, from substrate functionalization to catalyst production and model for C-H activation. A variation on this theme is rollover cyclometalation where a bidentate donor, such as 2,2'-bipyridine, upon C-H activation leads to a species with a free donor atom that is able to influence the properties, hence the reactivity, of the complex.We explored the similarities/differences between classical and rollover cyclometalation to check the importance of the second nitrogen donor by comparing suitably substituted 2,2'-bipyridines and 2-phenylpyridines; moreover the properties of the ligands were tuned to investigate the impact of stereoelectronic modification on the C-H activation process.The reactivity of the newly synthesized cyclometalates having formula [Pt(K2-NC)(CH3)(L)] (κ2-N,C = cyclometalated ligand, L = neutral ligand) was tested in different conditions: ligand substitutions, protonolysis, oxidative additions and reductive eliminations. A wide variety of mono and dinuclear complexes was obtained and fully characterized by NMR spectroscopy.Further insight has been sought fromin silicoexperiments using DFT calculations that led to a nice agreement with the chemistry observed
De-cyclometalation in organoplatinum(II) derivatives: a compared experimental-theoretical study
No full textPlatinum(II), palladium(II) and gold(III) adducts and cyclometalated derivatives of 6-methoxy-2,2′-bipyridine: A comparative study
Full text linkReaction of 6-methoxy-2,2′-bipyridine (bpy6OMe) with the electron-rich platinum(II) complex [Pt(Me)2(DMSO)2] gave the rollover cyclometalated complex [Pt(κ2-N,C-bpy6OMe-H) (Me) (DMSO)] under mild conditions. The occurrence of rollover cyclometalation was demonstrated by single crystal X-ray diffraction structure determination. In contrast, reaction of bpy6OMe with [Pt(Ph)2(DMSO)2] and [Pt(Cl)2(DMSO)2] under mild conditions gave only adduct species of the type [Pt(X)2(bpy6OMe)] (X = Ph, Cl). Under harsher conditions, activation of a C-H bond in the methoxy substituent yielded the terdentate cyclometalated complex [Pt (κ3-N,N,C-bpy6OMe-H)Cl]. Finally, reaction of Pd(II) and Au(III) derivatives invariably gave adduct species, with the exception of the dimeric complex [Pd(κ2-N,C-bpy6OMe-H) (OAc)]2, which, however, was not isolated in pure form. The reactivity of bpy6OMe, as emerged from this study, was compared with that of the corresponding disubstituted ligand 6,6′-dimethoxy-2,2′-bipyridine, previously studied by us
Chirality Generated by Hindered Rotations in Platinum(II)‐Pyridine Complexes
Full text linkThe reaction of trans-[Pt(DMSO)2MeCl] with 2-alkylsubstituted pyridines pyR affords a series of complexes of the type [Pt(pyR)(DMSO)MeCl]. These complexes are rare examples of organometallic square-planar complexes having four different ligands. Of the three possible geometric isomers, only the trans(N,S)-[Pt(pyR)(DMSO)MeCl] was observed in solution and isolated in the solid state. However, the lack of chirality typical to square planar geometry can be resolved in several ways one of which is described in this paper. 1H NMR studies reveal diastereotopicity of the DMSO methyls, as well as the CH2 protons in the ethyl- and neopentyl-pyridine complexes and the methyls in the isopropyl-pyridine derivative. This observation can be ascribed to restricted rotations around the Pt-N bond, thus resulting in the presence of planar chirality for these complexes. Additionally, large downfield shifts in the 1H NMR spectra of the alpha C-H protons in the pyridine substituents suggested the presence of PtMIDLINE HORIZONTAL ELLIPSISH-C interactions, confirmed via single crystal X-ray studies in the case of the isopropyl complex [Pt(pyip)(DMSO)MeCl] (4), which shows an apical PtMIDLINE HORIZONTAL ELLIPSISH interaction with a short 2.585 angstrom distance. DFT calculations shed light into the stability of these complexes and the influence of the pyridine-substituent on their chiral configuration.A series of Pt(II) complexes with 2-alkylsubstituted pyridines display planar chirality due to the influence of the pyridine substituent, which enhances the rotational barrier around the Pt-N bond generating a couple of enantiomers. The chiral conformation is also stabilized by the presence of an axial Pt & sdot;& sdot;& sdot;H-C interaction.imag
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Full text link“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
Full text linkWe 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
Electronic and steric effects in rollover C–H bond activation
No full textSteric and electronic factors in rollover C–H bond activation of substituted 2,2′-bipyridines, mediated by platinum(II), have been investigated by comparing the influence of two substituents, CH3 and CF3, on the progress of the reaction. The substituents were chosen to have similar steric hindrance but different electronic effects and were placed in position 6 (i.e., near one of the nitrogen atoms) or in position 5, which allows, in part, electronic and steric influence to be distinguished. The ligands studied, 6-methyl-2,2′-bipyridine, 5-methyl-2,2′-bipyridine, 6-trifluoromethyl-2,2′-bipyridine, and 5-trifluoromethyl-2,2′-bipyridine, were compared to unsubstituted 2,2′-bipyridine in the reaction with the electron-rich complex [Pt(Me)2(DMSO)2]. The electron-withdrawing CF3 group was found to have a significant effect in accelerating the cyclometalation reaction. The substituent in position 6 influences the stability of the intermediate adduct [Pt(N,N)(Me)2] (N,N = chelated bipyridine), as indicated by density functional theory calculations. The steric hindrance of substituted bipyridines was also evaluated by defining and measuring the angle ζ in [Pt(N,N)(Me)2] adducts. The presence of a substituent in position 6 causes destabilization of the adduct, acceleration of the cyclometalation reaction, and regioselectivity of C–H bond activation
Heterobimetallic rollover derivatives
No full textHeterobimetallic complexes with metal centers connected by a small delocalized ligand constitute an interesting class of compounds. Here we report that starting from the mononuclear platinum(II) rollover complexes [Pt(bipy-H)(L)Cl] (bipy-H = 2,2′-bipyridine C(3′)-N cyclometalated, L= DMSO, PPh3) a second rollover cyclometalation may produce a series of Pt(II)/Pd(II) heterobimetallic complexes where the two metals are linked by the planar, highly delocalized, doubly deprotonated 2,2′-bipyridine
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