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Role of cyclometalation in controlling the rates of ligand substitution at platinum(II) complexes
No full textThe rates of chloride for triphenylphosphine substitution have been measured in dichloromethane for a series of cyclometalated [Pt(N-N-C)Cl] complexes containing a number of terdentate N-N-C. anionic ligands, derived from deprotonated alkyl-, phenyl-, and benzyl-6-substituted 2,2'-bipyridines. These rates have been compared with those of the corresponding [Pt(N-N)(C)Cl] (N-N = 2,2'-bipyridine; C = CH3 or C6H5) complexes having the same set of donor atoms but less constrained arrangements of the ligands. The reactions of the cyclometalated compounds occur as a single-stage conversion from the substrate to the ionic pair [Pt(N-N-C)(PPh3)]Cl products. There is no evidence by H-1 and P-31{H-1} NMR spectroscopy for the formation of other Pt(II) species or of concurrent ring-opening processes. In contrast, ih the monoalkyl- or monoaryl-2,2'-bipyridine complexes, chloride substitution is followed by subsequent slower processes which involve the detachment of one arm of the chelated 2,2'-bipyridine, fast cis to hans isomerization of the cis-[Pt(PPh3)(2)(eta (1)-bipy)(R)](+) transient intermediate, and, eventually, the release of free bipy, yielding trans-[Pt(PPh3)(2)(R)Cl] (R = Me or Ph). All reactions are first-order with respect to complex and phosphine concentration, obeying the simple rate law k(obsd) =: k(2)[PPh3]. The values of the second-order rate constant k(2) do not seem particularly sensitive to the nature of the bonded organic moiety (alkyl or aryl), to its structure (cyclometalated or not), to the size of the ring, or to the number of alkyl substituents on it. The effects are those foreseen on the basis of an associative mode of activation. The only exception to this pattern of behavior is constituted by the complex [Pt(bipy(phi)-H)Cl] (bipy(phi) = 6-phenyl-2,2(bipyridine), which features a significant rate enhancement with respect to the analogue [Pt(bipy)(Ph)Cl] complex. The results of this work, together with those of a previous paper, Suggest that there is not a specific role of cyclometalation in controlling the reactivity, unless an in-plane aryl ring becomes part of the pi -acceptor system of the chelated 2,2'-bipyridine, behaving as a cyclometalated analogue of the nitrogen terdentate 2,2':6',2 " -terpyridine
Reactivity of tertiary phosphines toward a cyclometallated platinum(II) complex: evaluation of steric and electronic contributions
No full textAn extended series of phosphines (L), having widely different steric and electronic properties, was used to study the kinetics of chloride substitution on the cyclometallated complex [Pt(N-N-C)Cl] (N-N-CH = 6-(1-methylbenzyl)-2,2'-bipyridine). The final cationic products [Pt(N-N-C)(L)](+) were fully characterised by H-1 and P-31{H-1} NMR spectroscopy in CDCl3 solution. The values of the (1)J(PtP) coupling constants for these complexes showed interesting dependencies upon the size and the sigma-donor ability of the ligands. The reaction, in dichloromethane as solvent at 298.2 K, takes place by way of a direct bimolecular attack of the ligand on the substrate, with no evidence of a significant solvent contribution. The order of reactivity is that expected on the Oasis of the nucleophilic reactivity constants (n(pt)(P), derived from previous studies. The values of the second-order rate constants (k(2), M-1 s(-1)) can be resolved quantitatively into steric, electronic and aryl contributions of the phosphine ligands, by use of a four-parameter equation log k(2) alpha + beta(pK(a)') + gamma(theta) + delta(E-ar), where pK(a)' measures the sigma-donor ability of the phosphines, theta (Tolman's cone angle) is a measure of the size of the P-donor ligand, and (ar) is related to the number of aryl groups attached to the ligand phosphorus atom. This is the first reported example of an aryl effect contributing to the nucleophilicity of phosphines as entering ligands on square-planar complexes. No steric threshold is observed. Steric and electronic profiles for the substitution process can be plotted showing the dependence of rates on the size and the sigma-donor ability of the phosphines. The sensitivity of the rates to pK(a)' is significant (beta = 0.20 +/- 0.04) and this is related to the structural properties of the substrate. (C) 1997 Elsevier Science S.A
Ring Closure Kinetics of Bidentate Hemilabile P,N and P,S Ligands on a Platinum(II) Complex
No full textComplexes of the type cis-[PtPh2(CO)(eta(1)-P-N)] and cis[PtPh2(CO)(eta(1)-P-N)1, where bidentate phosphorus-nitrogen and phosphorus-sulfur ligands are bound to the metal centre in a monodentate fashion [P-N = Ph2PC5H4N (Ph2PPY), Ph2P(CH2)(2)C5H4N (ppye), Ph2P(O-C6H4)NMe2 (PNMe2), Ph2P(CH2),NMe2 (n = 2, 3, i.e., peNMe(2) and ppNMe(2)) and P-S = Ph2P(CH2)(2)SEt (P-SEt), Ph2P(CH2)(n)SPh (n = 1, 2, i.e., P-CH2SPh and P-SPh)], were prepared in situ by reaction of the hybrid ligands with cis- [PtPh2(CO) (SEt2)]. In each case, the first observed process was the fast substitution of diethyl sulfide by the phosphanyl group leading to the monosubstituted ring-open cis-[PtPh2(CO) eta(1)-P-X)] (X = N or S) complexes, which were characterised in solution by H-1 and P-31{H-1} NMR spectroscopy. These initially formed species undergo a slow ring closure process with extrusion of carbon monoxide and formation of the chelate [PtPh2(P-X)1 products, except in the case of the short-bite Ph2PPy and P-CH2SPh ligands and of ppNMe(2), where ring closure was not observed. The chelate complexes were isolated as solids from the reaction of the ligands with cis-[PtPh2(Me2SO)(2)], A single-crystal X-ray diffractometric study of cis-[PtPh2(P-SEt)J (18) was performed, The crystal packing showed linear chains originated by weak intermolecular Pt...H-C hydrogen bonding interactions. The chelation kinetics of P-X in the cis-[PtPh2(CO)(eta(1)-P-X)] complexes have been monitored in [D]chloroform by H-1 and P-31{H-1} NMR, The rates of ring closure were found to be strongly dependent on the nature (S or N) and steric hindrance of the chelating end of the monocoordinated bidentate P-X ligand, and on the size of the ring formed. In contrast, ring size plays a negligible role, if any, in the dechelation reactions of cis-[PtPh2(S-S)] [S-S = 1, 2-bis(phenylthio)ethane, dpte and 1,3-bis (phenylthio) propane, dptp] using diphosphanes (dppm and dppp) as reagents. These kinetic data, together with those of previous work, give useful insight into the factors controlling cyclisation reactions and the stability of the rings in square planar platinum(II) complexe
Effective cell uptake of nanoassemblies of a fluorescent amphiphilic cyclodextrin and an anionic porphyrin
No full textGoing 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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe 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
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