1,721,025 research outputs found
Schiff Base Metal Complexes for Second Order Nonlinear Optics
No full textThe research in the development of new materials suitable for NLO purposes has increased in the last few decades. Coordination metal complexes with tetradentate Schiff base ligands are a good target to aim to, since they are robust, chemically and physically
stable, and the degrees of variability that can lead to an enhancement of the NLO responses are great. All the chemical variations on the molecular structures (changes of the ligand skeleton and metals) studied are here reported, without going into details, in order to underline the fundamental role of the metal centre in the NLO properties and all the possible developments in the research in this specific field
Effect of crystal packing and coordinated solvent molecules on metal-ligand bond distances in linear trinuclear nickel compounds with bridging acetato and Schiff base ligands
No full textA combined experimental and theoretical study on the molecular structure of linear trinuclear nickel compounds of general formula [Ni3(μ-Zsaltn)2(μ-AcO)2(S)2] (Z = H, 1S, and Br, 2S, where S identifies coordinated solvent molecules) is here presented. In these complexes the three metal ions are linked together by bridging acetato ions and tetradentate Schiff base ligands, these last derived from the condensation of 1,3-diaminopropane with two equivalents of 5-Z-salicylaldehyde. Two solvent molecules S can also interact with the terminal nickel ions to complete an octahedral coordination environment. We report the synthesis of [Ni3(μ-Brsaltn)2(μ-AcO)2] (2), and its crystal structure with two coordinated dimethylformamide molecules, [Ni3(μ-Brsaltn)2(μ-AcO)2(DMF)2] (2DMF). Its comparison with the crystal structure of 2 without solvent, together with the series of compounds 1, 1DMF, 1MeOH, 1DMSO, 1Py and 1H2O (MeOH = methanol, DMSO = dimethylsulfoxide, Py = pyridine) available in the literature, allows recognising modification in Ni–O and Ni–N coordination bond lengths and peculiar intermolecular contacts between neighbour molecules. In order to discern among the contribution of solvent molecules, crystal packing and substituent Z (H or Br) on bond distances (and angles), we performed a systematic theoretical study on four representative derivatives, namely 1, 1DFM, 2 and 2DMF, with the M06 functional and a careful choice of the best basis set. The results indicate that, while the major modulating factor is the presence or absence of the sixth solvent molecule to the peripheral nickel(II) ion, even if less strongly coordinated with Ni–O/N bond distances > 2.15 Å π-π stacking interactions and C–H⋯π and/or C–Br⋯π non-bonded interactions affect the coordination geometry in a significant way
Copper(II) compounds with NNO tridentate Schiff base ligands: effect of subtle variations in ligands on complex formation, structures and magnetic properties
No full textThe formation and the magnetic properties of the copper(II) compounds [Cu(L1)(py)](ClO4) (1a) and [Cu(L2)(py)](ClO4) (2a), bearing the NNO tridentate Schiff base ligand L1 = (E)-2-((3-aminoethylimino)methyl)phenolate or L2 = (E)-2-((3-aminopropylimino)methyl)phenolate (obtained by monocondensation of salicylaldehyde, salH, and ethylenediamine, en, or 1,3-propylenediamine, tn, respectively) and pyridine (py) are presented. These complexes are converted into new mono-, di- and trinuclear derivatives, whose nature depends on the length of the diamine used and hence on the size of the corresponding
metallacycle. Pyridine can be substituted by a molecule of N,N-dimethylformamide (DMF) in compound 2a, leading to the mononuclear [Cu(L2)(dmf)](ClO4) (2b), while 1a undergoes only decomposition under similar conditions. Pyrazine does not act as bridging ligand between two copper centres by substitution of py, but its reaction with 2a yields either the dinuclear compound [Cu2(L2)2(ClO4)2] (2c), with exclusion of pyridine, or the trinuclear [Cu3(L2)3(l3-OH)](ClO4)2 (2d), where pyrazine acts as base generating OH, and it does not appear in the product. Reaction of 1a with pyrazine yields only the trinuclear [Cu3(L1)3(l3-OH)](ClO4)2 (1d). Also with 2,20-bipyridine (2,20-bpy) dinuclear complexes are not formed, but bpy acts as bidentate ligand to copper yielding the pentacoordinated mononuclear compounds [Cu(L1)(2,20-bpy)](ClO4) (1e) and [Cu(L2)(2,20-bpy)](ClO4) (2e). The crystal structures of compounds 2b, 2c and 1e have been solved and are reported. The magnetic susceptibilities vM(T) of 1a and 2a have been studied, showing the absence of any measurable Cu–Cu interaction for 1a (en, five-membered ring), while a weak but interesting intermolecular Cu–Cu ferromagnetic coupling (J = +0.96(3) cm1) through the short dimeric CuO contacts is detected for 2a (tn, six-membered ring). The X-band EPR spectrum of 2a in a frozen methanol solution at 70 K shows the hyperfine coupling of
mononuclear copper with the three coordinated 14N atoms, yielding seven narrow lines
Push-pull unsymmetrical substitution in nickel(II) complexes with tetradentate N2O2 Schiff base ligands: Synthesis, structures and linear-nonlinear optical studies
No full textNew push-pull (A-D) nickel(II) compounds of general formula [Ni(5-A-5'-D-saltn)] (3a-3l) with unsymmetrically-substituted N2O2 tetradentate Schiff base ligands are reported here. The ligands 5-A-5'-D-saltn(2-) (H(2)saltn = N,N'-bis(salicylidene) diaminopropane) possess differently-substituted salicylaldehyde ((A/D)sal) moieties condensed to 1,3-diaminopropane (tn), and carry either an electron acceptor (A = H, Br, or NO2) or donor (D = H, Me, or OMe) group in the para position with respect to the coordinated phenoxido oxygen atoms. These compounds could be obtained by template synthesis involving derivatives [Ni(Gsal)(2)(H2O)(2)], 1a-e (G = NO2, Br, H, Me and OMe, respectively) and [Ni(L-G)2], 2a-d (L-G-= (E)2-(( 3-aminopropylimino) methyl)-4-G-phenolate, G = NO2, Br, H and Me, respectively). Scrambling of the ligands and condensation to compounds 3 was suitably achieved by refluxing compounds 1 and 2 that carry the G groups suitable for the desired final A-D combination. Dinuclear intermediates [Ni-2(mu-L-G)2((')G'sal)(2)] (4a, b, e, f, g) were also detected and isolated. The single-crystal X-ray diffraction structures of [Ni(5'-OMe-saltn)]center dot CHCl3 (3c center dot CHCl3), [Ni(5-Br-5'-OMe-saltn)]center dot EtOH (3g center dot EtOH) and [Ni(5-NO2-saltn)] (3j) show different degrees of distortion around the central core, leading to saddle-like (3c), planar (3g) and step-like (3j) molecular conformations. DFT geometrical optimization of the compounds showed that, for isolated molecules, the saddle-like conformation is slightly more stable with respect to the other conformations. UV-visible absorption spectra showed structured absorption profiles at about 320-440 nm, whose intensity was amplified by the presence of the nitro group, and this was assigned to a convolution of one metal-to-ligand charge transfer and two intra-ligand charge transfer transitions by TDDFT computations. Surprisingly, UV-visible spectra of the derivatives with Br were comparable to the derivatives with Me, suggesting that in this case the behaviour of the halogen was as a weak electron donor group. The experimental investigation (through electric-field-induced second-harmonic and solvatochromic measurements) of the second-order NLO responses of compounds 3, in conjunction with the theoretical calculations, revealed that the observed NLO efficiency is determined by the combined effect of two almost orthogonal charge transfer directions within the molecules, one along the axis approximately bisecting the donor and the accepting moieties and the other along the A-D axis
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
New Silver(I) Coordination Polymer with Fe4 Single-Molecule Magnets as Long Spacer
Full text linkIn continuation of our work on supramolecular architectures of single-molecule magnets (SMMs) as a promising strategy in developing their magnetic performance, in this paper we report the synthesis and single crystal X-ray structure of the centered triangular tetrairon(III) SMM, [Fe4(PhpPy)2(dpm)6], Fe4 (Hdpm = dipivaloylmethane, H3PhpPy = 2-(hydroxymethyl)-
2-(4-(pyridine-4-yl)phenyl)propane-1,3-diol), and its assembly in the coordination polymer {[Fe4(PhpPy)2(dpm)6Ag](ClO4)}n, Fe4Ag, upon reaction with silver(I) perchlorate. Thanks to the presence of the pyridyl rings on the two tripodal ligands, Fe4 behaves as divergent ditopic linker, and due to the Fe4:AgClO4 1:1 ratio, Fe4Ag probably possesses a linear arrangement in which silver(I) ions are linearly coordinated by two nitrogen atoms, forming 1D chains whose positive charge is balanced by the perchlorate anions. The stabilization of such a polymeric structure can be ascribed to the long distance between the two donor nitrogen atoms (23.4 Å) and their donor power. Fe4Ag shows slow relaxation of the magnetization which follows a thermally activated process with Ueff/kB = 11.17(18) K, t0 = 2.24(17) 107 s in zero field, and Ueff/kB = 14.49(5) K, t0 = 3.88(8) 107 s in 1-kOe applied field, in line with what reported for tetrairon(III) SMMs acting as building blocks in polymeric structures
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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