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Synthesis and characterization of vanadium(IV) complexes with cis-inositol in aqueous solution and in the solid-state
No full textThe complex formation of vanadium(IV) with cis-inositol (ino) and the corresponding trimethyl ether 1,3,5-trideoxy-1,3,5-trimethoxy-cis-inositol (tmci) was studied in aqueous solution and in the solid-state. With increasing pH, the formation of [VO(H-2L)], [(VO)(2)L2H-5](-), [VO(H-3L)](-) (L = ino) or [(VO)(2)L2H-6](2-) (L = tmci), [V(H-3L)(2)](2-), and [VO(H-3L)(OH)(2)](3-) was observed. For the vanadium(IV)/ino system, [(VO)(2)L2H-7](3-) was observed as an additional dinuclear species. The formation constants of these complexes were determined by potentiometric titrations (25 degrees C, 0.1 M KCl). In addition, the vanadium(IV)/ino system was investigated by means of UV-vis spectrophotometric methods. EPR spectroscopy and cyclic voltammetry confirmed this complexation scheme. EPR measurements indicated the formation of three distinct isomers of the non-oxo complex [V(H(-3)ino)(2)](2-) in weakly basic solution. This type of isomerism, which is not observed for the vanadium(IV)/tmci system, was assigned to the ability of ino to bind the vanadium(IV) center with three alkoxo groups having either a 1,3,5-triaxial or an 1,2,3-axial-equatorial-axial arrangement. The structures of [V(H(-3)ino)(2)][K-2(ino)(2)]center dot 4H(2)O (1) and [Na6V(H(-3)ino)(2)](SO4)(2)center dot 6H(2)O (2) were determined by single-crystal X-ray analysis. In both compounds, the coordination of each ino molecule to the vanadium(IV) center via three axial deprotonated oxygen donors was confirmed. The centrosymmetric structure of the coordination spheres corresponds to an almost regular octahedral geometry with a twist angle of 60 degrees. The crystal structure of the potassium complex 1 represents an unusual 1:1 packing of [V(H(-3)ino)(2)](2-) dianions and [K-2(ino)(2)](2+) dications, in which both K+ ions have a coordination number of nine and are bonded simultaneously to a 1,3,5-triaxial and an 1,2,3-axial-equatorial-axial site of ino. In 2, the [V(H(-3)ino)(2)](2-) complexes are surrounded by six Na+ counterions that are bonded to the axial alkoxo oxygens and to the equatorial hydroxy oxygens of the cis-inositolato moieties. The six Na+ centers are further interlinked by bridging sulfate ions. According to EPR spectroscopy, the D-3d symmetric structure of the [V(H(-3)ino)(2)](2-) anion is retained in H2O, in dimethylformamide, and in a mixture of CHCl3/toluene 60:40 v/v
Complex Formation of Vanadium(IV) with 1,3,5-Triamino-1,3,5-trideoxy-cis-inositol and Related Ligands
No full textThe complex formation of vanadium(IV) with 1,3,5-triamino-1,3,5-trideoxy-cis-inositol (taci) and 1,3,5-trideoxy-1,3,5-tris(dimethylamino)-cis-inositol (tdci) was studied in aqueous solution and in the solid state. The formation constants of [(VO)-O-IV(taci)](2+), [(VO)-O-IV(tdci)](2+), and [V-IV(tdci)(2)](4+) and of the deprotonation product [V-IV(tdci)(2)H-1](3+) were determined (25 degreesC, 0.1 M KNO3). Cyclic voltammetry measurements established a reversible one-electron transfer for the [V-IV(tdci)(2)H-m]((4-m))/[V-III(tdci)(2)H-n]((3-n)) couple (0 less than or equal to n, m less than or equal to 6) with a strongly pH-dependent E-obs(1/2) (+0.15 V at pH 5, -0.57 V at pH 13.5). Slightly more negative potentials were measured for the taci complexes. An additional quasi-reversible electron transfer at strongly positive potentials (1.2-0.5 V) was assigned to the [V-V(taci)(2)H-m]((5-m))/[V-IV(taci)(2)H-n]((4-n)) couple. The structures of [V-IV(taci)(2)](SO4)(2).12H(2)O (1), [V-IV(tdci)(2)][(V4O12)-O-V].14.5H(2)O (2), and [V-IV(tdci)(2)H-1]Cl-3.15H(2)O (3) were determined by single-crystal X-ray analysis. The cations of 1-3 exhibit a structure of approximate D-3d symmetry. The vanadium centers have an almost regular octahedral geometry. The coordinated oxygen donors are deprotonated, and their protons are transferred to the amino groups which act as internal bases. For both ligands, EPR measurements support the formation of non-oxo V-IV complexes, with a d(z)(2) ground state, in aqueous solution above pH 7. The spectral features are indicative of hexacoordinated complexes with a geometry distorted toward a trigonal prism. Finally, a mechanism is proposed for the decomposition of [(VL2H-x)-L-IV]((4-x)) (L = taci and tdci) in strongly alkaline media (pH approximate to 13)
Formation in aqueous solution of a non-oxido V-IV complex with VN6 coordination. Potentiometric, ESI-MS, spectroscopic and computational characterization
No full textThe behaviour of the system formed by (VO2+)-O-IV ion with all-cis-2,4,6-trimethoxycyclohexane-1,3,5-triamine (tmca) was characterized in aqueous solution through the combined application of electron paramagnetic resonance (EPR) and UV-Vis spectroscopy, electrospray ionization mass spectrometry (ESI-MS), pH-potentiometry and DFT methods. The formation of an unusual non-oxido [V(tmcaH(-2))(2)] species with VN6 coordination, with the ligand in the bianionic form, was demonstrated. The geometry, EPR and UV-Vis spectra and electronic structure of [V(tmcaH(-2))(2)] were simulated with Gaussian 09 and ORCA software and the results were compared with those of similar oxido and non-oxido vanadium(IV) species formed by other polyamine and polyol related ligands, such as 1,3,5-triamino-1,3,5-trideoxy-cis-inositol (taci), 1,3,5-trideoxy-1,3,5-tris(dimethylamino)-cis-inositol (tdci), cis-inositol (ino) and 1,3,5-trideoxy-1,3,5-trimethoxy-cis-inositol (tmci). The results indicate that (VO2+)-O-IV species are formed in acid and weakly basic solution and that [V(tmcaH(-2))(2)] is observed above pH 10. In the non-oxido complex, DFT calculations suggest that the two -NH2 groups are in trans position and that the pre-organization of the ligands favours the metal complexation and the formation of the hexa-coordinated species with VN6 coordination
VIVO versus VIV complex formation by tridentate (O, Narom, O) ligands: Prediction of geometry, EPR 51V hyperfine coupling constants, and UV-Vis spectra
No full textSystems formed using the (VO2+)-O-IV ion with tridentate ligands containing a (O, N-arom, O) donor set were described. Examined ligands were 3,5-bis(2-hydroxyphenyl)-1-phenyl-1H-1,2,4-triazole (H(2)hyph(Ph)), 4-[3,5-bis(2-hydroxyphenyl)-1H-1,2,4-triazol-1-yl]benzoic acid (H(3)hyph(C)), 4-[3,5-bis(2-hydroxyphenyl)-1H-1,2,4-triazol-1-yl]benzenesulfonic acid (H(3)hyph(S)), and 2,6-bis(2-hydroxyphenyl)pyridine (H(2)bhpp), with H(3)hyph(C) being an orally active iron chelator that is commercially available under the name Exjade (Novartis) for treatment of chronic iron overload arising from blood transfusions. The systems were studied using EPR. UV-Vis, and IR spectroscopies, pH potentiometry, and DFT methods. The ligands bind vanadium with the two terminal deprotonated phenol groups and the central aromatic nitrogen to give six-membered chelate rings. In aqueous solution the main species were the mono- and bis-chelated (VO)-O-IV complexes, whereas in the solid state neutral non-oxido V-IV compounds were formed. [V(hyph(Ph))(2)] and [V(bhpp)(2)] are hexacoordinated, with a geometry close to the octahedral and a meridional arrangement of the ligands. DFT calculations allow distinguishing (VO)-O-IV and V-IV species and predicting their structure the V-51 hyperfine coupling constant tensor A, and the electronic absorption spectra. Finally, EPR spectra of several non-oxido V-IV species were compared using relevant geometrical parameters to demonstrate that in the case of tridentate ligands the V-51 hyperfine coupling constant is related to the geometric isomerism (meridional or facial) rather than the twist angle Phi, which measures the distortion of the hexacoordinated structure toward a trigonal prism
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
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
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