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Synthesis, characterization and substitution reaction studies of pyridyl N,N’-bidentate palladium(II) complexes. A kinetic and mechanistic study.
No full textMasters Degree. University of KwaZulu-Natal, PIetermaritzburg.The influence of structural as well as electronic properties of bidentate N,N chelates with different substituents on the mononuclear Pd(II) complexes were investigated. The complexes were synthesized and characterized by various spectroscopic methods such as 1H & 13C NMR, FT-IR, LC-MS, CHN and single x-ray crystallography. For the first set of complexes (Chapter 3), we studied the unexplored kinetics and mechanistic behaviour of N,N’-pyridyl Pd(II) complexes, viz. dichloro-(N-((pyridin-2-yl)methyl)aniline)palladium(II) (PdL1), dichloro-(4-fluoro-N-((pyridin-2-yl)methyl)aniline)-palladium(II) (PdL2), dichloro-(4-bromo-N-((pyridin-2-yl)methyl)aniline)-palladium(II) (PdL3), dichloro-(4-methoxy-N-((pyridin-2-yl)methyl)aniline)-palladium(II) (PdL4) and dichloro-(4-ethyl-N-((pyridin-2-yl)methyl)aniline)-palladium(II) (PdL5). The substitution behaviour of coordinated chloride atoms by three bio-relevant thiourea nucleophiles, viz. thiourea (TU), N,N’-dimethylthiourea (DMTU) and N,N,N’,N’-tetramethylthiourea (TMTU), of different steric demands was studied in a 0.1 M solution of ultra-pure water under pseudo-first order conditions. The reactions were studied as a function of concentration and temperature using standard Stopped-Flow and UV-Vis spectrophotometric technique. The substitution of the chloride atoms from the Pd metal by thiourea nucleophiles was a two-step reaction where the chloride trans to the pyridine ligand was substituted first, since the pyridine has a stronger trans effect compared to the amine group. The reactivity of mononuclear Pd(II) complexes containing bidentate N,N’-donor ligands with different substituents depends on the electronic effects of the complexes. The reactivity of the complexes increased with the presence of electron withdrawing substituents and decreased when an electron donating group was attached on the para position of the aniline moiety. The electron withdrawing groups influence the pull of electrons from the electron deficient amine that is coordinated to the metal center which results in the loss of electron density from the ligand moiety and increases the electrophilicity of the metal center and thus the substitution reaction. The reactivity of the nucleophiles depends on steric effects, with the bulky TMTU being the least reactive. The negative entropies and second order kinetics for all the substitution reactions support an associative mode of substitution mechanism. DFT calculations were performed to account for the observed reactivity of all the complexes studied. For the second set of novel Pd(II) complexes (Chapter 4), viz. bis[N-(4-bromophenyl)pyridine-2-carboxamidato] Palladium (Pd1) and Palladium(II) [N-(4-bromophenyl)-2-pyridinecarboxamide), pyridine chloride (Pd2), crystals were obtained and the structures were studied. Pd1 crystallizes in the monoclinic crystal system and in the P21/c space group, and Pd2 crystallizes in the orthorhombic system, with the space group Pbca
Substitution reactions of cis-platinum(II) complexes containing bidentate N,N-donor pyridinecarboxamide ligands with different substituents
Full text linkSubstitution reactions of [2-(pyridinecarboxamide)dichloride Pt(II)] [PtCl2], [N-phenyl-(2-pyridinecarboxamide)dichloride Pt(II)] [PhPtCl2], [N-(4-methylphenyl)-2-pyridinecarboxamide)dichloride Pt(II)] [CH3PhPtCl2], [N-(4-methoxyphenyl)-(2-pyridine-carboxamide)dichloride Pt(II)] [CH3OPhPtCl2] and [N-(4-fluorophenyl)-(2-pyridinecarboxamide)dichloride Pt(II)] [FPhPtCl2], with nucleophiles; thiourea (TU), N,N′-dimethylthiourea (DMTU) and N,N,N′,N′-tetramethylthiourea (TMTU) were studied under pseudo first-order conditions. The rates of substitution were investigated as a function of nucleophile concentration and temperature using stopped-flow and UV-visible absorption spectrophotometers. Substitutions of the two coordinated chloride ligands of the Pt(II) complexes occur consecutively, with the first substitution occurring opposite the coordinated pyridyl. The observed pseudo first-order rate constants regressed linearly with concentration of the incoming nucleophiles according to the equation kobs = k2[Nu]. The highest substitution rates were measured for PtCl2, which is attributed to the unsubstituted non-leaving carboxamide ligand. The Pt(II) center of this complex is the most electrophilic as a result of the strong withdrawal of electron density through π-resonance by the carboxamide group. The introduction of a 4′-substituted phenyl group on the amido N of the carboxamide reduces the reactivity of the complexes due to proportional electronic effects from the ancillary substituents on the phenyl ring as well as its conformational disposition with respect to the plane of the complex. The order of reactivity of studied nucleophiles is TU > DMTU > TMTU. The substitution is associatively activated as supported by the negative entropy of activation values for the reactions
Synthesis, characterization and the antimicrobial effect of diiodothiocyanate (I₂SCN-) on escherichia coli and enterococcus faecalis.
Full text linkMasters Degree. University of KwaZulu-Natal, Pietermaritzburg.The threat posed by antimicrobial resistance is rising to dangerously high levels worldwide. However, chemical species such as iodide-based interpseudohalogen are attracting attention as potential new class of antimicrobials. Previously, these compounds were ignored based on low availability of I- in most physiological fluids. Recently, it was shown that the peroxidase system LPO-H2O2-SCN-/I- produces a highly cytotoxic cyanogen iodide (ICN) which plays a key role in killing bacteria. Research efforts to find new chemical species with antimicrobial properties from non-enzymic reactions of iodine (I2/I-) with thiocyanate (SCN-) are currently underway. Herein, we describe the formation of the interpseudohalogen, diiodothiocyanate (I2SCN-) in two different media, i.e. water and acetonitrile. The formation of I2SCN- was confirmed by 13C NMR spectroscopy, UV-visible spectrophotometer, stopped-flow technique and Job’s plot. The biological activities of I2SCN- against strains of Escherichia coli and Enterococcus faecalis bacteria were determined by zone of inhibition plate tests and traditional optical density (OD) measurements. Reaction mixtures of I2 and SCN- at various ratios showed inhibition against both Gram-positive (E faecalis) and Gram-negative (E coli) strains of bacteria. The results indicate that I2SCN- was more effective against E coli compared to E faecalis due to differences in the structural features of the two bacteria. The E faecalis has a thicker cell wall compared to E coli which makes it difficult for any antibacterial compound to penetrate and kill the bacteria. Reaction mixtures of I2 and SCN- at various ratios showed antibacterial activity against both Gram-positive (E. faecalis) and Gram-negative (E. coli) strains of bacteria, with the equimolar solution being the most efficient. The I2SCN- compound was more effective against E. coli compared to E. faecalis. This is due to differences in the structural features of the two bacteria, E. faecalis has a thicker cell wall compared to E. coli which makes it difficult for antibacterial compounds to penetrate the bacteria
Tuning the π-backbonding and σ-trans effect of N^C^N coordinated Pt(II) complexes. Kinetic and computational study
No full textThe nucleophilic substitution reaction of cyclometallated complexes; [PtL2Cl] (L2 = 3,5-di(2-pyridinyl)-fluorobenzene), [PtL3Cl] (L3 = 2,4-di(2-pyridinyl)-fluorobenzene), and [PtL4Cl] (L4 = 3,5-di(2-pyridinyl)-toluene) with a series of neutral nucleophiles with different steric properties, thiourea (TU), N,N-dimethylthiourea (DMTU), and N,N,N′,N′-tetramethylthiourea (TMTU), was studied under pseudo-first-order conditions in methanol solution of an ionic strength of 0.1 M (0.09 M LiCF3SO3 and 0.01 M LiCl). The rate of substitution of the chloro ligand was studied as a function of nucleophile concentration and temperature using UV–visible and stopped-flow spectrophotometric techniques. The observed pseudo-first-order rate constants for the substitution reactions obey the rate law kobs = k2[Nu] + k−2. The reactivity of the investigated complexes when [PtL1Cl] is used as a reference follows the order [PtL2Cl] > [PtL3Cl] > [PtL4Cl] > [PtL1Cl]. The lability of the chloro group is dependent on the extent of π-backbonding and the σ-trans effect of the ligand backbone. [PtL2Cl] and [PtL3Cl], which have a common electron-withdrawing fluoride on the ligand trans to the leaving group, have a higher reaction rate compared to [PtL4Cl], which has an electron-donating methyl group attached to the ligand backbone. The position of the substituent on the phenyl group trans to the leaving group also influences the overlap of frontier molecular orbitals which result in controlling the reactivity of the fluoro complexes. In general, the results show that the nature of the substituent, either electron withdrawing or electron donating, results in an increase in the rate of substitution. Second-order kinetics and large negative activation entropies (ΔS#) support an associative substitution mechanism. The experimental data are supported by DFT calculations
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
Kinetics and mechanistic investigation of the substitution from dinuclear Pt(II) complexes bridged by N,N’-bis(2-pyridylcarboxamide)phenylenediamines
No full textThe rates of substitution of the chloride ligand in dinuclear complexes, [Pt2(N,N'-bis(2-pyridylcarboxamide)-1,3-phenylenediamine)Cl4] (1), [Pt2(N,N'-bis(2-pyridylcarboxamide)-1,4-phenylenediamine)Cl4] (2), [Pt2(N,N'-bis(3-isoquinolylcarboxamide)-1,3-phenylenediamine)Cl4] (3) and [Pt2(N,N'-bis(3-isoquinolylcarboxamide)-1,4-phenylenediamine)Cl4] (4), by three bio-relevant nucleophiles, thiourea (TU), N,N-dimethylthiourea (DMTU) and N,N,N’,N’-tetramethylthiourea (TMTU), were investigated. The mononuclear analogue [Pt(N-phenylpyridine-2-carboxamide)Cl2] (5) was included to compare the results. The kinetics of the reactions were studied under pseudo first-order conditions in a methanol solution (I = 0.1 M LiCl) as a function of concentration and temperature using the stopped-flow spectrophotometer. The observed pseudo first-order rate constants for the substitution reactions of all the complexes were calculated from two well-separated steps and obey the rate law kobs (1st/2nd) = k2 (1st/2nd)[Nu]. The reactivity of the complexes decreases in the order 2 > 1 > 4 > 3 > 5. The conformational symmetry of the complexes as controlled by the phenylenediamine bridges, steric hindrance due to the bridges as well as the σ-donor capacity of the coordinated groups around the metal centers influence the reactivity of the dinuclear complexes. The low enthalpy (ΔH#) and negative intrinsic entropy (ΔS#) values support an associative mechanism of substitution. The kinetic data are supported by DFT calculations
Tuning the π-backbonding and σ-trans effect of N^C^N coordinated Pt(II) complexes. Kinetic and computational study
No full textThe nucleophilic substitution reaction of cyclometallated complexes; [PtL2Cl] (L2 = 3,5-di(2-pyridinyl)-fluorobenzene), [PtL3Cl] (L3 = 2,4-di(2-pyridinyl)-fluorobenzene), and [PtL4Cl] (L4 = 3,5-di(2-pyridinyl)-toluene) with a series of neutral nucleophiles with different steric properties, thiourea (TU), N,N-dimethylthiourea (DMTU), and N,N,N′,N′-tetramethylthiourea (TMTU), was studied under pseudo-first-order conditions in methanol solution of an ionic strength of 0.1 M (0.09 M LiCF3SO3 and 0.01 M LiCl). The rate of substitution of the chloro ligand was studied as a function of nucleophile concentration and temperature using UV–visible and stopped-flow spectrophotometric techniques. The observed pseudo-first-order rate constants for the substitution reactions obey the rate law kobs = k2[Nu] + k−2. The reactivity of the investigated complexes when [PtL1Cl] is used as a reference follows the order [PtL2Cl] > [PtL3Cl] > [PtL4Cl] > [PtL1Cl]. The lability of the chloro group is dependent on the extent of π-backbonding and the σ-trans effect of the ligand backbone. [PtL2Cl] and [PtL3Cl], which have a common electron-withdrawing fluoride on the ligand trans to the leaving group, have a higher reaction rate compared to [PtL4Cl], which has an electron-donating methyl group attached to the ligand backbone. The position of the substituent on the phenyl group trans to the leaving group also influences the overlap of frontier molecular orbitals which result in controlling the reactivity of the fluoro complexes. In general, the results show that the nature of the substituent, either electron withdrawing or electron donating, results in an increase in the rate of substitution. Second-order kinetics and large negative activation entropies (ΔS#) support an associative substitution mechanism. The experimental data are supported by DFT calculations
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
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