Structural and mechanistic information on the nitrosation of model Fe(II) complexes containing a biomimetic S4N chelate

In order to provide insight into the reaction pathways of nitrogen oxide redox species with [Fe-S] models that may parallel those existing in biology, the reactivity of the iron-sulfur species, {[Fe(II)(S(4)NEt(2)N)]}(2) (1) and [Fe(II)(CH(3)CN)(S(4)NEt(2)N)] (2), where (S(4)NEt(2)N)(2-) = 2,6-bis(2-mercaptophenylthiomethyl)-4-diethylaminopyridine(2-), towards NO(+) (nitrosation) has been studied mechanistically in acetonitrile and compared with the corresponding reactions with NO (nitrosylation). For the nitrosation of 1, the reaction takes place in two steps that correspond to the nitrosation of the mononuclear (2) and dinuclear (1) complexes, respectively. For the corresponding carbonyl complex [Fe(II)(CO)(S(4)NEt(2)N)] (3), the nitrosation reaction occurs in a single step. The relative reactivity of the iron-sulfur species is approximately (1)/(2)/(3) = 1/20/10. Activation parameters for the nitrosation of 1 (ΔH(#) = 27 ± 1 kJ mol(-1), ΔS(#) = -111 ± 2 J K(-1) mol(-1), and ΔV(#) = -19 ± 2 cm(3) mol(-1)), 2 (ΔH(#) = 46 ± 2 kJ mol(-1), ΔS(#) = -22 ± 7 J K(-1) mol(-1), and ΔV(#) = -9.7 ± 0.4 cm(3) mol(-1)) and 3 (ΔH(#) = 38 ± 1 kJ mol(-1), ΔS(#) = -44 ± 4 J K(-1) mol(-1), and ΔV(#) = -7.8 ± 0.3 cm(3) mol(-1)) were determined from variable temperature and pressure studies. The significantly negative ΔS(#) and ΔV(#) values found for the nitrosation reactions are consistent with an associative mechanism. A comparative study of the reactivity of the iron-sulfur species 1 to 3 towards NO(+) and NO is presented.     

Understanding chemical reaction mechanisms in ionic liquids: successes and challenges

The focus of this article is an examination of chemical reaction mechanisms in ionic liquids. These mechanisms are compared with those pertaining to the same reactions carried out in conventional solvents. In cases where the mechanisms differ, attempts to provide an explanation in terms of the chemical and physicochemical properties of the reactants and of the components of the ionic liquids are described. A wide range of reactions from different branches of chemistry has been selected for this purpose. A sufficient background for student readers has been included. This tutorial review should also be of interest to kineticists, and to both new and experienced investigators in the ionic liquids field.     

Reactivity of a cytostatic active N,N-donor-containing dinuclear Pt(II) complex with biological relevant nucleophiles

A dinuclear platinum(II) complex that was recently investigated in our group was tested for its cytostatic activity and found to be active against HeLa S3 cells. The complex consists of a bidentate N,N-donor chelating ligand system in which the two platinum centers are connected by an aliphatic chain of 10 methylene groups. The complex [Pt(2)(N(1),N(10)-bis(2-pyridylmethyl)-1,10-decanediamine)(OH(2))(4)](4+) (10NNpy) is of further special interest, since only little is known about the substitution behavior of such dinuclear platinum complexes that contain a bidentate coordination sphere. The complex was investigated using different biologically relevant nucleophiles, such as thiourea (tu), L-methionine (L-Met), glutathione (GSH), and guanine-5'-monophosphate (5'-GMP), at two different pH values (2 and 7.4). The substitution of coordinated water by these nucleophiles was studied under pseudo-first-order conditions as a function of nucleophile concentration, temperature, and pressure, using stopped-flow techniques and UV-vis spectroscopy. The reactivity of 10NNpy with the selected nucleophiles was found to be tu ? 5'-GMP > L-Met > GSH at pH 2 and GSH > tu > L-Met at pH 7.4. The results for the dinuclear 10NNpy complex were compared to those for the corresponding mononuclear reference complex [Pt(aminomethylpyridine)(OH(2))(2)](2+), Pt(amp), studied before in our group, by which the effect of the addition of an aliphatic chain, an increase in the overall charge, and a shift in the pK(a) values of the coordinated water ligands could be investigated. The reactivity order for Pt(amp) was found to be tu > GSH > L-Met at pH 7.4.     

Mechanistic insight from activation parameters for the reaction of a ruthenium hydride complex with CO2 in conventional solvents and an ionic liquid

Detailed kinetic studies were performed on the reaction of [Ru(II)(terpy)(bpy)H](+) (terpy = 2,2',6',2″-terpyridine; bpy = 2,2'-bipyridine) with CO(2) in conventional solvents (water, methanol, and ethanol) and in the ionic liquid [emim][NTf(2)] ([emim] = 1-ethyl-3-methyl-imidazolium; [NTf(2)] = bistrifluoromethylsulfonylamide). Second-order rate constants and activation parameters (ΔH(?), ΔS(?), and ΔV(?)) were determined for the reaction in all solvents. The second-order rate constants correlate with the acceptor number of the solvent, whereas the activation parameters support the associative nature of the reaction. The results in water, especially the activation entropy (+14 ± 2 J K(-1) mol(-1)) and activation volume (-5.9 ± 0.6 cm(3) mol(-1)), differ significantly from those found for the other solvents.     

Thermodynamic and kinetic behaviour of [Pt(2-methylthiomethylpyridine)(OH2)2]2+

The diaqua complex [Pt(2-methylthiomethylpyridine)(OH(2))(2)](2+), Pt(mtp), was synthesized and investigated thermodynamically as well as kinetically. Spectrophotometric acid-base titrations were performed to determine the pK(a) values of the two coordinated water ligands. A low pK(a1) value of 3.15 was observed for the water molecule trans to the pyridine donor, whereas a pK(a2) value of 6.84 was found for the water molecule trans to the labilising sulphur donor. The substitution of coordinated water by a series of sterically hindered S-containing nucleophiles, viz. thiourea (tu), N,N'-dimethylthiourea (dmtu) and N,N,N',N'-tetramethylthiourea (tmtu), was studied under pseudo first-order conditions as a function of nucleophile concentration, pH (2, 4.75, 7.4), temperature and pressure, using stopped-flow techniques and UV-vis spectroscopy. In general the first substitution reaction takes place trans to the sulphur donor. At pH 2 the nucleophiles react in the order tu (634 ± 10) > dmtu (507 ± 5) ? tmtu (165 ± 3 M(-1) s(-1) at 25 °C), which is caused by steric hindrance. The second observed reaction involves two steps, viz. the displacement of the second water ligand and dechelation of the pyridine ring with the third-order rate constants 73.3 ± 0.8 (tu), 22.1 ± 0.1 (dmtu) and 6.8 ± 0.2 M(-2) s(-1) (tmtu) at 25 °C. At pH 4.75 the reactions are in general slower due to the presence of aqua-hydroxo species. The same order in reactivity was found, viz. tu (106 ± 1) > dmtu (72 ± 1) ? tmtu (14.1 ± 0.5 M(-1) s(-1) at 25 °C). No evidence for ring-dechelation could be observed under these conditions. At pH 7.4 the inert dihydroxo species is predominantly present in solution and consequently no substitution reaction was observed. Quantum chemical calculations were performed to support the interpretation and discussion of the experimental results.     

Characterization of surface and porous properties of synthetic hybrid lamellar silica

Synthetic lamellar silica and hybrid lamellar silicas have been prepared by liquid crystal templating, template extraction and silanization. The samples have been characterized by thermogravimetric analysis (TGA), carbon analysis, spectroscopy, X-ray diffraction (XRD) and nitrogen adsorption. The XRD analyses have shown that the lamellar periodic stacking is preserved for all samples. The quantity and type of organic molecules at the silica surface have been evaluated by carbon analysis, TGA and spectroscopy. The covalent grafting of the solvent used for extraction of the initial surfactant has been highlighted by these analyses. The nitrogen adsorption analyses have revealed three categories of pores and two types of samples. The initial lamellar silica exhibits a very low specific surface area and plate-like type of pores. The second type of samples is made up of the hybrid samples and the initial substrate from whom the surfactant has been extracted. These samples show a significantly higher specific surface area with interlamellar spaces corresponding to narrow-slit like mesopores around 4 nm. The nitrogen adsorption data analysis has highlighted the presence of micropores within the silica sheets. The difference of the specific surface is due to pore blocking by the surfactant impeding the access to nitrogen into interlamellar spaces and by the silanes covering the pores once the surface modified. The presence of micro and mesopores combined to a high BET specific surface of 612 m²/g makes these lamellar silicas interesting materials for catalysis applications.     

A Model for Flow‐enhanced Nucleation Based on Fibrillar Dormant Precursors

A model for flow‐enhanced nucleation is presented based on the concept of a polymer melt containing a fixed number of nucleation precursors with a fixed size distribution. Depending on the size, precursors can either be active (i.e. susceptible to nucleation, the characteristic time scale of which is governed by the deformation rate) and grow into a spherulite or remain dormant. The size distribution of precursors is derived by combining nucleation theory and experimentally determined quiescent spherulite number densities. Longitudinal precursor growth, causing activation of dormant precursors, is a function of molecular deformation: the stretch of high molecular weight chains. Both the eXtended Pom‐Pom and the Rolie‐Poly model are tested to calculate the molecular deformation. A quantitative agreement is found between simulations and experimental results.

     

Effect of pressure on an intramolecular electron-transfer reaction induced by pulse-radiolysis

Abstract

The pressure dependence of an intramolecular electron-transfer reaction between p-nitrobenzoate and the pentaamminecobalt (III) species was studied with the aid of a combined high-pressure pulse-radiolysis technique. The resulting volume of activation of +7.5±0.3 cm³mol^?1 is ascribed to an intrinsic volume increase due to the reduction of Co(III) to Co(II) and some desolvation of the coordinated ligand during the electron-transfer process. The results are compared with related data obtained using conventional kinetic techniques.