Over the past twenty years, polypyrrole has appeared as the most extensively studied conducting polymer. However, despite the volume of work already done in this area, there has been little focus put on the mechanism of polypyrrole synthesis, especially concerning the most efficient method, electropolymerization. Numerous analytical techniques have been used to study polypyrrole electrodeposition and/or doping. However, the mechanism itself is still a controversial subject as there is not one mechanism which is universally accepted. The mechanism proposed by Diaz is the one most commonly referred to in the literature although several other mechanisms are not lacking in support. The controversy lies in the initiation step as each mechanism proposes a different way of beginning the reaction, varying between electron transfer, proton transfer and direct radical pyrrole formation. Without considering the initiation step, there are many other factors including electrolyte, solvent, temperature and pH which can influence the reaction mechanism during the electropolymerization of pyrrole, thus impacting the characteristics of the polymer formed at the electrode. Dr Saïd Sadki is an associate Professor in Physical Chemistryand Electrochemistry at the University of Cergy Pontoise (France). He obtained his PhD degree from the University of Paris VI where he studied the fabrication of organic accumulators using polyparaphenylene and polypyrrole electrodes. His research interests include the synthesis and electrochemistry of conducting polymers for anti-corrosion coatings, electrochromic devices, sensors and actuators.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

No abstract

No abstract

Organophosphorus boranes 1−6 and amine pentacarbonyltungsten complexes 14−20 react under mild conditions, to afford by ligand exchange the corresponding W(CO)5(PR3) 21−26 derivatives in 63−92% yields. The use of piperazine 13 as a diamine tungsten substituent permits a tandem reaction which removes the borane group and leads to the formation of the corresponding organophosphorus tungsten complex. The stereochemistry of the formation of pentacarbonyltungsten complexes from tertiary chiral organophosphorus borane compounds proceeds with a high stereoselectivity and retention of configuration at the chiral P center.

No abstract

The kinetics have been studied of the substitution reactions of Ru,(CO),,L and RU,(CO),~L~ (L = PCy3 (Cy = C6Hl ,), P-n-Bu,, P(p-MeOC6H4),, P(p-CIC6H4),, P(OPh),, and etpb (P(OCH,),CEt) with AsPh, or various P-donor nucleophiles, L', in hexane. The reactions proceed through both [L'I-independent (CO dissociative) and [L'I-dependent (associative) paths. The values of the rate constants, k,, for the dissociative path for Ru,(CO),,L can be combined with other published data and are found to fit well to the equation: log k, = a + PLSL + yLOL. (PL and yL are constants that define how sensitive the values of log k, are to Bodner's electronic parameter, S,, and to Tolman's steric parameter, 0,, respectively.) Values of P, and y, are both positive, increasing electron donicity and increasing size of L both leading to increased rates. The steric effect is more important and accounts for ca. 60% of the decrease in the free energies of activation. The effects, on the fit and on the values of p, and y,, of using different electronic (Bartik's x values) and steric (Brown's ER values) parameters are examined and found to be small, and the question of which goodness-of-fit parameters should be used is considered. The kinetic effects of the substituents are only a small (ca. 20%) perturbation on the average free energies of activation, and the steric effect shows that use is made of only a few percent of the available ligand-ligand repulsion energy in accelerating the reactions. The second-order reactions of M,(CO),,L with Me3N0 (M = Ru or 0 s ) also fit an equation analogous to that given above and the values of P, and yL obtained for these assisted CO displacement reactions are compared with those for the unimolecular, unassisted, CO dissociation. Unassisted reactions of the Ru3(CO),oL2 clusters show values of P, and y, that are closely similar to those for Ru,(CO), ,L, but published data for CO dissociative reactions of some RU,(CO)~L, clusters lead to much larger positive values of P, and a large inverse steric effect, i.e., rates actually decrease substantially with increasing substituent size after electronic effects are allowed for. This effect is accounted for in terms of the different natures of the Ru,(CO),L, intermediates.Key words: metal carbonyls, ruthenium, clusters, kinetics, substituent effects.Resume : On a CtudiC la cinCtique des rkactions de substitution du Ru3(CO), ,L et Ru3(CO),oL, (L = PCy, (Cy = C6H, ,), P-n-Bu,, P(p-MeOC6H4),, P(p-C1C6H4),, P(OPh),, et etpb (P(OCH,),CEt) avec le AsPh, ou divers autres nucleophiles donneurs, L', dans I'hexane. Les reactions se produisent par des voies qui dependent (associatives) ou qui ne dependent pas (dissociative de CO) de la [L']. On peut combiner les valeurs des constantes de vitesse, k,, de la voie dissociative du RU,(CO)~ ,L avec d'autres donnCes publikes dans la litterature et on a trouvC qu'elles peuvent facilement s'ajuster i i l'equation : log k, = a + p,S, + yLO, (dans laquelle P, et y, sont des constantes qui definissent la sensibilite des valeu...

No abstract