Search

Critical thinking and analytical problem-solving skills in research involves using different search strategies. A proposed concept for an "Imploded Boolean Search" combines three unique identifiable field types to perform a search: keyword(s), numerical value(s), and a chemical structure or reaction. The object of this type of search is to help refine, narrow, and speed the searching process for finding relevant information, which would otherwise yield humanly unmanageable result sets. This search strategy is introduced to third year chemistry students in an information literacy workshop using SciFinder, Reaxys, and Web of Science Structure Search. The aim of the workshop is to help students develop hands-on familiarity on the use of chemistry databases and search strategies for scientific information retrieval. The "Imploded Boolean Search" adds a new skill set to a student's arsenal of search strategies. A survey and training exercise are used to assess the workshop.

Ethylene polymerization was studied using a variety of iminophosphonamide (P[N.sub.2]) complexes of zirconium. Bis(P[N.sub.2]) dichloride complexes [[P[h.sub.2]P[(NR').sub.2]].sub.2]Zr[X.sub.2[ (X = Cl; 1a: R' = p-tolyl; 1b: R' = Bn; 1c: R' = [C.sub.6][F.sub.5]) or dimethyl complexes (X = Me; 2a: R' = p-tolyl; 2b: R' = Bn) and cyclopentadienyl(P[N.sub.2])zirconium dichloride complexes [[η.sup.5] - [C.sub.5][R''.sub.5]][[R.sub.2] P[(NR').sub.2]]Zr[Cl.sub.2] (3a: R' = p-toyl, R = Ph, R'' = H; 3b: R' = Si[Me.sub.3], R = Et, R'' = H; 3c: R' = [C.sub.6][F.sub.5], R = Ph, R'' = H; 3e: R' = 3,5-[(C[F.sub.3]).sub.2]Ph, R = Ph, R'' = H; 3f: R' = 3,5-[(C[F.sub.3]).sub.2]Ph, R = Ph, R'' = Me) or dimethyl analogs [[η.sup.5] - [C.sub.5][R.sub.5]] [[R.sub.2]P[(NR').sub.2]]Zr[Me.sub.2] (4a: R' = p-toyl, R = Ph, 4b: R' = Si[Me.sub.3] R = Et) were evaluated under a range of conditions using methylaluminoxane (PMAO) activator. Complexes 1 and 2 behave as precursors to single-site polymerization catalysts under the conditions studied, while complexes 3 or dialkyls 4 show more complex behavior and formation of poly(ethylene) with a bimodal molecular weight distribution. In contrast, activation of dialkyl complexes 4 with [[Ph.sub.3]C][B[([C.sub.6][F.sub.5]).sub.4]] and polymerization in the presence of small amounts of PMAO or TIBAL as scavenger, led to single-site behavior. PMAO reacts with the neutral dialkyls via ligand abstraction to produce a number of P-containing species that may explain the multi-site behavior observed when using this activator. Dialkyls 4 react cleanly with [[Ph.sub.3]C][B[([C.sub.6] [F.sub.5]).sub.4]] in haloarene or even dichloromethane solution to furnish the corresponding cationic alkyls 5, which were characterized by multinuclear NMR spectroscopy. Fluxional dinuclear species are formed in the presence of excess dialkyl and these are susceptible to C--H activation to form μ-Me,μ-C[H.sub.2] complexes one of which could be isolated in pure form. The cationic alkyls initiate the polymerization of 1-hexene at room temperature in chlorobenzene solution, but extensive chain transfer occurs and the systems are not living. Key words: single site, early metal olefin polymerization catalysis. On a etudie la polymerisation de l'ethylene a l'aide d'une variete de complexes a base d'iminophosphonamide (P[N.sub.2]) du zirconium. Utilisant le methylaluminoxane (PMAO) comme activateur dans une grande variete de conditions, on a evalue les complexes avec le dichlorure de bis(P[N.sub.2]) [[P[h.sub.2]P[(NR').sub.2]].sub.2]Zr[X.sub.2] (X = Cl; 1a: R' = p-tolyl; 1b: R' = Bn; 1c: R' = [C.sub.6][F.sub.5]) ou les complexes dimethyles (X = Me; 2a: R' = p-tolyl; 2b: R' = Bn), et le dichlorure de cyclopentadienyl (P[N.sub.2])[[η.sup.5] - [C.sub.5][R''.sub.5]][[R.sub.2] P[(NR').sub.2]]Zr[Cl.sub.2] (3a: R' = p-toyl, R = Ph, R'' = H; 3b: R' = Si[Me.sub.3], R = Et, R'' = H; 3c: R' = [C.sub.6][F.sub.5], R = Ph, R'' = H; 3e: R' = 3,5-[(C[F.sub.3]).sub.2]Ph, R = Ph, R'' = H; 3f: R' = 3,5-[(C[F.sub.3]).sub.2]Ph, R = Ph, R'' = Me) ou de leurs analogues dimethyles [[η.sup.5] - [C.sub.5] [R.sub.5]][[R.sub.2]P[(NR').sub.2]]Zr[Me.sub.2] (4a: R' = p-toyl, R = Ph, 4b: R' = Si[Me.sub.3] R = Et). Dans les conditions etudiees, les complexes 1 et 2 se comportent comme des precurseurs de catalyseurs a site unique alors que les complexes 3 ou les dialkyles 4 presentent un comportement plus complexe avec une formation de poly(ethylene) presentant une distribution bimodale des poids moleculaires. Par ailleurs, l'activation des complexes de dialkyles avec du [[Ph.sub.3]C][B[([C.sub.6] [F.sub.5]).sub.4]] avec des polymerisations en presence de faibles quantites de PMAO ou de TIBAL comme piege conduit a un comportement a site unique. Le PMAO reagit avec les dialkyles neutres par le biais d'un enlevement de ligand conduisant a la formation de diverses especes contenant du phosphore qui peuvent expliquer le comportement multisite observe quand on utilise ce compose pour faire l'activation. Les complexes dialkyles 4 reagissent proprement avec le [[Ph.sub.3]C][B[([C.sub.6][F.sub.5]).sub.4]] en solution darts des halogenoarenes et meme dans le dichloromethane et ces reactions conduisent a la formation des complexes cationiques alkyles correspondants 5 qui ont ete caracterises par spectroscopie RMN multinucldaire. Il se forme des especes binucleaires fluxionnelles en presence d'exe's de derives dialkyles et ces especes sont susceptibles a une activation C--H conduisant a la formation de complexes μ-Me et μ-C[H.sub.2] desquels un a pu etre isole dans une forme pure. Les complexes alkyles cationiques initient la polymerisation du hex-1-ene a la temperature ambiante, en solution dans le chlorobenzene, mais il se produit beaucoup de transfert de chaine et les systemes ne sont pas vivants. Mots cles : precurseurs de catalyseurs a site unique de la polymerisation d'alcenes par des metaux de transition. [Traduit par la Redaction], Introduction The synthesis of novel group 4 complexes is of considerable interest in the context of olefin polymerization using 'non-metallocene' catalysts (1). A wide variety of complexes based on the [...]

A study was conducted to characterize a reaction that produces an electrophilic 14-electron titanium center in an environment sterically shielded from both C-H bonds and external coordinating agents. Bond distances and the solid-state structure was represented in the study. The nature and degree of agostic interactions were evaluated with an extended all-electron basis sets. Results indicated the presence of metal-to-saturated carbon atom contacts supported by a substantial orbital overlap.