Copolymerization of epoxides and carbon dioxide. Evidence supporting the lack of dual catalysis at a single metal site.

The chromium tetramethyltetraazaannulene catalyst system, Cr(tmtaa)Cl in the presence of a quaternary organic salt, has been employed to further investigate the mechanism of polycarbonate formation between cyclohexene oxide and carbon dioxide. Although the features of the proposed mechanism are generally well-accepted, the question of whether polymer chain propagation occurs from one side of the catalyst in a mono catalytic single-site fashion or from both sides of the N(4)-ligand plane in a dual catalytic manner. To probe this behavior, the ligand architecture was altered by the addition of a sterically encumbering strap (Cr(stmtaa)Cl), which effectively blocks one side of the complex and limits polymer chain growth to a single side of the catalyst.

An exploration of the coupling reactions of epoxides and carbon dioxide catalyzed by tetramethyltetraazaannulene chromium(III) derivatives: formation of copolymers versus cyclic carbonates.

The high catalytic activity of a tetramethyltetraazaannulene (tmtaa) chromium complex toward the copolymerization of cyclohexene oxide and carbon dioxide to discriminatively provide poly(cyclohexylene carbonate) has directed further studies into the capabilities of the catalyst system. Various [PPN]X (PPN(+) = bis(triphenylphosphoranylidene)ammonium) cocatalysts, where X = Cl, N(3), Br, CN, and OBzF(5), in the presence of (tmtaa)CrCl were examined for catalytic reactivity and selectivity for polycarbonate formation, achieving turnover frequencies of 1500 h(-1) at 80 degrees C in the case of PPNCl. The catalyst system was examined under varied pressures and found to be active even at 1 bar of CO(2) pressure.

(Tetramethyltetraazaannulene)chromium chloride: a highly active catalyst for the alternating copolymerization of epoxides and carbon dioxide.

A tetramethyltetraazaannulene complex incorporating a chromium(III) metal center has been shown to be highly active toward the copolymerization of cyclohexene oxide and carbon dioxide to afford poly(cyclohexene carbonate) in the presence of [PPN]N3 [PPN+=bis(triphenylphosphoranylidene)ammonium] as a cocatalyst. An asymptotical rate increase was observed, leveling at 2 equiv of cocatalyst with a maximum turnover frequency of 1300 h(-1) at 80 degrees C. A benefit of this new catalyst system over that of the previously studied less-active (salen)CrX system is that the (tmtaa)CrCl catalyst has a much lower propensity toward the formation of a cyclic carbonate byproduct throughout the copolymerization reaction.

A kinetic study of the ring-opening process in tungsten carbonyl complexes containing hemilabile metallodithiolate ligands.

The synthesis of the metallodithiolate derivative of tungsten pentacarbonyl from the reaction of photogenerated W(CO)(5)THF and Ni-1 ((1,5-bis(2-mercapto-2-methylpropane)-1,5-diazacyclooctanato)nickel(II)) is described, along with its crystal structure. In N,N-dimethylformamide solution, the pentacarbonyl exists in equilibrium with its tetracarbonyl analogue and carbon monoxide. The pentacarbonyl complex stereoselectively loses cis carbonyl ligands, as is apparent from (13)CO-labeling studies, where the thus-formed tetracarbonyl tungsten complex resulting from chelate ring-closure is preferentially (13)CO-labeled among the two mutually trans CO groups.