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Reduction of [IndCpMo(L)Br]Br (L = 1a, CO; 2, NCMe) with under the same conditions. With the aim of continuing the comparative study of the

DFT calculations were performed on Os 3 (CO) 10 (α-diimine) clusters, for α-diimine = DAB (1,4-diaza-1,3-butadiene), PYCA (σ-N,σ-NЈ-pyridine-2-carbaldehyde-imine), and BIPY (2,2Ј-bipyridine). Geometry optimizations were performed for several models of the possible isomers under different conditions. Axial isomers were always found to be the most stable, in agreement with the available X-ray determined structures. A full optimization of the geometry was required in order to improve the quality of the results, but the dis-

Multipurpose zinc carbene catalystThe cover picture features a dinuclear zinc-N-heterocyclic carbene (NHC) catalyst. In their Full Paper on p. 1357 ff., S. Dagorne, T. AvilØs, M. J. Calhorda et al. report that such zinc-NHC alkoxide species polymerize lactide to produce well-defined polylactic acid and, remarkably, also mediate the controlled degradation (depolymerization) of polylactic acid under mild conditions to produce methyl lactate as the major reaction product. DFT studies of the polymerization reaction highlight the beneficial effect of two zinc centers in close proximity. The work shows that biocompatible zinc(II) complexes can mediate the polymerization of lactide and the controlled degradation of polylactic acid under mild conditions (room temperature).

These complexes crystallize as the less common axial isomer and reach EC50 < 1.8 μM against HeLa cell lines.

Eine zuvor nicht erkannte Rutheniumcarbonat‐Zwischenstufe entsteht bei der Umsetzung eines allylischen tert‐Butoxycarbonats mit dem Rutheniumkomplex [Ru(Cp*)(CH3CN)3]PF6 (1) (siehe Schema). Der isolierbare Carbonat‐Allyl‐Komplex wirkt als Katalysator in der Alkylierung von Malonsäuredimethylester mit linearen und verzweigten Allylarylcarbonaten.

Mo(VI) complexes MoO2X2L2 (X=halide or Me, L neutral ligand) behave as catalysts for olefin epoxidation in the presence of t-butylhydroperoxide (TBHP). The active species results from OH activation of TBHP, which protonates one oxo group and leads to a seven coordinate complex, with a new OOR ligand. It was found that several Mo(II) complexes Cp'Mo(CO)3X (Cp'=C5R5, Cp* or C5H5, Cp) acted as precursors for the same reactions and the resulting Cp'MoO2X could also oxidize sulfides and sulfoxides, both with TBHP and H2O2 as oxidants. A review of the reaction mechanisms proposed for these reactions, by us and some other authors, and based on computational studies is given in this work. More than one active species can be found for the CpMoO(OH)( 1 -OOR)X intermediate, opening several competitive pathways. They differ by the O-H···O hydrogen bond formed between OH and one oxygen of the OOR ligand. This complex can also further react with oxidant to afford a peroxide complex CpMoO( 2 -O2)X, which can also promote oxidation reactions. The activation energies depend on hydrogen bond assistance, so that the Cl and Me derivatives of CpMoO2X behave differently (the peroxide complex has only been found active with Me), and on the steric constraints, more obvious when comparing Cp with Cp*. The preferred mechanism will thus depend on the specific substituents, but energy barriers are comparable.