The mechanism of the Heck reaction has been computationally studied for a carbenestabilized palladium catalyst employing a density functional method with a gradient-corrected exchange-correlation functional. The crucial steps of the reaction, insertion of the olefin into the Pd-aryl bond and -hydride elimination, have been investigated for a Pd catalyst ligated by two diaminocarbene ligands; these ligands have been chosen to model 1,3-dimethylimidazol-2-ylidene ligands which have been employed in experimental work. The reaction involves the cleavage of a Pd-halide bond and thus proceeds via positively charged complexes. For the insertion, the effect of a counterion has also been investigated; its presence does not alter the reaction mechanism since its influence on the reaction energies is small. The insertion step has also been studied for a newly proposed bidentate ligand which coordinates to the Pd center via one carbene as well as one phosphine group. In this system, a Pd-P bond is broken, leading to neutral intermediates. For both ligand systems, the calculated barriers to insertion are in the range 8.3-11.5 kcal/mol; the barrier to -hydride elimination is calculated as 9.0 kcal/mol.

Four controversies on the mechanism of the olefin epoxidation with Mimoun-type complexes, [MoO(O2)2(OPR3)], Herrmann-type complexes, [ReO(O2)2Me], and related inorganic peroxides have inspired industrial and academic researchers in the last three decades. First, is the oxygen transfer from the peroxo complex to the olefin concerted or stepwise? Second, does the oxidant act as an electrophile or a nucleophile? Third, is the mechanism of the stoichiometric reaction also valid for catalytic protocols? Fourth, how can stereochemical information be transferred between oxidant and substrate? In this Account, we discuss answers to the long-standing questions, focusing on recent contributions from quantum chemical calculations.

Methyltrioxorhenium(vii) (MTO) forms trigonal-bipyramidal adducts with pyridines and related Lewis bases. These complexes have been isolated and fully characterized, and two single-crystal X-ray structures are reported. The complexes react with H 2 O 2 to form mono-and bisperoxo complexes which were examined in situ by 1 H and 17 O NMR spectroscopy. A clear increase in electron deficiency at the Re center can be observed from the MTO complexes to the bisperoxo complexes in all cases examined. The activity of the bisperoxo complexes in olefin epoxidation depends on the Lewis bases, the redox stability of the ligands, and the excess of Lewis base used. Density functional calculations show that when the ligand is pyridine or pyrazole there are significantly stabilized intermediates and moderate energies of the transition states in olefin epoxidation. This ultimately causes an acceleration of the epoxidation reaction. In contrast, the catalytic performance is reduced when the ligand was a nonaromatic nitrogen base. The frontier orbital interaction between the olefin HOMO p(C ± C) and orbitals with s*(O ± O) character in the LUMO group of the Re-peroxo moiety controls the olefin epoxidation.

Oxygen transfer reactions mediated by transition metals, such as olefin epoxidation [1] and dihydroxylation, [2] are currently attracting much interest from both experimentalists and theoreticians. Many investigations, several of them of computational thrust, have unraveled details of olefin dihydroxylation as catalyzed by oxo complexes of the type MO 4

Epoxidation of olefins by TiIV peroxo and hydroperoxo functional group depends only weakly on the saturation of the coordination sphere of the Ti center. Substitution of (alkylperoxo) complexes was investigated using a hybrid DFT method (B3LYP). Reaction energies and activation methyl for hydrogen in a TiOOH group is found to slightly increase the activation barrier of epoxidation. The barriers for direct oxygen transfer to ethylene as a model olefin were computed for various model complexes to computational results give preference to reaction paths that involve TiOOR species. The factors governing the activity of compare the epoxidation activity of Ti(η 2 -O 2 ) and TiOOR (R = H, CH 3 ) moieties. The activity of complexes with a Ti(O 2 ) Ti(O 2 ) and TiOOR groups, in particular the effects of donor ligands, are discussed on the basis of a molecular orbital peroxo group is shown to be essentially quenched when the coordination sphere of the complex is saturated by strongly analysis. basic (σ-donor) ligands. In contrast, the activity of a TiOOH olefin epoxidation, e.g. (TPP)Ti(O 2 ) [10] (TPP ϭ tetraphe-

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The epoxidation of olefins by peroxo complexes of Cr(VI), Mo(VI) and W(VI) was investigated using the B3LYP hybrid density functional method. For the mono- and bisperoxo model complexes with the structures (NH(3))(L)M(O)(2)(-)(n)()(eta(2)-O(2))(1+)(n)() (n = 0, 1; L = none, NH(3); M = Cr, Mo, W) and ethylene as model olefin, two reaction mechanism were considered, direct oxygen transfer and a two-step insertion into the metal-peroxo bond. The calculations reveal that direct attack of the nucleophilic olefin on an electrophilic peroxo oxygen center via a transition state of spiro structure is preferred as significantly higher activation barriers were calculated for the insertion mechanism than for the direct mechanism. W complexes are the most active in the series investigated with the calculated activation barriers of direct oxygen transfer to ethylene decreasing in the order Cr > Mo > W. Barriers of bisperoxo species are lower than those of the corresponding monoperoxo species. Coordination of a second NH(3) base ligand to the mono-coordinated species, (NH(3))M(O)(2)(eta(2)-O(2)) and (NH(3))MO(eta(2)-O(2))(2), results in a significant increase of the activation barrier which deactivates the complex. Finally, based on a molecular orbital analysis, we discuss factors that govern the activity of the metal peroxo group M(eta(2)-O(2)), in particular the role of metal center.

Mixed metal clusters of sodium and cesium with gold have been generated in a supersonic expansion from the mixed vapor phase. Their tendency towards binary cluster formation, relative thermodynamic stability, and ionization potentials have been experimentally and computationally investigated. The properties of the NaxAu clusters may be understood within an electronic shell model based on delocalized cluster orbitals, whereas the characteristics of CsxAu are indicative of substantial ionic interactions. Relativistic density functional calculations have been performed to elucidate the cluster electronic structure and to rationalize observed properties which may not be accounted for by the jellium model. The properties of these finite-size clusters are shown to be related to the known bulk intermetallic compounds sodium–gold and cesium–gold (cesium aurid), respectively.