Molybdenum complexes oxygen ligands

Finally, highly dehydroxylated alumina were prepared by heating at 800 °C under vacuum and their interaction with [Mo(CO)e] followed by infrared [19]. At room temperature, the hexacarbonyl molybdenum complex is physisorbed and the interaction of one oxygen atom of a CO ligand with defective aluminium sites (tetrahedral and octahedral sites were identified)... 

Diperoxo(oxo)molybdenum(IV) complex bearing (S)-lactic acid piperidineamide as a chiral ligand has been used for the epoxidation of E-2-butene (Scheme 6B.8) and moderate enantiose-lectivity (49%) is achieved wherein the reaction is stoichiometric [16]. Two possible mechanisms have been proposed for this reaction. One mechanism includes coordination of an olefin prior to epoxidation, which makes the olefin electrophilic and facilitates the nucleophilic attack of the proximal oxygen atom of the peroxide on the olefin. The other one is that an olefin nucleophilically attacks the peroxo group of the molybdenum complex. 

One approach to limit dimer formation in model complexes involves the use of bulky ligands and weakly coordinating solvents. Holm and coworkers [196,198-201] have studied oxygen atom transfer reactions of 2,6-bis(2,2-diphe-nyl-2-thioethyl)pyridinate [2] molybdenum oxo complexes. In contrast to the structurally similar dithiocarbamate molybdenum complexes, the tendency of the 2,6-bis(2,2-dipheny 1-2-thioethy l)pyridinate MoIV monoxo complex to undergo di-... 

Sharpless et a . have conhmied this mechanism in part by labeling experiments which demonstrated that the epoxide oxygen is derived exclusively from the peroxo ligands of the complex and not from the oxo oxygen. However, the reactivity of the molybdenum complex toward olefins closely parallels that of peracids, for which a three-membered cyclic transition stale is favored. ... 

In order to avoid the unwanted dimerization, steric constraints were incorporated into the used ligands. This hinders the metal centres to approach each other, so either the equilibrium is shifted to the side of the monomeric molybdenum species or the dimerization completely prevented. One of the first bulky sulfur-containing ligands that was able to successfully promote oxygen atom transfer catalysis of its molybdenum complex was developed by Berg and Holm (Scheme 4.7 BuLNS = bis(4-tert-butylphenyl)-2-pyridylmethanthiolate). ... 

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