resumo
The dioxomolybdenum(VI) complex [MoO(2)Cl(2) {p-tolyl(CH(3)DAB)}] has been prepared in good yield by reaction of the solvent adduct MoO(2)Cl(2)(THF)(2) with one equivalent of the bidentate ligand NN-p-tolyl-2,3-dimethyl-1,4-diazabutadiene. Treatment of the dichloro complex with the Grignard reagent CH(3)MgCl gives the dimethyl derivative [MoO(2)(CH(3))(2)(p-tolyl(CH(3)DAB)}]. The complexes are highly active and selective catalysts for the homogeneous epoxidation of cyclooctene using tert-butyl hydroperoxide (TBHP) as the oxidant. In both cases, the initial activity is ca. 175 mol mol(-1) (Mo) h(-1) and cyclooctene oxide is obtained quantitatively within 4 h. It was possible to recover the dimethyl complex at the end of the reaction and reuse it in a second run with only a small decrease in activity. The complexes are also active and selective for the epoxidation of other olefins, such as 1-octene, 2-octene, cyclododecene and (R)-(+)-limonene, with TBHP. The, catalytic production of cyclooctene oxide was investigated in detail, varying either the reaction temperature or the initial concentrations of substrate, oxidant and catalyst precursor. Kinetic studies show that the catalyst precursor-oxygen donor complex formation is first-order in TBHP and in the metal complex [MoO(2)Cl(2){p-tolyl(CH(3)DAB)}]. A specific rate of 3:2 mol(-1) dm(3) s(-1) was found for catalyst formation at 25degreesC. Activation parameters for this reaction have also been measured (DeltaH(not equal)=48+/-3 kJ mol(-1), DeltaS(not equal)=-112+/-10 J mol(-1) K(-1)).
palavras-chave
LEWIS-BASE ADDUCTS; MOLYBDENUM COMPOUNDS; ALKENE EPOXIDATION; METAL-COMPLEXES; COORDINATION; DERIVATIVES; MECHANISM; OXIDATION; LIGANDS; DIOXOTUNGSTEN(VI)
categoria
Chemistry
autores
Valente, AA; Moreira, J; Lopes, AD; Pillinger, M; Nunes, CD; Romao, CC; Kuhn, FE; Goncalves, IS
nossos autores
Anabela Tavares Aguiar Valente
Investigador Principal
Isabel Maria de Sousa Gonçalves
Professor associado