N -Heterocyclic Carbene-Based Molybdenum and Tungsten Complexes as Efficient Epoxidation Catalysts with H 2 O 2 and tert
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N-Heterocyclic Carbene-Based Molybdenum and Tungsten Complexes as Efficient Epoxidation Catalysts with H2O2 and tert-Butyl Hydroperoxide V. V. Krishna Mohan Kandepi • Joa˜o M. S. Cardoso Beatriz Royo
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Received: 1 March 2010 / Accepted: 15 March 2010 / Published online: 30 March 2010 Ó Springer Science+Business Media, LLC 2010
Abstract The molybdenum and tungsten g3-allyl dicarbonyl complexes bearing N-heterocyclic carbene (NHC) ligands [M(g3-C3H5)Cl(CO)2(bis-NHCBz)] (M = Mo, W; bis-NHCBz = 1,10 -dibenzyl-3,30 -methylenediimidazoline2,20 -diylidene) have been prepared from the corresponding acetonitrile precursors [M(g3-C3H5)Cl(CO)2(NCMe)2] by treatment with free carbene. Their catalytic performance in epoxidation of cis-cyclooctene using H2O2 as oxidant has been studied. All complexes can be applied as catalysts precursors in olefin epoxidation displaying 100% selectivity for the formation of cyclooctene oxide. The tungsten acetonitrile precursor [W(g3-C3H5)Cl(CO)2(NCMe)2] displayed the highest catalytic activity achieving quantitative conversion of epoxide in 30 min. The molybdenum NHCbased compound [Mo(g3-C3H5)Cl(CO)2(bis-NHCBz)] displayed higher activity when the epoxidation reaction was performed using H2O2 as oxidant compared to tert-butyl hydroperoxide. Keywords N-heterocyclic carbenes Molybdenum Tungsten Epoxidation Hydrogen peroxide
1 Introduction In the last decade the use of N-heterocyclic carbenes (NHCs) as ancillary ligands for transition metal catalysts and as organic catalysts on their own has proved very fruitful [1]. Not only have NHCs yielded improved transition metal
V. V. Krishna Mohan Kandepi J. M. S. Cardoso B. Royo (&) Instituto de Tecnologia Quı´mica e Biolo´gica da Universidade Nova de Lisboa, Av. da Repu´blica (EAN), 2780-157 Oeiras, Portugal e-mail: [email protected]
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catalysts, but they have also lead to the isolation of unusual low-coordinate metal complexes, which often play a key role in catalytic processes [2]. In particular, NHCs emerge as effective ligands in late transition metal chemistry, in which they have found successful applications in homogeneous catalysis such as in olefin metathesis and C–C coupling reactions [2, 3]. Recently, also in the field of oxidation catalysis significant progress has been achieved. A variety of oxidation reactions have been studied using NHC catalysts, being the oxidation of alcohols and olefins the most explored. However, most of the work in the area of oxidation catalysis with NHCs has been performed with late transition metals [4]. With strong sigma-donor properties, NHCs are well suited to stabilize high-oxidation-state metal complexes. Nevertheless, the number of complexes that have been prepared to date is quite small and very little has been described concerning the oxidizing properties of these complexes [5]. In particular, the chemistry of high-valent molybdenum and tungsten-NHCs is almost unexplored [6]. The only examples of NHC-Mo(VI) and -W(VI) complexes were described by Herrmann and co-workers in 1996 [7]. Low-val
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