Epoxidation of olefins using a dichlorodioxomolybdenum(VI)-pyridylimine complex as catalyst Salete S. Balula a,b,c , Sofia M. Bruno a , Ana C. Gomes a , Anabela A. Valente a , Martyn Pillinger a , Isabel S. Gonçalves a,⇑ , Duncan J. Macquarrie c , James H. Clark c a Department of Chemistry, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal b Faculty of Science, Department of Chemistry and Biochemistry, REQUIMTE, University of Porto, 4169-007 Porto, Portugal c Clean Technology Centre, Department of Chemistry, University of York, York, YO105 DD, UK article info Article history: Received 28 October 2011 Received in revised form 12 January 2012 Accepted 13 January 2012 Available online 28 January 2012 Keywords: Dioxomolybdenum(VI) complex Pyridylimine ligand Catalysis Olefin epoxidation Silica supports Ionic liquids abstract The ligand N-(n-propyl)-2-pyridylmethanimine (pyim) and an immobilized analogue of this ligand (pyim-MTS) were prepared by the condensation reaction of 2-pyridinecarboxaldehyde with either n- propylamine or 3-aminopropyl groups covalently attached to a micelle-templated silica (MTS). Free and immobilized dioxomolybdenum(VI) complexes of the type MoO 2 Cl 2 (L 1 ) (L 1 = pyim (1), pyim-MTS) were then prepared by treatment of the organic ligand or ligand-silica with the solvent adduct MoO 2 Cl 2 (THF) 2 . MoO 2 Cl 2 (pyim) (1) is a highly active catalyst for the epoxidation of olefins (cyclooctene (Cy), cyclododecene, 1-octene, trans-2-octene, R-(+)-limonene) at 55 °C using tert-butylhydroperoxide (TBHP) as the oxidant under solvent-free conditions, giving the corresponding epoxides as the only reac- tion products. A turnover frequency of 1855 mol mol Mo 1 h 1 was measured for the epoxidation of Cy, and the epoxide (CyO) was formed quantitatively within 4.5 h. The MTS-supported complex was less active, and exhibited temperature-dependent leaching of active species. As an alternative approach to facilitating catalyst recycling, complex 1 was investigated with the ionic liquid (IL) 1-butyl-3-methylimi- dazolium tetrafluoroborate as solvent. The 1/IL phase could be reused, giving ca. 95% CyO yield in each run. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Metal-catalyzed olefin epoxidation is one of the most effective approaches for the preparation of epoxides, which are important building blocks for the production of bulk and fine chemicals [1–7]. The use of oxomolybdenum(VI) complexes as catalysts for this reaction has been extensively explored over the last 40 years, beginning with homogeneous molybdenum(VI) catalysts in the Halcon and Arco processes for the epoxidation of propylene [8–14]. Complexes containing the MoO 2 2+ core are the most com- mon [13], and include neutral species of the type MoO 2 X 2 (L 1 ) n [15–24], MoO 2 (L 2 ) n [25–28] and (g 5 -C 5 R 5 )MoO 2 X [10–12,29,30], and cationic complexes such as [MoO 2 X(L 2 )]Y [31] (where X = ha- lide, alkyl, OR, OSiR 3 , SCN, etc., L 1 and L 2 represent neutral and an- ionic ligands bearing donor atoms from groups 15 and 16, and Y = anion such as halide, BF 4  or PF 6  ). The catalytic performance of these complexes is governed to a large extent by their solubility and steric/electronic factors, which are all strongly influenced by the nature of the first-sphere ligands and their substituents. Com- plexes of the type MoO 2 Cl 2 (L 1 ) n proved to be among the most effi- cient catalysts for the liquid-phase epoxidation of olefins using tert-butylhydroperoxide (TBHP) as the mono-oxygen source. Of the numerous ligand types investigated, bidentate N-donor ligands such as substituted bipyridines [18,19], diazabutadienes [20–22] and pyrazolylpyridines [23,24] gave highly active and selective catalysts. One potentially promising family of ligands that have not, to the best of our knowledge, been studied in complexes of the type MoO 2 Cl 2 (L 1 ) n are pyridylimines. These are attractive due to their easy preparation (from the condensation of 2-pyridinecarboxalde- hyde with a primary amine) and the fact that their electronic properties are intermediate between those of the more classical li- gands such as bipyridine and phenanthroline, and the more flexible diazabutadienes [32]. Another advantage is that it is relatively straightforward to prepare inorganic support materials functional- ized with these ligands. In this way several pyridine-derived palla- dium [33–36], platinum [37], ruthenium [37], rhodium [38], manganese [39], cobalt [40], copper [41,42], (acetylacetona- to)dioxomolybdenum(VI) [43] and molybdenum tri- and tetracar- bonyl [32,44] complexes have been immobilized on silica supports and used as catalysts for various organic transformations, 0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2012.01.029 ⇑ Corresponding author. Tel.: +351 234 378190; fax: +351 234 370084. E-mail address: igoncalves@ua.pt (I.S. Gonçalves). Inorganica Chimica Acta 387 (2012) 234–239 Contents lists available at SciVerse ScienceDirect Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica