Journal of Molecular Catalysis A: Chemical 195 (2003) 83–94 Preparation and use of novel molybdenum-containing organic complexes as catalysts in the epoxidation of cyclohexene Stefan V. Kotov a , Tsonko M. Kolev b , Mariana G. Georgieva a,∗ a Inorganic Technology Building, University “Prof. Dr. As. Zlatarov”, 1 Prof. Yakimov St., 8010 Bourgas, Bulgaria b Institute of Organic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria Received 25 July 2002; accepted 15 October 2002 Abstract Novel molybdenum-containing organic complexes based on 3,4-dihydroxy-3-cyclobutene-1,2-dione (“squaric acid”) and some organic amines have been synthesized and characterized by infrared (IR) spectroscopy, thermogravimetric (TGA) and elemental analyses. The resulting molybdenum complexes were introduced as catalysts in the epoxidation of cyclohexene with t-butylhydroperoxide (TBHP) and showed high activities and selectivities, comparable to those of the commercially available molydenyl acetylacetonate (MoO 2 (acac) 2 ). © 2002 Elsevier Science B.V. All rights reserved. Keywords: Squaric acid; Molybdenum-containing complexes; Catalysts; Epoxidation 1. Introduction Molybdenum-containing complexes have long been known to be useful catalysts for the epoxidation of alkenes by organic hydroperoxides. The industrial im- plementation of this reaction was first achieved in the Halcon process, which utilized a soluble Mo com- plex and in the Arco process, which made use of a heterogeneous Ti-SiO 2 catalyst to effect the epoxida- tion of propylene to the synthetically useful propylene oxide precursor [1,2]. Since then, a number of solu- ble molybdenum-containing organic complexes with carboxylic acids, ketones, alcohols, glycols, amines, etc. have been synthesized and employed as homoge- neous catalysts for the epoxidation of various alkenes [3–7]. The preparation of polymer-supported catalysts for the epoxidation of alkenes has also received scien- ∗ Corresponding author. E-mail address: stekot@yahoo.com (M.G. Georgieva). tific attention since the late 1970s. Recently, a num- ber of chelating functional molybdenum-containing copolymers have been prepared and used as catalysts for the epoxidation of alkenes, mostly, cyclohexene [8–10]. Compared to the vast number of organic ligands employed so far for the preparation of molybdenum complexes and catalysts for the alkene epoxidation, 3,4-dihydroxy-3-cyclobutene-1,2-dione (squaric acid) holds unique position. Squaric acid is a strong diba- sic acid with a four-atom ring framework endowed with hydrogen bonding (donor as well as acceptor) and ionic organizational features. Therefore, it ap- pears particularly attractive as a template for generat- ing self-assemblies from polarizable cations in general and organic bases in particular. The squarate dianion possesses a square-planar structure (D 4h symmetry) and has been suggested as a member of an aromatic series of cyclic oxocarbon anions that are stabilized by delocalization of -electrons around the ring [11,12]. 1381-1169/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII:S1381-1169(02)00578-2