Stereochemical Influence of the Ligand on the Structure of Manganese Complexes: Implications for Catalytic Epoxidations Meenal D. Godbole, † Anna C. G. Hotze, § Ronald Hage, # Allison M. Mills, ‡ Huub Kooijman, ‡ Anthony L. Spek, ‡ and Elisabeth Bouwman* ,† Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden UniVersity, P.O. Box 9502, 2300 RA Leiden, The Netherlands, BijVoet Center for Biomolecular Research, NMR Department, Utrecht UniVersity, UnileVer R&D Vlaardingen, OliVier Van Noortlaan 120, 3133 AT Vlaardingen, The Netherlands, and BijVoet Center for Biomolecular Research, Crystal and Structural Chemistry, Utrecht UniVersity, Padualaan 8, 3584 CH Utrecht, The Netherlands Received July 22, 2005 Manganese complexes of the ligand HphoxCOOR (R ) H or Me) have been synthesized and characterized by X-ray analysis, ESI-MS, ligand-field spectroscopy, electrochemistry, and paramagnetic 1 H NMR. The ligands, chirally pure or racemic, influence the structures of the complexes formed. Manganese(III) complexes of the ligand HphoxCOOMe are square-pyramidal or octahedral with two ligands bound in a trans fashion in the solid state. The racemic ligand (RS-HphoxCOOMe) as well as the enantiopure ligand (R-HphoxCOOMe) forms manganese complexes with similar solid-state structures. Ligand-exchange reactions occur in solution giving rise to meso complexes as confirmed by ESI-MS and deuteration studies. The manganese(III) complex of R-HphoxCOOH is octahedral, with two dianionic ligands bound in a fac-cct fashion in a tridentate manner. The manganese(III) complex of RS- HphoxCOOH is also octahedral with two dianionic ligands now bound in a trans fashion in a didentate manner and with two water molecules occupying axial sites. The paramagnetic 1 H NMR spectra of the complexes have been interpreted on the basis of the relaxation times with the help of the inversion-recovery pulse technique. The binding of imidazole with the metal center depends on the chirality of the ligands in the metal complexes of HphoxCOOMe. Imidazole coordination was found to occur with the metal complex that contains two ligands with the same chirality (R and R)(R-1), while no imidazole coordination was found upon reaction with the metal complex that contains two ligands with opposite chirality (R and S)(RS-1). Epoxidation reactions of various alkenes with H 2 O 2 as the oxidant reveal that the complexes give turnover numbers in the range of 10-35, the epoxide being the major product. The catalytic activity depends on the additives used, and a clear base effect is observed. The turnover numbers have been found to be higher in the complexes where no binding of N-Meim is observed. The latter fact unambiguously shows that imidazole binding is not a prerequisite for higher turnover numbers, in contrast to the Mn-Schiff base catalysts. Introduction Single oxygen-transfer reactions are mediated by a variety of metallo-enzymes and biomimetic metal complexes. 1,2 The oxidation of olefins to epoxides is among the key reactions studied in organic chemistry as epoxides can be easily transformed into a large variety of compounds using suitable ring opening reagents. In bulk chemistry, stoichiometric reagents (viz., K 2 CrO 7 , KMnO 4 , and derivatives) are usually applied in these reactions. 3-5 However, the use of these reagents has two major disadvantages: it results in high chemical costs and a stoichiometric amount of (often toxic) * To whom correspondence should be addressed. E-mail: bouwman@ chem.leidenuniv.nl. † Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University. § Bijvoet Center for Biomolecular Research, NMR Department, Utrecht University. # Unilever R&D Vlaardingen. ‡ Bijvoet Center for Biomolecular Research, Crystal and Structural Chemistry, Utrecht University. (1) Costas, M.; Chen, K.; Que, L. Coord. Chem. ReV. 2000, 200, 517- 544. (2) Lane, B. S.; Burgess, K. Chem. ReV. 2003, 103, 2457-2473. (3) Sheldon, R. A. Top. Curr. Chem. 1993, 164, 21-43. (4) Sheldon, R. A. Chemtech 1991, 21, 566-576. (5) Cornils, B. Angew. Chem., Int. Ed. Engl. 1997, 36, 2057-2059. Inorg. Chem. 2005, 44, 9253-9266 10.1021/ic051223z CCC: $30.25 © 2005 American Chemical Society Inorganic Chemistry, Vol. 44, No. 25, 2005 9253 Published on Web 10/28/2005