FULL PAPER DOI: 10.1002/ejic.200801103 Adduct Formation of Dichloridodioxidomolybdenum(VI) and Methyltrioxidorhenium(VII) with a Series of Bidentate Nitrogen Donor Ligands Ahmad M. Al-Ajlouni, [a,b] Alev Günyar, [a] Ming-Dong Zhou, [a] Paul N. W. Baxter, [c] and Fritz E. Kühn* [a] Keywords: Molybdenum / Rhenium / N ligands / Oxido ligands / Stability constants The stability of a variety of bidentate N-base adducts of MoO 2 Cl 2 and (CH 3 )ReO 3 (MTO) was investigated in thf and CH 2 Cl 2 as solvents. The formation constants were deter- mined from the spectrophotometric data based on 1:1 adduct formation. The adduct formation constants for [MoO 2 Cl 2 L 2 ] (L 2 = bidentate nitrogen ligand) are 10 4 –10 6 times higher than those for [(CH 3 )ReO 3 L 2 ] with the same ligands under the same conditions. The adduct stability of both systems is very sensitive to the electronic nature of the ligands and increases with their donor ability. Hammett correlations of the forma- tion constants against σ give relatively large negative values for the reaction constants (ρ Re = –5.9, ρ Mo = –6.6). The stability is also governed by steric and strain factors. Thus, sterically hindered 6,6'-disubstituted-2,2'-bipyridines do not form ad- Introduction High-valent d 0 early transition metal oxides are well known for their catalytic oxidation activities in the presence of peroxides. [1] Among these, methyltrioxidorhenium(VII) (MTO) and dichloridodioxidomolybdenum(VI) complexes have been most widely utilized as catalysts or catalyst pre- cursors for oxygen-atom transfer reactions to a plethora of substrates. [2] MTO is very stable and has been used in aque- ous, semi-aqueous, and organic solutions under homogen- eous and heterogeneous conditions, [3] whereas dioxidomo- lybdenum(VI) complexes, such as MoO 2 X 2 (X = Cl, Br, or methyl), are coordinatively unsaturated (or coordinated to labile solvent molecules) and highly sensitive to moisture. [a] Molecular Catalysis, Catalysis Research Center, Technische Universität München, Lichtenbergstrasse 4, 85747 Garching bei München, Germany E-mail: fritz.kuehn@ch.tum.de [b] Department of Applied Chemical Sciences, Jordan University of Science and Technology, Irbid 22110, Jordan E-mail: aajlouni@just.edu.jo [c] Institut Charles Sadron, Université Louis Pasteur, 23 rue du Loess BP 20, 67083 Strassbourg, France E-mail: baxter@ics.u-strasbg.fr Supporting information for this article is available on the WWW under http://www.eurjic.org or from the author. Eur. J. Inorg. Chem. 2009, 1019–1026 © 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1019 ducts with MTO, and only 6,6'-dimethyl- and 6,6'-diphenyl- 2,2'-bipyridines form adducts with MoO 2 Cl 2 . However, these adducts are much less stable than other methyl derivatives of 2,2'-bipridine adducts. The steric strain between the two methyl groups in 3,3-dimethyl-2,2'-bipyridine influences the bipyridine planarity upon complexation and reduces the ad- duct stability. The thermodynamic parameters (enthalpy and entropy) were determined from temperature-dependence studies. The adduct stability is mainly due to the strongly exothermic binding of the nitrogen-bidentate ligand. The en- tropy change is small and has little effect on adduct stability. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) The MoO 2 X 2 moiety can be stabilized by adduct formation with relatively strong basic donor ligands, such as 2,2'-bi- pyridine and derivatives. Many papers have been published on the effect of nitrogen donor ligands on the stability and activity of MTO and, to a lesser extent, MoO 2 X 2 com- plexes. [4] Adducts of the composition [MoO 2 X 2 L 2 ] are formed with Lewis bases such as pyridine and 2,2'-bipyr- idine. [5,6] The first X-ray crystal structure of an [MoO 2 X 2 L 2 ]-type complex was reported in 1966. [7] [MoO 2 X 2 L 2 ] complexes are monomeric and display a dis- torted octahedral geometry, with the oxido ligands cis to each other in order to maximize donation into the empty t 2g set orbitals. [8] Such Mo VI -oxido complexes have been used as active catalysts for olefin epoxidation by organic peroxides, particularly homogeneous phase systems. [9,10] A series of papers has been published in recent years showing that complexes of the type [MoO 2 X 2 L 2 ] are excellent cata- lyst precursors for olefin epoxidation in the presence of tert- butylhydroperoxide (TBHP). [11] As regards the reaction mechanism, it has been generally agreed that formation of a Mo VI alkyl peroxide occurs followed by transfer of the distal oxygen atom of the alkyl peroxide rather than an ox- ido ligand. [12] The activity of d 0 M-oxido catalysts in olefin epoxidation depends on the Lewis bases, the redox stability of the ligands, and particularly on the stability of the ad-