Syntheses, Crystal Structures, Spectroscopic Properties, and Catalytic Aerobic Oxidations of Novel Trinuclear Non-Heme Iron Complexes

A series of 2,6-diacylpyridine ligand precursors 5a-d center dot HCl with different tether lengths between the carboxyl and pyridine moiety was prepared and converted into the correspondig trinuclear Fe-3(mu(3)-O) complexes 8a-d and 10. Under slow precipitation conditions a tetranuclear complex 9 was isolated instead of 8a. Single-crystal X-ray diffraction analyses were performed on ligands 5a-d and complexes 9 and 10. Characterization by X-ray absorption spectroscopy (XAS) proved a trinuclear Fe-3(mu(3)-O) core for complexes 8a-d. When complex 8a was submitted to Gif-type oxidations (O-2, Zn, pyridine, HOAc), Mossbauer and nuclear inelastic scattering (NIS) suggested the formation of mononuclear species. The trinuclear ferric complex 10 has an isosceles molecular structure, which is manifested in the Fe-57 Mossbauer spectrum by two quadrupole doublets with a 2:1 intensity ratio. The magnetic measurements reveal two moderate antiferromagnetic exchange interactions of -22.1 and -33.8 cm(-1). Spin concentrations of complex 10 were determined by EPR spectroscopy, which supports the Mossbauer and magnetic studies. Complexes 8-10 were employed in catalytic aerobic oxidations of adamantane 11, cyclohexene 19, and alpha-pinene 23. For adamantane 11, the oxidation of secondary C-H bonds to the corresponding ketone 14 is favored. In addition, adamantylpyridines 15-18 were isolated, thus supporting a radical pathway. A strong preference of allylic oxidation versus epoxidation was found for cyclohexene 19 and alpha-pinene 23. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)