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

Abstract

AbstractA series of 2,6‐diacylpyridine ligand precursors 5a–d·HCl with different tether lengths between the carboxyl and pyridine moiety was prepared and converted into the correspondig trinuclear Fe3(μ3‐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 Fe3(μ3‐O) core for complexes 8a–d. When complex 8a was submitted to Gif‐type oxidations (O2, Zn, pyridine, HOAc), Mössbauer 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 57Fe Mössbauer 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 Mössbauer and magnetic studies. Complexes 8–10 were employed in catalytic aerobic oxidations of adamantane 11, cyclohexene 19, and α‐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 α‐pinene 23. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)