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

Abstract

Keywords: Carboxylate ligands / EPR spectroscopy / EXAFS spectroscopy / Mössbauer spectroscopy / OxidationA 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 Fe 3 (µ 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 (µ 3 -O) core for complexes 8a-d. When complex 8a was submitted to Gif-type oxidations (O 2 , 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 57 Fe Mössbauer spectrum

Trinuclear Non‐Heme Iron Complexes Based on 4‐Substituted 2,6‐Diacylpyridine Ligands as Catalysts in Aerobic Allylic Oxidations

“…Unfortunately, X-ray crystal structures of the complexes 3a and 3c -3e could not be obtained. However, spectroscopic and analytical data (ESI-MS, IR, and elemental analysis) were in good agreement with our previously reported [Fe 3 (m 3 -O)] complex 3f (R ¼ H) [24].…”

“…In particular, the tether length had no influence on the nuclearity of the Fe complex. Furthermore, during catalytic aerobic oxidation, freeze-quench Mçssbauer and nuclear inelastic scattering data revealed that the precursor complexes 2 and 3 undergo cleavage to mononuclear species [24] [25], which is in agreement with Bartons original proposal of the catalytic cycle [21b]. We thus anticipated that substituents at C(4) of the pyridine reaction progress must be carefully monitored, and the reactions have to be stopped at incomplete conversion in order to avoid overreduction.…”

“…The results are summarized in Scheme 5 and the Table. For comparison, the known complex 3f (R ¼ H) with an unsubstituted pyridine moiety was studied as well [24]. The aerobic oxidations yielded the four products myrtenol (23), verbenone (24), myrtenal (25), and a-pinene oxide (26) poor conversion of the starting material 6 ). Irrespective of the substituent on the pyridine ligand in iron complexes 3a and 3c -3f, 24 was the major product, and the other allylic oxidation products, 23 and 25, were only obtained as minor products in a ratio of 1 : 5 to 1 : 10 relative to 24.…”

“…Pioneering work on aerobic oxidations was done by Barton and co-workers. The so-called Gif-type oxidations employ a trinuclear m-oxo iron catalyst Fe 3 (m 3 -O)(OAc) 6 We have recently developed a set of 2,6-diacylpyridine ligands 1 with different spacer lengths between the carboxylic acid and pyridine moieties, which were converted to the corresponding trinuclear [Fe 3 (m 3 -O)] complexes 2 or 3 depending on the type of counterion (Scheme 1) [24].…”

“…-A series of 4-substituted 2,6-diacylpyridine ligands 4a -4e has been synthesized and converted to the corresponding trinuclear [Fe 3 (m 3 -O)] complexes 3a and 3c -3e. The new complexes 3 were compared with complex 3f bearing a 4-unsubstituted pyridine ligand [24]. Complexes 3 were employed for the catalytic Giftype oxidation of a-pinene (22) to the allylic oxidation products myrtenol (23), verbenone (24), myrtenal (25), and the epoxide a-pinene oxide (26).…”