Iron(II) α‐Aminopyridine Complexes and Their Catalytic Activity in Oxidation Reactions: A Comparative Study of Activity and Ligand Decomposition

Abstract

AbstractNew well‐defined FeII complexes bearing bi‐ and tridentate α‐aminopyridine ligands were synthesized, and their catalytic activity in the oxidation of hydrocarbons and alcohols utilizing peroxide oxidants was investigated. The tridendate bis(picolyl)amine ligand 6 and its benzylated analogue 7 were converted into complexes [FeII(6)2]OTf2 (96 %, X‐ray; OTf=CF3SO3−) and [FeII(7)2]OTf2 (90 %). The bidentate aminopyridine ligand 8 was converted into [FeII(8)2(OTf)2] (93 %, X‐ray). The new complexes are catalytically active in the oxidation of secondary alcohols and benzylic methylene groups to the corresponding ketones, of toluene to benzaldehyde, and of cyclohexene to cyclohexene oxide (3 mol % catalyst, tBuOOH (4 equiv), RT, 2–6 h, 28 to 85 % yield of isolated product). The catalytic oxidation of cyclohexane with ROOH (R=H, tBu) to an alcohol/ketone mixture with low ratio revealed that these oxidations follow largely a radical mechanism, except when [FeII(6)2]OTf2 was employed and H2O2 was added slowly. Together with known bi‐ and tetradendate iron complexes, a comparative study showed slight reactivity differences for the newly prepared complexes, with the highest observed for [FeII(6)2]OTf2 and [FeII(7)2]OTf2. The reaction of the new complexes with peroxides was followed over time by UV/Visible spectroscopy; this revealed a fast reaction between the two reactants within minutes. Ligand‐decomposition pathways were investigated, and revealed that the NCH2 units of the complexes are rapidly oxidized to the corresponding amides NCO. The iron complex [FeII(6)2]OTf2 showed no decrease in catalytic activity and a moderate decrease in selectivity when first subjected to oxidative conditions similar to those employed in catalysis. Thus, oxidative ligand deterioration had a marginal effect on the catalytic activity of the iron complex [FeII(6)2]OTf2.