22026-13-7Relevant academic research and scientific papers
Iron(II) α-aminopyridine complexes and their catalytic activity in oxidation reactions: A comparative study of activity and ligand decomposition
Lenze, Matthew,Martin, Erin T.,Rath, Nigam P.,Bauer, Eike B.
, p. 101 - 116 (2013/05/22)
New 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 [Fe II(6)2]OTf2 was employed and H 2O2 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 [Fe II(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.
Polydentate pyridyl ligands and the catalytic activity of their iron(II) complexes in oxidation reactions utilizing peroxides as the oxidants
Lenze, Matthew,Sedinkin, Sergey L.,Bauer, Eike B.
, p. 161 - 171 (2013/06/27)
The paper describes the synthesis of iron(II) complexes bearing new polydentate N,O-coordinating pyridyl ligands and their catalytic application in oxidation reactions employing peroxides as the oxidants. The tridentate N,O,N (10) and N,N,O (11) ligands, the tetradentate N,N,O,N ligand 12 and the pentadentate N,N,N,O,N-coordinating ligand 16 were synthesized, and obtained as oils or solids in 74-93% isolated yields. The ligands were subsequently converted to the iron complexes [Fe(10)2](OTf)2, [Fe(11)2](OTf)2, [Fe(12)(OTf)2] and [Fe(16)(OTf)](OTf), which were obtained as tan powders in 90-94% yield and characterized by various instrumental techniques. Preliminary screening experiments revealed that all complexes are catalytically active in the oxidation of secondary alcohols and benzylic methylene groups to the corresponding ketones. Optimization experiments with the complex [Fe(12)(OTf)2] yielded a system that provided under mild condition ketones from benzylic methylene groups and secondary alcohols in 63-90% isolated yields (3 mol% catalyst loading, 3 equiv. H2O2 in CH 3CN for 2 h at room temperature). Similar conditions utilizing environmentally friendly acetone as the solvent and 4 equiv. tBuOOH resulted in 36-65% isolated yields for some of the substrates, indicating a somewhat lower catalytic activity in that solvent. For the complexes [Fe(10)2](OTf)2 (two tridentate ligands), [Fe(12)(OTf)2] (one tetradentate ligand) and [Fe(16)(OTf)](OTf) (one pentadentate ligand), the product formation for a test reaction was followed over time at significantly reduced catalyst loading to determine activities. Under these conditions, the complex [Fe(10)2](OTf)2 exhibited a somewhat lower catalytic activity compared to the other two complexes. Thus, the denticity seems to have an impact on catalytic activity although it is not dramatic, and a higher denticity appears to be beneficial for catalysis.
