Polydentate pyridyl ligands and the catalytic activity of their iron(II) complexes in oxidation reactions utilizing peroxides as the oxidants

Abstract

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 (3mol% catalyst loading, 3equiv. H2O2 in CH3CN for 2h at room temperature). Similar conditions utilizing environmentally friendly acetone as the solvent and 4equiv. 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.