Iron(II) Complexes with Tetradentate Bis(aminophenolate) Ligands: Synthesis and Characterization, Solution Behavior, and Reactivity with O₂

Tetradentate bis(aminophenolate) ligands H2salanX and H2bapenX (where X refers to the para-phenolate substituent = H, Me, F, Cl) react with [Fe{N(SiMe3)2}2] to form iron(II) complexes, which in the presence of suitable donor ligands L (L = pyridine or THF) can be isolated as the complexes [Fe(salanX)(L)2] and [Fe(bapenX)(L)2]. In the absence of donor ligands, either mononuclear complexes, for example, [Fe(salantBu,tBu)], or dinuclear complexes of the type [Fe(salanX)]2 are obtained. The dynamic coordination behavior in solution of the complexes [Fe(salanF)(L)2] and [Fe(bapenF)(L)2] has been investigated by VT 1H and 19F NMR spectroscopy, which has revealed equilibria between isomers with different ligand coordination topologies cis-α, cis-β and trans. Exposure of the iron(II) salanX complexes to O2 results in the formation of oxo-bridged iron(III) complexes of the type [{Fe(salanX)}2(μ-O)] or [{Fe(salanX)(L)}2(μ-O)]. The lack of catalytic activity of the iron(II) salan and bapen complexes in the oxidation of cyclohexane with H2O2 as the oxidant is attributed to the rapid formation of stable and catalytically inactive oxo-bridged iron(III) complexes.