Synthesis, Structure and Reactivity of a Mononuclear N,N,O-Bound Fe(II) α-Keto-Acid Complex

Abstract

A bulky, tridentate phenolate ligand (ImPh2NNOtBu) was used to synthesise the first example of a mononuclear, facial, N,N,O-bound iron(II) benzoylformate complex, [Fe(ImPh2NNOtBu)(BF)] (2). The X-ray crystal structure of 2 reveals that the iron centre is pentacoordinate (τ=0.5), with a vacant site located cis to the bidentate BF ligand. The Mössbauer parameters of 2 are consistent with high-spin iron(II), and are very close to those reported for α-ketoglutarate-bound non-heme iron enzyme active sites. According to NMR and UV-vis spectroscopies, the structural integrity of 2 is retained in both coordinating and non-coordinating solvents. Cyclic voltammetry studies show that the iron centre has a very low oxidation potential and is more prone to electrochemical oxidation than the redox-active phenolate ligand. Complex 2 reacts with NO to form a S=3/2 {FeNO}7 adduct in which NO binds directly to the iron centre, according to EPR, UV-vis, IR spectroscopies and DFT analysis. Upon O2 exposure, 2 undergoes oxidative decarboxylation to form a diiron(III) benzoate complex, [Fe2(ImPh2NNOtBu)2(μ2-OBz)(μ2-OH)2]+ (3). A small amount of hydroxylated ligand was also observed by ESI-MS, hinting at the formation of a high-valent iron(IV)-oxo intermediate. Initial reactivity studies show that 2 is capable of oxygen atom transfer reactivity with O2, converting methyl(p-tolyl)sulfide to sulfoxide.