Catalytic Oxidation of Methanol to Formaldehyde Catalyzed by Iron Complex with N3S3-type Tripodal Ligand.

ABSTRACT

A Fe3+ complex with N3S3-type tripod ligand, 1, reacts with O2 in CH3OH to generate formaldehyde, which has been studied structurally, spectroscopically, and electrochemically. Complex 1 crystallizes as an octahedral structure with crystallographic C3 symmetry around the metal, with Fe-N=2.2917(17) Å and Fe-S=2.3574(6) Å. UV-vis spectrum of 1 in CH3OH under Ar shows an intense band at 572 nm (ϵ 4,100 M-1cm-1), which shifts to 590 nm (ϵ 2,860 M-1cm-1) by the addition of O2, and a new peak appeared at 781 nm (ϵ 790 M-1cm-1). Such a spectral change is not observed in CH2Cl2. Cyclic voltammogram (CV) of 1 in CH2Cl2 under Ar gives reversible redox waves assigned to Fe2+/Fe3+ and Fe3+/Fe4+ couples at -1.60 V (ΔE=69 mV) and -0.53 V (ΔE=71 mV) vs Fc/Fc+, respectively. In contrast, in CH3OH, the reversible redox waves, albeit accompanied by a positive shift of the Fe2+/Fe3+ couple, are observed at -1.20 V (ΔE=85 mV) and -0.53 V (ΔE=64 mV) vs Fc/Fc+ under Ar. Interestingly, a catalytic current was observed for the CV of 1 in CH3OH in the presence of CH3ONa under Ar, when the sweep rate was slowed down.