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

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.