Catalytic Oxidation of Methanol to Formaldehyde Catalyzed by Iron Complex with N 3 S 3 -type Tripodal Ligand.

A Fe 3+ complex with N 3 S 3 -type tripod ligand, 1, reacts with O 2 in CH 3 OH to generate formaldehyde, which has been studied structurally, spectroscopically, and electrochemically. Complex 1 crystallizes as an octahedral structure with crystallographic C 3 symmetry around the metal, with Fe-N=2.2917(17) Å and Fe-S=2.3574(6) Å. UV-vis spectrum of 1 in CH 3 OH under Ar shows an intense band at 572 nm (ϵ 4,100 M -1 cm -1 ), which shifts to 590 nm (ϵ 2,860 M -1 cm -1 ) by the addition of O 2 , and a new peak appeared at 781 nm (ϵ 790 M -1 cm -1 ). Such a spectral change is not observed in CH 2 Cl 2 . Cyclic voltammogram (CV) of 1 in CH 2 Cl 2 under Ar gives reversible redox waves assigned to Fe 2+ /Fe 3+ and Fe 3+ /Fe 4+ couples at -1.60 V (ΔE=69 mV) and -0.53 V (ΔE=71 mV) vs Fc/Fc + , respectively. In contrast, in CH 3 OH, the reversible redox waves, albeit accompanied by a positive shift of the Fe 2+ /Fe 3+ 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 CH 3 OH in the presence of CH 3 ONa under Ar, when the sweep rate was slowed down.