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 O-2 in CH3OH 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) & Aring; and Fe-S=2.3574(6) & Aring;. UV-vis spectrum of 1 in CH3OH under Ar shows an intense band at 572 nm (& varepsilon; 4,100 M(-1)cm(-1)), which shifts to 590 nm (& varepsilon; 2,860 M(-1)cm(-1)) by the addition of O-2, and a new peak appeared at 781 nm (& varepsilon; 790 M(-1)cm(-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 (Delta E=69 mV) and -0.53 V (Delta E=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 (Delta E=85 mV) and -0.53 V (Delta E=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.