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

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