Fe(II) complexes of pyridine-substituted thiosemicarbazone ligands as catalysts for oxidations with hydrogen peroxide

The reaction of three [Fe^II(TSC)₂] complexes, where TSC is a pyridine-substituted thiosemicarbazone of the HDpT or HBpT families, with H₂O₂ in acetonitrile solution does not result in the accumulation of the corresponding [Fe^III(TSC)₂]⁺ complexes. Instead, a mixture of diamagnetic low-spin Fe^II species is generated. According to the MS spectra, those species result from the sequential addition of up to five oxygen atoms to the complex. This capability for the addition of oxygen atoms suggested that oxygen atom transfer to external substrates may be possible, and these TSC complexes were tested in the oxidation of thioanisole and styrene with H₂O₂. As hypothesized, the complexes are active in both the oxidation of thioanisole to its sulfoxide and styrene to benzaldehyde, with time scales indicating the participation of the species containing added oxygen atoms. Interestingly, the free thiosemicarbazone ligands and the [Zn(Dp44mT)₂] complex also catalyse the selective sulfoxidation of thioanisole, but they are ineffective in catalysing styrene oxidation to benzaldehyde. These findings open up new directions for the development of thiosemicarbazone-based metal catalysts for oxidation processes.