Reduction of Sulfur Dioxide to Sulfur Monoxide by Ferrous Porphyrin

The reduction of SO2 to fixed forms of sulfur can address the growing concerns regarding its detrimental effect on health and the environment as well as enable its valorization into valuable chemicals. The naturally occurring heme enzyme sulfite reductase (SiR) is known to reduce SO2 to H2S and is an integral part of the global sulfur cycle. However, its action has not yet been mimicked in artificial systems outside of the protein matrix even after several decades of structural elucidation of the enzyme. While the coordination of SO2 to transition metals is documented, its reduction using molecular catalysts has remained elusive. Herein reduction of SO2 by iron(II) tetraphenylporphyrin is demonstrated. A combination of spectroscopic data backed up by theoretical calculations indicate that FeIITPP reduces SO2 by 2e−/2H+ to form an intermediate [FeIII−SO]+ species, also proposed for SiR, which releases SO. The SO obtained from the chemical reduction of SO2 could be evidenced in the form of a cheletropic adduct of butadiene resulting in an organic sulfoxide. SO2 could be reduced to SO by an iron porphyrin via a [FeIII−SO]+ intermediate. This intermediate, which is proposed to be involved in sulfite reductase enzymes, has not been observed outside the protein matrix previously and was characterized by EPR, Mossbauer and resonance Raman spectroscopy. The SO formed could be transferred to an organic diene through a cycloaddition reaction.