Reduction of Sulfur Dioxide to Sulfur Monoxide by Ferrous Porphyrin.

The reduction of SO 2 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 SO 2 to H 2 S 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 SO 2 to transition metals is documented, its reduction using molecular catalysts has remained elusive. Herein reduction of SO 2 by iron(II) tetraphenylporphyrin is demonstrated. A combination of spectroscopic data backed up by theoretical calculations indicate that Fe II TPP reduces SO 2 by 2e - /2H + to form an intermediate [Fe III -SO] + species, also proposed for SiR, which releases SO. The SO obtained from the chemical reduction of SO 2 could be evidenced in the form of a cheletropic adduct of butadiene resulting in an organic sulfoxide.