Reduction of Sulfur Dioxide to Sulfur Monoxide by Ferrous Porphyrin

Abstract

Reduction of SO2 to fixed forms of sulfur can address the growing concerns regarding its detrimental effect on health and environment as well as enable its valorization into valuable chemicals. While the coordination of SO2 to transition metals are documented, its reduction using molecular catalysts has remained elusive. Alternatively, the naturally occurring heme enzyme sulfite reductase is known to reduce SO2 to H2S and is an integral part of the global sulfur cycle. However, its action is not yet mimicked in artificial systems outside of the protein matrix even after several decades of its structural elucidation. Here reduction of SO2 by iron (II) porphyrin, a synthetic analogue of heme, is demonstrated. A combination of spectroscopic and analytical methods indicates that SO2 is reduced by 2e -/2H + by Fe II TPP to form an intermediate [Fe III -SO] + species which releases SO. The SO obtained from the chemical reduction of SO2 could be valorized in the form of a Diels-Alder adduct of butadiene resulting in an organic sulfoxide.