O–S Bond Activation in Structures Isoelectronic with Ferric Peroxide Species Known in O–O‐Activating Enzymes: Relevance for Sulfide Activation and Sulfite Reductases

The interactions of H 2 S with Fe IV O porphyrin radical‐cation structures and of peroxide‐like H 2 OS systems with ferric heme have been investigated by DFT. The data are relevant for the formation of sulfheme in globins and for the catalytic cycle of sulfite reductases. Heme FeOS systems (and their protonated versions) feature relatively accessible potential energy surfaces for O–S bond formation and cleavage, and for the insertion of S at the meso position of the heme; O–S bond cleavage is thermodynamically favorable only in diprotonated versions of the FeOS unit, Fe IV O + H 2 S, or the ferric–HOSH isomer. The insertion at the meso position is reminiscent of heme oxygenase meso ‐hydroxylation via a ferric–hydroperoxo intermediate. Although axial imidazole and methylthiolate yield similar reaction profiles in all themodels studied, the Fe IV O + H 2 S reaction is another case of our previously advocated “thiolate obstruction”, in which thiolate hinders rather than favors reactions at the heme center.