Redox and "Antioxidant" Properties of Fe₂(μ-SH)₂(CO)₄(PPh₃)₂

The chemistry of Fe₂(μ-SH)₂(CO)₄(PPh₃)₂ (2^HH) is described with attention to S-S coupling reactions. Produced by the reduction of Fe₂(μ-S₂)(CO)₄(PPh₃)₂ (2), 2^HH is an analogue of Fe₂(μ-SH)₂(CO)₆ (1^HH), which exhibits well-behaved S-centered redox. Both 2^HH and the related 2^MeH exist as isomers that differ with respect to the stereochemistry of the μ-SR ligands (R = H, Me). Compounds 2^HH, 2^MeH, and 2 protonate to give rare examples of Fe-SH and Fe-S₂ hydrides. Salts of [H2]⁺, [H2^HH]⁺, and [H2^MeH]⁺ were characterized crystallographically. Complex 2^HH reduces O₂, H₂O₂, (PhCO₂)₂, and Ph₂N₂, giving 2. Related reactions involving 1^HH gave uncharacterizable polymers. The differing behaviors of 2^HH and 1^HH reflect stabilization of the ferrous intermediates by the PPh₃ ligands. When independently generated by the reaction of 2^HH with 2,2,6,6-tetramethyl-1-piperidinyloxy, 2* quantitatively converts to 2 or, in the presence of C₂H₄, is trapped as the ethanedithiolate Fe₂(μ-S₂C₂H₄)(CO)₄(PPh₃)₂. Evidence is presented that the Hieber-Gruber synthesis of 1 involves polysulfido intermediates [Fe₂(μ-S _n)₂(CO)₆]^2- ( n > 1). Two relevant experiments are as follows: (i) protonation of [Fe₄(μ-S)₂(μ-S₂)CO)₁₂]^2- gives 1 and 1^HH, and (ii) oxidation of 1^HH by sulfur gives 1.