Dioxygen activation by a dinuclear thiolate-ligated Fe(ii) complex

Dioxygen activation by Fe ^II thiolate complexes is relatively rare in biological and chemical systems because the sulfur site is at least as vulnerable as the iron site to oxidative modification. O ₂ activation by Fe ^II -SR complexes with thiolate bound trans to the O ₂ binding site generally affords the Fe ^IV ═O intermediate and oxidized thiolate. On the other hand, O ₂ activation by Fe(ii)-SR complexes with thiolate bound cis to the O ₂ binding site generates Fe ^III -O-Fe ^III or S-oxygenated complexes. The postulated Fe ^IV ═O intermediate has only been identified in isopenicillin N synthase recently. We demonstrated here that O ₂ activation by a dinuclear Fe ^II thiolate-rich complex produces a mononuclear Fe ^III complex and water with a supply of electron donors. The thiolate is bound cis to the postulated dioxygen binding site, and no Fe ^III -O-Fe ^III or S-oxygenated complex was observed. Although we have not detected the transient intermediate by spectroscopic measurements, the Fe ^IV ═O intermediate is suggested to exist by theoretical calculation, and P-oxidation and hydride-transfer experiments. In addition, an unprecedented Fe ^III -O ₂ -Fe ^III complex supported by thiolates was observed during the reaction by using a coldspray ionization time-of-flight mass (CSI-TOF MS) instrument. This is also supported by low-temperature UV-vis measurements. The intramolecular NHO═Fe ^IV hydrogen bonding, calculated by DFT, probably fine tunes the O ₂ -activation process for intramolecular hydrogen abstraction, avoiding the S-oxygenation at cis-thiolate.