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

Abstract

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. O₂ activation by Fe^II–SR complexes with thiolate bound trans to the O₂ binding site generally affords the Fe^IVO intermediate and oxidized thiolate. On the other hand, O₂ activation by Fe(II)–SR complexes with thiolate bound cis to the O₂ binding site generates Fe^III–O–Fe^III or S-oxygenated complexes. The postulated Fe^IVO intermediate has only been identified in isopenicillin N synthase recently. We demonstrated here that O₂ 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^IVO intermediate is suggested to exist by theoretical calculation, and P-oxidation and hydride-transfer experiments. In addition, an unprecedented Fe^III–O₂–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 NH⋯OFe^IV hydrogen bonding, calculated by DFT, probably fine tunes the O₂-activation process for intramolecular hydrogen abstraction, avoiding the S-oxygenation at cis-thiolate.