The reactivity of molecular oxygen and reactive oxygen species with [FeFe] hydrogenase biomimetics: reversibility and the role of the second coordination sphere

Abstract

The development of oxygen-tolerant H₂-evolving catalysts plays a vital role for a future H₂ economy. For example, the [FeFe] hydrogenase enzymes are excellent catalyst for H₂ evolution but rapidly become inactivated in the presence of O₂. The mechanistic details of the enzyme's inactivation by molecular oxygen still remain unclear. Here, two H₂-evolving diiron complexes [Fe₂(μ-SCH₂NHCH₂S)(CO)₆] (1^adt) and [Fe₂(μ-SCH₂CH₂CH₂S)(CO)₆] (2^pdt), inspired by the active site of [FeFe] hydrogenase, were investigated for their reactivity with molecular oxygen and reactive oxygen species. A one-electron reduced and oxygenated 1^adt species was identified and characterized spectroscopically, which can be directly generated by reacting with molecular oxygen and chemical reductants at room temperature but it is unstable and gradually decomposes. Interestingly, the whole process is reversible and the addition of protons can facilitate the deoxygenation process and prevent further degradation at room temperature. This new identification of intermediate species serves as a model for studying the reversible inactivation and degradation of oxygen-sensitive [FeFe] hydrogenases by O₂, and provides chemical precedence for such processes. In comparison, the complex lacking the nitrogen bridgehead, 2^pdt, exhibits reduced reactivity towards O₂ in the presence of reductants, highlighting that the importance of the second coordination sphere on modulating the oxygenation processes. These results provide new directions to design molecular electrocatalysts for proton reduction operated at ambient conditions and the re-engineering of [FeFe] hydrogenases for improving oxygen tolerance.