Multi-edge X-ray Absorption Spectroscopic Evidence for Sulphur-based Redox in [4Fe-4S] Molecular Clusters

Central to the dogma of Fe–S World Hypothesis for the Emergence of Biosphere is the transition from non-living Fe–S geochemistry to the molecular Fe–S biochemistry. While often assumed this being a trivial and spontaneous process; however, the chemical reality is a critical discontinuity of oxidation states of ferrous iron-based minerals and nanoparticles in low temperature hydrothermal environments and ferric iron-based prosthetic groups in metalloproteins. A recent ingenious synthetic effort enabled the access of the first four-electron transfer-series in [4Fe-4S] molecular cluster with the same aromatic thiolate ligand environment. This discovery opened up the possibility to probe a range of [4Fe-4S] cubanes from the geological all ferrous to the hypothetical all ferric cluster along with each intermediate oxidation state. Multi-edge X-ray absorption spectroscopic (meXAS) approach provided us with the insights to put forward the exclusive sulphur-based redox chemistry with only minor perturbations to the Fe centres. The meXAS results unambiguously prove that the Fe ions in the molecular cubanes remain essentially the same as in the mackinawite-like nanoparticles (n(FeS)(aq), 𝑛 < 150) formed in the precipitation reaction of Fe2+(aq) and HS–(aq) ions under ambient aqueous conditions. Furthermore, the non- innocent redox chemistry of [4Fe-4S] clusters underscores the intimate relationship between the Fe and S ions that give rise to the concept of chimera of chemical elements - as the building block of Fe–S clusters and nanoparticles - with tantalizing chemical properties that neither of its components can manifest.