X-Ray Spectroscopy and Structure Elucidation of Reactive Electrogenerated Tri-iron Carbonyl Sulfide Clusters

Abstract

The design and operation of electrosynthesis cells for generation of samples for X-ray absorption spectroscopy are described. Optimization of continuous-flow methods allows the generation of highly reducing species, which may be combined with spectroscopic validation of the composition of the electrogenerated solution. It is shown that the large sample volume (10 mL) of the 1–10 mM (in the absorbing element) solution required for such experiments can be reduced to ~100 μL using a strategy in which the in situ electrosynthesis cell is amenable to freeze-quenching and transfer to a beamline cryostat. The working electrode in this case doubles as the X-ray absorption spectroscopy sample cell. The application of these techniques is illustrated by the reduction chemistry of Fe3S2(CO)9, 3Fe2S. Spectra recorded in the near-edge region confirm that quantitative preparation of samples of 3Fe2S, 3Fe2S1– and 3Fe2S2– can be prepared by either approach, but samples of a more reduced form, identified as [Fe3S(CO)9]2–, could only be generated using continuous-flow electrosynthesis techniques. Differences in the structural chemistry of the 3Fe2S0/1–/2– redox series were examined from the perspective of their near-edge spectra and the structures of 3Fe2S1– and 3Fe2S2– forms were deduced by a combination of computational (density functional theory), spectroscopic and X-ray absorption fine-structure analyses. These show that addition of the first electron is predominantly localized in one of the Fe–Fe bonds; cleavage of the Fe–Fe bond by addition of a second electron to the Fe–Fe antibonding orbital is associated with a more substantial rearrangement of the molecule. The reduced compounds have structural similarities to the reduced dithiolate-bridged di-iron hexacarbonyl compounds and this is related to the weak electrocatalytic proton reduction exhibited by Fe3S2(CO)9. The methods described provide a strategy for the collection and analysis of experimental data directed towards structure elucidation of redox-activated solution-state complexes.