Electrochemical, Spectroscopic, and Computational Study of Bis(μ-methylthiolato)diironhexacarbonyl: Homoassociative Stabilization of the Dianion and a Chemically Reversible Reduction/Reoxidation Cycle

The
redox characteristics of (μ-SMe)2Fe2(CO)6 from the 1+ to 2– charge states are reported.
This [2Fe-2S] compound is related to the active sites of [FeFe]-hydrogenases
but notably without a linker between the sulfur atoms. The 1+ charge
state was studied both by ionization in the gas phase by photoelectron
spectroscopy and by oxidation in the solution phase by cyclic voltammetry.
The adiabatic ionization is to a cation whose structure features a
semibridging carbonyl, similar to the structure of the active site
of [FeFe]-hydrogenases in the same oxidation state. The reduction
of the compound by cyclic voltammetry gives an electrochemically irreversible
cathodic peak, which often suggests disproportionation or other irreversible
chemical processes in this class of molecules. However, the return
scan through electrochemically irreversible oxidation peaks that occur
at potentials around 1 V more positive than the reduction led to the
recovery of the initial neutral compound. The dependence of the CVs
on scan rate, IR spectroelectrochemistry of reduction and oxidation
cycles, chronocoulometry, and DFT computations indicate a mechanism
in which stabilization of the dianion plays a key role. Initial one-electron
reduction of the compound is accompanied in the same cathodic peak
with a second slower electron reduction to the dianion. Geometric
reorganization and solvation stabilize the [2Fe-2S]2– dianion such that the potential for addition of the second electron
is slightly less negative than that of the first (potential inversion).
The return oxidation peaks at more positive potentials follow from
rapid pairing of the dianion with another neutral molecule in solution
(termed homoassociation) to form a stabilized [4Fe-4S]2– dianion. Two one-electron oxidations of this [4Fe-4S]2– dianion result in regeneration of the initial neutral compound.
The implications of this homoassociation for the [FeFe]-hydrogenase
enzyme, in which the H-cluster active site features a [2Fe-2S] site
associated with a [4Fe-4S] cubane cluster via a thiolate bridge, are
discussed.