Electrochemical, Spectroscopic, and Computational Study of Bis(μ- methylthiolato)diironhexacarbonyl: Homoassociative Stabilization of the Dianion and a Chemically Reversible Reduction/Reoxidation Cycle Orrasa In-noi, † Kenneth J. Haller,* ,† Gabriel B. Hall, ‡ William P. Brezinski, ‡ Jacob M. Marx, ‡ Taka Sakamoto, ‡ Dennis H. Evans, § Richard S. Glass, ‡ and Dennis L. Lichtenberger* ,‡ † School of Chemistry, Institute of Science, Suranaree University of Technology, Nakhon Ratchasima 30000 Thailand ‡ Department of Chemistry and Biochemistry, The University of Arizona, Tucson, Arizona 85721, United States § Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States * S Supporting Information ABSTRACT: The redox characteristics of (μ-SMe) 2 Fe 2 (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. ■ INTRODUCTION The [2Fe-2S] cluster unit has important roles in biological chemistry, and the [2Fe-2S] unit also has a richly developed organometallic chemistry. Most relevant to the present study are the [2Fe-2S] clusters in the active sites of [FeFe]- hydrogenase enzymes that catalyze the reversible reduction of protons to hydrogen. 1-7 In organometallic chemistry, the structure of the [2Fe-2S] cluster, μ-S 2 Fe 2 (CO) 6 (1, Chart 1) was first published in 1965, 8-10 and some of the early chemistry of this class of complexes of the general form shown in Chart 1 was developed by Dietmar Seyferth. 11-14 More recently, the structural similarity of this class of complexes to the active site of [FeFe]-hydrogenases has inspired widespread investigations into the electrocatalytic reduction of protons to hydrogen by organometallic [2Fe-2S] clusters. 1,2,6,7,15 The oxidation and reduction properties of these species are central to their function. The majority of the mimics that have been studied to this time have a linker between the two sulfur atoms of the [2Fe- 2S] core, as depicted by the dashed line in Chart 1. 16 This linker can have both a direct functional role and a structural role in the reduction chemistry. The most notable functional role is incorporation of an amine in the linker that can act as a protonation site and proton relay to the [2Fe-2S] core. 17,18 The Special Issue: Organometallic Electrochemistry Received: April 18, 2014 Published: July 21, 2014 Article pubs.acs.org/Organometallics © 2014 American Chemical Society 5009 dx.doi.org/10.1021/om5004122 | Organometallics 2014, 33, 5009-5019