Steric effect of the dithiolato linker on the reduction mechanism of [Fe₂(CO)₆{μ-(XCH₂)₂CRR′}] hydrogenase models (X = S, Se)

Abstract

Studying the redox features of the [FeFe]-hydrogenase models is essential for understanding the function of the H cluster. The reduction of the [FeFe]-hydrogenase models of the type [Fe2(CO)6{μ-(XCH2)2E}] (X = S, Se) is described to occur either via sequential transfer of two electrons at and for the first and the second reduction steps, respectively, where , or via transfer of two electrons at the same applied potential due to potential inversion of the two reduction steps, i.e.. Typically, the phenomenon of potential inversion is observed when a structural change intervenes in the cathodic process stabilizing the reduced species. In this report, we investigate the mechanism of the cathodic process of series of models [Fe2(CO)6{μ-(XCH2)2E}] (X = S or Se and E = CH2, CHMe or CMe2) applying cyclic voltammetry. The studies herein show the remarkable influence of the steric bulk of E toward the cathodic process, such that only complexes with E = CMe2 are reduced with inverted potentials due to occurrence of an ECE mechanism (E = electrochemical process, C = chemical process) of reduction. Moreover, we describe the catalytic behaviour of these models toward reduction of protons using acetic acid, AcOH, as a proton source.

[FeFe]‐Hydrogenase H‐Cluster Mimics with Unique Planar μ‐(SCH2)2ER2 Linkers (E=Ge and Sn)

“…That is, stannylated model 4 a is the easiest to reduce in comparison with the models 1 , 2 , and 3 . This is due to reorganization and stabilization of the reduced species as previously shown in the case of electrochemical reduction of [FeFe]‐hydrogenase models with [2Fe‐2X] cores, where X=S, Se, or Te . The reduction of the [2Fe‐2X] core in this series of models becomes easier on going from X=S to Se to Te.…”

“…We have recently illustrated how the steric clash between the CMe 2 group and the proximal CO in 1 lowers the barrier of Fe(CO) 3 rotation during the reduction process and hence leads to an ECE reduction mechanism at slow scan rates. That the intervening core reorganization is a facile process during reduction of complexes 2 – 4 a can also be explained in terms of the steric bulkiness of their central EMe 2 groups of the dithiolato linkers; notwithstanding the planarity of 3 and 4 a (see Table for the barriers of Fe(CO) 3 rotation of the complexes in the ground Fe I Fe I states) …”

“…Glassy carbon electrode ( A =0.072 cm 2 ). The dashed line represents the current function expected for a one‐electron process assuming D ≈9×10 −6 cm 2 s −1 , a value calculated for various [FeFe]‐hydrogenase models …”

[FeFe]‐hydrogenase H‐cluster mimics mediated by mixed (S, Se) and (S, Te) bridging moieties: Insight into molecular structures and electrochemical characteristics

“…This indicates that the increase in the current of these two new reduction peaks of complexes 1 and 2 with increasing AcOH concentration is due to an electrocatalytic process for the proton reduction to H 2 . However, such catalytic behaviors are similar to those reported in the literature for other hexacarbonyl complexes …”

Carbon/Silicon Exchange at the Apex of Diphos‐ and Triphos‐Derived Ligands – More Than Just a Substitute?

“…In addition, the C/Si exchange was likewise utilized in bioinorganic chemistry, and the apex carbon atom in sulfur‐rich complex [{SCH 2 C(CH 3 ) 2 CH 2 S}Fe(CO) 6 ] ( 26a ) as a [FeFe] hydrogenase mimic has also been exchanged with silicon ( 26b , Figure ) . The C/Si replacement was shown to have only little influence in the overall structure.…”