Influence of the Dithiolate Bridge on the Oxidative Processes of Diiron Models Related to the Active Site of [FeFe] Hydrogenases

Abstract

Electrochemical studies of [Fe (CO) (κ -dmpe)(μ-dithiolate)] (dithiolate=adt , pdt) and density functional theory (DFT) calculations reveal the striking influence of an amine functionality in the dithiolate bridge on their oxidative properties. [Fe (CO) (κ -dmpe)(μ-adt )] (1) undergoes two one-electron oxidation steps, with the first being partially reversible and the second irreversible. When the adt bridge is replaced with pdt, a shift of 60 mV towards more positive potentials is observed for the first oxidation whereas 290 mV separate the oxidation potentials of the two cations. Under CO, oxidation of azadithiolate compound 1 occurs according to an ECE process whereas an EC mechanism takes place for the propanedithiolate species 2. The dication species [1-CO] resulting from the two-electron oxidation of 1 has been spectroscopically and structurally characterized. The molecular details underlying the reactivity of oxidized species have been explored by DFT calculations. The differences in the behaviors of 1 and 2 are mainly due to the presence, or not, of favored interactions between the dithiolate bridge and the diiron site depending on the redox states, Fe Fe or Fe Fe , of the complexes.

Electrochemical and Theoretical Investigations of the Oxidatively Induced Reactivity of the Complex [Fe2(CO)4(κ2‐dmpe)(μ‐adtBn)] Related to the Active Site of [FeFe] Hydrogenases

“…The molecular structure of [ 1 ‐(P(OMe) 3 ) 2 ](PF 6 ) 2 was determined by an X‐ray experiment (Figure , Table in the Experimental Section, and Table S1 in the Supporting Information). Its geometry is similar to that reported for [Fe 2 (CO) 5 (κ 2 ‐dmpe)(μ‐(SCH 2 ) 2 NBn)] 2+ and consists of two {Fe(CO) 2 P 2 } and {Fe(CO)P 2 } units linked by the two sulfur atoms of the adt Bn bridge . The amine of the dithiolate is coordinated in an axial position to the iron atom (Fe1) not bearing the dmpe ligand, with a N1−Fe1 distance of 2.083(4) Å, comparable with that observed (2.021(6) Å) in the dication [Fe 2 (CO) 5 (κ 2 ‐dmpe){μ‐adt Bn }] 2+ .…”

“…The cyclic voltammetry (CV) of [Fe 2 (CO) 4 (κ 2 ‐dmpe)(μ‐adt Bn )] ( 1 ) in MeCN/[NBu 4 ][PF 6 ] under Ar is substantially different from that recorded and described in CH 2 Cl 2 . This compound undergoes a partially reversible one‐electron oxidation in CH 2 Cl 2 whereas, in MeCN, the oxidation is a two‐electron process (Figure ) as is demonstrated by comparing the variation of the current function with the scan rate in both solvents (Figure S1 in the Supporting Information).…”

“…Previous computations regarding oxidation processes involving 1 revealed that pyramidal inversions (rotations) of both the {Fe(CO) 2 } and {FeP 2 } units are energetically viable upon one‐electron oxidation . It follows that the binding of the first P(OMe) 3 molecule can virtually take place at both metal centers.…”

“…Recently, we investigated electrochemically and theoretically the oxidation of [Fe 2 (CO) 4 (κ 2 ‐dmpe)(μ‐adt Bn )] (adt Bn =(SCH 2 ) 2 NCH 2 C 6 H 5 ; dmpe=Me 2 PCH 2 CH 2 PMe 2 ) ( 1 ) in the absence and in the presence of CO and we compared the results with those of the analogue [Fe 2 (CO) 4 (κ 2 ‐dmpe)(μ‐pdt)] ( 2 ) . The influence of the bridge could be assessed.…”

“…Whereas the oxidation of 2 under CO leads to an adduct that oxidizes at a potential more positive than that of 2 into 2 + , the adduct [ 1 ‐CO] + is easier to oxidize than the parent complex, DFT calculations showing that this ECE mechanism could be explained by the presence of a stabilizing Fe⋅⋅⋅N interaction in the elusive CO adduct (Scheme ). The dicationic species [Fe 2 (CO) 5 (κ 2 ‐dmpe)(μ‐κ 2 ( S , S ):κ 3 (S,S,N)‐adt Bn )] 2+ ([ 1 ‐CO] 2+ in Scheme ) was isolated and identified …”