[FeFe]-Hydrogenase H-Cluster Mimics with Various -S(CH2)nS- Linker Lengths (n = 2-8): A Systematic Study.

The effect of the nature of the dithiolato ligand on the physical and electrochemical properties of synthetic H-cluster mimics of the [FeFe]-hydrogenase is still of significant concern. In this report we describe the cyclization of various alkanedithiols to afford cyclic disulfide, tetrasulfide, and hexasulfide compounds. The latter compounds were used as proligands for the synthesis of a series of [FeFe]-hydrogenase H-cluster mimics having the general formulas [Fe2(CO)6{µ-S(CH2)nS}] (n = 4-8), [Fe2(CO)6{µ-S(CH2)nS}]2 (n = 6-8), and [Fe2(CO)6{(µ-S(CH2)nS)2}] (n = 6-8). The resulting complexes were characterized by 1H and 13C{1H} NMR and IR spectroscopic techniques, mass spectrometry, and elemental analysis as well as X-ray analysis. The purpose of this research was to study the influence of the systematic increase of n from 2 to 7 on the redox potentials of the models and the catalytic ability in the presence of acetic acid (AcOH) by applying cyclic voltammetry.

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

Analogues of the [2Fe-2S] subcluster of hydrogenase enzymes in which the central group of the three-atom chain linker between the sulfur atoms is replaced by GeR2 and SnR2 groups are studied. The six-membered FeSCECS rings in these complexes (E=Ge or Sn) adopt an unusual conformation with nearly co-planar SCECS atoms perpendicular to the Fe-Fe core. Computational modelling traces this result to the steric interaction of the Me groups with the axial carbonyls of the Fe2 (CO)6 cluster and low torsional strain for GeMe2 and SnMe2 moieties owing to the long C-Ge and C-Sn bonds. Gas-phase photoelectron spectroscopy of these complexes shows a shift of ionization potentials to lower energies with substantial sulfur orbital character and, as supported by the computations, an increase in sulfur character in the predominantly metal-metal bonding HOMO. Cyclic voltammetry reveals that the complexes follow an ECE-type reduction mechanism (E=electron transfer and C=chemical process) in the absence of acid and catalysis of proton reduction in the presence of acid. Two cyclic tetranuclear complexes featuring the sulfur atoms of two Fe2 S2 (CO)6 cores bridged by CH2 SnR2 CH2 , R=Me, Ph, linkers were also obtained and characterized.

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

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.

Ligand effects on the electrochemical behavior of [Fe2(CO)5(L){µ-(SCH2)2(Ph)P=O}] (L = PPh3, P(OEt)3) hydrogenase model complexes.

In this paper we study the influence of substituting one CO ligand in [Fe2(CO)6{µ-(SCH2)2(Ph)P=O}] (1) by better σ-donor L ligands affording [Fe2(CO)5(L){µ-(SCH2)2(Ph)P=O}] {L = PPh3 (2) and P(OEt)3 (3)} in relation to the steric interactions and the voltammetric behavior. Cyclic voltammetric investigations under N2 and CO showed remarkable differences in the electrochemical behaviour of complexes 2 and 3: (i) Complex 2 tends to expel PPh3 upon reduction whereas complex 3 exhibits chemical reversibility and (ii) Under CO, complex 3 reacts with CO affording a new compound P, which shows a reversible wave at E1/2 ∼ -0.9 V (vs. ferrocenium/ferrocene couple). The presence of CO assists the formation of 1 after electrochemically induced loss of PPh3 during the voltammetric experiment of 2. Using DFT calculations we provide an explanation for the difference in stabilities between the Fe-PPh3 and Fe-P(OEt)3 bonds.