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 GeR and SnR 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 Fe (CO) cluster and low torsional strain for GeMe and SnMe 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 Fe S (CO) cores bridged by CH SnR CH , R=Me, Ph, linkers were also obtained and characterized.

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 [Fe(CO){μ-S(CH)S}] (n = 4-8), [Fe(CO){μ-S(CH)S}] (n = 6-8), and [Fe(CO){(μ-S(CH)S)}] (n = 6-8). The resulting complexes were characterized by H andC{H} 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.

Synthetic models of the active site of [FeFe]-hydrogenase containing naphthalene monoimide (NMI) of peri-substituted dichalcogenides as bridging linkers have been prepared and characterized using different spectroscopic methods. The influence of the imide functionality and the chalcogen atoms on the redox properties and the catalytic behaviour of complexes 7-10 was studied using cyclic voltammetry. The results revealed that the imide functionality has improved the chemical stability of the reduced species and the replacement of the S atoms by Se caused a cathodic shift in the oxidation peaks. Moreover, the optical properties of compounds 1, 2, 4, and 5 and the respective diiron complexes 7-10 were investigated by UV-Vis absorption and fluorescence spectroscopy assisted by quantum chemical simulations. The structures of complexes 6-9 were confirmed by X-ray diffraction analysis.

The synthetic models of the active site of an [FeFe]-hydrogenase containing a Sn atom in the bridgehead of the diselenato ligand, namely [Fe(CO){μ-(SeCHSe)SnMe}], 3 and [Fe(CO){μ-(SeCH)SnMe}], 4 have been synthesized and characterized by different spectroscopic methods. The protonation properties of complex 4 have been investigated by monitoring the IR spectra in the carbonyl stretching region, H NMR in the hydride region as well as theSe{H} NMR upon addition of strong and moderate acids wherein the protonation of the active site of the [FeFe]-hydrogenase at one of its internal basic sites is considered an essential step in the catalytic cycle. Furthermore, we investigated the redox properties and the catalytic behaviour of complexes 3 and 4 in the presence of AcOH as a source of protons suggesting an ECE (E = electrochemical process, C = chemical process) mechanism.

Reactions of Fe3(CO)12 with cyclic diselenides or diselenocyanatoalkanes produced two different products, namely the dinuclear [Fe2(CO)6{μ‐(SeCH2)2(CH2)n}] (n= 5 (1 a), 6 (1 b), 7 (1 c), 8 (1 d)) and the tetranuclear models [Fe2(CO)6{μ‐(SeCH2)2(CH2)n}]2 (n=5 (2 a), 6 (2 b), 7 (2 c), 8 (2 d)). The resulting complexes were fully characterized by a variety of analytical techniques (NMR spectroscopy, elemental analysis, mass spectrometry) and by X‐ray structure determination of complexes 1 c, 1 d, 2 c and 2 d. Furthermore, we investigated the redox properties and the catalytic behaviour of complexes 1 a‐d in the presence of acetic acid (AcOH) as a source of protons.