Models for the Active Site in [FeFe] Hydrogenase with Iron-Bound Ligands Derived from Bis-, Tris-, and Tetrakis(mercaptomethyl)silanes

A series of multifunctional (mercaptomethyl)silanes of the general formula type RnSi(CH2SH)(4-n)(n = 0-2; R = organyl) was synthesized, starting from the corresponding (chloromethyl)silanes. They were used as multidentate ligands for the conversion of dodecacarbonyltriiron, Fe-3(CO)(12), into iron carbonyl complexes in which the deprotonated (mercaptomethyl)silanes act as p-bridging ligands. These complexes can be regarded as models for the [FeFe] hydrogenase. They were characterized by elemental analyses (C, H, S), NMR spectroscopic studies (H-1, C-13, Si-29), and single-crystal X-ray diffraction. Their electrochemical properties were investigated by cyclic voltammetry to disclose a new mechanism for the formation of dihydrogen catalyzed by these compounds, whereby one sulfur atom was protonated in the catalytic cycle. The reaction of the tridentate ligand MeSi(CH2SH)(3)with Fe-3(CO)(12) yielded a tetranuclear cluster compound. A detailed investigation by X-ray diffraction, electrochemical, Raman, Mossbauer, and susceptibility techniques indicates that for this compound initially [Fe2{mu-MeSi(CH2S)(2)CH2SH} (CO)(6)] is formed. This dinuclear complex, however, is slowly transformed into the tetranuclear species [Fe-4{mu-MeSi(CH2S)(3)}(2)(CO)(8)].