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 R(n)Si(CH(2)SH)(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 µ-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 ((1)H, (13)C, (29)Si), 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(CH(2)SH)(3) with Fe(3)(CO)(12) yielded a tetranuclear cluster compound. A detailed investigation by X-ray diffraction, electrochemical, Raman, Mössbauer, and susceptibility techniques indicates that for this compound initially [Fe(2){µ-MeSi(CH(2)S)(2)CH(2)SH}(CO)(6)] is formed. This dinuclear complex, however, is slowly transformed into the tetranuclear species [Fe(4){µ-MeSi(CH(2)S)(3)}(2)(CO)(8)].

Sila-substitution of alkyl nitrates: synthesis, structural characterization, and sensitivity studies of highly explosive (nitratomethyl)-, bis(nitratomethyl)-, and tris(nitratomethyl)silanes and their corresponding carbon analogues.

A series of analogous nitratomethyl compounds of carbon and silicon of the formula types Me(3)ElCH(2)ONO(2) (1a/1b), Me(2)El(CH(2)ONO(2))(2) (2a/2b), MeEl(CH(2)ONO(2))(3) (3a/3b), (CH(2))(4)El(CH(2)ONO(2))(2) (4a/4b), and (CH(2))(5)El(CH(2)ONO(2))(2) (5a/5b) were synthesized [El = C (a), Si (b); (CH(2))(4)El = (sila)cyclopentane-1,1-diyl; (CH(2))(5)El = (sila)cyclohexane-1,1-diyl]. All compounds were characterized by using NMR, IR, and Raman spectroscopy and mass spectrometry. In addition, the crystal structures of Me(2)C(CH(2)ONO(2))(2) (2a), (CH(2))(4)C(CH(2)ONO(2))(2) (4a), Me(2)Si(CH(2)ONO(2))(2) (2b), and (CH(2))(5)Si(CH(2)ONO(2))(2) (5b) were determined by single-crystal X-ray diffraction. The gas-phase structures of the C/Si analogues 1a and 1b were determined by electron diffraction and compared with the results of quantum chemical calculations at different levels of theory. The thermal stabilities of the C/Si pairs 1a/1b-5a/5b were investigated by using DSC. In addition, their friction and impact sensitivities were measured with standard BAM methods. The extreme sensitivities of the silicon compounds 1b-5b compared to those of the corresponding carbon analogues 1a-5a were discussed in terms of the structures of the C/Si analogues and possible geminal Si...O interactions.

Tripodal Binding Units for Self-Assembled Monolayers on Gold: A Comparison of Thiol and Thioether Headgroups.

Whereas thiols and thioethers are frequently used as binding units of oligodentate precursor molecules to fabricate self-assembled monolayers (SAMs) on coinage metal and semiconductor surfaces, their use for tridentate bonding configuration is still questionable. Against this background, novel tridentate thiol ligands, PhSi(CH(2)SH)(3) (PTT) and p-Ph-C(6)H(4)Si(CH(2)SH)(3) (BPTT), were synthesized and used as tripodal adsorbate molecules for the fabrication of SAMs on Au(111). These SAMs were characterized by X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The PTT and BPTT films were compared with the analogous systems comprised of same tripodal ligands with thioether instead of thiol binding units (anchors). XPS and NEXAFS data suggest that the binding uniformity, packing density, and molecular alignment of the thiol-based ligands in the respective SAMs is superior to their thioether counterparts. In addition, the thiol-based films showed significantly lower levels of contamination. Significantly, the quality of the PTT SAMs on Au(111) was found to be even higher than that of the films formed from the respective monodentate counterpart, benzenethiol. The results obtained allow for making some general conclusions on the specific character of molecular self-assembly in the case of tridentate ligands.

The sila-explosives Si(CH2N3)4 and Si(CH2ONO2)4: silicon analogues of the common explosives pentaerythrityl tetraazide, C(CH2N3)4, and Pentaerythritol Tetranitrate, C(CH2ONO2)4.

The reaction of tetrakis(chloromethyl)silane, Si(CH2Cl)4, with sodium azide afforded tetrakis(azidomethyl)silane (sila-pentaerythrityl tetraazide, Si(CH2N3)4 (1b)). Nitration of tetrakis(hydroxymethyl)silane, Si(CH2OH)4, with nitric acid resulted in the formation of tetrakis(nitratomethyl)silane (sila-pentaerythritol tetranitrate, Si(CH2ONO2)4 (2b)). Compounds 1b and 2b are extremely shock-sensitive materials and very difficult to handle. Spectroscopic data were obtained as good as sensitivity and safety allowed for umambiguous identification. Quantum chemical calculations (DFT) of the C/Si pairs C(CH2OH)4/Si(CH2OH)4, 1a/1b, and 2a/2b regarding the structures and electronic populations were performed.

Neutral hexa- and pentacoordinate silicon(IV) complexes with SiO(6) and SiO(4)N skeletons.

The neutral heteroleptic hexacoordinate silicon(IV) complexes 4 and 5 (SiO(6) skeletons) and the neutral pentacoordinate silicon(IV) complexes 7-9 (SiO(4)N skeletons) were synthesized, starting from the hexacoordinate precursor 2 and the pentacoordinate precursor 6, respectively. In these reactions, two monoanionic cyanato-N ligands are replaced by one dianionic bidentate O,O-chelate ligand. Compounds 4, 5, and 7-9 were characterized by single-crystal X-ray diffraction and solid-state and solution NMR spectroscopy. The chiral silicon(IV) complexes 4, 5, 7, and 8 were obtained as racemic mixtures, whereas 9 was isolated as a cocrystallizate consisting of the two diastereomers, (C,S)-9 and (A,S)-9 (ratio 1:1). The stereodynamics of 5 and 8 were studied by variable-temperature (1)H NMR experiments.