Monomeric Silicon Research Articles

Chemical and mineralogical characteristics of carbonated and uncarbonated cement pastes subjected to high temperatures

The fire-resistance of carbonated concrete under high temperatures is significant due to its direct exposure during an accidental fire. To evaluate the carbonation effect on fire-resistance of concrete, the chemical and mineralogical changes of uncarbonated and carbonated cement pastes subjected to high temperatures were thoroughly investigated in this research by employing micro-measurement methods including thermal-gravimetric analysis (TGA), X-ray diffraction (XRD) and 29Si nuclear magnetic resonance (29Si NMR). Uncarbonated cement paste results showed the decomposition of portlandite at 400 °C with the formation of lime, whilst the depolymerization of C–S–H happened simultaneously to generate monomeric silicon tetrahedron. Above 720 °C, all the C–S–H depolymerized to crystalline C2S. Carbonated cement pastes on the other hand showed that amorphous calcium carbonate and part of vaterite decomposed between the range of 400–600 °C, while the rest of the vaterite and calcite were decomposed at 600–720 °C. The individual content of calcium carbonate polymorph could not be obtained using a TGA curve. Besides, the calcium-modified silicate gel was significantly decomposed at 500 °C and completely depolymerized to crystalline C2S at 950 °C. In summary, carbonated pastes show better resistance to high temperatures with its heat absorption capacity 3.3 times as high as the uncarbonated sample, which delays the temperature development in the inner layer. Therefore, a reasonable carbonation process could help to improve the fire resistance of concrete to some extent.

Silicon in surface waters of the humid zone (the case of Karelian waterbodies)

We report a first investigation of the following dissolved forms of silicon in natural waters of Karelia: monomeric orthosilicic acid, polysilicic acids and silicon within dissolved organic compounds. A linear relationship was found between the concentration of the silicon-bearing organic compounds and indirect characteristics of organic matter content, with the correlation coefficient at 0.85. The seasonal variation of total dissolved silicon concentration and the ratio of its forms in different types of water bodies in Karelia were determined. The content of dissolved silicon was the lowest in spring and summer due to diatom plankton production. A technique has been suggested for tallying silicon within organic compounds in natural waters using persulfate oxidation in alkaline solution.

A Route to Base Coordinate Silicon Difluoride and the Silicon Trifluoride Radical.

Silicon difluoride (SiF2 ) is highly unstable at room temperature and condenses at this temperature rapidly to a polymeric material of unknown structure. Therefore, the isolation of a stable monomeric silicon difluoride species is a challenging task. The cyclic alkyl(amino) carbene (cAAC) coordinated silicon difluoride was isolated as (cAAC)2 SiF2 (2), synthesized from the reduction of cAAC-SiF4 (1) by using two equivalents of KC8 in the presence of one equivalent of cAAC. In the solid state, compound 2 is stable at room temperature for a long time under inert conditions. The reduction of compound 1 in the presence of one equivalent KC8 resulted in the first stable silicon trifluoride monoradical (cAAC)SiF3 (3).

A New Area in Main-Group Chemistry: Zerovalent Monoatomic Silicon Compounds and Their Analogues.

Monoatomic zerovalent main-group element complexes emerged very recently and attracted increasing attention of both theoretical and experimental chemists. In particular, zerovalent silicon complexes and their congeners (metallylones) stabilized by neutral Lewis donors are of significant importance not only because of their intriguing electronic structure but also because they can serve as useful building blocks for novel chemical species. Featuring four valence electrons as two lone pairs at the central atoms, such complexes may form donor-acceptor adducts with Lewis acids. More interestingly, with the central atoms in the oxidation state of zero, they could pave a way to new classes of compounds and functional groups that are otherwise difficult to realize. In this Account, we mainly describe our contributions in the chemistry of monatomic zerovalent silicon (silylone) and germanium (germylone) supported by a chelate bis-N-heterocyclic carbene (bis-NHC) ligand in the context of related species developed by other groups in the meantime. Utilizing the bis-NHC stabilized chlorosilyliumylidene [:SiCl]+ and chlorogermyliumylidene [:GeCl]+ as suitable starting materials, we successfully isolated silylone (bis-NHC)Si and germylone (bis-NHC)Ge, respectively. The electronic structures of the latter complexes established by theoretical calculations and spectroscopic data revealed that they are genuine metallylone species with electron-rich silicon(0) and germanium(0) centers. Accordingly, they can react with 1 molar equiv of GaCl3 to form Lewis adducts (bis-NHC)E(GaCl3) (E = Si, Ge) and with 2 molar equiv of ZnCl2 to furnish (bis-NHC)Si(ZnCl2)2. Conversion of the metallylones with elemental chalcogens affords isolable monomeric silicon(II) and germanium(II) monochalcogenides (bis-NHC)EX(GaCl3) (X = Se, Te), representing molecular heavier congeners of CO. Moreover, their reaction with elemental chalcogens can also yield monomeric silicon(IV) and germanium(IV) dichalcogenides (bis-NHC)EX2 (X = S, Se, Te) as the first isolable complexes of the molecular congeners of CO2. Moreover, (bis-NHC)Si could even activate CO2 to afford the monomolecular silicon dicarbonate complex (bis-NHC)Si(CO3)2 via the formation of SiO and SiO2 complexes as intermediates. Furthermore, starting with a chelate bis-N-heterocyclic silylene supported [:GeCl]+, we developed two bis-N-heterocyclic silylene stabilized germylone→Fe(CO)4 complexes. Our achievements in the chemistry of metallylones demonstrate that the characteristic of monatomic zerovalent silicon and its analogues can provide novel reaction patterns for access to unprecedented species and even extends the series of functional groups of these elements. With this, we can envision that more interesting zerovalent complexes of the main-group elements with unprecedented reactivity will follow in the near future.

An Isolable Silicon Dicarbonate Complex from Carbon Dioxide Activation with a Silylone.

The first isolable molecular silicon dicarbonate complex (bis-NHC)Si(CO3 )2 2 (bis-NHC=H2 C[{NC(H)=C(H)N(Dipp)}C:]2 , Dipp=2,6-iPr2 C6 H3 ) was synthesized by facile reaction of the bis-N-heterocyclic carbene stabilized silylone (bis-NHC)Si 1, bearing a zero-valent silicon atom, with carbon dioxide. The monomeric silicon dioxide complex (bis-NHC)SiO2 3 supported by the bis-NHC ligand was proposed as a key intermediate resulting from double oxygenation of the zero-valent silicon atom in 1 by two molar equivalents of CO2 under liberation of CO; its subsequent Lewis acid-base reaction with CO2 leads to 2 which has been fully characterized including an single-crystal X-ray diffraction analysis. Its electronic structure, spectroscopic data and the thermochemistry of the formation have been studied quantum-chemically.

An Isolable Silicon Dicarbonate Complex from Carbon Dioxide Activation with a Silylone

AbstractThe first isolable molecular silicon dicarbonate complex (bis‐NHC)Si(CO3)2 2 (bis‐NHC=H2C[{NC(H)=C(H)N(Dipp)}C:]2, Dipp=2,6‐iPr2C6H3) was synthesized by facile reaction of the bis‐N‐heterocyclic carbene stabilized silylone (bis‐NHC)Si 1, bearing a zero‐valent silicon atom, with carbon dioxide. The monomeric silicon dioxide complex (bis‐NHC)SiO2 3 supported by the bis‐NHC ligand was proposed as a key intermediate resulting from double oxygenation of the zero‐valent silicon atom in 1 by two molar equivalents of CO2 under liberation of CO; its subsequent Lewis acid–base reaction with CO2 leads to 2 which has been fully characterized including an single‐crystal X‐ray diffraction analysis. Its electronic structure, spectroscopic data and the thermochemistry of the formation have been studied quantum‐chemically.

From Silylone to an Isolable Monomeric Silicon Disulfide Complex

AbstractThe synthesis and characterization of the first bis‐N‐heterocyclic carbene stabilized monomeric silicon disulfide (bis‐NHC)SiS2 2 (bis‐NHC=H2C[{NC(H)C(H)N(Dipp)}C:]2, Dipp=2,6‐iPr2C6H3) is reported. Compound 2 is prepared in 89 % yield from the reaction of the zero‐valent silicon complex (′silylone′) 1 [(bis‐NHC)Si] with elemental sulfur. Compound 2 can react with GaCl3 in acetonitrile to give the corresponding (bis‐NHC)Si(S)S→GaCl3 Lewis acid–base adduct 3 in 91 % yield. Compound 3 is also accessible through the reaction of the unprecedented silylone‐GaCl3 adduct [(bis‐NHC)Si→GaCl3] 4 with elemental sulfur. Compounds 2, 3, and 4 could be isolated and characterized by elemental analyses, HR‐MS, IR, 13C‐ and 29Si‐NMR spectroscopy. The structures of 3 and 4 could be determined by single‐crystal X‐ray diffraction analyses. DFT‐derived bonding analyses of 2 and 3 exhibited highly polar SiS bonds with moderate pπ–pπ bonding character.

From Silylone to an Isolable Monomeric Silicon Disulfide Complex.

The synthesis and characterization of the first bis-N-heterocyclic carbene stabilized monomeric silicon disulfide (bis-NHC)SiS2 2 (bis-NHC=H2 C[{NC(H)C(H)N(Dipp)}C:]2 , Dipp=2,6-iPr2 C6 H3 ) is reported. Compound 2 is prepared in 89 % yield from the reaction of the zero-valent silicon complex ('silylone') 1 [(bis-NHC)Si] with elemental sulfur. Compound 2 can react with GaCl3 in acetonitrile to give the corresponding (bis-NHC)Si(S)S→GaCl3 Lewis acid-base adduct 3 in 91 % yield. Compound 3 is also accessible through the reaction of the unprecedented silylone-GaCl3 adduct [(bis-NHC)Si→GaCl3 ] 4 with elemental sulfur. Compounds 2, 3, and 4 could be isolated and characterized by elemental analyses, HR-MS, IR, (13) C- and (29) Si-NMR spectroscopy. The structures of 3 and 4 could be determined by single-crystal X-ray diffraction analyses. DFT-derived bonding analyses of 2 and 3 exhibited highly polar Si-S bonds with moderate pπ -pπ bonding character.

Elusive Silicon Oxides Unveiled

Using a new approach to marry silicon and oxygen, inorganic chemists have created the first monomeric silicon oxides: complexes of Si2O3 and Si2O4. This fundamental discovery, which might one day factor into how silicon-based electronics are made, is another feather in the caps of Gregory H. Robinson, Yuzhong Wang, Henry F. Schaefer III, and their colleagues at the University of Georgia. Although carbon and silicon sit together in group 14 of the periodic table, they display disparate behavior when it comes to forming oxides. Carbon monoxide and carbon dioxide—monomeric carbon oxides—are ubiquitous under normal room conditions. But simple silicon oxides don’t exist. Silicon oxide is a monomeric compound only at high temperature. And although silicon dioxide is one of the most abundant materials on Earth—as silica, quartz, and sand—it’s a covalent network of silicon atoms bound to neighboring oxygen atoms, rather than discrete molecules. Robinson’s group previously synthesized an unprecedented ...

Structural characterization of the lithium silicides Li15Si4, Li13Si4, and Li7Si3 using solid state NMR

Local environments and lithium ion dynamics in the binary lithium silicides Li(15)Si(4), Li(13)Si(4), and Li(7)Si(3) have been characterized by detailed variable temperature static and magic-angle spinning (MAS) NMR spectroscopic experiments. In the (6)Li MAS-NMR spectra, individual lithium sites are generally well-resolved at temperatures below 200 K, whereas at higher temperatures partial or complete site averaging is observed on the ms timescale. The NMR spectra also serve to monitor the phase transitions occurring in Li(7)Si(3) and Li(13)Si(4) at 235 K and 146 K, respectively. The observed lithium isotropic shift ranges of up to approximately 50 ppm indicate a significant amount of electronic charge stored on the lithium species, consistent with the expectation of the extended Zintl-Klemm-Busmann concept for the electronic structure of these materials. The (29)Si MAS-NMR spectra obtained on isotopically enriched samples, aided by double-quantum spectroscopy, are well suited for differentiating between the individual types of silicon sites within the silicon frameworks, and in Li(13)Si(4) their identification aids in the assignment of individual lithium sites via(29)Si{(7)Li} cross-polarization/heteronuclear correlation NMR. Variable temperature static (7)Li NMR spectra reveal motional narrowing effects, illustrating high lithium ionic mobilities in all of these compounds. Differences in the mobilities of individual lithium sites can be resolved by temperature dependent (6)Li MAS-NMR as well as (6)Li{(7)Li} rotational echo double resonance (REDOR) spectroscopy. For the compound Li(15)Si(4) the lithium mobility appears to be strongly geometrically restricted, which may result in a significant impediment for the use of Li-Si anodes for high-performance batteries. A comparison of all the (6)Li and (7)Li NMR spectroscopic data obtained for the three different lithium silicides and of Li(12)Si(7) previously studied suggests that lithium ions in the vicinity of silicon clusters or dimers have generally higher mobilities than those interacting with monomeric silicon atoms.

ChemInform Abstract: Synthesis, Characterization and Stereochemistry of Bis(acetylacetonato) diisothiocyanato Silane.

A hexacoordinated monomeric silicon(IV) complex, (acac)2Si(NCS)2 [where acac = (CH3COCHCOCH3)−] was isolated and characterized by elemental analyses, IR, multinuclei (1H, 13C and 29Si) NMR spectral data. Initially the complex has the cis-structure in solution which changes to an equilibrium mixture (1.8:1) of cis and trans isomers at room temperature in 24 h.

Formation of silicones mediated by the sponge enzyme silicatein-α

The sponge-restricted enzyme silicatein-α catalyzes in vivo silica formation from monomeric silicon compounds from sea water (i.e. silicic acid) and plays the pivotal role during synthesis of the siliceous sponge spicules. Recombinant silicatein-α, which was cloned from the demosponge Suberites domuncula (phylum Porifera), is shown to catalyze in vitro condensation of alkoxy silanes during a phase transfer reaction at neutral pH and ambient temperature to yield silicones like the straight-chained polydimethylsiloxane (PDMS). The reported condensation reaction is considered to be the first description of an enzymatically enhanced organometallic condensation reaction.

2,6‐(Trip)2H3C6 ](Cp*)Si: A Stable Monomeric Arylsilicon(II) Compound

Si takes a rest: A bulky sigma-bound terphenyl substituent and a pi-bound Cp* ligand enable the isolation and full characterization of the first aryl-substituted, monomeric silicon(II) compound 1, which can be regarded as the "resting state" of a true silylene containing a sigma-bound Cp* group. The conformation of the aryl group prevents aryl-Si pi back-bonding.

Silicateins, the major biosilica forming enzymes present in demosponges: Protein analysis and phylogenetic relationship

Silicateins are enzymes, which are restricted to sponges (phylum Porifera), that mediate the catalytic formation of biosilica from monomeric silicon compounds. The silicatein protein is compartmented in the sponges in the axial filaments which reside in the axial canals of the siliceous spicules. In the present study silicatein has been isolated from the freshwater sponge Lubomirskia baicalensis where it occurs in isoforms with sizes of 23 kDa, 24 kDa and 26 kDa. Since the larger protein is glycosylated we posit that it is a processed form of one of the smaller size forms. The silicatein isoforms are post-translationally modified by phosphorylation; at least four isoforms exist with p I's of 5.4, of 5.2, of 4.9 and of 4.7. Surprisingly silicatein not only mediates polymerization of silicate, but also displays proteolytic activity which is specific for cathepsin L enzymes, thus underscoring the high relationship of the silicateins to cathepsin L. The cDNAs from L. baicalensis for silicatein and cathepsin L, as well as the respective genes, were cloned. It was found that the five introns present in the sponge genes are highly conserved up to human cathepsin L. This analysis has been completed by sequencing of two silicatein genes (both for silicatein-α and -β) and of cathepsin L from another demosponge, Suberites domuncula. A comprehensive phylogenetic analysis with these new sequences shed new light upon the evolution of cathepsin L and silicatein families which occurred at the base of the metazoan phyla. It is concluded, that in parallel with the emergence of these enzymes at first the number of introns increased, especially in the coding region of the mature enzyme. Later in evolution the number of introns decreased again. We postulate that modification of the catalytic triad, especially of its first amino acid, is a suitable target for a chemical modulation of enzyme function of the silicateins/cathepsin L.

Chemical Transformations in the Course of Solid-Phase Synthesis of Fibrous Alkali-Free Fluoroamphibole

This paper reports on the results of investigations into the possible chemical transformations occurring in the course of the solid-phase synthesis of fibrous alkali-free fluoroamphibole from synthetic magnesium silicate dihydrate (MSDH) MgO · SiO2 · 2H2O in the MSDH–MgF2–NaCl model system in the concentration range of compositions corresponding to Mg-fluorocupfferite (Mg7Si8O22F2) at temperatures ranging from 60 to 1000°C. It is found that the formation of Mg-fluorocupfferite from MSDH under these conditions is a complex multistage chemical process. In the temperature range 60–700°C, dehydration, dehydroxylation of MSDH, and pyrohydrolysis of fluorides (MgF2, SiF4, etc.) proceed near the lower temperature boundary of the fluoroamphibole formation region (T ≥ 750°C). These processes result in the formation of compounds containing monomeric and simple polymeric silicon–oxygen anions [SiO4]4− and [SiO3]2−∞ (forsterite, fluoronorbergite, enstatite, etc.). In the temperature range 850–900°C, the transformation of intermediate compounds leads to the formation of the fluoroamphibole structure. The main elements of this structure are silicon–oxygen anions of the [Si4O11]6−∞ type.

Synthesis and characterization of ureidopropyl-MSU-X silica

Ureidopropyl-incorporated MSU-X (U-MSU-X) and bi-functionalized MSU-X silica materials containing ureidopropyl along with methyl (M-U-MSU-X) or phenyl (Ph-U-MSU-X) were synthesized by a “one-pot” co-condensation from monomeric silicon sources such as tetraethylorthosilicate and ureidopropyltriethoxysilane (UTES), methyltriethoxysilane, phenyltriethoxysilane. The so-produced materials were characterized by X-ray diffraction, FT-IR, 29SiNMR, HRTEM, thermogravimetry analysis, N 2 adsorption, bulk element analysis and synchrotron radiation small angle X-ray scattering. The surface and texture properties varied with the content of UTES and the change of organosiloxanes and the organic groups were distributed uniformly in the organo-MSU-X channels and formed interfacial layers.

Organic-functionalized molecular sieves (OFMSs):: II. Synthesis, characterization and the transformation of OFMSs containing non-polar functional groups into solid acids

Organic-functionalized molecular sieves (OFMSs) with a beta-type structure ( ∗BEA) containing intracrystalline phenethyl (PE) groups were synthesized and characterized by X-ray diffraction, thermogravimetric analysis, FT Raman spectroscopy, 29Si, 13C and 27Al solid-state nuclear magnetic resonance spectroscopy, X-ray photoelectron spectroscopy, bulk elemental analysis, scanning electron microscopy and physisorption techniques. The OFMSs are synthesized from monomeric silicon sources such as tetraethylorthosilicate and phenethyltrimethoxysilane and via the solid-state conversion of extracted, PE-functionalized MCM-41. Occluded structure-directing agent (tetraethylammonium fluoride; TEAF) is removed by solvent extraction techniques. By varying the extraction conditions, OFMSs with varying hydrophobicity and porosity are synthesized. Bulk and surface elemental analysis indicate that there is an even distribution of organic functionalities throughout the material at levels of PE incorporation below 5% (silicon basis). The phenethyl groups are sulfonated using SO 3 vapor to produce a microporous solid containing intracrystalline sulfonic acids.

Synthesis by FVT and Characterization of Unhindered Silanethiones

The retro-ene reaction of allylthio- and propargylthiosilanes led, under flash vacuum thermolysis (FVT) conditions, to unhindered silanethiones, characterized by their derivatives, and also directly by coupling of the FVT with gas-phase spectrometries. Monomeric silicon oxysulfide has been generated similarly. The unsubstituted silanethione was not obtained, but dehydrogenated into silicon monosulfide during FVT.

Synthesis by FVT and Characterization of Unhindered Silanethiones

The retro-ene reaction of allylthio- and propargylthiosilanes led, under flash vacuum thermolysis (FVT) conditions, to unhindered silanethiones, characterized by their derivatives, and also directly by coupling of the FVT with gas-phase spectrometries. Monomeric silicon oxysulfide has been generated similarly. The unsubstituted silanethione was not obtained, but dehydrogenated into silicon monosulfide during FVT.

Ultrasonic activation of SiO 2 and GeO 2 in basic solutions of diols

The sonication of silica gel in basic solutions of diols causes rapid breakdown of the inorganic polymer and the formation of monomeric silicon complexes. The triscatecholate complex Na 2[(C 6H 4O 2) 3Si] is formed cleanly and in good yield at room temperature. The germanium analogue Li 2[(C 6H 4O 2) 3Ge] may be formed similarly from GeO 2 but TiO 2 remains unaffected by ultrasound.