Synthesis and structure of <Emphasis Type="Italic">N</Emphasis>-(2-silatranylethyl)pyrazoles

Organosilicon pyrazole derivatives containing a 2-siltranylethyl fragment on the nitrogen atom were synthesized, and their structural and stereoelectronic parameters were determined by X-ray analysis and ¹H, ¹³C, and ²⁹Si NMR spectroscopy. The nature of interaction between the pyrazole and silatrane fragments was studied.     

Synthesis and molecular structure of 1-(2-pyridyloxy)silatrane

1-(2-Pyridyloxy)silatrane was synthesized by trans-etherification of 1-ethoxysilatrane with 2-hydroxypyridine as well as by the reaction of the latter with tetraethoxysilane and triethanolamine. Its structure was established by the XRD analysis of a single crystal, and in solution using the methods of ¹H, ¹³C, ²⁹Si NMR and IR spectroscopy.     

Synthesis and structure of <Emphasis Type="Italic">N</Emphasis>-(2-silatranylethyl)imidazoles

Organosilicon imidazole derivatives containing a 2-silatranylethyl [N(CH₂CH₂O)₃Si(CH₂)₂] group on the nitrogen atom were synthesized, and their steric and electronic structures, including the nature of interaction between the imidazole and silatrane fragments, were studied by X-ray analysis and ¹H, ¹³C, and ²⁹Si NMR spectroscopy.     

The effect of the C-methyl substitution on the molecular geometry of the silatrane skeleton: The crystal structures of 1-(4′-tolyl)-silatrane and 1-(4′-tolyl)-3,7,10-trimethylsilatrane

Unsubstituted (1) and 3,7,10-trimethyl substituted (2) 1-(4′-tolyl)-silatranes were synthesized. ¹H, ¹³C, ¹⁵N and ²⁹Si NMR spectra were recorded and assigned. The conformation of the 2 stereoisomers in solution were characterized by the NMR spectra. 1 is monoclinic, P2₁/c, with a = 13.454(2), b = 13.888(2), c = 14.345(2) Å, β = 99.10(2)°. The major stereoisomer fraction of 2 is orthorhombic, Pbca, with a = 12.168(2), b = 13.754(4), c = 20.260(3) Å. The structures were solved by direct methods and were refined by least squares to R values of 0.039 and 0.066 for 3523 and 2158 reflexions. The N→Si distances are 1: 2.169(2) (mean) and 2: 2.236(3) Å. The solid state conformation of the 2 major stereoisomer fraction is different from that in solution and this is the first example of a silatrane crystal structure where the silatrane moiety lacks C₃ symmetry.     

The influence of intermolecular interactions on the SiH bond vibration in silatrane and its c-substituents

Spectra of the silatrane HSi(OCHRCH₂)₃N and its 3,7,10-trimethyl- and -trifluoromethyl derivatives have been studied. The dependences of frequency and integral intensity of the SiH vibration on solvent polarity and temperature have been established. It has been shown that interaction of silatrane molecules with the solvent is universal in character. The phenomenon of the changing SiH vibration is explained by the possible formation of a three-centered orbital bending the atoms of hydrogen, silicon and nitrogen. In contrast to triethoxysilane, a dependence between v(SiH) and Σσ1 values of substituents in the cycle has been observed for silatranes.     

Crystal structure of 1-[N-2-aminoethyl)-aminopropyl]silatrane

The crystal structure of 1-[N-(2-aminoethyl)aminopropyl]silatrane has been determined by x-ray diffraction at room temperature. The Si←N bond distance (2.165(2) Å) is in the range observed for other 1-X-propylsilatranes (X = CN, OH, SH, Cl and SCN). The structure is partially disordered: the silatrane moiety displays a disorder that is typical for silatranes and the aminoethyl group terminating the planar chain linked to silicon is rotationally disordered.     

Synthesis of Novel Phosphoramide-Tegafur Derivatives Containing Aminopropylsilatrane

Abstract

A series of novel phosphoramide-tegafur derivatives containing γ-aminopropyl silatrane were synthesized via the condensation reactions of phosphoryl dichloride with N¹-(2-furanidyl)-N³-(hydroxyethyl)-5-fluorouracil, followed by condensation with γ-aminopropyl silatrane. The structures of the products were confirmed by ¹H NMR, ³¹P NMR, IR, MS, and elemental analysis. The results of preliminary bioassay showed that the new compounds had an inhibition effect against HCT-8 and Bel-7402 cell lines.     

Geminal 2J(29Si‐O‐29Si) couplings in oligosiloxanes and their relation to direct 1J(29Si‐13C) couplings

Absolute values of (79) geminal ²J(²⁹Si‐O‐²⁹Si) couplings were measured in an extensive series of (55) unstrained siloxanes dissolved in chloroform‐d. Signs of ²J(²⁹Si‐O‐²⁹Si) in some (9) silicon hydrides were determined relative to ¹J(²⁹Si‐¹H) which are known to be negative. It is supposed that positive sign of the ²J(²⁹Si‐O‐²⁹Si) coupling found in all studied hydrides is common to all siloxanes. Theoretical calculations for simple model compounds failed to reproduce this sign and so their predictions of bond length and angle dependences cannot be taken as reliable. Useful empirical correlations were found between the ²J(²⁹Si‐O‐²⁹Si) couplings on one side and the total number m of oxygen atoms bonded to the silicon atoms, sum of ²⁹Si chemical shifts or product of ¹J(²⁹Si‐¹³C) couplings on the other side. The significance of these correlations is briefly discussed. Copyright © 2011 John Wiley & Sons, Ltd.     

29Si and 13C NMR spectra of permethylpolysilanes

²⁹Si, ¹³C and ¹H NMR spectra are reported for the series of linear permethylpolysilanes Me(SiMe₂)_nMe where n = 1 to 6, for the cyclic permethylpolysilanes (Me₂Si)_n where n = 5 to 8, and for a few related compounds. For linear polysilanes the ²⁹Si and ¹³C chemical shifts can be accurately calculated from simple additivity relationships based on the number of silicon atoms in α, β, γ and δ positions. Adjacent (α) silicon atoms lead to upfield shifts in the ²⁹Si and ¹³C resonances, whereas more remote silicon atoms lead to downfield shifts. The ²⁹Si chemical shifts of the polysilane chains are linearly related to the ¹³C shifts of the carbon atoms attached to the silicon. The ²⁹Si and ¹³C resonances of the cyclic silanes deviate from this relationship. Ring current effects arising from σ delocalization are suggested as an explanation for the deviations. Proton-coupled ²⁹Si NMR spectra are reported for Me₃SiSiMe₃ and for (Me₂Si)_n, n = 5 to 7.     

Structural study of arylsilanes using 29Si NMR

²⁹Si NMR data (σ and ¹H, ²⁹Si coupling constants) of a series of arylsilanes were studied in relation to their molecular structure. It is shown that silicon chemical shift variation is too weak to allow empirical attribution. In contrast, ²⁹Si, ¹H coupling constants across the aromatic ring, especially the ³J, allow structural assignment. Generally, SPT experiments are sufficient to obtain these values. However, full attribution of polysilylated derivatives sometimes needs other experiments to identify all the silicons. A 2D J-resolved INEPT experiment is described and has proven to be an efficient method for ²⁹Si NMR signal identification. The usefulness of this approach is demonstrated by the complete analysis of all the ²⁹Si lines in a trisilyl derivative. An easy-to-use INEPT decoupled-type sequence is also proposed for a rapid characterization of polysilylated derivatives or mixture of isomers.     

Effects of active silicon uptake by rice on 29Si fractionation in various plant parts

Rice (Oryza sativa L.) accumulates large amounts of silicon which improves its growth and health due to enhanced resistance to biotic and abiotic stresses. Silicon uptake and loading to xylem in rice are predominantly active processes performed by transporters encoded by the recently identified genes Lsi1 (Si influx transporter gene) and Lsi2 (Si efflux transporter gene). Silicon deposition in rice during translocation to upper plant tissues is known to discriminate against the heavier isotopes ²⁹Si and ³⁰Si, resulting in isotope fractionation within the plant. We analyzed straw and husk samples of rice mutants defective in Lsi1, Lsi2 or both for silicon content and δ²⁹Si using isotope ratio mass spectrometry (IRMS) and compared these results with those for the corresponding wild‐type varieties (WT). The silicon content was higher in husk than in straw. All the mutant rice lines showed clearly lower silicon content than the WT lines (4–23% Si of WT). The δ²⁹Si was lower in straw and husk for the uptake defective mutant (lsi1) than for WT, albeit δ²⁹Si was 0.3‰ higher in husk than in straw in both lines. The effect of defective efflux (lsi2) differed for straw and husk with higher δ²⁹Si in straw, but lower δ²⁹Si in husk while WT showed similar δ²⁹Si in both fractions. These initial results show the potential of Si isotopes to enlighten the influence of active uptake on translocation and deposition processes in the plant. Copyright © 2009 John Wiley & Sons, Ltd.     

13C, 15N and 29Si nuclear magnetic resonance studies of some aminosilanes and aminodisilanes

¹³C, ¹⁵N (at natural abundance) and ²⁹Si NMR data (chemical shifts and coupling constants) are reported for aminosilanes R₂R′SiNHR¹ (1), bis(silyl)amines Me₂R′SiNHSiMe₃ (2), 1,2-bis(amino)-ethanes (3), bis(amino)silanes RR′Si(NHR¹)₂ (4), 1,2-bis(amino)tetramethyldisilanes (5) and 1,1,2,2-tetrakis(amino)dimethyldisilanes (6). The δ¹⁵N values depend more on the nature of the substituents R¹(H, alkyl, aryl) at the nitrogen atom (in the same way as for other amines) than on different substituents at the silicon atom. A linear correlation between ¹J(²⁹Si¹⁵N) and ¹J(²⁹Si¹³C) is proposed for silanes in which the SiN unit is replaced by the SiCH unit. This correlation comprises all ¹J(²⁹Si¹⁵N) values for aminosilanes R_4-nSi(N)n (n = 1–4) and—most likely—also for aminodisilanes, and it predicts ¹J(²⁹Si¹⁵N)>0 if the corresponding value |¹J(²⁹Si¹³C)|>25 Hz. For the first time a two-bond coupling across Si, ²J(²⁹Si ¹⁵N) = 6.9 Hz, has been observed for 6a. In the case of 6b (R¹ = ^sBu) all resonances for the diastereomers are resolved in the ¹⁵N and ²⁹Si NMR spectra in contrast to the ¹H and ¹³C NMR spectra.     

The crystal structures of m-trifluoromethylphenylsilatranone and p-fluorophenylsilatranone

The crystal structures of two silatranone derivatives are reported. The close N → Si approach (2.106(3) Å in m-trifluoromethylphenyl-, and 2.129(3) Å in p-fluorophenyl-silatranone) indicates strong dative acceptor bonds. For various silatrane derivatives and inverse relation has been revealed between the mean group electronegativity of the substituent R attached to silicon and the N → Si dative bond distance. In both structures there are long (1.72 Å) SiO bonds in the SiOCO moiety. The m-trifluoromethylphenyl derivative contains a disordered CF₃ group.     

Raman and 29Si MAS NMR spectroscopy of framework materials containing three-membered rings

Raman and ²⁹Si MAS NMR spectroscopies are evaluated for the identification of three-membered rings (3MR) in framework oxide materials. Raman and ²⁹Si MAS NMR spectra from the 3MR-containing materials euclase, phenakite, clinohedrite, willemite, lovdarite, VPI-7, ZSM-18 and dipotassium zinc tetrasilicate are presented. The Raman spectra from these materials do not exhibit common bands representing vibrational modes assignable to individual 3MR. The dense beryllosilicate and zincosilicate minerals exhibit ²⁹Si MAS NMR resonances indicative of silicon positioned in 3MR while the molecular sieves lovdarite and VPI-7 give ²⁹Si MAS NMR resonances that can be assigned to silicons located at the center of “spiro-5” units that are constructed from two 3MR. Silicon atoms located in isolated 3MR in the molecular sieves ZSM-18 and dipotassium zinc tetrasilicate do not exhibit ²⁹Si MAS NMR resonances that can be distinguished from those assigned to silicons residing in 4MR and larger.The ²⁹Si MAS NMR spectra from the new materials VPI-8, VPI-9 and VPI-10 do not exhibit ²⁹Si MAS NMR resonances indicative of “spiro-5” units. The presence of isolated 3MR in these materials cannot be ruled out from the ²⁹Si MAS NMR spectroscopic results.     

Solid-state NMR tools for the structural characterization of POSiSils: 29Si sensitivity improvement with MC-CP and 2D 29Si–29Si DQ-SQ at natural abundance

The ¹H–²⁹Si multiple-contact cross polarization (MC-CP) MAS NMR experiment is evaluated for the class of silicate-siloxane copolymers called POSiSils, that is, polyoligosiloxysilicones. It proves a reasonably good solution to tackle the challenge of recording quantitative ²⁹Si NMR data in experimental time much reduced compared with single pulse acquisition. In a second time, we report ²⁹Si–²⁹Si MC-CP double-quantum single-quantum (MC-CP-DQ-SQ) NMR experiment, which provides information about the through-space proximities between all silicon species despite the high degree of heterogeneity of this material. This work furthers the NMR tools for NMR crystallography for inorganic polymers, as it covers flexible polymers with different dimensionalities and long or heterogeneous relaxation characteristics at low ²⁹Si natural abundance.     

Ribbed-functionalized iron(ii) tris-dioximate clathrochelates with pendant fragments of various types: synthetic pathways,structures, and properties

Nucleophilic substitution of chlorine atoms in the iron(ii) hexachloride clathrochelate on treatment with thiolate anions afforded hexafunctionalized tris-dioximate complexes with the pendant n-butyl-, n-octylsulfide, silatrane, and captopryl functionalizing groups. These complexes were characterized by elemental analysis, IR, UV-Vis, ¹H and ¹³C NMR, and ⁵⁷Fe Mössbauer spectroscopies, and plasma-desorption mass spectrometry. The crystal and molecular structures of the n-butylsulfide and silatrane clathrochelates were established by X-ray diffraction analysis. Cyclic voltammetry study demonstrated that the Fe²⁺/Fe³⁺ redox process of the encapsulated iron ion is responsible for the electrochemical behavior of the prepared compounds in solution.     

29Si NMR Spektroskopie von Cyclopentasilanen

The structure and ²⁹Si chemical shifts of the halodimethylsilylnonamethylcyclopentasilanes Si₅Me₉SiMe₂X (1–4) and the halononamethylcyclopentasilanes Si₅Me₉X (5–8) (X = F, Cl, Br, I) have been assigned using ¹J(SiSi) and ²J(SiSi) coupling constants derived from ²⁹Si-INADEQUATE and ²⁹Si-INEPT—INADEQUATE NMR spectra. The compounds exhibit good correlation between chemical shift, ¹J(SiSi) and Pauling electronegativities.     

Structure investigations of solid organosilicon polymers by high resolution solid state 29Si NMR

The high resolution ²⁹Si NMR spectra of five solid silicon polymers of different structure have been studied and the ²⁹Si chemical shifts of characteristic structure units determined. ²⁹Si¹H cross-polarization in combination with high speed magic angle sample spinning and high power proton decoupling was used to achieve high resolution in the solid state spectra. Comparison of the latter with the results obtained in the liquid state clearly indicates that no special solid state effects on ²⁹Si chemical shifts arise and the relations between δ(Si) and the molecular structure, well known from investigations of liquids, can be used for interpretation of the solid state spectra. It is shown that high resolution solid state ²⁹Si NMR spectroscopy offers detailed information about the structural units of the siloxane resin framework, and this opens up new possibilities for structural determinations of solid organosilicon polymers.     

Synthesis and Characterization of Base‐catalyzed Phenyl Modified PDMS/PHMS Copolymers

A series of phenyl modified polydimethylsiloxane (PDMS) / polyhydrogenmethylsiloxane (PHMS) random copolymers containing both internal Si‐H and terminal SiH₂ and T (MeSiO_3/2) units was synthesized in one step through n‐BuLi‐catalyzed ring‐opening polymerization of cyclic comonomers and characterized by GPC, IR and ¹H and ²⁹Si NMR. Sequential microstructures of these copolymers were determined by ²⁹Si‐NMR spectroscopy. Epoxy‐modified polysiloxanes were prepared and used as comparable standards for the assignment of the NMR spectra. A hydride‐transfer mechanism has been proposed to account for the formation of terminal Si‐H and T group. Detailed sequential analyses and chemical shifts of ²⁹Si‐NMR for various siloxane units are reported for the first time.     

Crystal and molecular structure of 1-(3-ammoniopropyl)silatrane chloride

By single crystal X-ray diffraction the crystal and molecular structure of 1-(3-ammoniopropyl)silatrane chloride (I) is determined. One of three cations in the asymmetric unit of the crystal of I differs from the other two by the length of the coordination N → Si bond. The opposite electronic effects in two geometrically similar cations are transferred inductively through a three-carbon chain.