Boris Gostevskii

The synthesis and NMR spectra of new pentacoordinate silicon compounds, (OSi)- and (ClSi)-chloro[1-(1,1-dimethyl-2-acylhydrazonium)methyl]dimethylsilanes and (OSi)chloro-[2-(1,1-dimethyl-2-acylhydrazino)methyl]dimethylsilanes

Pentacoordinate silicon compounds containing a “chelate” six-membered silacarbofunctional ring, such as the chloro[1-(1,1-dimethyl-2-acylhydrazonium)methyl]-dimethylsilanes, Me2ClSiCH2NMe2NC(O)R with R = CF3 (Ib), C6H5 (Ic), 4′-MeC6H4 (Id), 4′-MeOC6H4 (Ie), 4′-NO2C6H4 (If) have been synthesized for the first time and were studied by NMR spectroscopy. In contrast to Ib in molecules of Ia (R = Me), IcIf the ClSi bond rather than OSi bond is the weaker component of the hypervalent OSiCl bond, which implies the possibility of Cl → Si coordinative interaction. An irreversible rearrangement of compounds I upon heating leads to (OSi)chloro[2-(1,1-dimethyl-2-acylhydrazino)methyl]dimethylsilanes, Me2ClSiCH2-N[C(O)R]NMe2 (II), and was used for preparative purposes.

Donor-stabilized silyl cations ☆: Part 7: Neutral hexacoordinate and ionic pentacoordinate silicon chelates with N-isopropylideneimino-acylimidato ligands

Abstract Silicon complexes with a new chelating donor group, the isopropylideneimino-acylimidato(N,O) moiety [OC(R)NNCMe2], have been prepared (9–12, 15), and their properties are compared with previously described silicon chelates (1, 2, and 16, respectively). The new ligand acts as a more powerful donor than the NNMe2 ligand, based on three criteria: (a) the new complexes form directly as pentacoordinate siliconium salts 9–12, i.e., the expected neutral hexacoordinate precursors ionize spontaneously; (b) comparison of crystallographic bond lengths with those of the NNMe2 complexes shows consistently shorter NSi coordination bonds and longer Sihalogen bonds in equally substituted new relative to the previously studied complexes; (c) while in the previous series 1, the dihalo complexes 20–22, 25, resisted ionization at any temperature or solvent, the dibromo-isopropylideneimino complexes 19a and 19b ionize reversibly upon decrease of temperature in chloroform solution. Steric congestion forces the trans-dihalo configuration on the dichloro and bibromo complexes 18, 19. The ionization-resistant complexes 17, 18 and the partly ionized 19 form stable ionic siliconium salts when their counterions are replaced by I−, BPh4−, or by reaction with strong Lewis acids, AlCl3 and AlBr3.

Silicon rehybridization and molecular rearrangements in hypercoordinate silicon dichelates

Hydrazide-based hypercoordinate silicon dichelates are remarkably flexible in terms of geometry and reactivity: this paper demonstrates how rather subtle constitutional changes result in dramatic geometrical and reactivity changes. A change of ligand-donor group from NMe2 to N=CMe2 changes the solid-state geometry from bicapped tetrahedral to octahedral. These two geometries are shown to coexist in solution in dynamic equilibrium. Hexacoordinate complexes are shown to dissociate to pentacoordinate compounds in two distinct ways: ionic or neutral, depending on substitution. Hexacoordinate dichelates with imino-donor groups undergo a skeletal rearrangement (intramolecular aldol-type condensation of imines), catalyzed by their dissociated halide counterions. However, even in the absence of counterions, silacyclobutane dichelates undergo a similar rearrangement.

The synthesis and structure of chiral compounds with two hypervalent silicon atoms, Si,Si′-substituted 1,2-bis(dimethylsilylmethyl)diacetylhydrazines

Abstract The multi-stage character of the reaction of 1,2-diacetylhydrazine bis- O , O ′-trimethylsilyl derivative with ClCH 2 SiMe 2 Cl to give bis-( O  Si )-1,2-bis(dimethylchlorosilylmethyl)-1,2-diacetylhydrazine (VIII) was established from NMR spectroscopy. Methanolysis of VIII in the presence of NEt 3 gives the Si , Si ′-dimethoxy derivative (IX). The corresponding Si , Si ′-difluoro derivative (XI) was obtained by treatment of IX with boron trifluoride etherate. The structure, the stereodynamic behaviour, and the exchange reactions of chiral molecules of VIII, IX and XI and their precursors have been studies by multinuclear NMR spectroscopy. Compounds VIII, IX, and XI are the first examples of substances that contain two hypervalent silicon atoms and an XSi(C) 3 O coordination centre.

Equilibrium between neutral hexacoordinate silicon complexes and ionic pentacoordinate siliconium salts through fast dissociation–recombination of the Si–Cl bond

Equilibrium between neutral hexacoordinate silicon complexes and ionic siliconium chlorides, which is highly temperature, solvent, counterion, ligand and substituent dependent, was observed by low temperature 29Si NMR and confirmed by crystal analysis.

Synthesis of 1-organyl-1-(trimethylsiloxy)-2-(dimethylamino)ethenes and new hypervalent silicon compounds based thereon

Abstract1-Organyl-1-(trimethylsiloxy)-2-(dimethylamino)ethenes were synthesized for the first time, and their reactions with trichloro(methyl)silane and trifluoro(phenyl)silane afforded silicon bis-chelates, methylbis-[2-(dimethylamino)-1-(organyl)ethenolato-N,O]siliconium chlorides and bis[1-(dimethylamino)-3,3-dimethylbut-1-en-2-olato-N,O]fluoro(phenyl)silicon(IV), respectively.