Silicenium Ions Research Articles

ChemInform Abstract: Electrochemical Approach to Germenium and Silicenium Ions.

The reduction potentials for carbonium, silicenium, and germenium ions were determined by cyclic voltammetry of neutral precursor compounds. The reduction potential shifted to the negative direction as the center element changed from C to Ge.

Isomerization of silicenium and germenium ions in the systems C4H11M+ (M = Si, Ge)

Equilibrium structures of the isomers and transition states of their interconversion in the system C4H11M+ (M = Si, Ge) have been obtained at theB3LYP level of theory using the cc-pVTZ basis set. The structures of these stationary points are close for Si and Ge; the most stable isomer in both systems is the tertiary cation (C2H5)(CH3)2M+, the second in energy is complex with ethylene [(CH3)2HM·C2H4]+. The secondary cation (C2H5)2HM+ is third in energy isomer, the height of the barrier of interconversion for these three cations being practically independent on M. However, for M = Ge a substantial decrease in the energy of isomeric forms corresponding to complexes with alkanes is observed. As a result, in the system C4H11Ge+ the fourth in energy is isomer [(C2H5)Ge·C2H6]+ rather than [(C2H5)H2Ge·C2H4]+ as for M = Si. Nevertheless, the height of the barriers for transition into these structures, although decreasing from M = Si to Ge, remain rather high, and the most favorable route of decomposition in both systems is the elimination of ethylene.

Heterolytic Bond Dissociation in Water: Why Is It So Easy for C4H9Cl But Not for C3H9SiCl?

The recently developed (Song, L.; Wu, W.; Zhang, Q.; Shaik, S. J. Phys. Chem. A 2004, 108, 6017-6024) valence bond method coupled to a polarized continuum model (VBPCM) is used to address the long standing conundrum of the heterolytic dissociation of the C-Cl and Si-Cl bonds, respectively, in tertiary-butyl chloride and trimethylsilyl chloride in condensed phases. The method is used here to compare the bond dissociation in the gas phase and in aqueous solution. In addition to the ground state reaction profile, VB theory also provides the energies of the purely covalent and purely ionic VB structures as a function of the reaction coordinate. Accordingly, the C-Cl and Si-Cl bonds are shown to be of different natures. In the gas phase, the resonance energy arising from covalent-ionic mixing at equilibrium geometry amounts to 42 kcal/mol for tertiary-butyl chloride, whereas the same quantity for trimethylsilyl chloride is significantly higher at 62 kcal/mol. With such a high value, the root cause of the Si-Cl bonding is the covalent-ionic resonance energy, and this bond belongs to the category of charge-shift bonds (Shaik, S.; Danovich, D.; Silvi, B.; Lauvergnat, D.; Hiberty, P. C. Chem.- Eur. J. 2005, 11, 6358). This difference between the C-Cl and Si-Cl bonds carries over to the solvated phase and impacts the heterolytic cleavages of the two bonds. For both molecules, solvation lowers the ionic curve below the covalent one, and hence the bond dissociation in the solvent generates the two ions, Me3E+ Cl- (E = C, Si). In both cases, the root cause of the barrier is the loss of the covalent-ionic resonance energy. In the heterolysis reaction of Si-Cl, the covalent-ionic resonance energy remains large and fully contributes to the dissociation energy, thereby leading to a high barrier for heterolytic cleavage, and thus prohibiting the generation of ions. By contrast, the covalent-ionic resonance energy is smaller for the C-Cl bond and only partially contributes to the barrier for heterolysis, which is consequently small, leading readily to ions that are commonly observed in the classical SN1 mechanism. Thus, the reluctance of R3Si-X molecules to undergo heterolysis in condensed phases and more generally the rarity of free silicenium ions under these conditions are experimental manifestations of the charge-shift character of the Si-Cl bond.

The unique reactivity of "super silyl" in organic synthesis: A theoretical study

We recently developed a new use of the super silyl group in organic reactions, and showed that it has unique reactivity. One of the reactions using this reagent is diastereoselective Mukaiyama aldehyde cross-aldol reaction catalyzed by a highly active Bronsted acid. This reaction produced syn-1,3-diol derivatives with high yield and high stereoselectivity. Certain features and outcomes of the reaction were not clear, so we decided to investigate the reaction mechanism through a theoretical study. In this paper, structures of super silyl species were observed as the subject of study. First 1 and 2 were investigated so that the effect of a TMS group adjacent to a silicenium ion could be determined. Interestingly, it was found that the TMS group stabilized the silicenium ion. It is presumed that the stability of the tris(trimethylsilyl) silicenium ion (TTMSS+) is related to the unique reactivity of super silyl group in theses reactions. We also investigated equivalent structures of silyl enol ethers (SEE) and found an unusual structure of 5. This structure is a result of unusual interactions between the silyl portion and enol ether (EE) portion; 1) C=C and Si-Si bonds, 2) lone pair of oxygen and Si-Si bond.

Some aspects of chemistry of the N→Si chelated aryloxydihydrosilanes R(ArO)SiH 2(RPh, ArO) and of the 2,2-diaryloxytrisilane (Me 3Si) 2Si(OAr) 2{ArO2,4,6-[(CH 3) 2NCH 2] 3C 6H 2O}

Reactions of the pentacoordinate aryloxydihydrosilane Ph(ArO)SiH 2 ( 1) with hydroxyl groups, carbonyl derivatives and PCl 5 resulted in formation of new alcoxylated, phenoxylated and halogenated hydrosilanes Ph(ArO)(H)X (X=Cl, OR, OAr, OC(O)R) respectively. Treatment of the hexacoordinated ( 4+ 2) diaryloxydihydrosilane (ArO) 2SiH 2 ( 2) with N-chloro- or N-bromosuccinimide, trimethylsilyltriflate or iodine yielded the novel stable silicenium ion [(ArO) 2(H)Si] +X −. Reactions of 2 with sulfur S 8 and the transition metal complex Fe(CO) 5 have resulted in the isolation of the new complexes (ArO) 2SiE [E=S, Fe(CO) 4]. Decomposition modes of the tetracoordinate 2,2-diaryloxytrisilane (Me 3Si) 2Si(OAr) 2 ( 3) on thermolysis and photolysis have also been studied; two pathways involving formation of the two silylenes (ArO)(Me 3Si): and (ArO) 2Si: have been identified by trapping experiments.

Pi-donation and stabilizing effects of pnicogens in carbenium and silicenium ions: a theoretical study of [C(XH2)3]+ and [Si(XH2)3]+ (X = N, P, As, Sb, Bi).

Quantum chemical calculations at the MP2 and CCSD(T) levels of theory are reported for cations of the general type [A(XH2)3]+ with A = C, Si and X = N, P, As, Sb, Bi. Population analysis, methyl stabilization energies (MSEs), and structural criteria were used to predict the p(pi)-donor ability of and the pi-stabilization energy exerted by this series of pnicogens. All of the substituents XH2 considered in these studies invariably stabilize the triply substituted carbenium as well as the silicenium ions. The calculated data show that the intrinsic p(pi)-donation of the group 15 atoms follows the order N < P < As < Sb < Bi. However, the trend of the stabilization energies is fully reversed. The intrinsic stabilization energies of the planar carbenium ions decrease monotonically from 161.2 kcal mol(-1) for X = NH2 to 98.0 kcal mol(-1) for X = BiH2. The effective stabilization of the pnicogens in the equilibrium structures, which also includes the energy-demanding pyramidalization of the XH2 substituents, follows the same trend, although the absolute numbers are reduced to 145.6 kcalmol(-1) for X = NH2 and 53.2 kcalmol(-1) for X = BiH2. This seemingly contrasting behavior of increasing p(pi) charge donation and decreasing stabilization has already been found for other substituents. Previous studies have shown that carbenium ions substituted by chalcogens up to the fourth row also stabilize C+ less effectively with respect to heavier substituents. Of the ions investigated in this study, only the silicenium ions that are stabilized by pnicogens from the third to the sixth row of the periodic system yield increased stabilizing energies that follow the corresponding intrinsic p(pi)-donor abilities of the respective substituent.

Bis[di(O→Si)chelatodi(2-oxopiperidinomethyl)chloromethyl- silicenium] Hexachlorodimercurate, the First Representative of Cation-Anion Complexes Containing Silicenium Ion with Three Si-C Bonds, Stabilized by O→Si Coordination

Bis[di(O→Si)chelatodi(2-oxopiperidinomethyl)chloromethyl- silicenium] Hexachlorodimercurate, the First Representative of Cation-Anion Complexes Containing Silicenium Ion with Three Si-C Bonds, Stabilized by O→Si Coordination

Study of ion/molecule reactions of ion derived from tert-butyl methyl ether and trimethylsilyl methyl ether

The ion/molecule reactions of the ions (CH 3) 2C +–OCH 3 ( 1a) and (CH 3) 2Si +–OCH 3 ( 2a), which are the main fragment ions in the electron ionization mass spectrum of tert-butyl methyl ether (MTBE, 1) and trimethylsilyl methyl ether (MTSE, 2), respectively, and of the protonated MTBE ( 1H +) and protonated MTSE ( 2H +), generated by chemical ionization, with MTBE or MTSE, with acetone and with ammonia have been studied by Fourier transform ion cyclotron resonance (FTICR) spectrometry. The carbenium ion 1a is practically unreactive toward these substrates whereas the silicenium ion 2a efficiently forms adduct ions with all three reactants. Clearly 1a is a very stable tertiary carbenium ion due to the resonance stabilization of the methoxy group so that it lacks significant electrophilic character, whereas 2a does not gain much stabilization by this substituent. This interpretation is supported by the results of ab initio calculations at the restricted Hartree-Fock/6-31G( d) level of the structures and stabilities of these ions, of the tert-butyl cation and of the trimethylsilicenium ion and of their adducts with H 2O, (CH 3) 2O, and NH 3. The reactions of the protonated silyl ether 2H + are straightforward and correspond to transfer of the trimethylsilyl group to the reactants, as expected for a silylation reagent. Silyl group transfer competes effectively with proton transfer even in the case of NH 3 as reactant. The typical reaction of the protonated tert-butyl ether 1H + with the three substrates is an efficient elimination of C 4H 8 to generate a proton bound heterodimer of the attacking reactant with methanol as the primary product ion. This reaction also takes place when NH 3 is the substrate, although in this case proton transfer is the main process. In view of the easy interconversion of the protonated ether 1H + with its isomeric ion/neutral complexes consisting of proton bound isobutene and methanol (β complex), as proven by Audier et al. [J. Orig. Chem. 60 (1995) 7198], it is suggested that this fast elimination process is in fact a ligand switching process of the β complex which is either present as a stable species in the gas phase of the FTICR cell or is generated from 1H + by electrostatic activation in the encounter complex of the reactants.

Clarification of the Nature of the “First Chiral and Highly Lewis Acidic Silyl Cationic Catalyst”. Concerning the Question of Siliconium Ions vs Silyl Cations1

Density functional theory (DFT)/IGLO NMR studies show that the “first chiral and highly Lewis acidic silyl cationic catalyst” (Jorgensen et al. J. Am. Chem. Soc. 1998, 120, 7637) is not a free tertiary alkylsilicenium ion or a somewhat coordinatively stabilized silyl cation but a de facto silylated acetonitrilium ion 4 which possesses some chirality and thus is capable of catalyzing reported reactions, however, with very low enantioselectivity. Attempts to find a minimum energy structure for the free silicenium ion 1 calculationally at the B3LYP/6-31G* level were unsuccessful and resulted instead in showing the stability of the derived intramolecularly silylated spirosilacyclopropylarenium ion 2 (a Cram-type aronium ion). In benzene solution a silylated benzenium ion 5 (Wheland intermediate) is obtained.

Hyperconjugative Effects in Carbenium and Silicenium Ions

Bond dissociation energy (R3M+−L) and bond length (R−M and M−L) trends in the R3ML+ series of cation−ligand (L) complexes for M = carbon and silicon, and R = H, CH3 and F are derived from density functional theory calculations using the hybrid B3LYP exchange−correlation potential. The ligands studied are NH3, H2O, HCN, H2CO, MeCN, Me2O, Me2CO, FCN, F2 O, F2CO, and NF3, where ligand binding to M is through the nitrogen or oxygen atom. For all ligand substrates, R3M+−L bond energies are calculated to decrease from carbenium to silicenium with R = H but to increase for R=methyl and fluorine. Also for these latter two cases, in going from the bare R3M+ cation to the ligand complexes, the R−M distances increase by more than twice as much for the carbenium than for the silicenium ions. These trends indicate the relative importance of a stabilizing R−M hyperconjugative interaction in the bare tert-butyl and trifluoromethyl cations compared with the other bare cations and all the cation−ligand complexes. Ab initio, multiconfiguration VBSCF calculations are carried out on model systems (AHn−MH2+; M = C, Si; AHn = CH3, SiH3, F), designed to mimic the R3M+ cations, in order to analyze the electronic structure of the R−M bond. The π bond component, representing the hyperconjugative interaction, is found to preferentially stabilize CH3CH2+ over SiH3CH2+, and FCH2+ relative to FSiH2+. The fluorosilicenium cation shows significant π donor effects. This analysis establishes the theoretical basis for the trends in energy and structural properties found for the R3M+ cations and cation−ligand complexes.

Stabilization of Organosilicenium Ions by means of Intramolecular Coordination of O, S, or P Ligands.

For the intramolecular stabilization of silicenium ions (R3Si+), O, S, and P donors as well as the known nitrogen-containing systems (as in 1 a) are suitable. The silyl cations in 1 a-d show a trigonal-bipyramidal structure; dynamic processes can be proved by NMR spectroscopy for 1 c, d. Calculations on model compounds document substantial differences in the bonding relationships and support the structural findings. Furthermore, preliminary experiments with 1 b-d indicate significant differences in reactivity.

Synthesis and crystal and molecular structures of cation-anionic complexes of five-coordinate silicon-containing disiloxane dications with lactamomethyl andN-methylacetamidomethyl C,O-chelating ligands

The reactions of bis(chloromethyl)dichlorosilane withN-trimethylsilyl lactams andN-trimethylsilyl-N-methylacetamide taken in a ratio of 1∶2 followed either by treatment with HgCl2 in the presence of atmospheric moisture or by hydrolysis under a wet atmosphere afforded cation-anionic complexes, which contain disiloxane dications of the general formula [L2SiOSiL2]2+ (L is the lactamomethyl orN-methylacetamidomethyl bidentate ligand) and hexachlorodimercurate, tetrachloromercurate, and hydroxonium trichloride counter-ions, respectively. X-ray diffraction analysis demonstrated that the disiloxane dications in these complexes contain two five-coordinate Si atoms and occur as silicenium ions stabilized through two O→Si coordination bonds. In the case of lactamomethyl ligands, the disiloxane dications exist as diastereomers whose bischelate ligands adopt opposite configurations, whereas whenN-methylacetamidomethyl ligands are present, the bischelate ligands adopt identical configurations. The first example of the presence of a hydroxonium trichloride ion as a counter-ion in the crystal has been found. It consists of the hydroxonium cation, which holds three Cl− anions through strong hydrogen bonds.

Mechanism of RF plasma induced fragmentation of SiCl4 and surface functionalization of polymeric substrates from SiClx species

The generation of extremely reactive species of silylium (silicenium) ions under plasma conditions opens up new ways for surface modification, even of the most inert polymeric surfaces. To control the plasma-induced chemistry and to tailor new molecular surface architecture, the mechanism of formation of the active species of the discharge must be understood. In this work the plasma-induced molecular fragmentation of SiCl4 was studied and the surface functionalization of cellulose paper (CP), polyester (PET), polypropylene (PP), and polytetrafluoroethylene (PTFE) substrates with silicon atom based active species was evaluated. Gas chromatograph mass spectroscopy (GC-MS) and low Energy Electron MS (LEEMS) measurements carried out both on SiCl4 and on the molecular mixture resulting from the recombination of plasma generated species indicate that the most predominant fragments are SiCl3+ cations accompanied by SiCl2+ and SiCl+ species. Survey and high-resolution electron spectroscopy for chemical analysis (ESCA) data collected from plasma functionalized surfaces show significant silicon and oxygen atom content regardless of the nature of the substrates. © 1996 John Wiley & Sons, Inc.

Density functional theory (DFT)/IGLO 29Si NMR study of trialkylsilylated arenium, bromonium, oxonium and nitrilium ions, comparison with experimental data and the question of persistent trialkylsilicenium ions in solution

Density functional theory (DFT)/IGLO 29Si NMR studies show that there is no significant silicenium ion nature in trialkylsilyl substituted arenium, bromonium, oxonium and nitrilium ions. High electrophilicity of trivalent silicon and its affinity for π-donor arene solvents, as well as for oxygen, halogen and nitrogen heteroatoms of solvents or counter ions, is responsible for the inability to observe long-lived trisubstituted silyl cations.

Unimolecular rearrangement of α-silylcarbenium ions. An ab initio study

The unimolecular rearrangements of hydrogen, methyl and phenyl groups at the Si atom in α-silylcarbenium ions have been investigated using an ab initio molecular orbital method. MP2/6–31 + G ∗//HF/6–31G ∗ calculations predict that all three groups migrate from the Si to an adjacent C α with no energy barrier. Thus, the silicenium ion is the only stable species in each potential energy surface. The conformation of the benzylsilicenium ion, (C 6H 5)CH 2−SiH 2 +, indicates that the phenyl ring is significantly bent toward the silyl cationic center in order to interact with the vacant 3p(Si +) orbital. In contrast to MP2 results, Hartree-Fuck calculations (both HF/3–21G ∗ and HF/6–31G ∗ levels) predict small energy barriers for 1,2-migrations of H and Me (1.4 kcal mol −1 for H migration, and 1.5 kcal mol −1 for Me migration, respectively, at the HF/6–31G ∗ level). This difference provides convincing evidence that the incorporation of electron correlation is of particular importance in describing the potential energy surface for the rearrangement of α-silylcarbenium ions to silicenium ions. The results of the calculations have also been applied to the possible rearrangement mechanism of α-chlorosilanes to chlorosilanes, assuming that the experimental conditions are favorable toward the generation of ionic species. Various factors which may govern the migratory aptitudes of various R groups, i.e. (1) activation energies, (2) overall reaction energies and (3) the conformational preference of reactants have been investigated. The calculated activation energy obtained, namely the energy for the generation of the silicenium ion and the C −1 ion from an α-chlorosilane, is consistent with the experimental migratory aptitude in the gas phase observed in mass spectrometers.

The radiation-induced polymerization of dimethylcyclosiloxanes in the liquid state: 3. Copolymerization of D 3 with D 4 and D 4 with D 5, reactivities and interpretation

The radiation-induced copolymerization of octamethylcyclotetrasiloxane (D 4) with hexamethylcyclotrisiloxane (D 3) and octamethylcyclotetrasiloxane with decamethylcyclopentasiloxane (D 5) was used to examine the relative reactivities between the three cyclic monomers, under super-dry conditions. The copolymerization results have shown very similar reactivities of the three monomers. These results were in agreement with previously reported monomer reactivities and homopolymerization activation energies, as well as with the production of similar quantities of cyclic products. A single reaction mechanism for the polymerization of D 3, D 4 and D 5 is proposed for both the propagation and the backbiting reactions. This mechanism assumes that backbiting (giving D 6, D 5 and D 4 but not D 3) involves, as intermediates, silicenium ions solvated by nearby oxygens of their own chain, and propagation by a much more reactive unsolvated (unpaired) silicenium ion in lower concentration.

Intramolecular Base Stabilization of Silicenium Ions: A New Route to Siliconium Ions

The silicon atom is pentacoordinate in cations of salts like 1, which can be prepared, for instance, in a redox reaction of the corresponding silane (with I2, X = I) or transformation with electrophiles like PhCoCl (X = Cl) or Ph3C+BF (X= BF4). These compounds were completely characterized; conductivity titrations confirm that they are ionic.

Hyperconjugative stabilization of silicenium ions: kinetics of hydride abstractions from .beta.-element-substituted silanes

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTHyperconjugative stabilization of silicenium ions: kinetics of hydride abstractions from .beta.-element-substituted silanesNils Basso, Solvig Goers, Eckhard Popowski, and Herbert MayrCite this: J. Am. Chem. Soc. 1993, 115, 14, 6025–6028Publication Date (Print):July 1, 1993Publication History Published online1 May 2002Published inissue 1 July 1993https://doi.org/10.1021/ja00067a018RIGHTS & PERMISSIONSArticle Views171Altmetric-Citations18LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (417 KB) Get e-AlertsSupporting Info (2)»Supporting Information Supporting Information Get e-Alerts

An NMR Study of the Formation of N-Silylacetonitrilium Ions by Using Tetrakis[3,5-bis(trifluoromethyl)phenyl]borate as Counteranion

Abstract The formation of adducts of silicenium ions with acetonitrile (N-silylacetonitrilium ions) was elucidated by means of NMR spectroscopy during the hydride abstraction from hydrosilanes by trityl TFPB in CD2Cl2 in the presence of excess acetonitrile. The results indicate that acetonitrile is not a non-nucleophilic solvent suitable for formation of free silicenium ions, but stabilizes the ions by complexation giving tetracoordinate silicon species in solution.

Kinetics of hydride-transfer reactions from hydrosilanes to carbenium ions. Substituent effects in silicenium ions

Rates of hydride transfer from hydrosilanes HSiR 1 R 2 R 3 with widely varying substitution to para-substituted diarylcarbenium ions have been measured in dichloromethane solution. Generally the reactions follow a second-order rate law, -d[Ar 2 CH + ]/dt=k 2 [Ar 2 CH + ] [HSiR 1 R 2 R 3 ], and k 2 is independent of the degree of ion-pairing and the nature of the counterion (exceptions are reported). The reaction rates are almost independent of solvent polarity