Four-coordinate Silicon Research Articles

Mapping the structural trends in zinc aluminosilicate glasses.

The structure of zinc aluminosilicate glasses with the composition (ZnO)x(Al2O3)y(SiO2)1-x-y, where 0 ≤ x < 1, 0 ≤ y < 1, and x + y < 1, was investigated over a wide composition range by combining neutron and high-energy x-ray diffraction with 27Al magic angle spinning nuclear magnetic resonance spectroscopy. The results were interpreted using an analytical model for the composition-dependent structure in which the zinc ions do not act as network formers. Four-coordinated aluminum atoms were found to be in the majority for all the investigated glasses, with five-coordinated aluminum atoms as the main minority species. Mean Al-O bond distances of 1.764(5) and 1.855(5) Å were obtained for the four- and five-coordinated aluminum atoms, respectively. The coordination environment of zinc was not observed to be invariant. Instead, it is dependent on whether zinc plays a predominantly network-modifying or charge-compensating role and, therefore, varies systematically with the glass composition. The Zn-O coordination number and bond distance were found to be 4.36(9) and 2.00(1) Å, respectively, for the network-modifying role vs 5.96(10) and 2.08(1) Å, respectively, for the charge-compensating role. The more open coordination environment of the charge-compensator is related to an enhanced probability of zinc finding bridging oxygen atoms as nearest-neighbors, reflecting a change in the connectivity of the glass network comprising four-coordinated silicon and aluminum atoms as the alumina content is increased.

Diradicaloid or Zwitterionic Character: The Non-Tetrahedral Unsaturated Compound [Si4 {N(SiMe3 )Dipp}4 ] with a Butterfly-type Si4 Substructure.

The reduction of the tribromoamidosilane {N(SiMe3 )Dipp}SiBr3 (Dipp=2,6-iPr2 C6 H3 ) with potassium graphite or magnesium resulted in the formation of [Si4 {N(SiMe3 )Dipp}4 ] (1), a bicyclo[1.1.0]tetrasilatetraamide. The Si4 motif in 1 does not adopt a tetrahedral substructure and exhibits two three-coordinate and two four-coordinate silicon atoms. The electronic situation on the three-coordinate silicon atoms is rationalized with positive and negative polarization based on EPR analysis, magnetization measurements, and DFT calculations as well as 29 Si CP MAS NMR and multinuclear NMR spectroscopy in solution. Reactivity studies with 1 and radical scavengers confirmed the partial charge separation. Compound 1 reacts with sulfur to give a novel type of silicon sulfur cage compound substituted with an amido ligand, [Si4 S3 {N(SiMe3 )Dipp}4 ] (2).

From Si(II) to Si(IV) and Back: Reversible Intramolecular Carbon-Carbon Bond Activation by an Acyclic Iminosilylene.

Reversibility is fundamental for transition metal catalysis, but equally for main group chemistry and especially low-valent silicon compounds, the interplay between oxidative addition and reductive elimination is key for a potential catalytic cycle. Herein, we report a highly reactive acyclic iminosilylsilylene 1, which readily performs an intramolecular insertion into a C═C bond of its aromatic ligand framework to give silacycloheptatriene (silepin) 2. UV-vis studies of this Si(IV) compound indicated a facile transformation back to Si(II) at elevated temperatures, further supported by density functional theory calculations and experimentally demonstrated by isolation of a silylene-borane adduct 3 following addition of B(C6F5)3. This tendency to undergo reductive elimination was exploited in the investigation of silepin 2 as a synthetic equivalent of silylene in the activation of small molecules. In fact, the first monomeric, four-coordinate silicon carbonate complex 4 was isolated and fully characterized in the reaction with carbon dioxide under mild conditions. Additionally, the exposure of 2 to ethylene or molecular hydrogen gave silirane 5 and Si(IV) dihydride 6, respectively.

Iron(II), Cobalt(II), Nickel(II), and Zinc(II) Silylene Complexes: Reaction of the Silylene [iPrNC(NiPr2 )NiPr]2 Si with FeBr2 , CoBr2 , NiBr2 ⋅MeOCH2 CH2 OMe, ZnCl2 , and ZnBr2.

Reaction of the donor-stabilized silylene [iPrNC(NiPr2 )NiPr]2 Si (1) with FeBr2 , CoBr2 , NiBr2 ⋅MeOCH2 CH2 OMe, ZnCl2 , and ZnBr2 afforded the respective transition-metal silylene complexes 4-8, the formation of which can be described in terms of a Lewis acid/base reaction (4, 5, 7, 8) or a nucleophilic substitution reaction (6). However, the reactivity profile of silylene 1 is not only based on its strong Lewis base character; the different coordination modes of the two guanidinato ligands (4-6 vs. 7 and 8) add an additional reactivity facet. The paramagnetic compounds 4 and 5 and the diamagnetic compounds 6⋅THF, 7, and 8⋅0.5 Et2 O were structurally characterized by single-crystal X-ray diffraction. In addition, compound 6⋅THF was studied by 15 N and 29 Si solid-state NMR spectroscopy, and 7 and 8 were characterized by NMR spectroscopic studies in the solid state (15 N, 29 Si) and in solution (1 H, 13 C, 29 Si). Compounds 4-8 represent very rare examples of FeII , CoII , NiII , and ZnII silylene complexes. Four-coordinate silicon(II) compounds with an SiN3 M skeleton (M=Fe, Co, Ni) and M in the formal oxidation state +2 (4-6) have not yet been reported, and five-coordinate silicon(II) compounds with an SiN4 Zn skeleton (7, 8) are also unprecedented.

Stable Four-Coordinate Guanidinatosilicon(IV) Complexes with SiN3El Skeletons (El = S, Se, Te) and Si=El Double Bonds.

To get information about the reactivity profile of the donor-stabilized guanidinatosilicon(II) complexes 2 and 3, a series of oxidative addition reactions was studied. Treatment of 2 and 3 with S8, Se, or Te afforded the respective four-coordinate silicon(IV) complexes 8-10 and 12-14, which contain an SiN3 El skeleton (El = S, Se, Te) with an Si=El double bond. Treatment of 2 with N2O yielded the dinuclear four-coordinate silicon(IV) complex 11 with an SiN3O skeleton and a central four-membered Si2O2 ring. Compounds 8-14 exist both in the solid state and in solution. They were characterized by elemental analyses, NMR spectroscopic studies in the solid state and in solution, and crystal structure analyses. The reactivity profile of 2 was compared with that of the structurally related bis[N,N'-diisopropylbenzamidinato(-)]silicon(II) (1), which is three-coordinate in the solid state and four-coordinate in solution (1'). In contrast, as shown by state-of-the-art relativistic DFT analyses and experimental studies, silylene 2 is three-coordinate both in the solid state and solution. The three-coordinate species 2 is 9.3 kcal mol(-1) more stable in benzene than the four-coordinate isomer 2'. The reason for this was studied by bonding analyses of 2 and 2', which were compared with those of 1 and 1'. The gas-phase proton affinities of the relevant species in solution (1' and 2) amount to 288.8 and 273.8 kcal mol(-1), respectively.

Formation of six-coordinated silicon in calcium phosphosilicate xerogels assisted by polyols at low temperature and pressure

Abstract Silicon is usually found to be four-coordinated when neighbored with oxygen. Six-coordinated silicon is only seen in samples at very specific composition or made at high temperature/pressure. In this study, we managed to synthesize calcium phosphosilicate xerogels containing six-coordinated silicon with the help of polyols by sol–gel method, without the need of treatment at high temperature or high pressure. Both phosphorus precursors and polyols were found to be essential for the formation of six-coordinated silicon species; in the absence of either species, only normal four-coordinated silicon was observed under otherwise identical conditions. Samples containing six-coordinated silicon sites were found to release silicon species faster than those without six-coordinated silicon sites upon dissolved in water, suggesting that six-coordinated silicon species have higher reactivity toward hydrolysis.

Synthesis, structure and reactivity of a donor-stabilised silylene with a bulky bidentate benzamidinato ligand.

The novel donor-stabilised silylene was prepared in a four-step synthesis, starting from bis(2,6-diisopropylphenyl)carbodiimide (Dipp-N[double bond, length as m-dash]C[double bond, length as m-dash]N-Dipp), and its reactivity was studied in a series of oxidative addition reactions and a nucleophilic substitution reaction. The three-coordinate silicon(ii) complex contains the bulky bidentate amidinato ligand Dipp-NC(Ph)N-Dipp(-) and a dimethylamido ligand. Treatment of with N2O afforded the dinuclear five-coordinate silicon(iv) complex (SiO2N3 skeletons), and the reaction with S8 yielded the dinuclear four-coordinate silicon(iv) complex (SiS2N2 skeletons). Treatment of with Se and Te afforded the respective four-coordinate silicon(iv) complexes (SiSeN3 skeleton) and (SiTeN3 skeleton), which contain an Si[double bond, length as m-dash]Se and Si[double bond, length as m-dash]Te double bond, respectively. The reaction of with the silyl azide Me3SiN3 yielded the four-coordinate silicon(iv) complex (SiN4 skeleton) with an Si[double bond, length as m-dash]N double bond, whereas the reaction with the alkyl azide PhSCH2N3 gave the four-coordinate silicon(iv) complex (SiSN3 skeleton), the first silicon(iv) complex with an unsubstituted methyleneamido ligand. The reaction of with [Fe(CO)5] afforded the four-coordinate silicon(ii) complex (SiFeN3 skeleton) with an Si-Fe bond. Compounds (and the precursors and (five-coordinate silicon(iv) complexes with an SiCl3N2 and SiCl2N3 skeleton, respectively) in the synthesis of ) were characterised by elemental analyses, crystal structure analyses and multinuclear NMR spectroscopic studies in the solid state and in solution.

Quantum-chemical calculations of NMR chemical shifts of organic molecules: X. Dynamic equilibrium effects in the 29Si NMR spectra of silacycloalkanes

Dynamic equilibria related to change of the coordination number of the silicon atom in bis[N-(dimethylamino) imidato-N′,O]silacycloalkanes in solution were studied by theoretical calculations and experimental measurement of the 29Si NMR chemical shifts. Silacyclopentane derivatives were found to exist in solution as mixtures of species with six- and five-coordinate silicon atoms, and silacyclohexane derivatives, as mixtures of five- and four-coordinate silicon compounds.

Quantum-chemical calculations of NMR chemical shifts of organic molecules: VI. Accuracy of DFT calculations of 29Si chemical shifts of four-coordinate silicon compounds

Systematic analysis of factors affecting the accuracy of DFT calculations of 29Si NMR chemical shifts in four-coordinate silicon compounds showed that the best agreement with the experimental values is attained using B1PW91 and PBE0 functionals in combination with the TZP basis set. In calculations of 29Si chemical shifts by quantum-chemical methods particular attention should be given to the contribution of relativistic spin-orbit interaction and conformational equilibrium.

Stable Silaimines with Three- and Four-Coordinate Silicon Atoms

The reactions of silylenes with organic azides are quite diverse, depending on the substituents of the silylene center and on the nature of the azide employed. Elusive silaimine with three-coordinate silicon atom L(1)SiN(2,6-Triip(2)-C(6)H(3)) (5) {L(1) = CH[(C═CH(2))(CMe)(2,6-iPr(2)C(6)H(3)N)(2)] and Triip = 2,4,6-triisopropylphenyl} was synthesized by treatment of the silylene L(1)Si (1) with a sterically demanding 2,6-bis(2,4,6-triisopropylphenyl)phenyl azide (2,6-Triip(2)C(6)H(3)N(3)). The reaction of Lewis base-stabilized dichlorosilylene L(2)SiCl(2) (2) {L(2) = 1,3-bis(2,6-iPr(2)C(6)H(3))imidazol-2-ylidene} with Ph(3)SiN(3) afforded four-coordinate silaimine L(2)(Cl(2))SiNSiPh(3) (6). Treatment of 2,6-Triip(2)C(6)H(3)N(3) with L(3)SiCl (3) (L(3) = PhC(NtBu)(2)) yielded silaimine L(3)(Cl)SiN(2,6-Triip(2)-C(6)H(3)) (7) possessing a four-coordinate silicon atom. The reactions of L(3)SiN(SiMe(3))(2) (4) with adamantyl and trimethylsilyl azide furnished silaimine compounds with a four-coordinate silicon atom L(3)(N(Ad)SiMe(3))SiN(SiMe(3)) (8) (Ad = adamantyl) and L(3)(N(SiMe(3))(2))SiN(SiMe(3)) (9). Compound 8 was formed by migration of one of the SiMe(3) groups. Compounds 5-9 are stable under inert atmosphere and were characterized by elemental analysis, NMR spectroscopy, and single-crystal X-ray studies.

ChromiumSilicon Multiple Bonds: The Chemistry of Terminal N‐Heterocyclic‐Carbene‐Stabilized Halosilylidyne Ligands

An efficient method for the synthesis of the first N-heterocyclic carbene (NHC)-stabilized halosilylidyne complexes is reported that starts from SiBr(4). In the first step, SiBr(4) was treated with one equivalent of the N-heterocyclic carbene 1,3-bis[2,6-bis(isopropyl)phenyl]imidazolidin-2-ylidene (SIdipp) to give the 4,5-dihydroimidazolium salt [SiBr(3)(SIdipp)]Br (1-Br), which then was reduced with potassium graphite to afford the silicon(II) dibromide-NHC adduct SiBr(2)(SIdipp)(2-Br) in good yields. Heating 2-Br with Li[CpCr(CO)(3)] afforded the complex [Cp(CO)(2)Cr=SiBr(SIdipp)] (3-Br) upon elimination of CO. Complex 3-Br features a trigonal-planar-coordinated silicon center and a very short CrSi double bond. Similarly, the reaction of SiCl(2)(SIdipp) (2-Cl) with Li[CpCr(CO)(3)] gave the analogous chloro derivative [Cp(CO)(2)Cr=SiCl(SIdipp)] (3-Cl). Complex 3-Br undergoes an NHC exchange with 1,3-dihydro-4,5-dimethyl-1,3-bis(isopropyl)-2H-imidazol-2-ylidene (IMe(2)iPr(2)) to give the complex [Cp(CO)(2)CrSiBr(IMe(2)iPr(2))(2)] (4-Br). Compound 4-Br features a distorted-tetrahedral four-coordinate silicon center. Bromide abstraction occurs readily from 4-Br with Li[B(C(6)F(5))(4)] to give the putative silylidene complex salt [Cp(CO)(2)Cr=Si(IMe(2)iPr(2))(2)][B(C(6)F(5))(4)], which irreversibly dimerizes by means of an Si-promoted electrophilic activation of one carbonyl oxygen atom to yield the dinuclear siloxycarbyne complex [Cp(CO)Cr{(μ-CO)Si(IMe(2)iPr(2))(2)}(2-)Cr(CO)Cp][B(C(6)F(5))(4)](2) (5). All compounds were fully characterized, and the molecular structures of 2-Br-5-Br were determined by single-crystal X-ray diffraction. DFT calculations of 3-Br and 3-Cl and their carbene dissociation products [Cp(CO)(2)Cr=Si-X] (X=Cl, Br) were carried out, and the electronic structures of 3-Br, 3-Cl and [Cp(CO)(2)Cr=Si-X] were analyzed by the natural bond orbital method in combination with natural resonance theory.

Si═X Multiple Bonding with Four-Coordinate Silicon? Insights into the Nature of the Si═O and Si═S Double Bonds in Stable Silanoic Esters and Related Thioesters: A Combined NMR Spectroscopic and Computational Study

The electronic structures and nature of silicon-chalcogen double bonds Si=X (X = O, S) with four-coordinate silicon in the unique silanoic silylester 2 and silanoic thioester 3 have been investigated for the first time, by (29)Si solid state NMR measurements and detailed DFT and ab initio calculations. (29)Si solid state NMR spectroscopy of the precursor silylene 1 was also carried out. The experimental and computational study of 2 and 3, which was also supported by a detailed computational study of smaller model systems with Si=O and Si=S bonds, provides a deeper understanding of the isotropic and tensor components of their NMR chemical shifts. The general agreement between the experimental NMR spectra and the calculations strongly support our previous NMR assignment deduced from experiment. The calculations revealed that in 2 delta((29)Si(=O))(iso) is shifted upfield relative to H(2)Si=O by as much as 175 ppm; the substituents are responsible for ca. 100 ppm of this shift, while the remaining upfield shift is caused by change in the coordination number from three to four at the Si=O moiety. The change in coordination number leads to a nearly cylindrical symmetry in the plane which is perpendicular to the Si=O molecular axis (delta(11) approximately delta(22)), in contrast to the significant anisotropy found in this plane in typical doubly bonded compounds. The change in r(Si=O) or in the degree of pyramidality at the Si=O center which accompanies the change in coordination number has practically no effect on the chemical shift. delta((29)Si(=S))(iso) in 3 is shifted downfield significantly relative to that in 2, and a similar trend is found in smaller models with Si=S vs those with Si=O subunits. This downfield shift can be explained by the smaller sigma-pi* energy difference in the Si=S bond, relative to that of the Si=O bond. The NMR measurements of 2 and 3 having a four-coordinate silicon-chalcogen moiety, and the calculations of their tensor components, their bond polarities, and their Wiberg bond indices revealed that the Si=X moieties in both 2 and 3 have a significant pi(Si=X) character; yet, in both molecules there is a substantial contribution from a zwitterionic Si(+)-X(-) resonance structure, which is more pronounced in 2.

Steering S−H and N−H Bond Activation by a Stable N-Heterocyclic Silylene: Different Addition of H2S, NH3, and Organoamines on a Silicon(II) Ligand versus Its Si(II)→Ni(CO)3 Complex

The strikingly different behavior of the ylide-like, N-heterocyclic silylene LSi: (5: L = CH[(C horizontal lineCH(2))CMe(NAr)(2)]; Ar = 2,6-(i)PrC(6)H(3)) versus its LSi-->Ni(CO)(3) complex 13 to activate E-H bonds (E = S, N) of small molecules is reported. Remarkably, conversion of 5 with hydrogen sulfide leads exclusively to the first isolable silathioformamide, L'Si( horizontal lineS)H (16: L' = CH[C(Me)NAr](2); Ar = 2,6-(i)PrC(6)H(3)) with a donor-supported Si horizontal lineS double bond and four-coordinate silicon. The latter result demonstrates the unusual ambivalent reactivity of 5 by combining two modes of reactivity involving S-H bond activation and subsequent 1,4- and 1,1-addition, respectively. In addition, 5 can serve as a ligand with well-balanced sigma-donor and pi-acceptor capabilities toward transition metals. This has been demonstrated by the isolable [Ni(0)(arene)] complexes 12a-e (arene = Me(n)C(6)H(6-n), n = 0-3), which are ideal precursors for the formation of the corresponding Ni(CO)(3) complex 13. The latter activates a S-H bond in hydrogen sulfide, too, but the presence of the Ni(CO)(3) moiety governs the formation of the complex 17, bearing an unprecedented beta-diketiminate silicon(II) thiol ligand: L'Si(SH): (L' = CH[C(Me)NAr](2); Ar = 2,6-(i)PrC(6)H(3)). Likewise, the Si(II)-->Ni(CO)(3) coordination in 13 steers exclusively 1,4-addition of ammonia, isopropylamine, and phenylhydrazine onto the silylene ligand 5, leading to the corresponding beta-diketiminate silicon(II) amide or hydrazide complexes L'Si(NHR)-->Ni(CO)(3) (23a-c: R = H, (i)Pr, N(H)Ph). IR measurements reveal that the carbonyl stretching frequencies of the Ni(CO)(3) moiety in 23a-c are shifted to even lower wavenumbers in comparison to those of NHCs or phosphines. In other words, the beta-diketiminate silicon(II) amide ligands in 23a-c represent the strongest donors in the series of N-heterocyclic silylenes reported as yet.

Synthesis and structure of silicon-containing N-methyland N-benzoylamides of diisopropylphosphoric acid

Diisopropyl N-benzoyl-N-(trimethylsilyl)phosphoramidate reacts with ClCH2SiMe2Cl under mild conditions to form diisopropyl N-benzoyl-N-[(chlorodimethylsilyl)methyl]phosphoramidate (III). Diisopropyl N-methyl-N-(trimethylsilyl)phosphoramidate with ClCH2SiMe2Cl affords an N-transsilylation product which does not rearrange into diisopropyl N-[(chlorodimethylsilyl)methyl]-N-methylphosphoramidate (XV) even under severe conditions (4 h, 130°C). Compound XV was prepared by the reaction of diisopropyl phosphorochloridate with N-[(methoxydimethylsilyl)methyl]-N-methylamine followed by treatment of diisopropyl N-[(methoxydimethylsilyl)methyl]-N-methylphosphoramidate with boron trichloride. Analysis of experimental and calculated 29Si chemical shifts points to a five-coordinate silicon atom in compound III and a fourcoordinate silicon atom in compound XV. According to B3LYP calculations with due regard to solvent effects, compound III is an isomer with a C=O→Si bond. By variation of substituents at silicon, phosphorus, and carbonyl carbon atoms, chelate structures with either C=O→Si or P=O→Si dative bonds can be obtained.

Stabilization of Planar Four-Coordinate Boron, Carbon, and Silicon Atoms in Borane Clusters: A Quantum-Chemical Study

Stability of a series of neutral and charged borane clusters containing planar four-coordinate boron centres was predicted by the density functional theory DFT (B3LYP/6-311+G**). The possibility of the modification of these systems to obtain new stable compounds with planar four-coordinate carbon and silicon atoms was examined. The calculations show that the substituted systems should be more stable than the isoelectronic boron analogs.

Local structure of six-coordinated silicon in Li2O-SiO2-P2O5 glasses by transmission XAFS experiments

Six-coordinated silicon [6]Si was observed by Si and P K-edge XAFS measurements in xLi2O-ySiO2-(100-y)P2O5 glasses (x = 25 and 33, y = 40, 45, 50, 55, and 60). Si K-edge EXAFS spectra of the glasses indicated the co-existence of [6]Si and four-coordinated silicon [4]Si. The proportion of octahedral [SiO66-] to [6]Si depends upon the alkali content and the [SiO66-]/[SiO44-] tetrahedral ratios were estimated by Si K-edge XANES spectra. The interatomic distances were 1.88–1.90 Å for Si-O and 1.55–1.61 Å for P-O and the amounts of [SiO66-] were 11.8–46.9%.

Reactivity of N-(chlorodimethylgermyl)methyl and N-(chlorodimethylsilyl)methyl derivatives of lactams and amides toward Grignard reagents

N-[(Chlorodimethylgermyl)methyl]lactams and -amides containing a five-coordinate germanium atom react with Grignard reagents chemoselectively by the Ge-Cl bond to form four-coordinate germanium compounds. The method of competitive reactions was used to establish that respective five-coordinate germanium and silicon compounds are almost equally reactive toward Grignard reagents but much more reactive than model four-coordinate germanium and silicon compounds.

Steady-State and Time-Resolved Spectroscopic Characteristics of Novel Silicon-Containing Organopolymers

The spectroscopic characteristics of novel π-conjugated polymers containing four-coordinated silicon, acetylene groups and either 1,4-biphenylene or 2,7-fluorene in the main chain were investigated by steady-state and picosecond laser spectroscopy. The spectral features of absorption, fluorescence excitation spectra, fluorescence lifetime, and fluorescence polarization were explained by the existence of two kinds of inhomogeneously broadened electronic states formed in the disordered polymeric chain. The dynamics of photoinduced absorption was measured in the 400–900 nm spectral range with picosecond time resolution. The long-wavelength band with λmax ∼ 710 nm was ascribed to excited-state absorption from higher-lying electronic states created in short polymeric segments with essential conformational distortion of the subunits. The short-wavelength band with λmax ∼ 580 nm and a shoulder at 500 nm was interpreted as photoinduced absorption from a lower-lying state arisen in more planar, longer π-conjugated segments populated via direct excitation and energy migration between disordered segments of the polymeric chain. For the fluorene-containing polymer, the smaller Stokes shift and the greater degree of fluorescence polarization are consistent with more extensive electron delocalization along the backbone.

Intramolecularly donor-stabilized silenes: Part 3. Synthesis and structure of 1-[2,6-bis(dimethylaminomethyl)phenyl]-1,2,2-tris(trimethylsilyl)silene

1-[2,6-Bis(dimethylaminomethyl)phenyl]-1,2,2-tris(trimethylsilyl)silene ( 9) was prepared by treatment of (dichloromethyl)tris(trimethylsilyl)silane ( 3) with 2,6-bis(dimethylaminomethyl)phenyllithium (molar ratio 1:2). 9 is a crystalline compound, indefinitely stable at room temperature. The X-ray structural analysis of 9 characterized the compound as an intramolecularly amine-stabilized silene with four-coordinate silicon, i.e. only one of the two amine groups of the bis(dimethylaminomethyl)phenyl substituent is coordinated to the electrophilic silicon center (SiN distance 2.035(3) Å). Thus, the silene silicon atom is pyramidalized (sum of angles 348.49°), whereas the configuration at the silene carbon atom is trigonal planar (sum of angles 359.9°). The SiC bond length was found to be 1.759(3) Å. Reaction of 9 with water or methanol led to the addition of these nucleophiles to the SiC bond, producing the silanol 12 and the methoxysilane 13, respectively. The reaction pathway leading to 9 is discussed.

Main-group-element calix[4]arenes: variable coordination and conformational isomerism at phosphorus and silicon.

Treatment of calix[4]arene (1b) with trichloromethylsilane yields two conformers of calix[4]SiMe(OH), the cone 2b-C and the partial cone 2b-PC. These are isolated and structurally characterized, and their thermodynamic activation parameters are determined in solution [Ea = 117(3) kJ/mol, delta H = 5(4) kJ/mol]. Similar parameters are found for the p-tert-butylcalix[4]arene analogues 2a-C and 2a-PC. Deprotonation of 2b with butyllithium yields calix[4]SiMe(OLi) (8b). The structure of 8b is solvent dependent: 8b contains a five-coordinate silicon in THF and a four-coordinate silicon in benzene. Similar behavior is found for the p-tert-butylcalix[4]arene analogues. The five-coordinate phosphorus analogue of the anion in 8, p-tert-butylcalix[4]PMe (11a), is synthesized from the phosphonium triflate salt p-tert-butylcalix[4]PMe(OH)OTf (10a(OTf)) via treatment with butyllithium. The structure of 11a shows the geometry around phosphorus to be very close to a pure trigonal bipyramid. The X-ray structure of 10a(OTf) cannot be obtained, but its iodide analogue 10a(T) is synthesized and structurally characterized. The cation in 10a(I) adopts the partial cone conformation in the solid state, similar to 2b-PC. Treatment of 8b with methyl trifluoromethanesulfonate yields the methyl phenyl ether calix[4]SiMe(OMe) (3b). This species and its p-tert-butyl analogue 3a are structurally characterized. Both adopt the partial cone conformation. The SiMe group in 3b can be removed via treatment with fluoride to yield the monomethylated calix[4]arene 4b in 50-60% overall yield based on 1b.