A quantum chemical study of silsesquioxanes: H8Si8O12, Me8Si8O12, H@Me8Si8O12, He@Me8Si8O 12 + , and He@Me8Si8O12

Quantum chemical PBE0 and B3LYP/cc-pVTZ, PBE0, B3LYP, RHF and MP2/6-31G(d,p) methods are employed to calculate the structural parameters of octa(silsesquioxane) H₈Si₈O₁₂ and octa(methylsilsesquioxane) Me₈Si₈O₁₂. These molecules and complexes H@Me₈Si₈O₁₂, He@Me₈Si₈O ₁₂ ⁺ , and He@Me₈Si₈O₁₂ have highly symmetric (O _h ) equilibrium configurations. With the use of the PBE0 method and a cc-pVTZ multicenter basis set common for the complex and its components coincidence is achieved between the calculated polarizability of a free He atom and the experimental value of 0.21 ų and the polarizability depression of 0.17 ų was found for He@Me₈Si₈O₁₂. In order to avoid the false conclusion about molecular symmetry the calculations of the structure of silsesquioxanes must be performed with sufficiently high accuracy (Int = ultrafine and Opt = tight in the use of the GAUSSIAN program).     

Monosubstitution von Octa(hydridosilasesquioxan) H8Si8O12 zu R′H7Si8O12 mittels Hydrosilylierung Kurzmitteilung

Monosubstitution of Octa(hydridosilasesquioxane) H₈Si₈O₁₂ to R′H₇Si₈O₁₂ by Hydrosilylation Hydrosilylation of hex-1-ene and of styrene by octa(hydridosilasesquioxanes) catalyzed by hexachloroplatinic acid leads to the first monosubstituted octasilasesquioxane R′H₇Si₈O₁₂ molecules.     

A quantum chemical study of the structure of dodecasilsequioxane H12Si12O18

Quantum chemical B3LYP/cc-pVTZ, PBE0/cc-pVTZ, and MP2(full)/6-311G(d,p) methods are used to calculate the structural parameters of dodecasilsequioxane H₁₂Si₁₂O₁₈ and the H₁₂Si₁₂O ₁₈ ⁺ cation. According to DFT/cc-pVTZ calculations the energy of H₁₂Si₁₂O₁₈ (D _6h ) is 1.3–1.7 kcal/mol higher than the energy of H₁₂Si₁₂O₁₈ (D _2d ). A reduction of the basis set results in a greater energy difference of H₁₂Si₁₂O₁₈ isomers. For the cation ² B _2u and ² B ₁ electronic states are obtained, which correspond to symmetric equilibrium structures H₁₂Si₁₂O ₁₈ ⁺ (D _6h ) and (D ₂) respectively. For the He@H₁₂Si₁₂O₁₈ endocomplex the D _2d symmetry is obtained; for He₂@H₁₂Si₁₂O₁₈ the D _2h symmetry; and for H₂@H₁₂Si₁₂O₁₈ the D _6h symmetry.     

Normal coordinate analysis of H8Si8O12

Normal coordinate analysis of the fundamental vibrations of H₈Si₈O₁₂ has been carried out. Because of the octahedral symmetry, the 78 vibrational degrees of freedom lead to 33 different vibrations, six of which are infrared active, 13 are Raman active and 14 are inactive. From the internal coordinates one gets 116 symmetry coordinates. We describe a straightforward method for determining the internal symmetry coordinates of any molecular system. Internal coordinates, symmetry force constants, the full set of orthonormal symmetry coordinates as well as the 38 redundant orthonormal symmetry coordinates of H₈Si₈O₁₂ are tabulated. The potential energy distribution analysis shows that most of the fundamental vibrations can be very well interpreted in terms of the internal vibrations ν(SiH), ν(SiO), δ(SiH), δ(OSiO) and δ(SiOSi) which makes it easy to compare them with vibrations observed in other silsesquioxanes and similar silicon compounds.     

Why is the Si-X stretching frequency of X8Si8O12 (X = H,CH3, Cl) much higher than that of XSi(OSiME3)3?

The totally symmetric stretching frequencies u₅(Si-X) of the octasilases-quioxanes X₈Si₈O₁₂ are distinctively larger than those of XSi(OSiMe₃)₃ for X = H, CH₃, Cl whereas u₅(Si-H) of H₈Si₈O₁₂ and of H₁₀Si₁₀O₁₅ differ very little. Inspection of the experimental data and molecular orbital calculations of the EHMO-ASED type show that this striking discrepancy is caused by the differences in the X-Si-O-Si conformations. The cage structure of X₂Si₈O₁₂ requires the anti X-Si-O-Si conformation whereas the syn conformation is the stable one in XSi(OSiMe₃)₃. The Si-H bond order decreases from anti to syn caused by a decreasing interaction of the H-ls orbital with the Si-O-pπ type orbitals.     

Silsesquioxane chemistry, 14.[1] A highly crowded disiloxane: Synthesis and structure of the bis(silsesquioxanyl) ether (Cy7Si8O12)2O

The bis(silsesquioxanyl) ether derivative (Cy₇Si₈O₁₂)₂O (4, Cy =c-C₆H₁₁) has been prepared for the first time by controlled hydrolysis of Cy₇Si₈O₁₂Cl (2) in the presence of triethylamine and structurally characterized by X-ray diffraction.     

Selective Hydrosilylation of Alkynes with Octaspherosilicate (HSiMe2O)8Si8O12

Comprehensive studies on platinum‐catalyzed hydrosilylation of a wide range of terminal and internal alkynes with spherosilicate (HSiMe₂O)₈Si₈O₁₂ ( 1 a ) were performed. The influence of the reaction parameters and the types of reagents and catalysts on the efficiency of the process, which enabled the creation of a versatile and selective method to synthesize olefin octafunctionalized octaspherosilicates, was studied in detail. Within this work, twenty novel 1,2‐(E)‐disubstituted and 1,1,2‐(E)‐trisubstituted alkenyl‐octaspherosilicates ( 3 a – m , 6 n – t ) were selectively obtained with high yields, and fully characterized (¹H, ¹³C, ²⁹Si NMR, FTIR, MALDI TOF or TOF MS ES⁺ analysis). Moreover, the molecular structure of the compound (Me₃Si(H)C=C(H)SiMe₂O)₈Si₈O₁₂ ( 3 a ) was determined by X‐ray crystallography for the first time. The developed procedures are the first that allow selective hydrosilylation of terminal silyl, germyl, aryl, and alkyl alkynes with 1 a , as well as the direct introduction of sixteen functional groups into the 1 a structure by the hydrosilylation of internal alkynes. This method constituted a powerful tool for the synthesis of hyperbranched compounds with a Si?O based cubic core. The resulting products, owing to their unique structure and physicochemical properties, are considered novel, multifunctional, hybrid, and nanometric building blocks, intended for the synthesis of star‐shaped molecules or macromolecules, as well as nanofillers and polymer modifiers. In the presented syntheses, commercially available reagents and catalysts were used, so these methods can be easily repeated, rapidly scaled up, and widely applied.     

Spectroscopically tracing the reactions of octahydridosilsesquioxane (H8Si8O12) with organic molecules

This article studies the reactions and mechanisms of H₈Si₈O₁₂ (T₈H₈) molecules with n-propanol, acetone, allyl alcohol, n-butylamine, allylamine, acetic acid, and 1-octene in air, at room temperature, and without catalysts. The reaction between T₈H₈ and n-propanol involves both the highly polarized Si O and Si H bonds and results in cage breakage and forming Q⁴ and Q³ structures with  OC₃H₇ in the reaction product. T₈H₈ also reacts with acetone, and the resultant product possesses Si OCH(CH₃)₂. Allyl alcohol is less reactive to cause T₈H₈ decomposition, and the resultant product contains Si OCH₂CHCH₂ and Si OCH₂(CH₂)₃CHCH₂. However, it is found that basically T₈H₈ does not react with acetic acid and 1-octene. In the reactions of T₈H₈ with n-butylamine and allylamine, the resultant products contain Si NH(CH₂)₃CH₃ and Si NHCH₂CHCH₂, respectively. For the reaction with T₈H₈, allylamine is less active than n-butylamine. Possible mechanisms for the T₈H₈ reactions are discussed.     

[{μ‐Cy8Si8O13}2Ca(DME)Ca(THF)2] – The First Metallasilsesquioxane Derivative of a Heavier Alkaline Earth Metal

[{μ‐Cy₈Si₈O₁₃}₂Ca(DME)Ca(THF)₂] ( 2 ), the first metallasilsesquioxane derivative of a heavier alkaline earth metal, has been prepared by a reaction of Cy₇Si₇O₉(OH)₃ ( 1 ) with metallic Ca in liquid ammonia / THF followed by recrystallization from DME. In the course of the reaction ligand rearrangement under formation of the (Cy₈Si₈O₁₃)^? dianion takes place. In the dinuclear calcium complex 2 the anionic silsesquioxane cages act as bridging ligands. The Ca²⁺ ions are unsymmetrically coordinated by THF and DME molecules.     

Silsesquioxane Chemistry. 12 [1] Preparation and Complexation of a Novel Silsesquioxanyl Phosphine Ligand

The reaction of the monofunctional closo‐silsesquioxane silanol derivative Cy₇Si₈O₁₂OH ( 3 , Cy = c‐C₆H₁₁) with tris(dimethylamino)phosphine afforded the novel silsesquioxanyl phosphine ligand Cy₇Si₈O₁₂OP(NMe₂)₂ ( 4 ) in virtually quantitative yield. The complexes [Cy₇Si₈O₁₂OP(NMe₂)₂]₂PtCl₂ ( 5 ) and [Cy₇Si₈O₁₂OP(NMe₂)₂]₂Mo(CO)₄ ( 6 ) were obtained in excellent yields upon treatment of 4 with (COD)PtCl₂ (COD = 1, 5‐cyclooctadiene) and (NBD)Mo(CO)₄ (NBD = norbornadiene), respectively. An attempted preparation of the bis(silsesquioxanyl)phosphine (Cy₇Si₈O₁₂O)₂P(NMe₂) led to the formation of the known disiloxane derivative (Cy₇Si₈O₁₂)₂O ( 7 ), instead.     

Silyl‐functionalized Silsesquioxanes: New Building Blocks for Larger Si‐O‐Assemblies,including the First Si‐Si‐Bonded Silsesquioxanes

Novel silyl‐functionalized silsesquioxane building blocks have been prepared by treatment of Cy₇Si₇O₉(OH)₃ ( 1 , Cy = c‐C₆H₁₁) with hexachlorodisilane or hexachlorodisiloxane, respectively, in the presence of triethylamine. Reactions in a 1:1 molar ratio afforded the trichlorosilyl‐functionalized silsesquioxane derivatives Cy₇Si₈O₁₂SiCl₃ ( 2 ) and Cy₇Si₈O₁₂OSiCl₃ ( 3 ). Related bis(silsesquioxanes), (Cy₇Si₈O₁₂)₂ ( 4 ) and (Cy₇Si₈O₁₂)₂O ( 5 ) are accessible in a similar manner by employing a 2:1 molar ratio of the reactands. Compound 1 also served as a starting material in the preparation of the partially closed silsesquioxane cages Cy₇Si₇O₁₁(OH)SiMe₂ ( 6 ) and Cy₇Si₇O₁₁(OH)Si(OEt)₂ ( 7 ), while the related condensation product Cy₇Si₇O₁₀(OSiMe₃) ( 9 ) was made by AlCl₃‐catalyzed elimination of water from Cy₇Si₇O₉(OH)₂OSiMe₃ ( 8 ). The molecular structure of 9 was determined by X‐ray diffraction.     

[TMPA]4[Si8O20] · 34 H2O and [DDBO]4[Si8O20] · 32 H2O are heteronetwork clathrates with 1,1,4,4-tetramethylpiperazinium (TMPA) and 1,4-dimethyl-1,4-diazoniabicyclo[2.2.2]octane (DDBO) guest cations

[TMPA]₄[Si₈O₂₀] · 34 H₂O ( 1 ) and [DDBO]₄[Si₈O₂₀] · 32 H₂O ( 2 ) have been prepared by crystallization from aqueous solutions of the respective quaternary alkylammonium hydroxide and SiO₂. The crystal structures have been determined by single-crystal X-ray diffraction. 1 : Monoclinic, a = 16.056(2), b = 22.086(6), c = 22.701(2) Å, β = 90.57(1)° (T = 210 K), space group C2/c, Z = 4. 2 : Monoclinic, a = 14.828(9), b = 20.201(7), c = 15.519(5) Å, β = 124.13(4)° (T = 255 K), space group P2₁/c, Z = 2. The polyhydrates are structurally related host-guest compounds with three-dimensional host frameworks composed of oligomeric [Si₈O₂₀]^8? anions and H₂O molecules which are linked via hydrogen bonds. The silicate anions possess a cube-shaped double four-ring structure and a characteristic local environment formed by 24 H₂O molecules and six cations (TMPA, [C₈H₂₀N₂]²⁺, or DDBO, [C₈H₁₈N₂]²⁺). The cations themselves reside as guest species in large, irregular, cage-like voids. Studies employing ²⁹Si NMR spectroscopy and the trimethylsilylation method have revealed that the saturated aqueous solutions of 1 and 2 contain high proportions of double four-ring silicate anions. Such anions are also abundant species in the saturated solution of the heteronetwork clathrate [DMPI]₆[Si₈O₁₈(OH)₂] · 48.5 H₂O ( 3 ) with 1,1-dimethylpiperidinium (DMPI, [C₇H₁₆N]⁺) guest cations.     

Infrared and Raman spectra of octa(hydridosilasesquioxanes)

We report the NIR FT-Raman spectrum of H₈Si₈O₁₂ and the fundamental IR modes of D₈Si₈O₁₂. The fundamental modes are assigned according to a normal coordinate analysis and in terms of internal vibrations.     

The diphyllosilicate Rb2(VO)2[Si8O19]

Dirubidium divanadyl phyllooctasilicate, Rb₂(VO)₂[Si₈O₁₉], is the first known anhydrous diphyllosilicate containing V^IV. The structure consists of silicate double layers which are separated by [V₂O₈]⁸⁻ dimers and is related to that of the compounds A₂Cu₂[Si₈O₁₉] (A = Rb or Cs), although the title compound crystallizes in a noncentrosymmetric orthorhombic space group. The silicate double layers contain four tetrahedrally coordinated Si sites in general positions and 12 O sites, nine in general positions and the other three on mirror planes. The vanadyl dimers have two square‐pyramidally coordinated V sites (site symmetry m). There are two different 10‐ and 12‐fold coordinated Rb sites with site symmetry m, one of which is a split position located between the dimers in the interlayer space, while the other is in a channel within the silicate layer.     

Erstmaliger Einbau eines späten Übergangsmetalls in ein Silsesquioxan‐Gerüst: Synthese und strukturelle Charakterisierung von Cy7Si7O12Fe(tmeda)

Silsesquioxane Chemistry. VIII First Incorporation of a Late Transition Metal into a Silsesquioxane Cage: Synthesis and Structural Characterization of Cy₇Si₇O₁₂Fe(tmeda) Cy₇Si₇O₁₂Fe(tmeda) ( 3 , Cy = cyclohexyl, tmeda = N,N,N′,N′‐tetramethylethylenediamine) has been prepared by reacting in situ generated Cy₇Si₇O₉(OLi)₃ ( 2 ) with anhydrous FeCl₃ followed by treatment with tmeda. Pale yellow crystalline 3 represents the first example of a metallasilsesquioxane in which a late transition metal is incorporated into a silsesquioxane cage. The compound crystallizes as discrete monomers in which the coordination sphere around Fe is saturated by addition of a chelating tmeda ligand ( 3 : monoclinic space group Cc, Z = 4, lattice dimensions at –100 °C: a = 2469.0(2), b = 1529.9(2), c = 1010.4(2) pm, β = 116.68(1)°, R = 0.052).     

Infrared and Raman spectra of the trimethylsilyl ester of the cubic octameric silicate, (Si8O20)[Si(CH3)3]8, at very high pressures

Infrared and FT-Raman spectra have been recorded, at ambient temperature, for the solid trimethylsilyl ester of the cubic octameric silicate, (Si₈O₂₀)[SiCH₃)₃]₈ (Q₈M₈), at various pressures up to ∼50 kbar with the aid of a diamond-anvil cell. The vibrational spectroscopic data indicate that there is pressure-induced structural change occurring at ∼20 kbar. Moreover, the central Si₈O₂₀ framework (D_4h symmetry) is remarkably resistant to applied pressure, as are other high-symmetry systems (e.g., C₆₀, I_h symmetry), while the trimethylsilyl side-chains are more easily compressed. The pressure sensitivity (dν/dp), 1.35 cm⁻¹/kbar, of the ν(C–H) mode of the trimethylsilyl groups in the low-pressure phase is by far the greatest.     

Silicon buckyballs versus prismanes: Influence of spatial confinement on the structural properties and optical spectra of the Si18H12 and Si19H12 clusters

The genetic algorithm is combined with the density functional theory to predict how the cylindrical spatial confinement affects the structural characteristics and optical adsorption spectra of the low‐energy Si₁₈H₁₂ and Si₁₉H₁₂ isomers. Retrieved ground states (minimum energy states) of Si₁₈H₁₂ and Si₁₉H₁₂ isomers significantly differ from the earlier proposed “ultrastable” aromatic molecular systems and prismanes. According to our calculations, they are represented by the almost spherical endohedral buckyballs. In contrast to pure silicic clusters (Si₁₈ or Si₁₉), the most of Si atoms in the low‐energy Si₁₈H₁₂ and Si₁₉H₁₂ are four‐coordinated. Spatial confinement results in more oblong structures with the different optical spectra. Prismanes under confinement get some energy advantages over the spherical structures, but despite this they do not become the most energetically favorable ones. Thus, the current results warrant however further research on the spatial confinement of silicic prismanes, as our study suggests challenges in devising the method of their synthesis without additional chemical techniques.     

Thermal Methane Activation by [Si2O5].+ and [Si2O5H2].+: Reactivity Enhancement by Hydrogenation

The thermal reactions of methane with the oxygen‐rich cluster cations [Si₂O₅]^?+ and [Si₂O₅H₂]^?+ have been examined using Fourier transform–ion cyclotron resonance (FT‐ICR) mass spectrometry in conjunction with state‐of‐the‐art quantum chemical calculations. In contrast to the inertness of [Si₂O₅]^.+ towards methane, the hydrogenated cluster [Si₂O₅H₂]^.+ brings about hydrogen‐atom transfer (HAT) from methane with an efficiency of 28 % relative to the collision rate. The mechanisms of this process have been investigated in detail and the reasons for the striking reactivity difference of the two cluster ions have been revealed.     

Über die Reaktion des käfigartigen Kieselsäurederivats [(CH3)2HSi]8Si8O20 mit ungesättigten organischen Verbindungen

Reaction of the Cage-like Silicic Acid Derivative [(CH₃)₂HSi]₈Si₈O₂₀ with Unsaturated Organic Compounds By ²⁹Si, ¹H, and ¹³C NMR investigations were shown that the eight HSi?groups of the double four-ring silicic acid derivative [(CH₃)₂HSi]₈Si₈O₂₀ react with the following unsaturated compounds: vinylcyclohexene, allyl glycidyl ether, methyl methacrylate, octadecene-1, and styrene. The resulting oily products are soluble in organic solvents. The compounds were characterized by the chemical shifts of the ²⁹Si, ¹H, and ¹³C NMR signals. Their formulae are [C₆H₉(CH₂)₂Si(CH₃)₂]₈Si₈O₂₀, [CH₃OOCCH(CH₃)CH₂Si(CH₃)₂]₈Si₈O₂₀, [CH₃(CH₂)₁₇Si(CH₃)₂]₈Si₈O₂₀ and [C₆H₅(CH₂)₂Si(CH₃)₂]₈Si₈O₂₀, and [C₆H₅CH(CH₃)Si(CH₃)₂]₈ Si₈O₂₀, respectively. Mainly the addition reactions do not follow the Markovnikov rule.     

Li8Si8, Li10Si9, and Li12Si10: Assemblies of Lithium-Silicon Aromatic Units

We report the global minima structures of Li₈Si₈, Li₁₀Si₉, and Li₁₂Si₁₀ systems, in which silicon moieties maintain structural and chemical bonding characteristics similar to those of their building blocks: the aromatic clusters T_d−Li₄Si₄ and C_2v−Li₆Si₅. Electron counting rules, chemical bonding analysis, and magnetic response properties verify the silicon unit‘s aromaticity persistence. This study demonstrates the feasibility of assembling silicon-based nanostructures from aromatics clusters as building blocks.