Jan Tillmann

Chloride-Induced Aufbau of Perchlorinated Cyclohexasilanes from Si2Cl6: A Mechanistic Scenario

A surprisingly simple preparative procedure, addition of Si2Cl6 to a solution of [nBu4N]Cl in CH2Cl2, leads to the formation of the chloride-complexed cyclic dianions [Si6Cl12⋅2Cl](2-), [(SiCl3)Si6Cl11⋅2Cl](2-), or [1,y-(SiCl3)2Si6Cl10⋅2Cl](2-) (y = 1, 3, 4), depending on the stoichiometric ratio of the reactants and the reaction temperature (25-85 °C). Below -40 °C the open-chain oligosilane chloride adducts [Si3Cl9](-), [Si3Cl10](2-), [Si4Cl11](-), and [Si6Cl15](-) are formed, again depending on the reaction conditions chosen. All species were characterized by X-ray crystallography. The underlying reaction mechanism is elucidated by DFT calculations. It incorporates all experimental findings and involves a few key elementary steps: 1) chloride-induced liberation of SiCl3(-) or higher silanides, 2) their addition to neutral silanes yielding larger oligosilane chloride adducts, 3) dimerization of larger silanides to (substituted) cyclohexasilane dichloride adducts with inverse sandwich structure.

One‐Step Synthesis of a [20]Silafullerane with an Endohedral Chloride Ion

Silicon analogues of the most prominent carbon nanostructures, namely, hollow spheroidals such as C60 and the fullerene family, have been unknown to date. Herein we show that discrete Si20 dodecahedra, stabilized by an endohedral guest and valence saturation, are accessible in preparative yields through a chloride-induced disproportionation reaction of hexachlorodisilane in the presence of tri(n-butyl)amine. X-ray crystallography revealed that each silicon dodecahedron contains an endohedral chloride ion that imparts a net negative charge. Eight chloro substituents and twelve trichlorosilyl groups are attached to the surface of each cluster in a strictly regioregular arrangement, a thermodynamically preferred substitution pattern according to quantum-chemical assessment. Our results demonstrate that the wet-chemical self-assembly of a complex, monodisperse Si nanostructure is possible under mild conditions starting from simple Si2 building blocks.

Lewis acidity of Si6Cl12 and its role as convenient SiCl2 source.

The free cyclohexasilane Si6Cl12 (1) was obtained in 66% yield from the corresponding Cl(-) diadduct [nBu4N]2[1·2Cl] and AlCl3 in C6H6. The substituted cyclohexasilane 1,1-(Cl3Si)2Si6Cl10 (2), however, cannot be liberated from [nBu4N]2[2·2Cl] under comparable reaction conditions. Instead, a mixture of several products was obtained, from which the oligosilane Si19Cl36 (3) crystallized in low yields. X-ray crystallography revealed 3 to consist of two Si5 rings, bridged by one silicon atom. Compound 1 possesses Lewis acidic sites above and below the ring centroid. Competition experiments reveal that their corresponding acid strengths are comparable to that of BCl3. The reaction of 1 with 6 equiv of 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (Idipp) leads to a complete breakdown of the cyclic scaffold and furnishes the dichlorosilylene adduct Idipp-SiCl2.

Tetraarylethylenes from Diarylmethanones and Hexachlorodisilane: The "Sila-McMurry" Reaction.

Hexachlorodisilane reacts with diarylmethanones (aryl=C6 H5 , 4-MeC6 H4 , 4-tBuC6 H4 , 4-ClC6 H4 , 4-BrC6 H4 ) to furnish the corresponding tetraarylethylenes in good yields. The reductive conversion requires temperatures of about 160 °C and reaction times of 60-72 h. In the initial step, the Lewis-basic carbonyl groups likely generate low-valent [SiCl2 ] as an analogue of [TiCl2 ] in the classical McMurry reaction. The coupling sequence further proceeds via benzopinacolones, which have been isolated as key intermediates.

A structural study of Si6-ring-containing [Si6Cl14](2-) chlorosilicates.

The crystal structures of four substituted-ammonium dichloride dodecachlorohexasilanes are presented. Each is crystallized with a different cation and one of the structures contains a benzene solvent molecule: bis(tetraethylammonium) dichloride dodecachlorohexasilane, 2C8H20N(+)·2Cl(-)·Cl12Si6, (I), tetrabutylammonium tributylmethylammonium dichloride dodecachlorohexasilane, C16H36N(+)·C13H30N(+)·2Cl(-)·Cl12Si6, (II), bis(tetrabutylammonium) dichloride dodecachlorohexasilane benzene disolvate, 2C16H36N(+)·2Cl(-)·Cl12Si6·2C6H6, (III), and bis(benzyltriphenylphosphonium) dichloride dodecachlorohexasilane, 2C25H22P(+)·2Cl(-)·Cl12Si6, (IV). In all four structures, the dodecachlorohexasilane ring is located on a crystallographic centre of inversion. The geometry of the dichloride dodecachlorohexasilanes in the different structures is almost the same, irrespective of the cocrystallized cation and solvent. However, the crystal structure of the parent dodecachlorohexasilane molecule shows that this molecule adopts a chair conformation. In (IV), the P atom and the benzyl group of the cation are disordered over two sites, with a site-occupation factor of 0.560 (5) for the major-occupied site.

Two ammonium tetrachloridoaluminate salts: how a twinned orthorhombic structure emulates a trigonal crystal system

We have determined the crystal structures of two tetrachloridoaluminate salts. Tetrabutylammonium tetrachloridoaluminate benzene hemisolvate, (C16H36N)[AlCl4]·0.5C6H6, (I), crystallizes with discrete cations, anions and solvent molecules. The benzene molecule is located on a centre of inversion. The structure of the benzene-free polymorph has been determined previously. Tetraethylammonium tetrachloridoaluminate, (C8H20N)[AlCl4], (II), also crystallizes with discrete cations and anions, and forms crystals which appear trigonal but are actually orthorhombic. With the additional reflections of the second and third domains of this nonmerohedral twin, a trigonal lattice is emulated, although the correct crystal system is orthorhombic.

Exhaustively Trichlorosilylated C1 and C2 Building Blocks: Beyond the Müller–Rochow Direct Process

The Cl--induced heterolysis of the Si-Si bond in Si2Cl6 generates an [SiCl3]- ion as reactive intermediate. When carried out in the presence of CCl4 or Cl2C═CCl2 (CH2Cl2 solutions, room temperature or below), the reaction furnishes the monocarbanion [C(SiCl3)3]- ([A]-; 92%) or the vicinal dianion [(Cl3Si)2C-C(SiCl3)2]2- ([B]2-; 85%) in excellent yields. Starting from [B]2-, the tetrasilylethane (Cl3Si)2(H)C-C(H)(SiCl3)2 (H2B) and the tetrasilylethene (Cl3Si)2C═C(SiCl3)2 (B; 96%) are readily available through protonation (CF3SO3H) or oxidation (CuCl2), respectively. Equimolar mixtures of H2B/[B]2- or B/[B]2- quantitatively produce 2 equiv of the monoanion [HB]- or the blue radical anion [B•]-, respectively. Treatment of B with Cl- ions in the presence of CuCl2 furnishes the disilylethyne Cl3SiC≡CSiCl3 (C; 80%); in the presence of [HMe3N]Cl, the trisilylethene (Cl3Si)2C═C(H)SiCl3 (D; 72%) is obtained. Alkyne C undergoes a [4+2]-cycloaddition reaction with 2,3-dimethyl-1,3-butadiene (CH2Cl2, 50 °C, 3d) and thus provides access to 1,2-bis(trichlorosilyl)-4,5-dimethylbenzene (E1; 80%) after oxidation with DDQ. The corresponding 1,2-bis(trichlorosilyl)-3,4,5,6-tetraphenylbenzene (E2; 83%) was prepared from C and 2,3,4,5-tetraphenyl-2,4-cyclopentadien-1-one under CO extrusion at elevated temperatures (CH2Cl2, 180 °C, 4 d). All closed-shell products were characterized by 1H, 13C{1H}, and 29Si NMR spectroscopy; an EPR spectrum of [ nBu4N][B•] was recorded. The molecular structures of [ nBu4N][A], [ nBu4N]2[B], B, E1, and E2 were further confirmed by single-crystal X-ray diffraction. On the basis of detailed experimental investigations, augmented by quantum-chemical calculations, plausible reaction mechanisms for the formation of [A]-, [B]2-, C, and D are postulated.

Analysis of geometric parameters and packing considerations for triphenylboroxine derivatives, with tris(pentafluorophenyl)boroxine as an example.

Molecules of the title compound [systematic name: 2,4,6-(pentafluorophenyl)-1,3,5,2,4,6-trioxatriborinane], C(18)B(3)F(15)O(3), are located on crystallographic twofold rotation axes which run through the boroxine and one of the pentafluorophenyl rings. The boroxine ring (r.m.s. deviation = 0.027 Å) and the pentafluorophenyl rings (r.m.s. deviations = 0.004 and 0.001 Å) are essentially planar. The dihedral angles between the boroxine and the two symmetry-independent benzene rings are 8.64 (10) and 8.74 (12)°. The two benzene rings are mutually coparallel [dihedral angle = 0.80 (11)°]. The packing shows planes of molecules parallel to ( ̅201), with an interplanar spacing of 2.99 Å. Within these planes, all the molecules are oriented in the same direction, whereas in neighbouring planes the direction is inverted. Short B···F contacts of 3.040 (2) and 3.1624 (12) Å occur between planes. The geometric parameters of the boroxine ring in the title compound agree well with those of comparable boroxine structures, while the packing reveals some striking similarities and differences.

Tetra-butyl-ammonium tris-(methyl-sulfanylmeth-yl)phenyl-borate.

In the title molecular salt, C16H36N+·C12H20BS3 −, three of the four n-butyl chains show a trans conformation, whereas the fourth has the C—C—C—C torsion angle in a gauche conformation [−77.8 (5)°]. In the crystal, molecules are packed in layers parallel to the (101) plane.