Isolable Si=B Analogue of a Vinyl Halide: A Building Block for Facile Access toward Silicon-Boron Multiple Bonded Species.

Compared to the outstanding development in the synthesis of Si-B single bonded species, borylsilanes and their application to organic synthesis, the chemistry of Si=B double bonded species, borasilenes and boratasilenes have only made little progress, first of all, due to the difficulties in accessing such double bonds. Herein we report the synthesis of the first Si=B analogue of a vinyl halide, a bromoboratasilene, via formal borylene insertion to the coordination sphere of a monoatomic Si(0) complex, using a dihaloborane as the borylene source. The treatment of bromoboratasilene toward neutral or anionic Lewis bases gives access to new boratasilenes, all of which were proved to possess significant Si=B double bond character by XRD analysis and DFT calculations. These results demonstrate exciting strategies to synthesize new types of Si=B double bonded species which should further progress the chemistry of boron, silicon-containing molecules.

Partial and Complete Replacement of the Phenyl Group(s) of 1,1,3,3-Tetraphenyl-2,2,4,4-tetrakis(trimethylsilyl)cyclotetrasilane by Chlorine Atom(s).

The four phenyl groups of 1,1,3,3-tetraphenyl-2,2,4,4-tetrakis(trimethylsilyl)cyclotetrasilane were partially replaced by chlorine atom(s) by successive treatment with trifluoromethanesulfonic acid and lithium chloride. The replacement of all phenyl groups was carried out with hydrogen chloride in the presence of aluminum chloride. The structural features of the resulting mono-, di-, and tetrachlorocyclotetrasilanes were compared by X-ray crystallography.

Synthesis, Structure and Electronic Properties of a Stable π-Type 3-Electron-2-Center-Bonded Species: A Silicon Analogue of a Bicyclo[1.1.0]butane Radical Anion.

σ-Type 3-electron-2-center (3e-2c) bonds have been extensively studied as one of the key bonding motifs in radical chemistry and some biological systems. "π-Type 3e-2c-bonded species" that contain a 3e-2c π-bond without an underlying σ-bond framework, however, have been unexplored so far both theoretically and experimentally. Herein, we report the synthesis of the first stable π-type 3e-2c-bonded species, a silicon analogue of a bicyclo[1.1.0]butane radical anion. This compound exhibits an extremely long bridgehead Si-Si bond (3.0638(8) Å) and a strong near-IR absorption at 922 nm in solution which arises from a HOMO→SOMO [π(Si-Si)→π*(Si-Si)] transition. DFT calculations revealed a π-type bonding interaction between the two bridgehead silicon atoms with an unpaired electron mainly delocalized across the corresponding π*-type orbital, which introduces a novel bonding motif for constructing π-electron systems.

A Spiropentasiladiene Radical Cation: Spin and Positive Charge Delocalization across Two Perpendicular Si═Si Bonds and UV-vis-NIR Absorption in the IR-B Region.

Spiroconjugation, that is, through-space orbital interactions between two perpendicular π orbitals, is a key concept in the contemporary molecular design of spirocyclic π-electron systems. We synthesized spiropentasiladiene radical cation salt 1 as a dark-green solid via the one-electron oxidation of the stable spiropentasiladiene 2. Characterization of the molecular structure combined with theoretical studies indicated that the spin and positive charge are delocalized across the two perpendicular Si═Si double bonds of 1. Two π(Si═Si) orbitals are split into HOMO and SOMO with a small energy gap owing to the second-order Jahn-Teller distortion and steric repulsion between bulky alkyl groups upon one-electron oxidation. In the UV-vis-NIR spectrum, the longest-wavelength absorption band of 1 (λmax = 1972 nm) covers the IR-B region (1400-3000 nm; 0.89-0.41 eV) despite having the smallest possible spiroconjugation motif. The unprecedented absorption band in the IR region was assigned to the HOMO → SOMO transition that arises from the delocalized π-orbitals in the spirocyclic Si5 skeleton.

Dialkylboryl-Substituted Cyclic Disilenes Synthesized by Desilylation-Borylation of Trimethylsilyl-Substituted Disilenes.

π-Electron systems of silicon have attracted attention because of their narrow HOMO-LUMO gap and high reactivity, but the structural diversity remains limited. Herein, new dialkylboryl-substituted disilenes were synthesized by the selective desilylation-borylation of the corresponding trimethylsilyl-substituted disilenes. The dialkylboryl-substituted disilenes were fully characterized by a combination of NMR spectroscopy, MS spectrometry, single-crystal X-ray diffraction analysis, and theoretical calculations. The longest-wavelength absorption bands of boryldisilenes were bathochromically shifted compared to the corresponding silyl-substituted disilenes, indicating a substantial conjugation between π(Si=Si) and vacant 2p(B) orbitals. In the presence of 4-(dimethylamino)pyridine (DMAP), the dialkylboryl groups in the boryl-substituted disilenes were easily converted to trimethylsilyl groups, suggesting the dialkylboryl-substituted disilenes in the presence of a base serve as the surrogates of disilenyl anions (disilenides).

A Six-Coordinate Silicon Dihydride Embedded in a Porphyrin: Enhanced Hydride-Donor Properties and the Catalyst-Free Hydrosilylation of CO2.

The catalyst-free hydrosilylation of CO2 under mild conditions remains limited. Herein, we report the synthesis, characterization, and reactivity of 5,10,15,20-tetraphenylporphyrinato(dihydrido)silicon(IV) (1) as a six-coordinate silicon dihydride. The Si-H moiety of 1 reacts with polar double bonds and CO2 in the absence of a catalyst to afford hydrosilylated products. Combining the hydrosilylation with subsequent transformation furnishes formic acid from CO2 . Computational studies indicate that the hydride-donor properties of 1 are exceptionally high for a neutral silicon hydride, and that the direct hydride transfer from silicon to carbon is a pivotal step in the hydrosilylation of CO2 with 1.

Trialkylphosphines Having a Bulky Phosphacyclopentane Backbone: Structural and Redox Properties Depending on the Exocyclic Alkyl Groups and EPR Observation of a Persistent Trialkylphosphine Radical Cation.

Bulky phosphines and their redox properties have received increased attention in the view of useful auxiliary ligands for transition metal catalysts and Lewis-base components of frustrated Lewis pairs for chemical transformations. Herein we report the synthesis, structure, and properties of a series of trialkylphosphines 2R (R = methyl, ethyl, isopropyl, tert-butyl, 1-adamantyl) that possess the bulky 2,2,5,5-tetrakis(trimethylsilyl)-1-phosphacyclopentane as a structural backbone. Among these phosphines, 2Ad, which contains an adamantyl moiety, has a very large buried volume (%Vbur) for a trialkylphosphine (62.0) and shows a quasi-reversible oxidative wave at a lower oxidation potential (-0.12 V in CH2Cl2, vs ferrocene/ferrocenium couple) by cyclic voltammetry. The reaction of 2Ad with AgPF6 afforded a cationic silver aquo complex [Ag(2Ad)(H2O)]+[PF6]-, whereas the reaction with NOSbF6 gave a persistent phosphine radical cation [2Ad]•+. Based on the EPR spectra and DFT studies, the spin and positive charge of [2Ad]•+ are localized on the phosphorus atom.

An Isolable Silicon Analogue of a Ketone that Contains an Unperturbed Si=O Double Bond.

Despite tremendous efforts to synthesize isolable compounds with an Si=O bond, silicon analogues of ketones that contain an unperturbed Si=O bond have remained elusive for more than 100 years. Herein, we report the synthesis of an isolable silicon analogue of a ketone that exhibits a three-coordinate silicon center and an unperturbed Si=O bond, thus representing the first example of a genuine silanone. Most importantly, this silanone does not require coordination by Lewis bases and acids and/or the introduction of electron-donating groups to stabilize the Si=O bond. The structure and properties of this unperturbed Si=O bond were examined by a single-crystal X-ray diffraction analysis, NMR spectroscopy, and theoretical calculations. Bimolecular reactions revealed high electrophilicity on the Si atom and high nucleophilicity on the O atom of this genuine Si=O bond.

A Tetrasilicon Analogue of Bicyclo[1.1.0]but-1(3)-ene Containing a Si=Si Double Bond with an Inverted Geometry.

Bicyclo[1.1.0]tetrasil-1(3)-ene 1, a tetrasilicon analogue of bicyclo[1.1.0]but-1(3)-ene that contains a formal double bond between bridgehead silicon atoms in an inverted geometry, was synthesized and isolated in the form of thermally stable orange crystals. The distance between the bridgehead Si atoms in 1 is much longer than those in typical Si=Si bonds, but still shorter than that of a previously reported pentasila[1.1.1]propellane. DFT calculations suggest that the bridgehead bond in 1 comprises a σ bond with an inverted geometry and a π bond. This notion is supported by the UV/Vis spectrum of 1, which exhibits several absorption bands in the UV/Vis region. While 1 is stable toward typical trapping agents for Si=Si double bonds, 1 reacts with carbon tetrachloride to furnish a hexachlorotetrasilane.

Facile Skeletal Rearrangement of Polycyclic Disilenes with Bicyclo[1.1.1]pentasilanyl Groups.

The unexpected formations of fused polycyclic disilenes 2 a and (E),(Z)-3 b by the reduction of the 1,2-dibromodisilanes 5 a (R=Me) and 5 b (R=iPr) bearing bicyclo[1.1.1]pentasilanyl (BPS) groups is reported. The disilenes 2 a and (E),(Z)-3 b were characterized by a combination of NMR spectroscopy and X-ray diffraction analysis (XRD). The reduction of 5 b in the presence of 2,3-dimethyl-1,3-butadiene provided an ene adduct of the disilene 1 b bearing BPS groups, which suggested that an initial product of the reduction of 5 b was the disilene 1 b. Thermal reactions of 2 a and (E),(Z)-3 b afforded the highly strained saturated silicon clusters 4 a and 4 b. A computational study suggested that the transformation of 1 to 2, 3, or 4 can involve silyldisilene-disilanylsilylene rearrangement reactions and insertion reactions of a silylene into a Si-Si bond.

Pentasila-1,4-diene: Homoconjugation between Si═Si Double Bonds via a SiMe(2) Unit.

Although the synthesis of several bis(disilenes) has already been reported, the number of reported conjugation modes between the Si═Si double bonds remains limited. Herein, we report the properties of the stable pentasila-1,4-diene 1, which was obtained from the reaction of two equivalents of disilenide 4 with dichlorodimethylsilane. The π(Si═Si)→π*(Si═Si) absorption band of 1 is considerably broadened and red-shifted compared to those of the corresponding monodisilene and hexasila-1,5-diene, but blue-shifted relative to those of typical tetrasila-1,3-dienes. The bathochromic shift and the broadening of the absorption band in 1 should be attributed to the homoconjugation between Si═Si double bonds through the SiMe2 unit.

An Isolable Potassium Salt of a Borasilene-Chloride Adduct.

Among the variety of isolable compounds with multiple bonds involving silicon, examples of compounds that contain silicon-boron double bonds (borasilenes) still remain relatively rare. Herein, we report the synthesis of the potassium salt of a chloride adduct of borasilene 1 ([2]- ), which was obtained as an orange crystalline solid. Single-crystal X-ray diffraction analysis and reactivity studies on [2]- confirmed the double-bond character of the Si=B bond as well as the reduced Lewis acidity, which is due to the coordination of Cl- to the boron center. A thermal reaction of [2]- afforded a bicyclic product by formal intramolecular C-H insertion across the Si=B bond of 1, which was corroborated by a theoretical study.

Benzodisilacyclobutadienes: 8π-Electron Systems with an Antiaromatic Silicon Ring.

Benzodisilacyclobutadienes 2 a-c were isolated as blue to green crystalline solids from the reaction of stable disilyne 1 and 1,2-dibromobenzenes in the presence of potassium graphite. In the solid state, substantial bond alternation was observed within the benzene rings of 2 a-c. In hexane, 2 a-c showed remarkable bathochromic shifts of the π→π* (HOMO→LUMO) absorption bands at 625-670 nm. NMR spectra and theoretical calculations indicated that the diamagnetic ring currents of the benzene rings of 2 a-c are considerably reduced by contributions from the antiaromatic 1,2-disilacyclobutadienes. In their entirety, the obtained results indicate that 2 a-c represent 8π-electron systems that contain an antiaromatic 1,2-disilacyclobutadiene.

Isolation and Characterization of Radical Anions Derived from a Boryl-Substituted Diphosphene.

Radical anions of a diphosphene with two boryl substituents were isolated and characterized by single-crystal X-ray diffraction, electron spin resonance (ESR), and UV/Vis absorption spectroscopy as well as DFT calculations. Structural analysis of the radical anions revealed an elongation of the P=P bond and a contraction of the B-P bonds relative to the neutral diphosphene. The UV/Vis spectra of these radical anions showed a strong absorption in the visible region, which was assigned to SOMO-related transitions on the basis of DFT calculations. The ESR spectra revealed that the hyperfine coupling constant with the phosphorus nuclei is the smallest that has been reported thus far. The results of the DFT calculations furthermore suggest that this should be attributed to a soaking of electron spin to the vacant p orbitals of the boryl substituents.

A Two-Coordinate Cyclic (Alkyl)(amino)silylene: Balancing Thermal Stability and Reactivity.

A crystalline two-coordinate cyclic (alkyl)(amino)silylene (1) was successfully synthesized and isolated. Its 29 Si NMR and UV/Vis spectra indicate that the electronic properties of 1 fall between those of cyclic dialkylsilylenes and diaminosilylenes. At very low temperature, the color of a solution of 1 turned from colorless to yellow, which was monitored by UV/Vis spectroscopy. DFT calculations supported the hypothesis that head-to-head dimers (disilenes) with a very long Si-Si distance are formed at such low temperatures. Although 1 is thermally stable, it readily undergoes cycloadditions, Si-H insertions, and photochemical reactions with benzene similar to dialkylsilylenes. At higher temperatures, 1 is also susceptible to intermolecular benzylic C-H insertion reactions, as well as unprecedented dehydrogenation reactions with cyclohexa-1,4-diene and 9,10-dihydroanthracene to afford benzene and anthracene, respectively.

A Potassium Diboryllithate: Synthesis, Bonding Properties, and the Deprotonation of Benzene.

A potassium diboryllithate (B2 LiK) was synthesized and structurally characterized. DFT calculations, including NPA and AIM analyses of B2 LiK, revealed ionic interactions between the two bridging boryl anions and Li(+) and K(+) . Upon standing in benzene, B2 LiK deprotonated the solvent to form a hydroborane and a phenylborane. On the basis of DFT calculations, a detailed reaction mechanism, involving deprotonation and hydride/phenyl exchange processes, is proposed. An NBO analysis of the transition state for the deprotonation of benzene suggests that the deprotonation should be induced by the coordination of benzene to the K(+) .

Persistent Dialkylsilanone Generated by Dehydrobromination of Dialkylbromosilanol.

A persistent dialkylsilanone was synthesized by the dehydrobromination of a dialkylbromosilanol with tris(trimethylsilyl)silyl potassium in solution at -80 °C: It was characterized by NMR and IR spectroscopy, and was tested in several reactions. In (29) Si NMR spectrum in [D8 ]toluene, the signal due to the unsaturated silicon nuclei was observed at 128.7 ppm. Reactions of the dialkylsilanone with water and mesitonitrile oxide gave a silanediol and a [2+3] cycloadduct, respectively. The silanone remains intact in [D8 ]toluene below -80 °C for at least two days, while it undergoes unprecedented isomerization to give a siloxysilene by means of 1,3-silyl migration at higher temperatures.

An Isolable Radical Anion of an Organosilicon Cluster Containing Only σ Bonds.

The radical anion of octa-tert-butyloctasilacubane was generated and isolated. The EPR spectrum showed the satellites due to the tertiary (13)C nuclei of the eight tert-butyl groups. The X-ray crystallographic analysis showed that the Si-Si bonds are shortened and the Si-C bonds are elongated compared with those of octa-tert-butyloctasilacubane. These results are well explained by the distribution of an unpaired electron in the singly occupied molecular orbital (SOMO).

Isolable 2,3-disila-1,3-butadiene from a double sila-Peterson reaction.

A new 2,3-disila-1,3-butadiene (3) was synthesized as pale yellow crystals by a double sila-Peterson reaction of 1,1,2,2-tetrasilyl-1,2-dilithiodisilane with two equivalents of 2-adamantanone at 0 °C. In the solid state, the two Si=C bonds adopt a synclinal conformation with a dihedral angle of 76.8(1)°. UV/Vis spectra show two distinct absorption bands, assignable to the π(Si=C)→π*(Si=C) transition, at 371 and 322 nm and the former is considerably redshifted compared with that of structurally similar silenes. The 2,3-disilabutadiene isomerizes to the corresponding 1,3-disilabicyclo[1.1.0]butane with the activation parameters of ΔH(≠) = 74.5±5.4 kJ mol(-1) and ΔS(≠) = -71.1±17.1 J mol(-1) K(-1). DFT studies suggest that the isomerization proceeds through a conrotatory route rather than a disrotatory route. H2O and 9,10-phenanthrenequinone added across each Si=C bond in the 2,3-disila-1,3-butadiene. The UV/Vis spectrum and reactivity of 3 suggest that the interaction between the two Si=C bonds in 3 would be significant but rather small compared with that between Si=Si bonds in a synclinal tetrasilabutadiene.