Metal-catalyzed valence isomerization of a methylene(thioxo)phosphorane to a thiaphosphirane.

We explored coordination chemistry associated with valence isomerization between a methylene(thioxo)phosphorane (MTP) and a thiaphosphirane. For this purpose, we developed the selective synthesis of a MTP by eliminating chlorodimethylphenylsilane from the corresponding chlorophosphine sulfide. Treatment of the MTP with an equivalent amount of pentafluorophenylgold(I) complex resulted in the formation of a thiaphosphirane gold(I) complex, which likely proceeds via an η2-P,C-MTP gold complex. The MTP undergoes a valence isomerization catalyzed by copper(I) chloride to furnish a transition metal-free thiaphosphirane. Computational studies proposed a plausible mechanism involving a Cu-assisted cyclization reaction.

An Isolable THF-Coordinated Dialkylgermanone.

A stable dialkylgermanone was generated by mixing a solid of the corresponding dialkylgermylene and gaseous N2O. While the dialkylgermanone is marginally persistent in solution and gradually converts to its head-to-tail dimer at room temperature, the addition of THF to the dialkylgermanone provided an isolable THF-coordinated dialkylgermanone. The THF-coordinated dialkylgermanone reacts with H2O, THF, and B(C6F5)3 similar to the corresponding base-free two-coordinate dialkylsilanone. The dialkylgermanone undergoes deoxygenation in the presence of triphenylphosphine to provide the corresponding germylene and olefination upon treatment with phosphaylide Ph3PCHPh to afford the corresponding Ge=C bond compound (germa-Wittig reaction).

Coordination-Induced Migration of Aryl Groups in the Reactions of a Dialkylsilanone with a Series of Triarylboranes.

Although carbonyl compounds are activated by the coordination of a Lewis acid to the carbonyl oxygen atom, a similar activation process of R2 Si=O species remains unclear. We report herein the reactions of a silanone (1, Scheme 1) with a series of triarylboranes to afford the corresponding boroxysilanes. Experimental results and computational studies indicate that the electrophilicity of the unsaturated silicon atom is enhanced by the complexation of 1 with triarylboranes, and the subsequent aryl migration from the boron atom to the electrophilic silicon atom proceeds.

Cyclobutenylidene: A Multifaceted Two-Coordinate Carbon Species Obtained via Skeletal Editing of a Cyclopropenylidene.

C4H4 isomers not only serve as a basis to understand the chemical properties of hydrocarbons but are possible intermediates in combustion and organic reactions in outer space. Cyclobutenylidene (CBY), an elusive C4H4 isomer, is often proposed as a key intermediate in transition-metal-catalyzed metathesis and cycloaddition reactions between carbon-carbon multiple bonds. The geometrical structure of cyclobutenylidene predicted by calculations had been debated as whether it should be regarded as a carbocyclic carbene or a strained bridgehead alkene. Here, we report the synthesis of a crystalline cyclobutenylidene derivative, namely, a 3-silacyclobut-2-en-4-ylidene (SiCBY) via "carbene-to-carbene ring-expansion" reaction of an isolable diaminocyclopropenylidene induced by a silicon analogue of a carbene (silylene). The SiCBY exhibits multifaceted electronic properties which are corroborated by its extremely strong electron-donating properties and ambiphilic reactivity toward small gaseous molecules and C-H bonds. This result introduces an exciting strategy as well as a molecular motif to access low-valent carbon species with unusual electronic properties.

Frustrated Lewis pair-ligated tetrelenes.

The reactivity of [PB{SiX2}] (X = Cl, Br; PB = 1,2-iPr2(C6H4)BCy2; E = Si, Ge) adducts is described, with an initial focus on reduction attempts to access [PB{E}]x species; however, in all cases only free PB ligand was formed as the soluble product. Moreover, computations were performed to evaluate the energy penalty associated with EX2 dissociation from the PB chelates. Moving up the periodic table, the formal methylene adduct [PB{CH2}] was isolated and its reactivity was compared with its heavier element congeners of [PB{EH2}]. We also introduce new phosphine-borane frustrated Lewis pair (FLP) chelates and explore preliminary coordination chemistry with these ligands.

Twisted push-pull disilenes obtained by direct 1,2-hydro/chloroborylation of a silylone.

Overcrowded 1-amino-2-boryldisilenes were isolated as blue crystals by the direct hydro/chloroborylation of a cyclic (alkyl)(amino)silylene-coordinated monatomic silicon complex (silylone) with 9-BBN (BBN = borabicyclo[3.3.1]nonane) hydride/chloride. The extreme twisting and push-pull effect around the SiSi bonds resulted in strong polarization of the SiSi bond. The disilenes exhibit longest-wavelength absorptions above 600 nm due to the charge-transfer-type π → π* transitions and undergo reversible 1,3-hydride/halide migration in solution reflecting the electrophilic terminal silicon atoms.

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.

π-Conjugated species with an unsupported Si-Si π-bond obtained from direct π-extension.

1,3-Diethynylbicyclo[1.1.0]tetrasilanes that contain an unsupported bridgehead Si-Si π-bond and ethynyl π-moieties were synthesized by the reaction of a 1,3-dihalobicyclo[1.1.0]tetrasilane with the corresponding lithium acetylide. The substantial bathochromic shift of the longest-wavelength absorption band observed for the phenylethynyl-substituted bicyclo[1.1.0]tetrasilane in the solid state compared to that of the octynyl-substituted derivative suggested the presence of effective π-conjugation between the unsupported Si-Si π and ethynyl π units.

Conformationally Switchable Silylone: Electron Redistribution Accompanied by Ligand Reorientation around a Monatomic Silicon.

Complexes that could be switched between two electronic states by external stimuli have attracted much attention for their potential application in molecular devices. However, a realization of such a phenomenon with low-valent main-group element-centered complexes remains challenging. Herein, we report the synthesis of cyclic (alkyl)(amino)silylene (CAASi)-ligated monatomic silicon(0) complexes (silylones). The bis(CAASi)-ligated silylone adopts a π-localized ylidene structure (greenish-black color) in the solid state and a π-delocalized ylidene structure (dark-purple color) in solution that could be reversibly switched upon phase transfer (ylidene [L: → :Si = L ↔ L = Si: ← :L]). The observed remarkable difference in the physical properties of the two isomers is attributed to the balanced steric demand and redox noninnocent character of the CAASi ligand which are altered by the orientation of the two terminal ligands with respect to the Si-Si-Si plane: twisted structure (π-localized ylidene) and planar structure (π-delocalized ylidene). Conversely, the CAASi/CDASi-ligated heteroleptic silylone (CDASi = cyclic dialkylsilylene) only exhibited the twisted π-localized ylidene structure regardless of the phase. The synthesized silylones also proved themselves as monatomic silicon surrogates. Thermolysis of the silylones in the presence of an ethane-1,2-diimine afforded the corresponding diaminosilylenes. Analyses of the products suggested a stepwise mechanism that proceeds via a disilavinylidene intermediate.

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 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.

Cleavage of C-O and C-H bonds in ethers by a genuine Si[double bond, length as m-dash]O bond.

An isolable three-coordinate dialkylsilanone without substantial electronic stabilization reacts with several ethers resulting in the cleavage of the C-O or C-H and C-O bonds in the ethers which have not been observed for the hitherto-known electronically stabilised isolable Si[double bond, length as m-dash]O species. The formation of the Lewis base (DMAP and MTHP) complexes of the dialkylsilanone and the DFT calculations elucidated that the coordination of the ethereal oxygen atom to the Lewis acidic Si atom of the genuine Si[double bond, length as m-dash]O bond is a key interaction for the reaction.

Frustrated Lewis Pair Chelation as a Vehicle for Low-Temperature Semiconductor Element and Polymer Deposition.

The stabilization of silicon(II) and germanium(II) dihydrides by an intramolecular Frustrated Lewis Pair (FLP) ligand, PB, i Pr2 P(C6 H4 )BCy2 (Cy=cyclohexyl) is reported. The resulting hydride complexes [PB{SiH2 }] and [PB{GeH2 }] are indefinitely stable at room temperature, yet can deposit films of silicon and germanium, respectively, upon mild thermolysis in solution. Hallmarks of this work include: 1) the ability to recycle the FLP phosphine-borane ligand (PB) after element deposition, and 2) the single-source precursor [PB{SiH2 }] deposits Si films at a record low temperature from solution (110 °C). The dialkylsilicon(II) adduct [PB{SiMe2 }] was also prepared, and shown to release poly(dimethylsilane) [SiMe2 ]n upon heating. Overall, this study introduces a "closed loop" deposition strategy for semiconductors that steers materials science away from the use of harsh reagents or high temperatures.

Intermolecular C-H Activation at the Allylic/Benzylic and Homoallylic/Homobenzylic Positions of Cyclic Hydrocarbons by a Stable Divalent Silicon Species.

Direct activation of inert C(sp3 )-H bonds by main group element species is yet a formidable challenge. Herein, the dehydrogenation of cyclohexene and 1,2,3,4-tetrahydronaphthalene through the allylic/benzylic and homoallylic/homobenzylic C-H bond activation by cyclic (alkyl)(amino)silylene 1 in neat conditions is reported to yield the corresponding aromatic compounds. As for the reaction of cyclohexene, allylsilane 3 and 7-silanorbornene 4 were also observed, which could be interpreted as a direct dehydrogenative silylation reaction of monoalkenes at the allylic positions. Experimental and computational studies suggest that the dehydrogenation of cyclohexene at the homoallylic position was accomplished by a combination of silylene 1 and radical intermediates such as hydrosilyl radical INT1 or cyclohexenyl radical H, which are generated in the initial step of the reaction.

High performance global exploration of isomers and isomerization channels on quantum chemical potential energy surface of H5 C2 NO2.

High performance global exploration of isomers and isomerization channels on the quantum chemical potential energy surface (PES) is performed for H5 C2 NO2 by using the scaled hypersphere search-anharmonic downward distortion following (SHS-ADDF) method. A multi-node operation, NeoGRRM, has achieved high performance exploration calculations for the large system by submitting SHS-ADDF sub-jobs into many cores in parallel and unifying the results of sub-jobs into the total lists of the main-job. Global exploration of equilibrium (EQ) and transition-state structures at the level of B3LYP/6-31G(d) gave 3210 EQs and 23278 TSs. Nine compounds were found in the low energy regions of 0-100 kJ/mol; the lowest energy compound is N-methylcarbamic acid, the second is methyl carbamate, and the third is glycine (the most fundamental amino acid). Interconversion pathways between the conformers of each of the low energy compounds were surveyed. Isomerization channels around glycine were explored in detail. The lowest energy barriers around some of the EQs turned to be negative after zero-point energy corrections. This indicates that those structures cannot exist as independent structures because they spontaneously collapse into more stable structures. The global PES search showed various interesting dissociating channels which indicate synthon reaction pathways in the reverse directions.

Interconversion between a planar 1,3-dichlorobicyclo[1.1.0]tetrasilane and a (chloro)(chlorosilyl)cyclotrisilene.

Heavy element analogues of bicyclo[1.1.0]butane have attracted much attention as they often exhibit skeletal isomerisations that are not observed for all-carbon bicyclo[1.1.0]butanes. The reaction of a bicyclo[1.1.0]tetrasil-1(3)-ene with CCl4 at low temperature provided orange crystals of a 1,3-dichlorobicyclo[1.1.0]tetrasilane containing a planar Si4 ring with a π-type bridgehead Si-Si bond. The equilibrium between the bicyclotetrasilane and an isomer, 1-chloro-2-(chlorosilyl)cyclotrisilene in solution, which is an unprecedented interconversion mode of Si4R6 isomers, was identified by NMR spectroscopy and also the formation of its 4-(dimethylamino)pyridine (DMAP) complex.

Naphtho- and anthra-disilacyclobutadienes.

Naphthodislacyclobutadiene 3 and anthradisilacyclobutadiene 4 were synthesized and characterized. In these compounds, the diatropic ring current on the benzene ring adjacent to the disilacyclobutadiene moiety decreases while that on the other benzene rings remains intact. The longest-wavelength absorption bands of 3 and 4 were hypsochromically shifted compared to those of benzodisilacyclobutadiene 1a despite the extended π-electron systems.

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.

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.