Isolable Tetragold(0) Clusters with Polarity-Tunable exo-Au-Au Bond via Intramolecular σ-Aromatization.

Intramolecular π-aromatization is a trait of many organic compounds that enhances the stability of their structures and polarizes related C-C π bonds. In contrast, rare study is focused on this phenomenon in metal clusters. Many existing homometallic clusters exhibit aromaticity, often characterized by nonpolar metal-metal bonds and a high degree of symmetry. However, synthesizing low-symmetric homometallic clusters with high-polar metal-metal bonds is challenging due to their limited thermodynamic stability. Herein, we report a facile strategy for the synthesis of [Au(µ2 -ER2 )]3 -AuPMe3 (E=Ge, Sn; R2 =1,1,4,4-tetrakis(trimethylsilyl)butane-1,4-diyl) clusters and reveal a novel stabilization mode, intramolecular σ-aromatization. Our electronic structure analyses show that these low-symmetric clusters possess a ten-electron σ-aromatic system, which is achieved via intramolecular σ-aromatization. Moreover, the strength of σ-aromaticity gives rise to a polarity-tunable exo-Au-Au bond.

A Stable Cyclic (R2 SnAu)3 Anion Having In-Plane σ-Möbius Aromaticity.

A cyclic (R2 SnAu)3 anion (3- , R2 Sn=2,2,5,5-tetrakis(trimethylsilyl)-1-stannacyclopentane-1,1-diyl) has been synthesized as a stable blue salt with K+ (THF)6 through the reaction of stable dialkylstannylene 1 with R'3 PAuCl (R'=Et, Ph) followed by the reduction with KC8 . Crystallographic and NMR analysis shows that the six-membered (SnAu)3 ring of 3- is planar and highly symmetric with an equal distance of six Au-Sn bonds. A UV/Vis spectrum of 3- in hexane reveals an intense absorption maximum at 598 nm. While cyclic Au3 - with four valence electrons is known as unstable anti-aromatic anion, 3- with three divalent tin ligands is stable σ aromatic anion with an unprecedented Möbius orbital array as predicted by the perturbation MO and CCSD analysis of 3- .

Cation-Triggered Stannate(II)/Stannylenoid/Stannylene Conversion.

The reaction of dipotassio-tetrasilan-1,4-diide (4) with anhydrous SnCl2 at low temperature results in the formation of a five-membered cyclic potassio chlorostannate(II) ([(18-C-6)K](1)). By careful cation exchange reactions, it was transformed into the sodium chlorostannylenoid 2 (by using Na2 [B12 Cl12 ]) or into the non-stabilized cyclic bissilylstannylene 3 (through applying Li[Al(OC(CF3 )3 )4 ]). The increasing Lewis basicity of the bissilylstannylene 3 was analyzed by combined methods of DFT calculations and NMR spectroscopy and substantiated by the synthesis of the corresponding iron carbonyl complexes 7 and 8 from the chlorostannate 1 and the stannylene 3, respectively.

Strait Gate: Special Issue on Advances in Silicon Chemistry.

Manufacturing high-purity element silicon and organic polysilicones are two major silicon industries, supporting the basis of the modern electronic industry and our daily lives [...].

Reactions of an Isolable Dialkylsilylene with Aroyl Chlorides. A New Route to Aroylsilanes.

The reactions of isolable dialkylsilylene 1 with aromatic acyl chlorides afforded aroylsilanes 3a-3c exclusively. Aroylsilanes 3a-3c were characterized by ¹H-, 13C-, and 29Si-NMR spectroscopy, high-resolution mass spectrometry (HRMS), and single-crystal molecular structure analysis. The reaction mechanisms are discussed in comparison with related reaction of 1 with chloroalkanes and chlorosilanes.

1,3-Diazasilabicyclo[1.1.0]butane with a Long Bridging N-N Bond.

A 1,3-diazasilabicyclo[1.1.0]butane (1) is synthesized as thermally stable crystals by using the cycloaddition reaction of an isolable dialkylsilylene with aziadamantane. The bridge N-N bond length of 1 (1.70 Å) is the longest among those of known N-N singly-bonded compounds, including side-on bridged transition-metal dinitrogen complexes. The compound 1 is intact in air but moisture sensitive. No reaction occurs with hydrogen, even under pressure at 0.5 MPa. Irradiation of 1 with light gives an isomer quantitatively by N-N and adamantyl C-C bond cleavage. The origin of the remarkable N-N bond elongation is ascribed to significant interaction between a Si-C σ* and Ν-Ν π and σ orbitals as determined by DFT calculations of model compounds.

Reversible Stannylenoid Formation from the Corresponding Stannylene and Cesium Fluoride.

A fluorostannylenoid (Cs+ [R2 SnF]- (9), R2 =(TMS)2 CCH2 CH2 C(TMS)2 ) was prepared by reacting a stable dialkylstannylene (R2 Sn (8), R2 =(TMS)2 CCH2 CH2 C(TMS)2 ) with cesium fluoride at room temperature in THF. While 9 is stable in THF and DME, removal of the solvent leads to the regeneration of stannylene 8. No reaction occurred when 8 was treated with CsF in a hydrocarbon solvent. Addition of dibenzo-21-crown-7 ether to the THF solution of stannylenoid 9 followed by usual workup affords the corresponding crystalline stannylenoid crown ether complex, the X-ray structural analysis of which revealed a fluorine-bridged contact ion-pair structure. The reaction of 9 with excess phenylacetylene gives the corresponding di(phenylethynyl)stannane.

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.

Elusive 2H-1,2-oxasiletes through reactions of an isolable dialkylsilylene with diazocarbonyl compounds.

The reactions of isolable dialkysilylene 1 with 2-diazo-1,2-diphenylethanone and ethyl 2-diazo-2-phenylacetate gave elusive silacycles, 2H-1,2-oxasiletes 2 and 3, respectively, in high yields. Because these reactions occur at low temperatures of ca.-30 °C, initial complexation of the silylene to the carbonyl oxygen of the diazocarbonyl compounds is suggested to trigger dinitrogen elimination followed by cyclization. In contrast, a six-membered cyclic diazo compound 8 and 1-sila-2,3-diazabicyclo[3.3.0]oct-3-ene 10 were obtained in good yields by the reaction of 1 with less reactive ethyl 2-diazo-3-oxo-3-phenylpropanoate 7 and trimethylsilyldiazomethane 9. Molecular structures of 2, 3, 8 and 10 were determined by X-ray crystallography.

New isolable dialkylsilylene and its isolable dimer that equilibrate in solution.

The new isolable dialkylsilylene 3 bearing a bidentate alkyl substituent was synthesized. Recrystallization of silylene 3 gave yellow crystals of 3 and orange-red crystals of tetraalkyldisilene 4, a dimer of 3. In the solid state, 3 exists as a monomer with a closest distance of 6.745 Å between dicoordinate silicon atoms, while disilene 4 has a remarkably long Si═Si double bond distance of 2.252 Å. An equilibrium between 3 and 4 in solution was observed by NMR and UV-vis spectroscopies, and the thermodynamic parameters of the equilibrium were estimated to be ΔH = -36 ± 3 kJ mol(-1) and ΔS = -170 ± 15 J mol(-1) K(-1). Analysis of the percent buried volume, a measure of the steric demand around the divalent silicon, showed that the flexible steric bulkiness of the alkyl substituent of 3 and 4 allows the reversible dimerization of silylene 3 to disilene 4 and the isolation of both species.

Bonding and structure of disilenes and related unsaturated group-14 element compounds.

Structure and properties of silicon-silicon doubly bonded compounds (disilenes) are shown to be remarkably different from those of alkenes. X-Ray structural analysis of a series of acyclic tetrakis(trialkylsilyl)disilenes has shown that the geometry of these disilenes is quite flexible, and planar, twist or trans-bent depending on the bulkiness and shape of the trialkylsilyl substituents. Thermal and photochemical interconversion between a cyclotetrasilene and the corresponding bicyclo[1.1.0]tetrasilane occurs via either 1,2-silyl migration or a concerted electrocyclic reaction depending on the ring substituents without intermediacy of the corresponding tetrasila-1,3-diene. Theoretical and spectroscopic studies of a stable spiropentasiladiene have revealed a unique feature of the spiroconjugation in this system. Starting with a stable dialkylsilylene, a number of elaborated disilenes including trisilaallene and its germanium congeners are synthesized. Unlike carbon allenes, the trisilaallene has remarkably bent and fluxional geometry, suggesting the importance of the π-σ* orbital mixing. 14-Electron three-coordinate disilene-palladium complexes are found to have much stronger π-complex character than related 16-electron tetracoordinate complexes.

Trinuclear anionic gold clusters bridged by dialkylsilylene and dialkylgermylene.

While the chemistry of gold clusters has been developed rapidly, the knowledge of trinuclear gold clusters with low-valent gold remains scarce. Herein, we report the synthesis, structural characterization, and density functional theory (DFT) calculations of trinuclear anionic gold clusters 3a and 3b ligated by dialkylsilylene (1a) and dialkylgermylene (1b), respectively.

Structure and reactivity of germylene-bridged digold complexes.

The bonding between gold and main-group metallic elements (M) featuring Auδ--Mδ+ polarity, has been studied recently. The gold in the bonds is expected to have the oxidation number of -1, and hence, nucleophilic. However, the knowledge of the reactivity of the gold-metal bonds remains limited. Here, we report digold-substituted germanes of the form of R'2Ge(AuPR3)(AuGeR'2) (3a; R = Me, 3b; R = Et), featuring two Au-Ge(IV) and one Au-Ge(II) bonds. DFT calculations of 3a revealed the existence of high-lying σ(Ge-Au) type HOMO and low-lying LUMO with germylene pπ nature. A pendular motion of AuPR3 group between Ge(IV) and Ge(II) of 3 occurs in the NMR time scale, suggesting that the Ge(II) center has an enhanced electrophilicity to be attacked by the nucleophilic gold (-I) atom. 3a reacts with nucleophilic Cl- and electrophilic MeOTf reagents at Ge(II) and Ge(IV) centers, respectively.

Diverse reactions of a fluorostannylenoid towards ethynes.

Fluoro(dialkyl)stannylenoid 2 exhibits unique reactivity towards ethynes with acetylenic hydrogen and those with trimethylsilyl groups, though the corresponding free dialkylstannylene 1 is inactive against those ethynes. Stannylenoid 2 reacts smoothly with gaseous ethyne and phenylethyne at room temperature, giving the corresponding diethynylstannanes, di(phenylethynyl)stannane 3 and diethynylstannane 6, respectively, in good yields with the concomitant evolution of H2. Trimethylsilyl-substituted ethynes such as 1-trimethylsilyl-(2-phenyl)ethyne and 1,2-bis(trimethylsilyl)ethyne react similarly to give 3 and bis(trimethylsilylethynyl)stannane 8, respectively. Rather unexpectedly, the reaction of 2 with (trimethylsilyl)ethyne affords 1,2-bis(ethenylstannyl)ethyne 7 in a good yield. The reactions of 2 with methyl and ethyl propynoates give the same products 4 and 5 as those obtained during the reaction of dialkylstannylene 1 without CsF. Pathways involving the nucleophilic attack of cesium acetylide to an ethyne-complexed stannylene were proposed, while the detailed mechanisms remain unknown. The structure of 7 was studied by single crystal X-ray diffraction analysis.

A stable 1,2-disilacyclohexene and its 14-electron palladium(0) complex.

A novel stable cyclic disilene, 1,2-diphenyl-3,3,6,6-tetrakis(trimethylsilyl)-1,2-disilacyclohexene (1), was synthesized via the reaction of the corresponding 1,4-dilithiobutane with phenylchlorosilane. This new strategy would be applicable to the synthesis of stable cyclic disilenes having rather small residual substituents on unsaturated silicon atoms. A stable tricoordinate Y-shaped disilene palladium complex, 2, that was synthesized by the reaction of 1 with bis(tricyclohexylphosphine)palladium was found to have the strongest pi-complex character among known disilene palladium complexes.

Anthryl-substituted trialkyldisilene showing distinct intramolecular charge-transfer transition.

1-Naphthyl-, 9-phenanthryl-, and 9-anthryl-substituted trialkyldisilenes 1-3 were synthesized as the first stable disilenes with single polycyclic aromatic substituents, allowing elucidation of the unprecedented intramolecular charge transfer interaction between disilene pi and aromatic pi systems. Anthryl-substituted disilene 3 having a low-lying pi*(aryl) LUMO showed a distinct ICT absorption band due to the charge transfer from a disilene pi donor to an aromatic pi acceptor.

Unusual bonding in trisilaallene and related heavy allenes.

The synthesis, X-ray and spectroscopic analysis, and unusual bonding and structure among heavy group-14 element congeners of allene (heavy allenes) [R(2)M=M'=MR(2); M = M' = Si (1a), M = Si, M' = Ge (1b), M = Ge, M' = Si (1c), and M = M' = Ge (1d)] are comparatively discussed. Using DFT calculations and PMO theory, the origin of the bent and fluxional skeleton of the heavy allenes is ascribed to the Jahn-Teller distortion associated with the effective pi-sigma* mixing (pi-sigma* distortion). This type of distortion is suggested to be a key concept for qualitative description of the unusual bonding of unsaturated compounds of heavy main-group elements.

Remarkable substituent effects on the activation energy of silylene insertion into silicon-chlorine bonds.

Insertion reactions of dimethylsilylene (Me(2)Si:) into the silicon-chlorine bond of various substituted chlorosilanes have been computationally studied by using DFT calculations with a 6-31++G(d,p) basis set. All of the insertions investigated herein were exothermic (more than 40 kcal mol(-1)) and proceeded via three-membered cyclic transition states (TS) with one substituent of a chlorosilane in the ring plane and two other substituents out of the plane. Among the two possible concerted insertion pathways, the pathway via a TS with a silylene lone-pair orbital approaching a chlorosilane silicon atom was preferred. Important nucleophilic interactions of silylene lone-pair orbitals with anti-bonding sigma orbitals of Si-Y (in plane) were supported by carrying out a natural bond orbital (NBO) analysis of the reaction. A radical pathway through the initial abstraction of chlorine from chlorosilane by the silylene cannot compete with the preferred concerted pathway. Systematic calculations for SiMe(2) insertion into various chlorosilanes (YR(2)SiCl; Y (in-plane substituent) = H, Me, NH(2), OH, F, SiH(3), PH(2), SH and Cl; R (out-of-plane substituent) = H, Me, iPr and tBu) have revealed remarkable substituent effects on activation free energies (DeltaG(double dagger)). In-plane (Y) substituent effects were mostly electronic. Electron-withdrawing substituents accelerate the insertion through enhanced nucleophilic interaction between silylene lone-pair orbitals with the sigma*(Si-Y) orbital at the TS. The DeltaG(double dagger) values correlate with the sigma(I) constants as a scale of the inductive effect of Y. Out-of-plane (R) substituent effects are mainly steric, and bulky substituents increase the activation free energy . Correlation of the DeltaG(double dagger) values with Taft's steric substituent constants (E(s)) was observed. There was no significant difference in the out-of-plane substituent effects between the electron-withdrawing Cl and the electron-donating Me groups.

Introduction of clutch function into a molecular gear system by silane-silicate interconversion.

Introduction of the clutch-declutch mechanism into a new gear system, bis(4-methyl-9-triptycyl)difluorosilane 1, is achieved by the reversible attachment of fluoride ion giving the corresponding fluorosilicate 2. Although the phase isomers of 1 (1(dl) and 1(meso)) cannot be separated because of the equilibrium via a slow gear slippage process (DeltaH(double dagger) = 17.2 +/- 0.2 kcal x mol(-1) and DeltaS(double dagger) = 0.9 +/- 0.9 cal x mol(-1) x K(-1)), 1 works as meshed molecular gears in solution at room temperature. On the other hand, silicate 2 in the solid state has quite an unusual TBP structure having two organic triptycyl groups at the apical positions and three electronegative fluorine atoms at the equatorial positions against the Muetterties rule. Rotation of the two triptycyl groups around Si-C bonds in 2 is facile and independent to each other in solution. Silicate 2 is reverted to the corresponding silane mixture by treating with excess water.