Importance of π–σ* Orbital Mixing in Bonding and Structure of Bicyclo[1.1.0]tetrasilane and Related Compounds

The structural diversity of bicyclo[1.1.0]­tetrasilane has been analyzed using perturbation MO theory and DFT calculations. Among its four planar-<i>cis</i> (<i>C</i>_2<i>v</i>), planar-<i>trans</i> (<i>C</i>_2<i>h</i>), long-bond (<i>C</i>_2<i>v</i>), and short-bond isomers (<i>C</i>_2<i>v</i>) found theoretically, the first two are derived as the results of second-order Jahn–Teller distortion of the <i>D</i>_2<i>h</i> isomer associated with the π–σ* orbital mixing. The last two are formed through orbital mixing in an isomer with a folded ring. The orbital mixing of the two σ*­(Si–H) orbitals at the 1- and 3-silicon atoms into a π-type orbital between the atoms plays a crucial role in stabilizing the system by causing pyramidalization at the silicon atoms and by introducing the inverted σ-bond nature between the atoms. The π–σ* orbital mixing model is applied to predicting the structure of related compounds

New Isolable Dialkylsilylene and Its Isolable Dimer That Equilibrate in Solution

The new isolable dialkylsilylene <b>3</b> bearing a bidentate alkyl substituent was synthesized. Recrystallization of silylene <b>3</b> gave yellow crystals of <b>3</b> and orange-red crystals of tetraalkyldisilene <b>4</b>, a dimer of <b>3</b>. In the solid state, <b>3</b> exists as a monomer with a closest distance of 6.745 Å between dicoordinate silicon atoms, while disilene <b>4</b> has a remarkably long SiSi double bond distance of 2.252 Å. An equilibrium between <b>3</b> and <b>4</b> in solution was observed by NMR and UV–vis spectroscopies, and the thermodynamic parameters of the equilibrium were estimated to be Δ<i>H</i> = −36 ± 3 kJ mol^–1 and Δ<i>S</i> = −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 <b>3</b> and <b>4</b> allows the reversible dimerization of silylene <b>3</b> to disilene <b>4</b> and the isolation of both species

New Isolable Dialkylsilylene and Its Isolable Dimer That Equilibrate in Solution

The new isolable dialkylsilylene <b>3</b> bearing a bidentate alkyl substituent was synthesized. Recrystallization of silylene <b>3</b> gave yellow crystals of <b>3</b> and orange-red crystals of tetraalkyldisilene <b>4</b>, a dimer of <b>3</b>. In the solid state, <b>3</b> exists as a monomer with a closest distance of 6.745 Å between dicoordinate silicon atoms, while disilene <b>4</b> has a remarkably long SiSi double bond distance of 2.252 Å. An equilibrium between <b>3</b> and <b>4</b> in solution was observed by NMR and UV–vis spectroscopies, and the thermodynamic parameters of the equilibrium were estimated to be Δ<i>H</i> = −36 ± 3 kJ mol^–1 and Δ<i>S</i> = −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 <b>3</b> and <b>4</b> allows the reversible dimerization of silylene <b>3</b> to disilene <b>4</b> and the isolation of both species

Siloxy-Substituted Cyclopentadiene Showing Aggregation-Enhanced Emission: An Application of Cycloaddition of Isolable Dialkylsilylene

Cycloaddition of an isolable dialkylsilylene converted nonemissive 2,3,4,5-tetraphenylcyclopentadienone to an emissive siloxycyclopentadiene, which shows aggregation-enhanced emission behavior with a light blue fluorescence (λ_em = 474 nm, Φ_F = 0.11) in the solid state rather than in solution

Siloxy-Substituted Cyclopentadiene Showing Aggregation-Enhanced Emission: An Application of Cycloaddition of Isolable Dialkylsilylene

Cycloaddition of an isolable dialkylsilylene converted nonemissive 2,3,4,5-tetraphenylcyclopentadienone to an emissive siloxycyclopentadiene, which shows aggregation-enhanced emission behavior with a light blue fluorescence (λ_em = 474 nm, Φ_F = 0.11) in the solid state rather than in solution

Insertion of an Isolable Dialkylstannylene into C–Cl Bonds of Acyl Chlorides Giving Acyl(chloro)stannanes

The reactions of isolable dialkylstannylene <b>1</b> with 1-adamantanoyl, 2,2-dimethylpropanoyl, benzoyl, and substituted benzoyl chlorides afford the corresponding acyl­(chloro)­stannanes in good yields. Similar reactions with more reactive acetyl and propanoyl chlorides do not give the corresponding insertion products but the corresponding dichlorostannane by the overreaction. The benzoyl­(chloro)­stannane reacts with acetyl chloride to afford the corresponding 1,2-dione and the dichlorostannane quantitatively. Acyl­(chloro)­stannanes obtained were fully characterized by multinuclear NMR spectroscopy, high-resolution mass spectrometry, and by single-crystal X-ray diffraction studies

Reactions of an Isolable Dialkylstannylene with Carbon Disulfide and Related Heterocumulenes

The reaction of isolable dialkylstannylene <b>1</b> with an excess amount of CS₂ produces an isomeric mixture of 3,3′-distanna-2,2′,4,4′-tetra­thia­bicyclo­butylidene <b>8</b> and 3,7-distanna-2,4,6,8-tetra­thia­bicyclo­[3.3.0]­oct-1(5)-ene <b>9</b> with a ratio depending on the reaction conditions. Compounds <b>8</b> and <b>9</b> are separated by column chromatography and characterized by NMR spectroscopy and X-ray crystallography. Detailed investigation of the reaction has revealed that the initial product is <b>8</b>, which isomerizes to <b>9</b> irreversibly under the catalytic influence of <b>1</b> as a Lewis acid. The above view is supported by the theoretical DFT calculations. Treatment of <b>1</b> with ArNCO [Ar = 2,6-<i>i</i>Pr₂C₆H₃] affords the corresponding carbamoyl­(hydroxyl)­stannane <b>11</b> via the hydrolysis of the corresponding sila­aziridinone formed by the [1 + 2] cycloaddition reaction of <b>1</b> with the NC double bond of the isocyanate. Stannylene <b>1</b> reacts with ArNCS, giving a mixture of complex products, while <b>1</b> does not react with CO₂

Reactions of an Isolable Dialkylstannylene with Propynoates and Benzyne

The reactions of stable monomeric dialkylstannylene <b>1</b> with methyl and ethyl propynoates give the corresponding 1:2 adducts, alkenyl­(alkynyl)­stannane <b>2</b> and <b>3</b> in high yields, while <b>1</b> does not react with parent acetylene or common mono- and disubstituted acetylenes such as phenylacetylene, trimethylsilylacetylene, diethyl 2-butynedioate, etc. Notably, <b>2</b> and <b>3</b> have the <i>Z</i>-configuration of the alkenyl moieties, in contrast to similar adducts obtained by the known reactions of silylenes with terminal acetylenes. It is suggested that the formation of a carbonyl oxygen-coordinate cyclic zwitterion as a key intermediate is essential for the reactions. Stannylene <b>1</b> adds to in situ generated benzyne, forming a 1:1 adduct having a unique 3-stanna-1-silaindane ring system

Reactions of an Isolable Dialkylstannylene with Propynoates and Benzyne

The reactions of stable monomeric dialkylstannylene <b>1</b> with methyl and ethyl propynoates give the corresponding 1:2 adducts, alkenyl­(alkynyl)­stannane <b>2</b> and <b>3</b> in high yields, while <b>1</b> does not react with parent acetylene or common mono- and disubstituted acetylenes such as phenylacetylene, trimethylsilylacetylene, diethyl 2-butynedioate, etc. Notably, <b>2</b> and <b>3</b> have the <i>Z</i>-configuration of the alkenyl moieties, in contrast to similar adducts obtained by the known reactions of silylenes with terminal acetylenes. It is suggested that the formation of a carbonyl oxygen-coordinate cyclic zwitterion as a key intermediate is essential for the reactions. Stannylene <b>1</b> adds to in situ generated benzyne, forming a 1:1 adduct having a unique 3-stanna-1-silaindane ring system