An Anionic Amido-Substituted Seven-Vertex Siliconoid Cluster.

The silanide [Si4 {N(SiMe3 )Dipp}3 ]- (1) transforms into the anionic siliconoid cluster [Si7 {N(SiMe3 )Dipp}3 ]- (2) with four unsubstituted silicon atoms as a contact ion pair with [K([18]crown-6)] in C6 D6 at room temperature within five weeks. Anion 2 was investigated by natural population analysis and visualization of intrinsic atomic orbitals. Magnetically induced current-density calculations of 2 revealed two distinct strong diatropic vortices that sum up in one direction and create a strongly shielded apical silicon atom in 2.

Synthesis and Reactivity of a Neutral Homocyclic Silylene.

Isolation of the neutral homocyclic silylene 2 is possible via amine ligand abstraction with potassium graphite (KC8 ) and subsequent reaction with SiMe3 Cl from a bicyclic silicon(I) amide J. This reaction proceeds via an anionic homoaromatic silicon ring compound 1 as an intermediate. The twofold-coordinated silicon atom in the homocyclic silylene 2 is stabilized by an allyl-type π-electron delocalization. 2 reacts in an oxidative addition with two equivalents of MeOH and in cycloadditions with ethene, phenylacetylene, diphenylacetylene and with 2,3-dimethyl-1,3-butadiene to afford novel functionalized ring compounds.

Reactivity of the Bicyclic Amido-Substituted Silicon(I) Ring Compound Si4 {N(SiMe3 )Mes}4 with FLP-Type Character.

The bicyclic amido-substituted silicon(I) ring compound Si4 {N(SiMe3 )Mes}4 2 (Mes=Mesityl=2,4,6-Me3 C6 H2 ) features enhanced zwitterionic character and different reactivity from the analogous compound Si4 {N(SiMe3 )Dipp}4 1 (Dipp=2,6-i Pr2 C6 H3 ) due to the smaller mesityl substituents. In a reaction with the N-heterocyclic carbene NHC Me 4 (1,3,4,5-tetramethyl-imidazol-2-ylidene), we observe adduct formation to give Si4 {N(SiMe3 )Mes}4 ⋅ NHC Me 4 (3). This adduct reacts further with the Lewis acid BH3 to yield the Lewis acid-base complex Si4 {N(SiMe3 )Mes}4 ⋅ NHC Me 4 ⋅ BH3 (4). Coordination of AlBr3 to 2 leads to the adduct 5. Calculated proton affinities and fluoride ion affinities reveal highly Lewis basic and very weak Lewis acidic character of the low-valent silicon atoms in 1 and 2. This is confirmed by protonation of 1 and 2 with Brookharts acid yielding 6 and 7. Reaction with diphenylacetylene only occurs at 111 °C with 2 in toluene and is accompanied by fragmentation of 2 to afford the silacyclopropene 8 and the trisilanorbornadiene species 9.

Cycloadditions with a Stable Charge-Separated Cyclobutadiene-Type Amido-Substituted Silicon Ring Compound.

Reductive debromination of {N(SiMe3 )2 }SiBr3 with Rieke magnesium results in the formation of the five-vertex silicon cluster with one bromine substituent Si5 {N(SiMe3 )2 }5 Br, 1, and the cyclobutadiene analogue 2 in a 1:1 ratio. The latter features a planar four-membered silicon ring with a charge-separated electronic situation. Two silicon atoms in 2 are trigonal planar and the other two trigonal pyramidal. In cycloadditions with ethylene, diethylacetylene, 1,5-cyclooctadiene, and 2,3-dimethyl-1,3-butadiene cyclic unsaturated ring compounds (3-6) were formed at room temperature in quantitative reactions. Two of the products (3 and 6) show photochemical isomerization with LED light (λ=405 nm) to afford saturated ring compounds 4 e and 6'.

Reactions of a Silicon-Based Cyclic Silylone with Chalcogens.

Tetrasilasilylone N is composed of a silicon(0) atom, two NHC-coordinated silylenes, and a silaimine (Si═N) group. It reacts rapidly and selectively with the chalcogens selenium and tellurium to afford corresponding dichalcogenides 1 and 2 in high yields of 69-79%. In these compounds, one chalcogen atom is connected to the silicon(0) atom with a short bond in the range of Si═E double bonds. The second chalcogen atom adopts the bridging position between the Si(0) atom and the Si═N group. Using only 1 equiv of tellurium affords the selective formation of monotelluride 3 in 84% yield. In this monotelluride, the tellurium atom adopts a bridging position between the silylone functionality and the Si═N group.

Facile Access to an NHC-Coordinated Silicon Ring Compound with a Si=N Group and a Two-Coordinate Silicon Atom.

Reaction of the bicyclo[1.1.0]tetrasilatetraamide Si4 {N(SiMe3 )Dipp}4 1 (Dipp=2,6-diisopropylphenyl) with 5 equiv of the N-heterocyclic carbene NHCMe4 (1,3,4,5-tetramethylimidazol-2-ylidene) affords a bifunctional carbene-coordinated four-membered-ring compound with a Si=N group and a two-coordinate silicon atom Si4 {N(SiMe3 )Dipp}2 (NHCMe4 )2 (NDipp) 2. When 2 reacts with 0.25 equiv sulfur (S8 ), two sulfur atoms add to the divalent silicon atom in plane and perpendicular to the plane of the Si4 ring, which confirms the silylone character of the two-coordinate silicon atom in 2.

Diradicaloid or Zwitterionic Character: The Non-Tetrahedral Unsaturated Compound [Si4 {N(SiMe3 )Dipp}4 ] with a Butterfly-type Si4 Substructure.

The reduction of the tribromoamidosilane {N(SiMe3 )Dipp}SiBr3 (Dipp=2,6-iPr2 C6 H3 ) with potassium graphite or magnesium resulted in the formation of [Si4 {N(SiMe3 )Dipp}4 ] (1), a bicyclo[1.1.0]tetrasilatetraamide. The Si4 motif in 1 does not adopt a tetrahedral substructure and exhibits two three-coordinate and two four-coordinate silicon atoms. The electronic situation on the three-coordinate silicon atoms is rationalized with positive and negative polarization based on EPR analysis, magnetization measurements, and DFT calculations as well as 29 Si CP MAS NMR and multinuclear NMR spectroscopy in solution. Reactivity studies with 1 and radical scavengers confirmed the partial charge separation. Compound 1 reacts with sulfur to give a novel type of silicon sulfur cage compound substituted with an amido ligand, [Si4 S3 {N(SiMe3 )Dipp}4 ] (2).

A highly unsaturated six-vertex amido-substituted silicon cluster.

Thermal treatment of the bicyclo[1.1.0]tetrasilatetraamide [Si4{N(SiMe3)Dipp}4] 1 resulted in the formation of a highly unsaturated six-vertex silicon cluster [Si6{N(SiMe3)Dipp}4] 2 with only four amine-substituents and two ligand-free silicon atoms. In solution, a major and a minor conformer of this cluster are in equilibrium according to multinuclear NMR spectroscopy, lineshape analysis, DFT calculations and molecular dynamics simulations. The bonding situation in the highly unsaturated cluster features lone pair type character at the ligand-free silicon atoms and partial single and double bond character in the upper butterfly-shaped ring of 2. This allows to consider 2 as the silicon analogue of a butalene isomer.